Heredity , also called inheritance or biological inheritance , is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction , the offspring cells or organisms acquire the genetic information of their parents. Through heredity, variations between individuals can accumulate and cause species to evolve by natural selection . The study of heredity in biology is genetics .
137-408: In humans, eye color is an example of an inherited characteristic: an individual might inherit the "brown-eye trait" from one of the parents. Inherited traits are controlled by genes and the complete set of genes within an organism's genome is called its genotype . The complete set of observable traits of the structure and behavior of an organism is called its phenotype . These traits arise from
274-443: A Scandinavian population, the estimated selection coefficient was 0.09-0.19. This implies that the cultural practice of raising cattle first for meat and later for milk led to selection for genetic traits for lactose digestion . Recently, analysis of natural selection on the human genome suggests that civilization has accelerated genetic change in humans over the past 10,000 years. Food processing Culture has driven changes to
411-401: A brown to a green. Although hazel mostly consists of brown and green, the dominant color in the eye can either be brown/gold or green. This is why hazel eyes can be mistaken as amber, and why amber is often counted as hazel in studies, and vice versa. The combination can sometimes produce a multicolored iris, i.e. an eye that is light brown/amber near the pupil and charcoal or dark green on
548-419: A combination of the two. For example, food preferences can result from genetic preferences for sugary or fatty foods and socially-learned eating practices and taboos. Content biases are sometimes called "direct biases." Context biases result from individuals using clues about the social structure of their population to determine what cultural variants to adopt. This determination is made without reference to
685-468: A consequence from the complex cognition that developed. Yet this is still an example of a cultural shift in diet and the resulting genetic evolution. Further criticism comes from the controversy of the archaeological evidence available. Some claim there is a lack of evidence of fire control when brain sizes first started expanding. Wrangham argues that anatomical evidence around the time of the origin of Homo erectus (1.8 million years ago), indicates that
822-425: A darkened, occasionally black region encircling the iris resulting from a manifestation of the optical properties of the corneal limbus . Limbal rings are not present in all individuals, and their thickness and prominence may correlate with health or youthfulness, and contributes to facial attractiveness . Although people with lighter eye color are generally more sensitive to light because they have less pigment in
959-509: A day chewing. This frees up time which can be used for hunting. A raw diet means hunting is constrained since time spent hunting is time not spent eating and chewing plant material, but cooking reduces the time required to get the day's energy requirements, allowing for more subsistence activities. Digestibility of cooked carbohydrates is approximately on average 30% higher than digestibility of non-cooked carbohydrates. This increased energy intake, more free time and savings made on tissue used in
1096-478: A gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism. However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by multiple interacting genes within and among organisms. Developmental biologists suggest that complex interactions in genetic networks and communication among cells can lead to heritable variations that may underlie some of
1233-450: A golf ball or throwing baseballs. In another study, people with darker eyes performed better at hitting racquetballs. There are also other studies that challenge these findings, and more study is needed to verify these results. Dual inheritance theory Dual inheritance theory ( DIT ), also known as gene–culture coevolution or biocultural evolution , was developed in the 1960s through early 1980s to explain how human behavior
1370-466: A highly sexually dimorphic eye color. Studies from various populations in Europe have shown that men are substantially more likely to have blue eyes than women. The inheritance pattern followed by blue eyes was previously assumed to be a Mendelian recessive trait, though this has been shown to be incorrect. Eye color inheritance is now recognized as a polygenic trait , meaning that it is controlled by
1507-417: A lightening of their eye color by adulthood. Children with green eyes often experienced a darkening of their eye color. It was also found that 11% of the children's mothers experienced an eye color change during the same period, with most developing lighter eyes, relative to their original color at the time of their child's birth. Almost all mammals have brown or darkly-pigmented irises. In humans, brown
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#17327761450511644-437: A long history of raising cattle for milk. Until around 7,500 years ago, lactase production stopped shortly after weaning, and in societies which did not develop dairying, such as East Asians and Amerindians, this is still true today. In areas with lactase persistence, it is believed that by domesticating animals, a source of milk became available while an adult and thus strong selection for lactase persistence could occur; in
1781-497: A lower energetic cost than those with larger organs. Cooking is notable because the energy available from food increases when cooked and this also means less time is spent looking for food. Humans living on cooked diets spend only a fraction of their day chewing compared to other extant primates living on raw diets. American girls and boys spent on average 7 to 8 percent of their day chewing respectively (1.68 to 1.92 hours per day), compared to chimpanzees, who spend more than 6 hours
1918-460: A mode of inheritance is also achieved primarily through statistical analysis of pedigree data. In case the involved loci are known, methods of molecular genetics can also be employed. An allele is said to be dominant if it is always expressed in the appearance of an organism (phenotype) provided that at least one copy of it is present. For example, in peas the allele for green pods, G , is dominant to that for yellow pods, g . Thus pea plants with
2055-400: A mostly similar environment but genes have insufficient time to change to changes in the environment. While other species have social learning, and thus some level of culture, only humans, some birds and chimpanzees are known to have cumulative culture. Boyd and Richerson argue that the evolution of cumulative culture depends on observational learning and is uncommon in other species because it
2192-496: A mutation of the gene coding for the enzyme ATPase 7B , which prevents copper within the liver from entering the Golgi apparatus in cells. Instead, the copper accumulates in the liver and in other tissues, including the iris of the eye. This results in the formation of Kayser–Fleischer rings , which are dark rings that encircle the periphery of the iris. Eye color outside of the iris may also be symptomatic of disease. Yellowing of
2329-409: A part of early Lamarckian ideas on evolution. During the 18th century, Dutch microscopist Antonie van Leeuwenhoek (1632–1723) discovered "animalcules" in the sperm of humans and other animals. Some scientists speculated they saw a "little man" ( homunculus ) inside each sperm . These scientists formed a school of thought known as the "spermists". They contended the only contributions of the female to
2466-644: A part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). It is a result of the relative excess or lack of pigment within an iris or part of an iris, which may be inherited or acquired by disease or injury . This uncommon condition usually results due to uneven melanin content. A number of causes are responsible, including genetic, such as chimerism , Horner's syndrome and Waardenburg syndrome . A chimera can have two different colored eyes just like any two siblings can—because each cell has different eye color genes. A mosaic can have two different colored eyes if
2603-573: A population. This effect should be especially strong in small populations. A model by Hahn and Bentley shows that cultural drift gives a reasonably good approximation to changes in the popularity of American baby names. Drift processes have also been suggested to explain changes in archaeological pottery and technology patent applications. Changes in the songs of song birds are also thought to arise from drift processes, where distinct dialects in different groups occur due to errors in songbird singing and acquisition by successive generations. Cultural drift
2740-450: A result of a prestige bias, where individuals that forgo reproduction to gain more influence in industrial societies are more likely to be chosen as cultural models. People have defined the word "culture" to describe a large set of different phenomena. A definition that sums up what is meant by "culture" in DIT is: Culture is socially learned information stored in individuals' brains that
2877-405: A review of competing evolutionary perspectives on human behavior, Laland and Brown see DIT as the best candidate for uniting the other evolutionary perspectives under one theoretical umbrella. Two major topics of study in both sociology and cultural anthropology are human cultures and cultural variation. However, Dual Inheritance theorists charge that both disciplines too often treat culture as
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#17327761450513014-406: A series of 221 photographs of Spanish subjects, 16.3% of the subjects were determined to have blue-gray eyes. Like blue eyes, gray eyes have a dark epithelium at the back of the iris and a relatively clear stroma at the front. One possible explanation for the difference in appearance between gray and blue eyes is that gray eyes have larger deposits of collagen in the stroma, so that the light that
3151-448: A similar relationship to what ecology and evolutionary biology have in the biological sciences. HBE is more concerned about ecological process and DIT more focused on historical process. One difference is that human behavioral ecologists often assume that culture is a system that produces the most adaptive outcome in a given environment. This implies that similar behavioral traditions should be found in similar environments. However, this
3288-409: A solid gold hue. Even though amber is similar to gold, some people have russet- or copper-colored amber eyes that are mistaken for hazel, though hazel tends to be duller and contains green with red/gold flecks, as mentioned above. Amber eyes may also contain amounts of very light gold-ish gray. The eyes of some pigeons contain yellow fluorescing pigments known as pteridines . The bright yellow eyes of
3425-410: A specific skill as in the skill bias.) A "similarity bias" results when individuals are more likely to imitate cultural models that are perceived as being similar to the individual based on specific traits. Frequency-dependent biases result when an individual is biased to choose particular cultural variants based on their perceived frequency in the population. The most explored frequency-dependent bias
3562-448: A static superorganic entity that dictates human behavior. Cultures are defined by a suite of common traits shared by a large group of people. DIT theorists argue that this doesn't sufficiently explain variation in cultural traits at the individual level. By contrast, DIT models human culture at the individual level and views culture as the result of a dynamic evolutionary process at the population level. Evolutionary psychologists study
3699-431: A strong yellowish/golden or russet/coppery tint, which may be due to a yellow pigment called lipochrome (also found in green eyes). Amber eyes should not be confused with hazel eyes. Although hazel eyes may contain specks of amber or gold, they usually tend to have many other colors, including green, brown, and orange. Also, hazel eyes may appear to shift in color and consist of flecks and ripples, while amber eyes are of
3836-607: A study published in Human Genetics that a mutation in the 86th intron of the HERC2 gene, which is hypothesized to interact with the OCA2 gene promoter , reduced expression of OCA2 with subsequent reduction in melanin production. It has been proposed that blue eyes may have been adaptive to shorter day lengths at higher latitudes, as blue eyes increase intraocular light scattering, which suppresses melatonin release from
3973-574: A total of ten genes, allowing the explanation of about 50% of eye color variation. Blue eyes with a brown spot, green eyes, and gray eyes are caused by an entirely different part of the genome. A 1997 study of White Americans found that eye color may be subject to change in infancy, and from adolescence to adulthood. 17% of children experienced a change of eye color by adulthood. Of those children, 50% of developed lighter eyes as they got older. The other 50% developed darker eyes. Generally, children with hazel and light brown eyes tended to experience
4110-429: A variant form, causes the pink eye color and hypopigmentation common in human albinism . (The name of the gene is derived from the disorder it causes, oculocutaneous albinism type II.) Different SNPs within OCA2 are strongly associated with blue and green eyes as well as variations in freckling , mole counts, hair and skin tone . The polymorphisms may be in an OCA2 regulatory sequence , where they may influence
4247-400: A variety of ideas about heredity: Theophrastus proposed that male flowers caused female flowers to ripen; Hippocrates speculated that "seeds" were produced by various body parts and transmitted to offspring at the time of conception; and Aristotle thought that male and female fluids mixed at conception. Aeschylus , in 458 BC, proposed the male as the parent, with the female as a "nurse for
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4384-511: A while, like genes do. Ideas are transformed every time they are passed from mind to mind, because the sender's message is being interpreted by evolved inference systems in the receiver. It is useful for some applications to note, however, that there are ways to pass ideas which are more resilient and involve substantially less mutation, such as by mass distribution of printed media. There is no necessary contradiction between evolutionary psychology and DIT, but evolutionary psychologists argue that
4521-888: A younger generation; and horizontal transmission , passing traits between members of the same population. The next significant DIT publication was Robert Boyd and Peter Richerson 's 1985 Culture and the Evolutionary Process . This book presents the now-standard mathematical models of the evolution of social learning under different environmental conditions, the population effects of social learning, various forces of selection on cultural learning rules, different forms of biased transmission and their population-level effects, and conflicts between cultural and genetic evolution. The book's conclusion also outlined areas for future research that are still relevant today. In their 1985 book, Boyd and Richerson outlined an agenda for future DIT research. This agenda, outlined below, called for
4658-400: Is X-inactivation , which can result in a number of heterochromatic traits, such as calico cats . Trauma and certain medications, such as some prostaglandin analogues , can also cause increased pigmentation in one eye. On occasion, a difference in eye color is caused by blood staining the iris after injury. The limbal ring is also a feature of the iris contributing to eye color, appearing as
4795-587: Is a difference in academic lineage. Memetics as a label is more influential in popular culture than in academia. Critics of memetics argue that it is lacking in empirical support or is conceptually ill-founded, and question whether there is hope for the memetic research program succeeding. Proponents point out that many cultural traits are discrete, and that many existing models of cultural inheritance assume discrete cultural units, and hence involve memes. Israeli psychologist Liane Gabora has criticised DIT. She argues that traits that are not transmitted by way of
4932-502: Is a product of two different and interacting evolutionary processes: genetic evolution and cultural evolution . Genes and culture continually interact in a feedback loop: changes in genes can lead to changes in culture which can then influence genetic selection, and vice versa. One of the theory's central claims is that culture evolves partly through a Darwinian selection process, which dual inheritance theorists often describe by analogy to genetic evolution. 'Culture', in this context,
5069-436: Is also associated with an increased risk of ARMD progression. A gray iris may indicate the presence of a uveitis , and an increased risk of uveal melanoma has been found in those with blue, green or gray eyes. However, a study in 2000 suggests that people with dark brown eyes are at increased risk of developing cataracts and therefore should protect their eyes from direct exposure to sunlight. Wilson's disease involves
5206-519: Is also observed in an early computer model of cultural evolution. Cultural traits may be gained in a population through the process of individual learning. Once an individual learns a novel trait, it can be transmitted to other members of the population. The process of guided variation depends on an adaptive standard that determines what cultural variants are learned. Understanding the different ways that culture traits can be transmitted between individuals has been an important part of DIT research since
5343-615: Is also understood to have many potential biases , including success bias (copying from those who are perceived to be better off), status bias (copying from those with higher status), homophily (copying from those most like ourselves), conformist bias (disproportionately picking up behaviors that more people are performing), etc. Understanding social learning is a system of pattern replication, and understanding that there are different rates of survival for different socially learned cultural variants, this sets up, by definition, an evolutionary structure: cultural evolution. Because genetic evolution
5480-525: Is by far the most common eye color, with approximately 79% of people in the world having it. Brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed. In many parts of the world, it is nearly the only iris color present. Brown eyes are common in Europe , East Asia , Southeast Asia , Central Asia , South Asia , West Asia , Oceania , West Africa and
5617-533: Is capable of affecting behavior. This view of culture emphasizes population thinking by focusing on the process by which culture is generated and maintained. It also views culture as a dynamic property of individuals, as opposed to a view of culture as a superorganic entity to which individuals must conform. This view's main advantage is that it connects individual-level processes to population-level outcomes. Genes affect cultural evolution via psychological predispositions on cultural learning. Genes encode much of
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5754-490: Is common in cultural evolution. Analysis of small groups in New Guinea imply that cultural group selection might be a good explanation for slowly changing aspects of social structure, but not for rapidly changing fads. The ability of cultural evolution to maintain intergroup diversity is what allows for the study of cultural phylogenetics. In 1876, Friedrich Engels wrote a manuscript titled The Part Played by Labour in
5891-488: Is complicated. As of 2010 , as many as 16 genes have been associated with eye color inheritance. Some of the eye-color genes include OCA2 and HERC2 . The earlier belief that blue eye color is a recessive trait has been shown to be incorrect, and the genetics of eye color are so complex that almost any parent-child combination of eye colors can occur. Eye color is an inherited trait determined by multiple genes . These genes are sought by studying small changes in
6028-412: Is defined as 'socially learned behavior', and 'social learning' is defined as copying behaviors observed in others or acquiring behaviors through being taught by others. Most of the modelling done in the field relies on the first dynamic (copying), though it can be extended to teaching. Social learning , at its simplest, involves blind copying of behaviors from a model (someone observed behaving), though it
6165-402: Is growing evidence that there is transgenerational inheritance of epigenetic changes in humans and other animals. The description of a mode of biological inheritance consists of three main categories: These three categories are part of every exact description of a mode of inheritance in the above order. In addition, more specifications may be added as follows: Determination and description of
6302-465: Is ineffective when it is rare in a population. They propose that the environmental changes occurring in the Pleistocene may have provided the right environmental conditions. Michael Tomasello argues that cumulative cultural evolution results from a ratchet effect that began when humans developed the cognitive architecture to understand others as mental agents. Furthermore, Tomasello proposed in
6439-593: Is more adaptive to a given environment. Random variation arises from errors in the learning, display or recall of cultural information, and is roughly analogous to the process of mutation in genetic evolution. Cultural drift is a process roughly analogous to genetic drift in evolutionary biology. In cultural drift, the frequency of cultural traits in a population may be subject to random fluctuations due to chance variations in which traits are observed and transmitted (sometimes called "sampling error"). These fluctuations might cause cultural variants to disappear from
6576-514: Is not always the case. A study of African cultures showed that cultural history was a better predictor of cultural traits than local ecological conditions. Memetics , which comes from the meme idea described in Dawkins's The Selfish Gene , is similar to DIT in that it treats culture as an evolutionary process that is distinct from genetic transmission. However, there are some philosophical differences between memetics and DIT. One difference
6713-426: Is people with the inherited trait of albinism , who do not tan at all and are very sensitive to sunburn . Heritable traits are known to be passed from one generation to the next via DNA , a molecule that encodes genetic information. DNA is a long polymer that incorporates four types of bases , which are interchangeable. The Nucleic acid sequence (the sequence of bases along a particular DNA molecule) specifies
6850-485: Is reflected from the epithelium undergoes Mie scattering (which is not strongly frequency-dependent) rather than Rayleigh scattering (in which shorter wavelengths of light are scattered more). This would be analogous to the change in the color of the sky, from the blue given by the Rayleigh scattering of sunlight by small gas molecules when the sky is clear, to the gray caused by Mie scattering of large water droplets when
6987-485: Is relatively well understood, most of DIT examines cultural evolution and the interactions between cultural evolution and genetic evolution. DIT holds that genetic and cultural evolution interacted in the evolution of Homo sapiens . DIT recognizes that the natural selection of genotypes is an important component of the evolution of human behavior and that cultural traits can be constrained by genetic imperatives. However, DIT also recognizes that genetic evolution has endowed
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#17327761450517124-408: Is that developmental biology (' evo-devo ') played little part in the synthesis, but an account of Gavin de Beer 's work by Stephen Jay Gould suggests he may be an exception. Almost all aspects of the synthesis have been challenged at times, with varying degrees of success. There is no doubt, however, that the synthesis was a great landmark in evolutionary biology. It cleared up many confusions, and
7261-417: Is that memetics' focus is on the selection potential of discrete replicators (memes), where DIT allows for transmission of both non-replicators and non-discrete cultural variants. DIT does not assume that replicators are necessary for cumulative adaptive evolution. DIT also more strongly emphasizes the role of genetic inheritance in shaping the capacity for cultural evolution. But perhaps the biggest difference
7398-424: Is the "conformity bias." Conformity biases result when individuals attempt to copy the mean or the mode cultural variant in the population. Another possible frequency dependent bias is the "rarity bias." The rarity bias results when individuals preferentially choose cultural variants that are less common in the population. The rarity bias is also sometimes called a "nonconformist" or "anti-conformist" bias. In DIT,
7535-478: Is the third-rarest natural eye color after green and gray, occurring in 5% of the world's population. People with amber-colored eyes are found in Europe , and in fewer numbers in the Middle East , North Africa , and South America . Hazel eyes are due to a combination of Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer. Hazel eyes often appear to shift in color from
7672-413: Is therefore a major factor in the expression of eye color genotypes. One study suggested that women's higher levels of the sex hormone estrogen may explain why women tend to have darker eyes than men. People of European descent show the greatest variety in eye color of any population worldwide. Recent advances in ancient DNA technology have revealed some of the history of eye color in Europe. Through
7809-518: The Americas . Light or medium-pigmented brown eyes can also be commonly found in Europe , among the Americas , and parts of Central Asia , West Asia , South Asia , and East Africa . Light brown eyes bordering amber and hazel coloration are more common in Europe , but can also be observed in East Asia , Southeast Asia , North Africa and East Africa . Amber eyes are a solid color with
7946-587: The Homo lineage. In DIT, the evolution and maintenance of cultures is described by five major mechanisms: natural selection of cultural variants, random variation, cultural drift, guided variation and transmission bias. Differences between cultural phenomena result in differential rates of their spread; similarly, cultural differences among individuals can lead to differential survival and reproduction rates of individuals. The patterns of this selective process depend on transmission biases and can result in behavior that
8083-513: The Moravian monk Gregor Mendel who published his work on pea plants in 1865. However, his work was not widely known and was rediscovered in 1901. It was initially assumed that Mendelian inheritance only accounted for large (qualitative) differences, such as those seen by Mendel in his pea plants – and the idea of additive effect of (quantitative) genes was not realised until R.A. Fisher 's (1918) paper, " The Correlation Between Relatives on
8220-849: The Netherlands and the United Kingdom , and have also been observed to be very common among the Low Saxon -speaking populations of northern Germany. Green eyes probably result from the interaction of multiple allelic variants of OCA2 and other genes. They may have been present in southern Siberia during the Bronze Age . Green eyes are most common in Northern , Western , and Central Europe . Around 8–10% of men and 18–21% of women in Iceland and 6% of men and 17% of women in
8357-408: The Netherlands have green eyes. Among European Americans , green eyes are most common among those of recent Celtic and Germanic ancestry, occurring in about 16% of people with those backgrounds. The green color is caused by the combination of: 1) an amber or light brown pigmentation in the stroma of the iris (which has a low or moderate concentration of melanin), and 2) a blue shade created by
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#17327761450518494-453: The great horned owl are thought to be due to the presence of the pteridine pigment xanthopterin within certain chromatophores (called xanthophores) located in the iris stroma. In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin , also known as lipochrome. Many animals such as canines, domestic cats, owls, eagles, pigeons, and fish have amber eyes, whereas in humans this color occurs less frequently. Amber
8631-473: The iris pigment epithelium (located on the back of the iris), the melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma. The appearance of blue, green, and hazel eyes results from the Tyndall scattering of light in the stroma, a phenomenon similar to Rayleigh scattering which accounts for the blue sky. Neither blue nor green pigments are present in
8768-542: The pineal gland , perhaps reducing psychological depression (which is linked to the short day length of higher latitudes). Blue eyes are predominant in northern and eastern Europe, particularly around the Baltic Sea . Blue eyes are also found in Southern Europe , Central Asia , South Asia , North Africa , and West Asia . Approximately 8% to 10% of the global population have blue eyes. A 2002 study found
8905-407: The sclera (the "whites of the eyes") is associated with jaundice , and may be symptomatic of liver diseases such as cirrhosis or hepatitis . A blue coloration of the sclera may also be symptomatic of disease. Aniridia is a congenital condition characterized by an extremely underdeveloped iris, which appears absent on superficial examination. Normally, there is a thick layer of melanin on
9042-479: The 1960s, Donald T. Campbell published some of the first theoretical work that adapted principles of evolutionary theory to the evolution of cultures. In 1976, two developments in cultural evolutionary theory set the stage for DIT. In that year Richard Dawkins's The Selfish Gene introduced ideas of cultural evolution to a popular audience. Although one of the best-selling science books of all time, because of its lack of mathematical rigor, it had little effect on
9179-567: The 1970s. Transmission biases occur when some cultural variants are favored over others during the process of cultural transmission. Boyd and Richerson (1985) defined and analytically modeled a number of possible transmission biases. The list of biases has been refined over the years, especially by Henrich and McElreath. Content biases result from situations where some aspect of a cultural variant's content makes them more likely to be adopted. Content biases can result from genetic preferences, preferences determined by existing cultural traits, or
9316-406: The 80s that there are some disparities between the observational learning mechanisms found in humans and great apes - which go some way to explain the observable difference between great ape traditions and human types of culture (see Emulation (observational learning) ). Although group selection is commonly thought to be nonexistent or unimportant in genetic evolution, DIT predicts that, due to
9453-526: The DNA difference happens to be in an eye-color gene. There are many other possible reasons for having two different-colored eyes. For example, the film actor Lee Van Cleef was born with one blue eye and one green eye, a trait that reportedly was common in his family, suggesting that it was a genetic trait. This anomaly, which film producers thought would be disturbing to film audiences, was "corrected" by having Van Cleef wear brown contact lenses. David Bowie , on
9590-518: The Homo lineage started to increase well before the advent of cooking due to increased consumption of meat and that basic food processing (slicing) accounts for the size reduction in organs related to chewing. Cornélio et al. argues that improving cooperative abilities and a varying of diet to more meat and seeds improved foraging and hunting efficiency. It is this that allowed for the brain expansion, independent of cooking which they argue came much later,
9727-572: The Origin of Species and his later biological works. Darwin's primary approach to heredity was to outline how it appeared to work (noticing that traits that were not expressed explicitly in the parent at the time of reproduction could be inherited, that certain traits could be sex-linked , etc.) rather than suggesting mechanisms. Darwin's initial model of heredity was adopted by, and then heavily modified by, his cousin Francis Galton , who laid
9864-456: The Rayleigh scattering of reflected light. Green eyes contain the yellowish pigment lipochrome . There is no intrinsically blue pigmentation either in the iris or in the vitreous body . Rather, blue eyes result from structural color in combination with certain concentrations of non-blue pigments. The iris pigment epithelium is brownish black due to the presence of melanin . Unlike brown eyes, blue eyes have low concentrations of melanin in
10001-500: The Supposition of Mendelian Inheritance " Mendel's overall contribution gave scientists a useful overview that traits were inheritable. His pea plant demonstration became the foundation of the study of Mendelian Traits. These traits can be traced on a single locus. In the 1930s, work by Fisher and others resulted in a combination of Mendelian and biometric schools into the modern evolutionary synthesis . The modern synthesis bridged
10138-473: The Transition from Ape to Man , accredited as a founding document of DIT; “The approach to gene-culture coevolution first developed by Engels and developed later on by anthropologists…” is described by Stephen Jay Gould as “…the best nineteenth-century case for gene-culture coevolution.” The idea that human cultures undergo a similar evolutionary process as genetic evolution also goes back to Darwin . In
10275-407: The alleles in an organism. Eye color Eye color is a polygenic phenotypic trait determined by two factors: the pigmentation of the eye 's iris and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris . In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of melanin in
10412-449: The amount of deference shown to a potential cultural model by other individuals. A "skill bias" results when individuals can directly observe different cultural models performing a learned skill and are more likely to imitate cultural models that perform better at the specific skill. A "success bias" results from individuals preferentially imitating cultural models that they determine are most generally successful (as opposed to successful at
10549-663: The analysis of ancient DNA, a 2020 study published in Experimental Dermatology suggested that the common gene for blue eye color likely originated in the Near East and arrived in Europe around 42,000 years ago, after the exodus out of Africa . There is evidence that as many as 16 different genes could be responsible for eye color in humans; however, the main two genes associated with eye color variation are OCA2 and HERC2 , and both are localized in chromosome 15 . The gene OCA2 (OMIM: 203200 ), when in
10686-424: The back of the iris. Even people with the lightest blue eyes, with no melanin on the front of the iris at all, have dark brown coloration on the back of it, to prevent light from scattering around inside the eye. In those with milder forms of albinism , the color of the iris is typically blue but can vary from blue to brown. In severe forms of albinism, there is no pigment on the back of the iris, and light from inside
10823-442: The changes that occur, and innate behaviour is able to deal with the constant environment. In fast changing environments cultural learning would not be useful because what the previous generation knew is now outdated and will provide no benefit in the changed environment, and hence individual learning is more beneficial. It is only in the moderately changing environment where cultural learning becomes useful since each generation shares
10960-518: The content of the variant. There are two major categories of context biases: model-based biases, and frequency-dependent biases. Model-based biases result when an individual is biased to choose a particular "cultural model" to imitate. There are four major categories of model-based biases: prestige bias, skill bias, success bias, and similarity bias. A "prestige bias" results when individuals are more likely to imitate cultural models that are seen as having more prestige . A measure of prestige could be
11097-420: The control of fire and hence cooking occurred. At this time, the largest reductions in tooth size in the entirety of human evolution occurred, indicating that softer foods became prevalent in the diet. Also at this time was a narrowing of the pelvis indicating a smaller gut and also there is evidence that there was a loss of the ability to climb which Wrangham argues indicates the control of fire, since sleeping on
11234-462: The development of DIT. Also in 1976, geneticists Marcus Feldman and Luigi Luca Cavalli-Sforza published the first dynamic models of gene–culture coevolution. These models were to form the basis for subsequent work on DIT, heralded by the publication of three seminal books in the 1980s. The first was Charles Lumsden and E.O. Wilson's Genes, Mind and Culture . This book outlined a series of mathematical models of how genetic evolution might favor
11371-683: The development of both theoretical models and empirical research. DIT has since built a rich tradition of theoretical models over the past two decades. However, there has not been a comparable level of empirical work. In a 2006 interview Harvard biologist E. O. Wilson expressed disappointment at the little attention afforded to DIT: "...for some reason I haven't fully fathomed, this most promising frontier of scientific research has attracted very few people and very little effort." Kevin Laland and Gillian Ruth Brown attribute this lack of attention to DIT's heavy reliance on formal modeling. "In many ways
11508-483: The digestive system allowed for the selection of genes for larger brain size. Despite its benefits, brain tissue requires a large amount of calories, hence a main constraint in selection for larger brains is calorie intake. A greater calorie intake can support greater quantities of brain tissue. This is argued to explain why human brains can be much larger than other apes, since humans are the only ape to engage in food processing. The cooking of food has influenced genes to
11645-431: The direct control of genes include the inheritance of cultural traits , group heritability , and symbiogenesis . These examples of heritability that operate above the gene are covered broadly under the title of multilevel or hierarchical selection , which has been a subject of intense debate in the history of evolutionary science. When Charles Darwin proposed his theory of evolution in 1859, one of its major problems
11782-438: The evolution of culture is dependent on the evolution of social learning. Analytic models show that social learning becomes evolutionarily beneficial when the environment changes with enough frequency that genetic inheritance can not track the changes, but not fast enough that individual learning is more efficient. For environments that have very little variability, social learning is not needed since genes can adapt fast enough to
11919-428: The evolved architecture of the human mind. They see it as composed of many different programs that process information, each with assumptions and procedures that were specialized by natural selection to solve a different adaptive problem faced by our hunter-gatherer ancestors (e.g., choosing mates, hunting, avoiding predators, cooperating, using aggression). These evolved programs contain content-rich assumptions about how
12056-461: The expression of the gene product, which in turn affects pigmentation. A specific mutation within the HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes. Other genes implicated in eye color variation are SLC24A4 and TYR . A 2010 study of eye color variation in hue and saturation values using high-resolution digital full-eye photographs found three new loci for
12193-452: The extent that, research suggests, humans cannot live without cooking. A study on 513 individuals consuming long-term raw diets found that as the percentage of their diet which was made up of raw food and/or the length they had been on a diet of raw food increased, their BMI decreased. This is despite access to many non-thermal processing, like grinding, pounding or heating to 48 °C. (118 °F). With approximately 86 billion neurons in
12330-593: The extremely low quantities of melanin , allowing the blood vessels to show through. In addition, flash photography can sometimes cause a " red-eye effect ", in which the very bright light from a flash reflects off the retina, which is abundantly vascular, causing the pupil to appear red in the photograph. Although the deep blue eyes of some people such as Elizabeth Taylor can appear purple or violet at certain times, "true" violet-colored eyes occur only due to albinism. Eyes that appear red or violet under certain conditions due to albinism occur in less than 1 percent of
12467-448: The eye can pass through the iris to the front. In these cases, the only color seen is the red from the hemoglobin of the blood in the capillaries of the iris. Such albinos have pink eyes, as do albino rabbits, mice, or any other animal with a total lack of melanin. Transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation. The ocular albino also lacks normal amounts of melanin in
12604-441: The field. Since one of the goals of DIT is to explain the distribution of human cultural traits, ethnographic and ethnologic techniques may also be useful for testing hypothesis stemming from DIT. Although findings from traditional ethnologic studies have been used to buttress DIT arguments, thus far there have been little ethnographic fieldwork designed to explicitly test these hypotheses. Herb Gintis has named DIT one of
12741-416: The form of homologous chromosomes , containing a unique combination of DNA sequences that code for genes. The specific location of a DNA sequence within a chromosome is known as a locus . If the DNA sequence at a particular locus varies between individuals, the different forms of this sequence are called alleles . DNA sequences can change through mutations , producing new alleles. If a mutation occurs within
12878-464: The framework for the biometric school of heredity. Galton found no evidence to support the aspects of Darwin's pangenesis model, which relied on acquired traits. The inheritance of acquired traits was shown to have little basis in the 1880s when August Weismann cut the tails off many generations of mice and found that their offspring continued to develop tails. Scientists in Antiquity had
13015-403: The fundamental unit of life is the cell, and not some preformed parts of an organism. Various hereditary mechanisms, including blending inheritance were also envisaged without being properly tested or quantified, and were later disputed. Nevertheless, people were able to develop domestic breeds of animals as well as crops through artificial selection. The inheritance of acquired traits also formed
13152-460: The gap between experimental geneticists and naturalists; and between both and palaeontologists, stating that: The idea that speciation occurs after populations are reproductively isolated has been much debated. In plants, polyploidy must be included in any view of speciation. Formulations such as 'evolution consists primarily of changes in the frequencies of alleles between one generation and another' were proposed rather later. The traditional view
13289-644: The genes themselves and in neighboring genes, called single-nucleotide polymorphisms or SNPs. The total number of genes that contribute to eye color is unknown, but there are a few likely candidates. A study in Rotterdam (2009) found that it was possible to predict eye color with more than 90% accuracy for brown and blue using just six SNPs. In humans, eye color is a highly sexually dimorphic trait. Several studies have shown that men are more likely to have blue eyes than women, while women are more likely to have darker eye colors (green and brown eyes) than men. Sex
13426-515: The genetic information: this is comparable to a sequence of letters spelling out a passage of text. Before a cell divides through mitosis , the DNA is copied, so that each of the resulting two cells will inherit the DNA sequence. A portion of a DNA molecule that specifies a single functional unit is called a gene ; different genes have different sequences of bases. Within cells , the long strands of DNA form condensed structures called chromosomes . Organisms inherit genetic material from their parents in
13563-400: The ground needs fire to ward off predators. The proposed increases in brain size from food processing will have led to a greater mental capacity for further cultural innovation in food processing which will have increased digestive efficiency further providing more energy for further gains in brain size. This positive feedback loop is argued to have led to the rapid brain size increases seen in
13700-449: The human brain and 60–70 kg body mass, an exclusively raw diet close to that of what extant primates have would be not viable as, when modelled, it is argued that it would require an infeasible level of more than nine hours of feeding every day. However, this is contested, with alternative modelling showing enough calories could be obtained within 5–6 hours per day. Some scientists and anthropologists point to evidence that brain size in
13837-442: The human digestive systems making many digestive organs, such as teeth or stomach, smaller than expected for primates of a similar size, and has been attributed to one of the reasons why humans have such large brains compared to other great apes. This is due to food processing. Early examples of food processing include pounding, marinating and most notably cooking. Pounding meat breaks down the muscle fibres, hence taking away some of
13974-433: The human iris or vitreous humour . This is an example of structural color , which depends on the lighting conditions, especially for lighter-colored eyes. The brightly colored eyes of many bird species result from the presence of other pigments, such as pteridines , purines , and carotenoids . Humans and other animals have many phenotypic variations in eye color. The genetics and inheritance of eye color in humans
14111-628: The human species with a parallel evolutionary process of cultural evolution. DIT makes three main claims: The human capacity to store and transmit culture arose from genetically evolved psychological mechanisms. This implies that at some point during the evolution of the human species a type of social learning leading to cumulative cultural evolution was evolutionarily advantageous. Social learning processes give rise to cultural evolution. Cultural traits are transmitted differently from genetic traits and, therefore, result in different population-level effects on behavioral variation. Cultural traits alter
14248-517: The information needed to form the human brain. Genes constrain the brain's structure and, hence, the ability of the brain to acquire and store culture. Genes may also endow individuals with certain types of transmission bias (described below). Culture can profoundly influence gene frequencies in a population. Lactase persistence One of the best known examples is the prevalence of the genotype for adult lactose absorption in human populations, such as Northern Europeans and some African societies, with
14385-400: The interaction of the organism's genotype with the environment . As a result, many aspects of an organism's phenotype are not inherited. For example, suntanned skin derives from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to people's children. However, some people tan more easily than others, due to differences in their genotype: a striking example
14522-576: The interactions of several genes. In 2008, a team of researchers from the University of Copenhagen located a single mutation that causes the phenomenon of blue eyes. The research was published in the Journal of Human Genetics . The same DNA sequence of the OCA2 gene among blue-eyed people suggests they may have a single common ancestor. The researchers hypothesized that the OCA2 mutation responsible for blue eyes arose in an individual who lived in
14659-433: The iris to protect them from sunlight, there is little to no evidence that eye color has a direct impact on vision qualities such as visual acuity . However, there is a study that found that dark-eyed people perform better at "reactive-type tasks", which suggests they may have better reaction times. People with light-colored eyes, however, performed better at so-called "self-paced tasks", which include activities like hitting
14796-445: The iris. Gray is the second-rarest natural eye color after green, with 3% of the world's population having it. As a result of heterochromia iridum , it is also possible to have two different eye colors. This occurs in humans and certain breeds of domesticated animals and affects less than 1 percent of the world's population. The eyes of people with severe forms of albinism may appear red under certain lighting conditions owing to
14933-483: The job from the mouth, teeth and jaw. Marinating emulates the action of the stomach with high acid levels. Cooking partially breaks down food making it more easily digestible. Food enters the body effectively partly digested, and as such food processing reduces the work that the digestive system has to do. This means that there is selection for smaller digestive organs as the tissue is energetically expensive, those with smaller digestive organs can process their food but at
15070-512: The lasting effect of this book. The second 1981 book was Cavalli-Sforza and Feldman's Cultural Transmission and Evolution: A Quantitative Approach . Borrowing heavily from population genetics and epidemiology , this book built a mathematical theory concerning the spread of cultural traits. It describes the evolutionary implications of vertical transmission , passing cultural traits from parents to offspring; oblique transmission, passing cultural traits from any member of an older generation to
15207-433: The laws of heredity through compiling data on family phenotypes (nose size, ear shape, etc.) and expression of pathological conditions and abnormal characteristics, particularly with respect to the age of appearance. One of the projects aims was to tabulate data to better understand why certain traits are consistently expressed while others are highly irregular. The idea of particulate inheritance of genes can be attributed to
15344-492: The mechanics in developmental plasticity and canalization . Recent findings have confirmed important examples of heritable changes that cannot be explained by direct agency of the DNA molecule. These phenomena are classed as epigenetic inheritance systems that are causally or independently evolving over genes. Research into modes and mechanisms of epigenetic inheritance is still in its scientific infancy, but this area of research has attracted much recent activity as it broadens
15481-477: The modern era or past environments, can explain human behavior. When faced with a trait that seems maladaptive, some sociobiologists try to determine how the trait actually increases genetic fitness (maybe through kin selection or by speculating about early evolutionary environments). Dual inheritance theorists, in contrast, will consider a variety of genetic and cultural processes in addition to natural selection on genes. Human behavioral ecology (HBE) and DIT have
15618-735: The most complex and potentially rewarding of all approaches, [DIT], with its multiple processes and cerebral onslaught of sigmas and deltas, may appear too abstract to all but the most enthusiastic reader. Until such a time as the theoretical hieroglyphics can be translated into a respectable empirical science most observers will remain immune to its message." Economist Herbert Gintis disagrees with this critique, citing empirical work as well as more recent work using techniques from behavioral economics . These behavioral economic techniques have been adapted to test predictions of cultural evolutionary models in laboratory settings as well as studying differences in cooperation in fifteen small-scale societies in
15755-643: The nature of cultural inheritance, it may be an important force in cultural evolution. Group selection occurs in cultural evolution because conformist biases make it difficult for novel cultural traits to spread through a population (see above section on transmission biases). Conformist bias also helps maintain variation between groups. These two properties, rare in genetic transmission, are necessary for group selection to operate. Based on an earlier model by Cavalli-Sforza and Feldman, Boyd and Richerson show that conformist biases are almost inevitable when traits spread through social learning, implying that group selection
15892-488: The next generation were the womb in which the homunculus grew, and prenatal influences of the womb. An opposing school of thought, the ovists, believed that the future human was in the egg, and that sperm merely stimulated the growth of the egg. Ovists thought women carried eggs containing boy and girl children, and that the gender of the offspring was determined well before conception. An early research initiative emerged in 1878 when Alpheus Hyatt led an investigation to study
16029-696: The northwestern part of the Black Sea region in Europe sometime between 6,000 and 10,000 years ago, during the Neolithic period. However, more recent ancient DNA research has identified human remains much older than the Neolithic period which possess the OCA2 mutation for blue eyes. It is now believed that the OCA2 allele responsible for blue eyes dates back to the migration of modern humans out of Africa roughly 50,000 years ago, and entered Europe from western Asia. Eiberg and colleagues suggested in
16166-428: The other hand, had the appearance of different eye colors due to an injury that caused one pupil to be permanently dilated. Another hypothesis about heterochromia is that it can result from a viral infection in utero affecting the development of one eye, possibly through some sort of genetic mutation. Occasionally, heterochromia can be a sign of a serious medical condition. A common cause in females with heterochromia
16303-499: The outer part of the iris (or vice versa) when observed in sunlight. Definitions of the eye color "hazel" vary: it is sometimes considered to be synonymous with light brown or gold, as in the color of a hazelnut shell. Around 18% of the US population and 5% of the world population have hazel eyes. 55.2% of Spanish subjects in a series of 221 photographs were judged to have hazel eyes. Hazel eyes are found in Europe , most commonly in
16440-400: The pair of alleles either GG (homozygote) or Gg (heterozygote) will have green pods. The allele for yellow pods is recessive. The effects of this allele are only seen when it is present in both chromosomes, gg (homozygote). This derives from Zygosity , the degree to which both copies of a chromosome or gene have the same genetic sequence, in other words, the degree of similarity of
16577-500: The prevalence of blue eye color among the white population in the United States to be 33.8% for those born from 1936 through 1951, compared with 57.4% for those born from 1899 through 1905. As of 2006 , one out of every six Americans, or 16.6% of the total US population, has blue eyes, including 22.3% of whites. The incidence of blue eyes continues to decline among American children. Of Slovenes , 56% have blue/green eyes. In
16714-410: The process of niche construction is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effect that modifies and feeds back into the selection regime of subsequent generations. Descendants inherit genes plus environmental characteristics generated by the ecological actions of ancestors. Other examples of heritability in evolution that are not under
16851-473: The psychology implicit in many DIT models is too simple; evolved programs have a rich inferential structure not captured by the idea of a "content bias". They also argue that some of the phenomena DIT models attribute to cultural evolution are cases of "evoked culture"—situations in which different evolved programs are activated in different places, in response to cues in the environment. Sociobiologists try to understand how maximizing genetic fitness, in either
16988-426: The retina as well, which allows more light than normal to reflect off the retina and out of the eye. Because of this, the pupillary reflex is much more pronounced in albino individuals, and this can emphasize the red eye effect in photographs. Heterochromia ( heterochromia iridum or heterochromia iridis ) is an eye condition in which one iris is a different color from the other (complete heterochromia), or where
17125-436: The scope of heritability and evolutionary biology in general. DNA methylation marking chromatin , self-sustaining metabolic loops , gene silencing by RNA interference , and the three dimensional conformation of proteins (such as prions ) are areas where epigenetic inheritance systems have been discovered at the organismic level. Heritability may also occur at even larger scales. For example, ecological inheritance through
17262-471: The selection of cultural traits and how cultural traits might, in turn, affect the speed of genetic evolution. While it was the first book published describing how genes and culture might coevolve, it had relatively little effect on the further development of DIT. Some critics felt that their models depended too heavily on genetic mechanisms at the expense of cultural mechanisms. Controversy surrounding Wilson's sociobiological theories may also have decreased
17399-800: The sky is cloudy. Alternatively, it has been suggested that gray and blue eyes might differ in the concentration of melanin at the front of the stroma. Gray eyes can also be found among the Algerian Shawia people of the Aurès Mountains in Northwest Africa, in the Middle East / West Asia , Central Asia , and South Asia . In the Iliad , the Greek goddess Athene is said to have gray eyes (γλαυκῶπις). Under magnification, gray eyes exhibit small amounts of yellow and brown color in
17536-479: The social and physical environments under which genetic selection operates. For example, the cultural adoptions of agriculture and dairying have, in humans, caused genetic selection for the traits to digest starch and lactose , respectively. As another example, it is likely that once culture became adaptive, genetic selection caused a refinement of the cognitive architecture that stores and transmits cultural information. This refinement may have further influenced
17673-457: The stroma of the iris, which lies in front of the dark epithelium. Longer wavelengths of light tend to be absorbed by the dark underlying epithelium, while shorter wavelengths are reflected and undergo Rayleigh scattering in the turbid medium of the stroma. This is the same scattering that accounts for the blue appearance of the sky. The result is a " Tyndall blue" structural color that varies with external lighting conditions. Blue eyes are
17810-444: The two major conceptual theories with potential for unifying the behavioral sciences, including economics, biology, anthropology, sociology, psychology and political science. Because it addresses both the genetic and cultural components of human inheritance, Gintis sees DIT models as providing the best explanations for the ultimate cause of human behavior and the best paradigm for integrating those disciplines with evolutionary theory. In
17947-402: The way culture is stored and the biases that govern its transmission. DIT also predicts that, under certain situations, cultural evolution may select for traits that are genetically maladaptive. An example of this is the demographic transition , which describes the fall of birth rates within industrialized societies. Dual inheritance theorists hypothesize that the demographic transition may be
18084-572: The world and other people work. When ideas are passed from mind to mind, they are changed by these evolved inference systems (much like messages get changed in a game of telephone). But the changes are not usually random. Evolved programs add and subtract information, reshaping the ideas in ways that make them more "intuitive", more memorable, and more attention-grabbing. In other words, "memes" (ideas) are not precisely like genes. Genes are normally copied faithfully as they are replicated, but ideas normally are not. It's not just that ideas mutate every once in
18221-482: The world's population. The most important role of melanin in the iris is to protect the eyes from the sun's harmful rays. People with lighter eye colors, such as blue or green, have lessened protection from the sun, and so need greater protection from the sun's rays than those with darker eye colors. Those with lighter iris color have been found to have a higher prevalence of age-related macular degeneration (ARMD) than those with darker iris color; lighter eye color
18358-524: The young life sown within her". Ancient understandings of heredity transitioned to two debated doctrines in the 18th century. The Doctrine of Epigenesis and the Doctrine of Preformation were two distinct views of the understanding of heredity. The Doctrine of Epigenesis, originated by Aristotle , claimed that an embryo continually develops. The modifications of the parent's traits are passed off to an embryo during its lifetime. The foundation of this doctrine
18495-413: Was based on the theory of inheritance of acquired traits . In direct opposition, the Doctrine of Preformation claimed that "like generates like" where the germ would evolve to yield offspring similar to the parents. The Preformationist view believed procreation was an act of revealing what had been created long before. However, this was disputed by the creation of the cell theory in the 19th century, where
18632-514: Was directly responsible for stimulating a great deal of research in the post- World War II era. Trofim Lysenko however caused a backlash of what is now called Lysenkoism in the Soviet Union when he emphasised Lamarckian ideas on the inheritance of acquired traits . This movement affected agricultural research and led to food shortages in the 1960s and seriously affected the USSR. There
18769-416: Was the lack of an underlying mechanism for heredity. Darwin believed in a mix of blending inheritance and the inheritance of acquired traits ( pangenesis ). Blending inheritance would lead to uniformity across populations in only a few generations and then would remove variation from a population on which natural selection could act. This led to Darwin adopting some Lamarckian ideas in later editions of On
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