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What is Gene-Culture Co-Evolution?

What is it?

Gene Culture Co-Evolution is a theory which proposes that genetic and cultural processes interact with one another in order to shape the genome. It is theorized that the human genome, and human behaviors as a result, is a product of the interactions of genetic evolution and cultural evolution.

The field of gene-culture co-evolution was brought to light over 30 years ago and these researchers view genes and culture to be two forms of inheritance that interact with each other with progeny that acquire not only genetic influences from their ancestors, but also cultural legacy. The genetics are expressed throughout development and they influence what the individual learns. Culture, however, will be transmitted through behaviors and particular artifacts that will migrate throughout populations and will modify selection pressures that will influence and shape individuals within populations.

Why is it being studied?

It is thought that, by studying gene-culture co-evolution, we can gain a more thorough understanding of human evolution. There have been recent studies that show, anthropologically, how cultural practices have modified environments and environmental conditions. Allele frequency has been seen to change as a result of these cultural practices and their environmental modifications.

It has recently been seen that genes have experienced positive selection; it has been suggested that this selection is a result of a response to human culture. For example, dairy farming created an environment that positively selected for the spread of alleles for adult tolerance to lactose. This positive selection is thought to result from cultural farming practices. If this positive selection happened in India, for example, then would it also happen in America where the farming practices are so different?

What has it told us so far?

This idea of culturally driven selection, and the data corresponding to it, is consistent with two types of evolutionary analysis: the gene-culture co-evolutionary theory, which has been previously defined, and the niche-construction theory, which looks at the evolutionary impact of the environmental modifications by the individuals or organisms. These models help to explain and understand “the evolution of learning, culture, language, intelligence, cooperation, sex differences and mating systems.”1

Analyses of the gene-culture co-evolutionary theory and the niche-construction theory have helped to confirm that genes and culture can potentially co-evolve and alter evolution. These dynamics are usually faster and have a stronger influence on evolution; they also work over a wider range of conditions. These characteristics lead some to think that gene-culture co-evolution may play a bigger role in human evolution than the conventional evolutionary theories; it could be “the dominant role of human evolution.”1

There have been many instances where we have assumed that human evolution has been shaped by modifications to our external environment; this idea focuses on the assumption that the changes in the environment were a result from events that were beyond human control. For example, theories have said that the Homo species have followed a global trend towards, “cooler, drier climates, which pushed an arboreal ape out of contracting forests into savannah.”1 Another example is heat stress being the reason for the evolution of bipedality, hairless skin, and sweating.1 These examples do not consider the impact of culture and they do not consider the gene-culture-coevolutionary theory. Little consideration has been given to this theory, which is why interest has blossomed in this field.

Models of Gene–culture Co‑evolution

When discussing gene-culture coevolution, culture is defined as behavior learned through social interactions. This notional branch of population genetics speculates that both genes and culture are inherited. Inclinations caused by genes impact what organisms learn. Interactions between an evolving culture and evolving genes are analyzed using gene-culture coevolution. Gene-culture analyses are used to examine inherited personality traits, behavioural traits, and to further investigate topics such as the evolution of language in humans. These analyses are based on speculative genes and speculated functions. Surprisingly, there is often a correlation between recently discovered genes and the analyses.

Selection can be impacted by cultural practices, and it occurs faster than biological evolution that does not involve culture. In East Asia, a gene that is associated with thick hair has recently undergone positive selection. One possible reason this gene, called ectodysplasin A receptor (EDAR) gene, has undergone this change could be due to gene-culture coevolution sexual selection. Culturally learned preferences in regards to thickness of hair have caused some individuals to select partners with thick hair. Using gene-culture coevolution, mathematical models can be used to quantify the changes in allele frequencies due to sexual selection.

Niche‑construction Theory

The Niche-construction theory indicates that organisms are able to change the way that natural selection occurs in their environment. These organisms are one of many factors that are involved in the evolution of other species, as well as their own.


How does this influence evolution?

Niche-construction can impact the evolutionary result, even without culture. An example of the impact niche construction has on evolution can be seen when alleles remain in a population because of niche construction modifications. These changes have allowed organisms to survive in environments that normally would have led to their extinction. When cultural practices cause niche construction, the modifications that result can be very influential, in comparison to niche-construction caused by other factors that do not relate to culture. The niche construction theory has generated the idea that a component of human evolution is gene-culture coevolution. For instance, the domestication of animals by humans , which caused humans to be exposed to animal pathogens, influenced natural selection.

What is counteractive niche-construction?

The niche construction discussed in the previous section is classified as inceptive niche construction. Inceptive niche construction describes a change caused by an organism. When something in the environment causes a change that organisms respond to, it is called counteractive niche-construction. The two types of niche construction are able to work together. In order for humans to occupy a new environment, inceptive niche construction may be necessary. However, in order to survive in the new environment, humans may need to lessen the impact caused by selective pressures using counteractive niche construction. This illustrates how the two types of niche construction work together. Selection caused by cold environments would result in the survival of genes best suited to colder climates. Humans responded to these selection pressures in various ways, including seeking shelter, building fires and developing clothing to keep them warm. This prevented the selection of genes that were not favoured by the climate. This example demonstrates how counteractive niche construction protects organisms from environmental changes that they are not suited for. If human counteractive niche construction depends on culture, the existence of deleterious alleles in the gene pool would be more likely, when compared to their presence in the gene pool when counteractive niche construction was not dependent on culture. This prediction provides an explanation for genetic differences in humans and animals. Humans have a greater ability to respond to environmental changes than animals, allele frequencies in humans may clash with the environment the human is found in. Relating this once again to a cold climate, the animals that occupy the climate have visible modifications, whereas humans may not have any modifications.

Anthropological Evidence

A strong source of evidence of gene-culture co-evolution comes from anthropological and archaeological data.1 Anthropologists study modern and recent populations and their cultural practices, then analyze how these practices have increased selective pressure for particular genotypes within the population. This evidence is important because it not only implicates alleles that are being selected for or against in a population, but also the cultural practices that create these selection environments. There are many examples of cross-discipline studies of culture and genes that give evidence for gene-culture co-evolution.

One of the most compelling examples of an allele that has been maintained through gene-culture co-evolution is the hemoglobin S (Hbs) allele which causes sickle-cell anemia.1 Populations of Kwa speakers of West Africa used a farming technique unique to their cultural group in which they cleared trees from forests to plant yams. This had the unintended effect of reducing drainage of the forest after rains, increasing the amount of standing water near the Kwa speakers' living area. This caused mosquitoes and thus malaria to increase, introducing a strong selective pressure on the population. The Kwa speakers who employed this farming practice saw an increase in the number of individuals with sickle cell anemia since the heterozygous HbS genotype confers malaria resistance. It is clear that the cultural practice of clearing trees led to this gene selection, since populations living near the Kwa speakers who do not clear trees do not have the same increased mutation of the HbS gene.

Many of the culturally caused gene selection events now lead to disease in modern populations. This can be seen in the increase in sickle-cell anemia in some African populations, as well as high rates of Type 2 diabetes in Polynesian populations due to long and difficult ocean voyages, and salt-sensitive hypertension due to "Out of Africa" migration events and differing diet among migratory populations.1

Genetic Evidence

There is abundant genetic evidence that the human genome is still evolving, with researchers having developed ways to scan the human genome for alleles under recent positive selection.1 These scans are not always completely clear in their results, and must be interpreted with caution. However, they do provide a starting point for discovering the genes that have been under selective pressure in humans in the last 100,000 years. After these genes have been isolated, they can then be matched to their phenotype in humans. This is the point at which it becomes possible to assess if the gene has been affected by cultural practices by comparing human populations with different practices or simply taking note of cultural practices that could have an effect on the selection of the gene. This evidence can be compelling, but it is a difficult process to begin with selected genes and try to discover their cultural connections.

One way to facilitate the discovery of connections between culture and evolution is to find categories of genes that are overrepresented in screens for positive selection.1 Some of these include heat shock genes, which likely evolved through dispersal of human populations, and pathogen resistance genes, which likely came from the switch in human populations from a hunter-gatherer to an agricultural lifestyle. The genes abnormal spindle, microcephaly associated (ASPM) and microcephalin 1 (MCPH1) appear in positive selection screens and have a distinctive geographic distribution.1 These alleles co-vary with linguistic tone, meaning that the alleles prevalent in a population may have influenced its spoken language, and vice-versa.

Researchers have also been successful in proposing theoretical models of gene-culture co-evolution and then finding evidence of selection on those genes in human populations. Examples of this include the rise of dairy farming and the associated lactose tolerance, the development of sign language in the deaf community and the connexin deafness gene, and the rise of language concurrent with the spread of genes associated with language.1

Dietary changes have caused many alleles to be selected for in populations.1 An example of this is the human amylase gene, which processes starch. Humans in populations that eat many foods rich in starches have more copies on average of the salivary amylase gene, allowing them to break down and digest their diet more easily. Another example is selection of the alcohol dehydrogenase gene cluster in East Asian populations. Many of them have developed a sensitivity to alcohol, a mutation which could have grown in the population as a defense against alcoholism. Genetic evidence can be misleading at times, but when combined with anthropological understanding of human cultures can be powerful evidence for gene-culture co-evolution.

Dairy Farming and Lactose Tolerance

The rise of lactose tolerance in conjunction with dairy farming shows a relationship between diet and genetic evolution is currently the best-studied example of gene-culture co-evolution.1 In most humans, lactase activity disappears after childhood when the child no longer uses milk as their main food source. However, in some populations from northern Europe, parts of Africa and parts of the Middle East, lactase activity persists into adulthood. Northern Europeans have a single nucleotide polymorphism (SNP) 14kb upstream of the lactase gene which confers lactose resistance, and dairy farming African and Middle Eastern populations have SNPs in similar locations.1 These populations consume animal milk after childhood, which seems to have caused lactose resistance to experience strong positive selection over time. Other populations that eat fermented milk products but do not drink milk by itself have intermediate levels of lactose resistance, suggesting an intermediate selective pressure driving the evolution of their lactase persistance.

Saqqara-dairying.jpg

An ancient Egyptian wall painting, dating from around 2350 B.C.E., which depicts a cow being milked.2

The idea that dairy farming practices led to the rise of lactose resistance in these populations is known as the "culture historical hypothesis."1 Various studies have supported this hypothesis, including extraction of DNA from human ancestors. The DNA of ancient Neolithic Europeans does not contain the lactase mutation, suggesting that levels of lactose resistance in this population were low or nonexistent 7,000-8,000 years ago. This refutes the hypothesis that lactose resistance allowed this population to begin consuming milk, and supports the opposing conclusion that consuming milk caused selection of the lactase persistance mutation.1

A study by Feldman and Cavalli-Sforza lends further evidence that lactose resistance is selected for by learned cultural practices.1 They constructed a theoretical model in which lactase persistance was controlled by a single gene. Whether or not this gene rose to fixation in the population depended largely upon whether the children of those who drank milk also grew up to drink milk. If the children did not continue the cultural practice of milk drinking, it is statistically unrealistic that lactose resistance would accumulate in the population. This study and many others support the idea that modern human evolution is strongly affected by culture, and that interdisciplinary investigation of the subject is needed to fully understand both the historical and modern consequences of culture.

Conclusion

The information provided above provides evidence supporting that humans are still evolving. The human genome is still evolving and research suggests our most recent evolutionary traits have been driven by our culture. It plausible that through the entire history of the human species gene-culture co-evolution has occurred and acted on many human traits."1 The challenge researches face in supporting this statement is that the regions of the genome that have undergo recent selective change only a few have shown to be linked to an adaptive phenotype. Of the few adaptive phenotypes discovered even fewer have can be associated with selection pressures that can be linked to culture. While identify a section of a genome that has undergone change due to selective pressure caused from human culture can be difficult, researchers have been able to do it. The best example of this will be the lactose-tolerance gene. The development of the lactose-tolerance gene in humans shows not only that gene-culture co-evolution exist but how it was established."1 When humans expanded out of Africa and began to inhibit all habitable locations on Earth, human culture quickly diversified among the societies living in these locations. This can be seen with the increase frequency of the sickle cell anemia gene in Africa and Asian societies that cultivate yams. Human culture is cumulative, as our culture advances it is being built upon the tools, technologies and beliefs previously created. Each generation of human will have more culture than the previous suggesting that as culture has increased over time its importance on the human genome has also increased. In the future researchers can evaluate the dependency of one trait on another using phylogenetic methods to determine if cultural or non-cultural factors selected upon the gene. "1 This technique was used to show the lactose gene co-evolved with dairy farming and not latitude. The growing significance of gene-cultural evolution will require models of human evolution that do not factor in gene-culture associations to be replaced so it will be factored in. Using unbiased genome-wide scans could provide new cases of gene-culture co-evolution to explore by theoreticians. "1 In time researchers will be able to expand upon the ideas of gene-culture co-evolution and niche construction by integrating theory and empirical data. This will allow us to better understand the process of human evolution and the causes of genetic variation. "1 As human culture continues to grow and change it will continuously cause the human genome to evolve to be better adapted to its new cultural surrounding, so the human species will never stop evolving.

References

1. Laland, Kevin, Odling-Smee, John, and Sean Myles. "How culture shaped the human genome: bringing genetics and the human sciences together." Nature Reviews 11(2010): 137-148. Web. Feb. 2016.

2. Hirst, K. Kris. "Dairy Farming - The Ancient History of Producing Milk." About Education. June 22 2015. Web. Feb. 2016.