Effect of Behaviour on Genetic Evolution

Dual inheritance or gene-culture co-evolution theory claim that behaviour at an individual or a group level can affect which genetic traits are transmitted to the next generation. Behaviour can affect genetic evolution in two key ways; via the adaptability driver or Baldwin effect and niche construction.

This essay will discuss these methods of gene-culture co-evolution and some key criticisms of the theory. Memes or culturgens, (the unit of culture) are transmitted via social learning in a great many species but for the purposes of this essay I will focus on humans. Hominins have been transmitting cultural knowledge since at least the advent of tool use in our ancestors 2.5 million years ago (Laland & Brown 2011) and this has had a profound influence on the evolution of our species.

The Baldwin effect or adaptability driver theory explains the benefit of traits “being expressed spontaneously without being learned” (Bateson 2006; 342) Bateson explains that while the majority of individuals in a population may be capable of acquiring a trait via social learning, any individuals who are predisposed to the behaviour without expending energy and time learning would be at an advantage.

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He also notes that the adaptability driver is a more likely theory for the evolution of more complex behavioural traits than the behaviour developing without a social learning origin. This is because behaviours with several steps, such as Galapagos woodpecker finches picking up cactus spines and then probing for insect larvae with them, are unlikely to have spontaneously occurred at all let alone in the correct order. (Bateson 2006). In this scenario the socially learned behaviour of probing for insect larvae with cactus spines has created a selection pressure that did not exist before which in turn affects the genetic evolution of the group,

A key way in which behaviour can affect genetic evolution is niche construction. Niche construction can work in two ways to do this; by creating a new environmental pressure (inceptive niche-construction) or by buffering from an environmental pressure (counter-active niche construction). Organisms achieve these new niches by either altering their environment (perturbation) or by moving to a novel environment, (relocation) (Odling-Smee et al. 2003).

This modern take on evolutionary theory suggests that the behaviour of an individual or group of individuals is often at least partially responsible for the state of their environment and therefore the selection pressures acting upon them. In light of this, niche-construction theorists have suggested that organisms receive two inheritances which affect which genes are passed on to the next generation; a genetic and an ecological inheritance (Laland et al. 2007).

Types of activities and decisions of individuals can spread through social learning to dramatically alter the environment and therefore create a new niche which will then exert selection pressures on the inhabitants. Counter-active niche-construction is extremely obvious in humans who manage to survive in particularly hostile climates due to the use of fur and synthetic fibres as clothing among other cultural innovations. This particular example shows both the perturbation method (in the case of the dramatic change our ancestors experienced during the ice-age) and relocation as our ancestors spread out from East Africa to areas with diverse climatic stresses across the globe.

One particularly notable example of inceptive niche-construction through perturbation in humans is the case of dairy farming and lactose tolerance in adults. Lactase persistence, in contrast to standard mammalian development, continues into adulthood in over 70% of people in northern European populations and certain African tribes (Holden & Mace 1997).

This phenomenon correlates with a long history of agriculture or pastoralism; in other words the appearance of easily accessible nutrition through milk in adulthood. This behaviour is understood to have emerged more than once and have spread between populations (Myles et al. 2005). Hunter-gatherer communities, without this nutritional option, such as the indigenous peoples of Oceania and the Americas show no evidence of the trait (Holden & Mace 1997).

This is clear evidence of humans creating a niche for themselves by the socially learned behaviour of agriculture or pastoralism. This behaviour created a new selection pressure; increased fitness for those individuals who were able to obtain nutrition from milk. A combination of a maximum likelihood model and the fact that adult lactose tolerance seems never to have evolved in a non-milking society (Holden & Mace 1997) suggests that this adaptation did follow the spread of the behaviour rather than occur first and lead to agriculture and pastoralism. So this socially learned behaviour did in fact affect the genetic inheritance of these dairying populations.

Other behavioural innovations have been suggested to have affected the genetic evolution of mankind from the invention of effective projectile hunting weapons which allowed a more energy efficient, less robust body shape to thrive, to the social exile of those who cannot control 'anti-social impulses' and violate their society's moral standards (Richerson & Boyd 2005). However, some of these examples may not be ideal examples of gene-culture co-evolution.

Lactase persistence in a population without the trait has been estimated to need 290 generations to reach modern, northern European levels (Holden & Mace 2005) and very few, if any, moral standards last that long in one community with such obvious fitness benefits to conformists. Less controversial correlations can be seen between human physiology and dietary innovations beyond dairying.

Our ancestors' use of fire to cook food seems to have triggered a wealth of evolutionary changes in our bodies. Humans have distinctly smaller lips, mouths, teeth, jaw muscles, stomachs and intestines than our great ape relatives (Wrangham 2009). This has been attributed to the reduced energy output in consuming cooked food over raw. The human jaw muscle alone shows how dramatic this change is. The MYH16 gene for the predominant myosin heavy chain (MYH) which codes for the muscle fibres in the prominent jaw muscles of other primates was 'inactivated' in the days of early Homo. The reduction is dramatic enough that the type II fibres in our masticatory muscles are eight times smaller than macaques, our raw food eating, primate relatives (Stedman et al. 2004).

The lack of these large muscles allowed for increased cranial capacity and brain size to develop (Stedman et al. 2004) and so cooking as a behaviour arguably had a long reaching effect on human intellectual capability as well as affecting the evolution of the shape of the mandibular musculature.

Niche construction appears to alter the selection pressures on individuals and the course of their genetic evolution but it has also been suggested that behaviour can affect selection at a group level. While group selection at a genetic level is problematic at many levels, from the issue of cheats to exogamy and migration eroding any strong genetic differences between groups (Laland & Brown 2011) there is the possibility of selection of cultural traits.

Human prosociality has been suggested as one example of this (Laland & Brown 2011, Henrich 2003). This brings us back to the Baldwin effect or adaptability driver. If fitness is gained through conforming to social rules and fitness is lost through the costs of punishment, individuals who have a 'learning bias' towards these particular prosocial behaviours will expend less in the learning of the behaviours and be more likely to avoid punishment, thus increasing their fitness (Henrich 2003).

One key criticism of gene-culture co-evolutionary theory is timing. Modern humans have been described as 'stone agers in the fast lane' (Boyd Eaton et al. 1988). This is based on a concept of adaptive lag between our genetics and our environment which assumes that we are still optimally adapted to our ancestral environment of the African savannah. Critics such as Adenzato (2000) have claimed that genetic evolution moves too slowly to be affected by something as rapid as cultural change.

This argument can be attacked from two sides; firstly genetic mutations in humans have increased rapidly since the advent of cultural practices. Genetic evolution has not at all been at a standstill in Homo sapiens; in fact studies have shown a great deal of recent mutations, (Wang et al. 2006, Voight et al. 2006) and even an increase in the rate of evolution (Hawks et al. 2007). Genes relating to pathogen response and neuronal function are over-represented in those that have undergone recent changes (Wang et al. 2006).

This is highly likely to be a result of increased pathogen stresses in a larger population with close contact with animals and a complicated social structure. Genetic change is not therefore too slow to be affected by cultural change.

There are however, examples of cultural change being too delayed to save a population from extinction, wiping out all the genetic information within it. The example of Greenland Vikings failing to adapt culturally to the Ice Age in the fifteenth century (McGovern 1981 quoted in Barrett, Dunbar & Lycett 2002) shows that cultural change does not have a set rate any more so than genetic evolution; both are flexible enough to interact with each other.

Another important criticism of the gene-culture co-evolutionary theory is the idea of proximate and ultimate causes; if an organism's behaviour can be linked to its genetics then it becomes merely the proximate cause while the ultimate cause remains natural selection. However human niche-construction shows the failings of this argument. There are no genes for cooking, dairying or many of the other cultural practices that have been shown to have affected our genetic evolution. (Laland et al. 2007) Behaviour can therefore be reasonably said to influence genetic evolution rather than just being considered a proximate cause.

It would seem that despite the traditional standpoint that, 'adaptation is always asymmetrical; organisms adapt to their environment, never vice versa' (Williams 1992; 484 quoted in Laland et al. 2007) organisms, and most evidently humans, do in fact alter the selection pressures acting upon them. From buffering climatic stresses to dietary innovations, recent human evolution shows clear evidence of socially learned behaviour affecting the course of our recent genetic evolution.

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