The modern understanding of evolution by natural selection is known as neo-Darwinism. It does not contradict the great Darwin’s original theory, but instead merges it with a more recent understanding of inheritance and genetics.
Neo-Darwinism came to the fore in the 1960s through the work of John Maynard Smith and William Hamilton (later popularized by Richard Dawkins, pictured). Its most obvious contribution was the correction of a misunderstanding that had grown up around Darwin’s original theory. As the theory became widely accepted in the early 20th century, it was assumed that natural selection acted at the species level, with adaptations made for ‘the good of the species’. How natural selection did this was not clear, and the unit of selection was corrected to the individual body, which was more in keeping with Darwin’s original idea. Today, evolutionary biology sees a body as a machine produced by genes to ensure their survival. Thus, natural selection is really only at work at the level of the gene.
There are many reasons why humans are altruistic, or selfless, to one another. It might be due to a moral code, to boost personal virtue or to maximize benefit for the majority despite a personal cost. However, other animals also behave altruistically. For example, a worker bee will sting an attacker and condemn itself to death in the process; a meerkat sentinel (opposite) will bark a warning to its mob when it sees a predator, attracting attention to itself, while giving the others a chance to escape; and a monarch butterfly will risk being eaten by a bird – and make its assailant sick from the toxins in its body – in order to teach that bird to stay clear of related butterflies in future. How does this behaviour match up with the brutal struggle for survival at any cost that underlies natural selection? The only reason required is that parents and offspring share genes, so do other members of a family or social group, and indeed all members of a species to a lesser extent. Animal altruism may hinder, or even kill, an individual, but it also ensures the survival of many more copies of the genes it shares with others.
Also called the ‘coefficient of relationship’, relatedness is a way of putting a numerical value on how closely related individual organisms are in terms of the genes they share. An organism that reproduces asexually will share 100 per cent of its genes with its offspring, and so they have a relatedness of 1. Sexually reproducing organisms pass on half of their genes to their offspring, and although they may not be expressed in the phenotype, they are still present in the genotype, giving parent and child a relatedness of 0.5. The same is true of siblings; they have a 50 per cent chance of inheriting the same genes from their parents and so they have a relatedness of 0.5. Half-siblings and grandparents have a relatedness of 0.25, while first cousins are 0.125 related. Second cousins share about 3 per cent of your genes, which in the human gene pool makes them as closely related as a complete stranger. Relatedness has its place in human laws to avoid inbreeding (0.125 is usually the limit) but it also helps to understand behaviours in the wider animal world.
Made famous by Richard Dawkins’ 1976 book of the same name, the ‘selfish gene’ concept is frequently misinterpreted. It is not a declaration that our genetics determines our personality, giving us a licence to behave as selfishly as we like because it is the ‘natural’ thing to do. Nor is it saying that genes are constantly calculating – by some supernatural intelligence – the best outcome for themselves and acting accordingly.
Instead, the phrase ‘selfish gene’ was intended to sum up the central tenets of neo-Darwinism: that natural selection works at the level of genes; that the drive for survival is the result of a gene’s drive to be replicated in ever greater numbers; and that all animal behaviours, even seemingly altruistic ones, can be explained in these terms. Genes do not exist in order to produce the phenotype (the organism’s body and behaviours). In fact, the reverse is true: the phenotype is a means by which the genes can ensure their survival and maximize their abilities to replicate themselves.
Watching over her pups, this mother capybara (one of the world’s largest rodents) is also protecting her genes.
Altruistic behaviour in animals is the result of selfish genes ensuring their survival. Parents protect their offspring from attack, and brothers and sisters help each other raise young. This is called ‘kin selection’, where an individual works to boost the reproductive success of their kin, and so replicate their own genes by proxy. But what if unrelated, non-kin animals could see that they shared at least one gene? Would they then help each other out? This is the question considered by a thought experiment called the ‘green-beard effect’. Imagine a gene that gives the carrier a green beard. The gene also makes its carrier recognize other individuals with the green-beard gene and behave altruistically towards them. Such a system would be very advantageous to the gene, but does it exist in nature? There are a few possible examples, mostly in microorganisms, but the system is rare because it is prone to cheating: a mutant green-beard allele that precludes altruistic behaviour would get all the benefits with none of the costs, and rapidly replace the original version of the gene.
Taking risks to aid people that look like you is a flawed genetic strategy – only cheaters benefit.
Another modernizing aspect of neo-Darwinism is its use of game theory in understanding the evolution of animal behaviours, especially those used in conflict resolution. Game theory is an arm of probability, the mathematics of chance. It was developed in the 1940s to help predict human behaviours in economic and military scenarios, but can be used in a simpler form to show how species evolve stable behavioural strategies.
The Hawk-Dove ‘game’ considers the benefits of being belligerent (a hawk) or a pacifist (dove). It pits two individuals against each other in a conflict over food or a mate. If a hawk meets a dove, he always wins the prize. If a hawk meets another hawk, he has a 50 per cent chance of winning (or losing). If a dove meets a hawk he never fights, wins nothing but loses nothing either. If two doves meet, they share the resource and get half each. The chances of these four scenarios occurring depends on the frequency of hawks and doves in the population. A hawk among doves does well, but so too does a dove among hawks.
The emphasis that neo-Darwinism places on the power of the gene to influence evolution has led to a belief that genes are the single factor in the development of an organism’s body and its behaviours. People think that having a certain gene will inexorably lead them to have a certain trait, saying things like ‘it’s in my genes’ to explain away their behaviours and other features. This is a mistaken belief known as ‘genetic determinism’.
Classical Darwinism refers to the interplay between an organism’s body and behaviour and the environment. Darwin knew nothing of Mendelian genetics, and when the two ideas were fused, a shorthand emerged that seemed to suggest that an organism’s genes were the de facto instructions for building a body. In fact, it was always understood that every gene interacted with the environment to develop a unique body. The question was how much of the end result is nature (the gene) and how much is nurture (the environment) – see here.
German biologist August Weismann (1834–1914) was the first great advocate of genetic determinism, and is often seen as a pioneer of the ‘selfish gene’ theory.
The field of psychology known as behaviourism seeks to understand the way animals, including humans, learn, and aims to understand the motivations of an animal through its observable actions. One technique used to investigate learning is ‘operant conditioning’, in which a test animal is taught to perform tasks through a system of reward and punishment. This technique’s most vocal exponent was American psychologist B.F. Skinner, who designed a chamber that could be used to house his test subjects. Skinner mostly used pigeons, which were rewarded when they performed correctly, reinforcing that behaviour. Skinner taught pigeons to perform complex sequences of tasks just as well as any more ‘intelligent’ test subject. His conclusion was that learning was a purely physical process that did not require any mental component – even in humans! This radical notion went unchallenged for 20 years until the late 1960s, when the first physical trace of a mental memory was isolated in the brain, proving a link between physical and mental phenomena.
