Under the Microscope: The principal theory in biology is the theory of evolution. Heredity is central to how evolution works. writes Prof William Reville
Darwin and Wallace, who proposed the theory of evolution by natural selection in 1859, did not know the mechanism whereby heredity works, except it seemed clear that the hereditary information is carried from generation to generation by physical carriers. Gregor Mendel essentially discovered these carriers, later named genes, in 1866. James Watson and Francis Crick worked out the chemical structure of the gene in 1953.
Certain aspects of how evolution works, for example how it can accommodate altruistic behaviour, were deeply puzzling. In 1964, the British biologist William Hamilton proposed a theory called "kin selection" to explain altruistic selection. It became clear that evolution could be most clearly explained by looking at it from the perspective of selection acting principally on genes and changing their frequency in the population. This idea was brilliantly proclaimed by the British zoologist Richard Dawkins who wrote The Selfish Gene in 1976.
Biological and evolutionary success depends on how successfully you breed - how many offspring you leave. This in turn is determined by the traits that fit you to live successfully in the environment. The fitter members of a species will breed more successfully than the less fit and will produce more offspring thereby spreading their successful traits in the population relative to the less fit traits of their breeding competitors.
In this way, nature automatically selects (natural selection) for traits that enhance individual fitness.
How does altruism emerge in a scheme where success is all about leaving the maximum number of offspring? Altruism often puts the bearer at risk of an earlier death, thereby minimising their potential to bear offspring. The trait may encourage the bearer to engage in risky behaviour for the good of the group - eg birds that cry out to warn others despite thereby identifying themselves and putting themselves at great risk.
Darwin tried to account for this problem by proposing group selection. This means that sometimes evolution works at the level of the group rather than the individual, selecting and protecting traits that, although detrimental to the individual, benefit the group. This idea was popular for a long time but it doesn't point towards a mechanism.
The persistence of altruism was eventually explained by the British theoretical biologist William Hamilton. He remembered a witticism coined by the British physiologist JBS Haldane, who said: "I would be willing to lay down my life for two brothers or eight first cousins." Haldane's saying has a genetic explanation. Each sibling inherits a copy of their parents' genes. If one sibling sacrifices his/her life to save two siblings, two copies of the parents' genes survive. From the genes' "point of view" this altruistic behaviour is a better overall result than the result of the alternative self-interested-no-sacrifice scenario where the two siblings die and only one copy of the parents' genes survives. Larger numbers are needed in order to get an equivalent result with cousins because cousins are less closely related to the altruistic individual.
Once an altruistic trait arises it tends to spread because this behaviour promotes the survival of individuals who carry the gene(s) that underlies the trait. Hamilton devised a rule which says that, in addition to taking account of the relatedness of the relevant individuals, altruistic behaviour is sensible from the genetic point of view if the benefit to the recipient of the altruistic act exceeds the cost to the altruist. This is called "Hamilton's rule".
Richard Dawkins was a student of Hamilton. He quickly saw the power and elegance of a gene's-eye view of evolution and determined to explain evolution to a worldwide audience using this approach. He coined the term "selfish gene", which is often misunderstood to mean that our genes consciously control us. Genes are not conscious, but the way they are propagated from generation to generation can be more easily understood if we consider them to be acting in their own self-interest.
Evolution eventually produced self-conscious thinking creatures. Because humans can think, we can decide to behave in many different ways and therefore we are not fully dictated to by our genes. Human behaviour could never be fully explained by genetics alone.
Not only do humans inherit genes, we also inherit culture. In The Selfish Gene, Dawkins proposed a parallel between genetic inheritance and cultural inheritance. Just as the gene is the unit of genetic inheritance, Dawkins proposes the "meme" as the unit of cultural inheritance. Memes are passed from parents to children by imitation and often oppose the "goals" of the selfish gene - eg children who devote their lives to care of sick parents, overriding the urge to set up their own families. Dawkins would class religion and politics as memes inherited from parents.
According to memetics, the "science" of memes, our minds and culture are formed by natural selection acting on memes, just as organisms are designed by natural selection acting on genes. Some memes survive because they are genuinely useful, like arts and sport, others use a variety of tricks to get themselves copied. Exploitative memes have "copy me" instructions backed up by threats and promises. Dawkins classifies religion as an exploitative meme, with believers under orders to pass on the belief under threat of damnation and promise of salvation. However, the whole idea of memes is controversial, with many difficulties to be resolved. More in 25 Big Ideas By Robert Matthews.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC - http://understandingscience.ucc.ie