The theory of evolution by natural selection is the central unifying theory in biology. It provides an explanatory principle that illuminates the relationships that exist in the vast diversity of biological life. Everything in biology must be explicable in the light of the theory of evolution.
A great many things are readily explained by evolution, but some things are not. For example, for many years biologists were puzzled as to how evolution could explain selfless behaviour in animals. How do we explain the evolution of altruism?
Altruistic behaviour is readily seen in animals. For example, ground squirrels and many birds sound warning calls that alert their companions to the presence of a predator, and in the process render themselves more conspicuous. Many animals spend valuable time grooming one another. Female honey bees do not reproduce: instead they work for the queen. How did this seemingly generous behaviour evolve in an evolutionary world that rewards selfish behaviour?
Natural selection is the primary mechanism of the evolutionary process. This works by unconsciously selecting traits in an organism that confer a reproductive advantage. If some individuals in a group possess heritable traits that make them better able to survive and copulate in their environment than their fellows, they will reproduce more successfully and, in this way, their favourable traits will spread throughout the species.
Individuals' characteristics compete with one another and those best suited to the environment replace those that are less well suited. Altruistic behaviour must confer an advantage if it is to be selected. But it is difficult to understand how this behaviour initially becomes established in particular cases where the bearer of the altruistic traits puts himself at great extra risk, for example by sounding a warning cry, and thereby tending to have himself and his traits eliminated through being pounced on by an enemy.
A broader understanding of the evolutionary process is used to explain the general evolution of altruistic behaviour. In this picture, bodies are defined as structures produced by successful genes. The function of bodies is to transmit these genes to future generations. In sociobiology, the central principle of fitness maximisation holds that living things unconsciously act so as to maximise their success in transmitting genes into the future.
GENES that are common in a species are genes that have successfully propagated themselves over many generations because they confer certain advantages that promote the success of the species. A "genes-eye" view of evolution sees Darwinian fitness (the success of an individual at producing offspring) as a special case of the more general process of fitness maximisation by which genes try to transmit copies of themselves into the future.
Thus, whereas parental genes are certainly contained in offspring, copies of these genes are also present in relatives. The closer the relative the greater the chance that copies are present. It has frequently been found that, when animals seem to behave altruistically as individuals they are also being genetically selfish since their behaviour enhances the prospects of the copies of their genes contained in the bodies of relatives.
The peculiar genetic system of the honey bee workers makes them more closely related to their sisters than they would be to their offspring if they were to reproduce. Therefore, by helping their mother (the queen) to reproduce, they are doing more to propagate their own genes into the future than if they were to seek to produce offspring of their own.
The central principle of fitness maximisation in sociobiology is used to explain a wide variety of animal behaviour, e.g. mate selection. Males produce a large number of small sex cells (sperm) whereas females produce a small number of large sex cells (eggs). Consequently, in most species maximum fitness is achieved through different behaviour strategies for males and females.
The sex that provides the smaller investment in rearing offspring (normally the male) is selected to compete among themselves for access to the sex providing the greater investment (usually female). Males therefore maximise their fitness by loudly advertising their sexual wares whereas females are selected to be relatively quiet selectors of the most attractive wares on offer.
Species where the male invests more in parenting than the female, show a reversal of usual competitive behaviour, with the female tending to be larger, and more showy and aggressive than the male.
Females are the primary caregivers to offspring in most vertebrates. This correlates not only with their greater investment in rearing offspring but also with the greater "confidence" they have of being genetically related to their offspring. Male fitness is therefore enhanced by having the largest possible number of successful copulations, whereas female fitness concentrates on nurturing the offspring.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC.
The Irish Times has recently published a collection of Dr Reville's articles. Understanding the Natural World, illus. paperback, price £14.99, can be bought at all good bookshops. Further information is available by faxing inquiries to (01) 671-8446.