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Natural Selection and Evolution

Updated Wednesday, 9th November 2005

Tim Halliday explores natural selection and evolution in the animal world

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Elephant in Kruger National Park Copyrighted  image Icon Copyright: Production team

In the Rules of Life series, and its accompanying CD, Aubrey Manning looks at the behaviour of animals and describes many new discoveries about the way that this is beautifully adapted to meet the challenges which animals face in their daily lives. The scientific study of animal behaviour, or ethology, was founded some 50 years ago by Niko Tinbergen and Konrad Lorenz. Their most significant contribution was to bring together the study of natural history with an understanding of evolution by natural selection. Since then, the numerous studies that have been made of animal behaviour have revealed that animals do many things that challenge many of the ideas that people had about natural selection 50 years ago. Natural selection theory has developed enormously since that time, largely as the result of animal behaviour studies, and a number of popular misconceptions about evolution have been revealed to be false as a result.

One such misconception is embodied in the phrase ‘survival of the fittest’, first coined by Herbert Spencer, and often used to encapsulate the way natural selection works. It is misleading, because ‘survival’ is only part of the story. Evolutionary change comes about because some individuals are more successful in reproduction and so pass on their genes to succeeding generations. As a result, the characteristics that make individuals successful in reproduction become more common in the population. It is thus reproduction that is important in evolution, and survival becomes simply a means to that end.

The importance of this point becomes apparent when we appreciate that, for most animals, reproduction is a very costly and sometimes dangerous activity, as illustrated many times in the series. To engage effectively in reproductive activity requires a great deal of energy and other resources that could otherwise be put towards survival. This is apparent in the common observation that animals show a greatly reduced growth rate, or stop growing altogether, when they reach sexual maturity. The resources that they derive from feeding are diverted from growth to reproduction.

A second common misconception is that natural selection acts ‘for the good of the species’. There are numerous examples of animal behaviour that show that this cannot be the case. For example, African lions live in prides, consisting of several females and their cubs, controlled by a group of two or three males. Other mature males are excluded from living in a pride. From time to time, a coalition of excluded males is formed and they attempt to take over a pride, by attacking and driving away the current pride-holders. If they succeed in doing this, their first act is to kill any cubs in the pride that are still being suckled by their mothers. To kill the young of one’s own species cannot benefit the species. The adaptive value of this behaviour for males is that, deprived of the cubs they have been suckling, the females very quickly come on heat and can conceive cubs fathered by the new pride-holders.

This example illustrates another common misconception about evolution, that, when mating, males and females are acting cooperatively. While it is in the interests of both parents that reproduction is successful, the way that that success comes about is not necessarily the same for the two sexes. For male lions, it is of no reproductive benefit to them to guard females that give birth to cubs fathered by other males. Infanticide is of benefit to them because it ensures that cubs born in the pride are their offspring. Infanticide is costly for females because the considerable time and resources that they have put into producing cubs is wasted. There is thus a conflict between the sexes, even though they have to behave cooperatively if either is to reproduce at all.

The interplay of cooperation and conflict is also apparent in the relationship between parents and offspring. For animals that produce more than one young at a time, it is usually to the advantage of the parent to share food more or less equally among its progeny. For each individual progeny, however, it is to its advantage if it receives more food than its siblings. There is thus a great deal of competition among progeny. This takes a bizarre form in the European Fire Salamander, and in a species of shark described in one of the programmes. In these animals, the young develop within the mother and, as they grow, they eat one another until only one, very large young is left. It may be that producing one offspring at a time is a good strategy from the mother’s point of view, or it could be that she would have higher reproductive success if she produced more; it may be, however that she has no choice in the matter.

The key to understanding evolution by natural selection is to think of it, not in terms of an individual’s survival, but in terms of its effectiveness in passing on its genes, what in the series is referred to as ‘genetic accounting’. Natural selection favours those individuals that pass on the most copies of their genes. This enables us to explain many aspects of animal behaviour that are difficult to explain purely in terms of survival. For example, in many species, particularly among birds, certain adult individuals do not breed themselves, but help other adults to do so, for example by feeding their young. In almost all cases, helpers turn out to be close relatives of the parents they are helping and so they are, in an indirect way, helping to spread those genes that they share with their relatives.





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