3.5 Natural selection
Darwin summarised his theory of natural selection in the introduction to The Origin of Species as follows:
As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form. [The italics are mine.]
The important principle that Darwin first established here was that variation is quite separate from the direction of evolutionary change. Characteristics do not arise in response to an organism's needs. This notion is explored in course S182_2 where I draw attention to the way that statements implying a purpose for evolution are described as teleological and ought to be avoided. Doing so is by no means easy, for even the most respectable scientists and TV pundits use teleological statements to make points more forceably! But in my view there is a real danger that this practice will lead to misunderstanding of how natural selection works in those encountering these far-from-easy ideas for the first time. For instance, picking up the example I referred to earlier on, there is no 'arms race' in the sense of predator and prey actively involved in developing ever more cunning adaptations to outwit one another. Rather a variation that is 'in any manner profitable' can be naturally selected as individuals within a population come under pressure that affects their survival. The variant must already be in existence to be available for selection.
Yet we do see specific, reciprocal adaptations between species, eloquently described by DA [pp. 61-62 and pp. 122-123]. There are seeds that are protected from predators by their tough coats and, on the other hand, we find rodents with sharp teeth capable of cracking these tough nuts. The evolutionary processes that give rise to such specific forms of adaptation are called coevolution. In general, coevolution comprises a series of adaptive changes between two interdependent species, such that a change in one may, in time, favour the establishment of an adaptive response in the other. (Note this concept is very different from the one introduced in S182_2 as convergent evolution, where similar adaptations arise independently in unrelated organisms - e.g. the golden and marsupial moles.)
DA's account then goes on to give the vital clue that ought to alert us to the inappropriateness of the 'arm's race' analogy. He says 'So grasses have not only achieved a truce with those animals that feed on them' [p. 123].
How does DA's statement reveal the limitation of the 'arms race' analogy? In the human 'arms race' each side works to develop weapons of an increasingly destructive nature. In the natural situation, this kind of escalation would lead to the destruction, i.e. the extinction, of one of the species involved to the detriment of the other species. Instead of extinction, we observe that some species, especially pathogenic (disease-causing) organisms, show adaptations that allow coexistence. Much evidence from the long-term study of diseases shows that over time the pathogenic organism becomes less virulent. A much quoted example is the coevolution of the parasitic myxoma virus that gives rise to the disease of myxomatosis in rabbits. The virus is endemic in South America but the local rabbits do not usually die after becoming infected. When the disease was introduced into rabbit populations in Australia and the UK in the 1950s, it nearly wiped out the rabbit populations. Over the next few years after the population crash, rabbit numbers built up again as a consequence of both diminished virulence of the pathogen and increased resistance to the virus. Both organisms were observed to have changed. After the introduction of the virus, which is spread by biting mosquitoes, the rabbit population was reduced to a tiny percentage of its original size. 99.8% of the rabbits died.
What reasons can you suggest to account for the survival of 0.2% of the rabbit population?
(1) These surviving rabbits might not have been infected with the virus.
(2) These rabbits might have had a natural resistance to the virus.
(3) These rabbits might have been infected with a less virulent form of the virus.
If the virus killed all the rabbits that it infected, it would not survive. There would be no means by which it could be transferred to the few uninfected rabbits. So if all the surviving rabbits were uninfected, we would have seen a subsequent build-up of the rabbit population but no evidence of the myxoma virus. Thus suggestion (1) cannot on its own account for the observed facts.
Both suggestions (2) and (3) are needed to account for the situation that we find today. They depend on there having been variation within both the pathogenic population and the rabbit population. In each case there was a very dramatic 'struggle for survival' - examples of indirect competition.
The story of the introduction of myxomatosis to these rabbit populations demonstrates natural selection at work. The selection pressure (the strength of selection) was very strong but you can appreciate that those rabbits who survived must have had a natural resistance to the virus. In the absence of the virus, this characteristic would have no particular value. Only with the introduction of the virus does the characteristic become adaptive. Such a characteristic is termed a preadaptation. A characteristic can only be designated a preadaptation with hindsight. The possession of a preadaptation is entirely fortuitous; selection cannot plan for the future. The outcome of the myxomatosis story was not predictable. Had there been no suitable variants, the whole of the rabbit populations in Australia and the UK would have been wiped out, as was the intention of those who introduced the virus.
Can you now suggest one reason why it is difficult to poison rats?
There is variation in the rat population and if there are any rats that have a natural resistance to the poison they will survive and breed, while susceptible rats leave no offspring. The prodigious reproductive output of these resistant rats will ensure that numbers rapidly build up again and many of the offspring will inherit the characteristic that conferred resistance.