The structure of evolutionary theory
Darwin and Wallace are often associated with the ‘discovery’ of evolution, but this is not what they did. Ideas about evolution – often referred to as transformationism – had been around for many years, and were present in classical thought. the problem was proposing a mechanism by which it could work. Several scientists – including Darwin’s own grandfather, Erasmus, had proposed evolutionary theories, but the failed to convince. Independently Charles Darwin and Alfred Wallace proposed a workable mechanism – natural selection.
Natural selection simply means differential reproductive success; that is, that given a population of reproducing organisms, and given that individuals within that population have different numbers of offspring, then natural selection is the mechanism that determines this differential reproductive and survivorship rate. This mechanism lies at the heart of evolutionary theory. In a sense, it is the bit of the theory that does all the work, sorting out the individuals in each generation and thus determining the direction of evolutionary change. While this is the heart of the theory of evolution, the mechanism of natural selection rests on the presence of certain conditions.
- The first of these is that organisms reproduce. If there is no reproduction, then the game of life would have to start afresh with each deceased generation.
- The second condition is that there should be some mode of inheritance – that is, offspring should resemble their parents more than they do the population as a whole. This is the field of genetics. If information that determines the characteristics of the parent can be transmitted to offspring, then those features which enhance the survival and reproductive potential of the parents will occur more frequently in each subsequent generation, dependent upon the number of offspring.
- There must, thirdly, be variation within the population. Even if the first two conditions are fulfilled, if each individual in the population is phenotypically and genetically identical, then natural selection cannot operate. Differential survival will have no effect because all individuals are the same, and so each generation will be identical.
- Finally, there must be competition. Imagine a world in which the first three conditions outlined above were fulfilled. However, if the resources needed to support all the populations were infinite, then there will be no differential reproduction. An individual could have all the offspring possible, and so there would be no change from one generation to another, just a constant and everlasting expansion. Clearly, though, such a world does not exist. If resources are limited, then not all individuals will survive and reproduce or will reproduce at different rates of success.
Given the conditions of reproduction, variation and inheritance, then those individuals who are better adapted to acquiring the resources necessary to survive and reproduce, will leave more offspring, and those offspring will carry the feature of their parents that gave them this competitive edge. These then are the conditions under which natural selection must occur – it is in fact a logical necessity deriving from the observations of these conditions.
If these conditions occur, then evolution (change through time) must be a consequence. It is useful to distinguish in this way between evolution and natural selection. Natural selection is the mechanism of change, dependent upon certain conditions. Evolution is the outcome of those conditions, and evolutionary patterns will vary if those conditions vary. The fact that they do accounts for the enormous diversity of forms of life and the pattern of evolution itself.
Apart from evolution – change through time – there is another consequence of natural selection – adaptation. In its simplest meaning this refers to the goodness of fit between an organism and its environment. The better fitted an individual is to its environment, then the better adapted it is. Adaptation is a consequence of natural selection because it is those individuals who are better adapted to their environment who will leave more offspring, and given the other conditions, then over time a population will come to be adapted to its environment.
Characterising evolution in this way – conditions, mechanisms and consequences – is helpful for knowing how to develop specific theories and explanations in evolution.