The theory of evolution an outline
Briefly, the theory of evolution in its classical form is based on a few near-commonplaces. (This "classical form" was fully articulated in the 1930s.)
- In any population of a species there is variety - no two individuals are alike (even identical twins are exactly alike only in their genomes).
- At least some of this variation is genetic. In chapter 11 we surveyed the ways in which genetic variation can be generated.
- Variation means that some individuals in the population are better able to survive and leave offspring than others, though the differences might be very slight.
- As a result, the variant genes that favour survival become relatively more abundant in the offspring generation. This is the process known as "natural selection".
- Over a series of generations, therefore, the genes for more successful variants become progressively more abundant in the population's gene pool25.
- However, the individual's survival chances depend on an unlimited number of environmental factors (climate, food resources, predation, etc.). Therefore, whether a particular variant of a gene favours survival and reproduction is a matter of time and place. It depends on the nature of the ecosystem.
- In other words, gene variants that are good for survival in one ecosystem might not be good in another. Therefore, populations of the same species in two different ecosystems diverge genetically over time. And the gene pool of a population within one geographical area changes as environmental conditions change.
Viable scientific theories change with time because they adapt to new discoveries - much as viable biological species change over generations because they adapt to changing environments26. Since its inception in the 19th century, the theory of evolution by natural selection has undergone serial modifications. Its most radical adaptations have been the assimilation of classical genetics and, more recently, molecular biology. It continues to develop amid healthy and sometimes heated scientific debate. We shall say more about this aspect of the history of science elsewhere. Here, we concentrate on the theory itself.
25 "Gene pool" is to population as "genome" is to individual. It means the sum total of all the genes (each in all its variant forms) in all living individuals in the population.
26 More than one philosopher of science has explored this analogy between the evolution of scientific theories and the evolution of species. Stephen Toulmin has made particular use of it.
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