Before you read this, I suggest you read post 21.8.
We usually think that when individual animals can breed, to produce fertile offspring, that they belong to the same species. If they can’t – they belong to different species (see post 23.5).
So where do individual species come from? The English biologist Charles Darwin (1809-1882) developed a theory to answer this question that he first explained in an unpublished essay and later in his book “The Origin of Species”. But, before the book was published, another English biologist, Alfred Wallace (1823-1913) sent Darwin an essay describing the same ideas. Darwin and Wallace presented their ideas at a meeting of the Linnean Society in 1858 but they were mostly ignored. But, after the publication of Darwin’s book, in 1859, the theory of evolution received considerable attention and started a serious of controversies that continue.
Darwin’s theory consists of two parts:
- creation of new species from existing species
- natural selection.
To make my explanation easier to understand, I am going to use the idea of the gene. But Darwin and Wallace had never heard of genes which makes their development of the theory truly remarkable. Darwin was guided by his observations on the appearance of different species in the Galapagos Islands, in the Pacific Ocean, and by his reading, especially the work of the English economist Thomas Malthus (1766-1834).
New individuals inherit half of their genes from one parent and half from the other, as described in post 21.8. So each individual (apart from identical twins) has a unique set of genes (genotype) and, therefore, a characteristic appearance and behaviour (phenotype). The diversity of these individual characteristics is increased by mutation.
How does this diversity of characteristics lead to new species? Suppose that a population becomes separated into two groups A and B, so that they are unable to interbreed. Over many generations, continuing diversity means that the pattern in which the genes in group A are combined will have changed, leading to changes in appearance, metabolism and behaviour. Changes will also occur in group B but they won’t be the same changes. Now suppose that groups A and B are now so different that they are no longer capable of interbreeding – the result will be that A and B are different species.
How could the two populations become isolated? The simplest idea is by the creation of new geographical barriers (mountains, rivers, formation of a new islands) that keep them apart.
If new species arise by random mixing of genes, why does each species appear to be so well adapted to its environment. For example, polar bears store more vitamin D in their livers than do most animals because of the lack of sunshine and food during the winter where they live (see post 16.36). Darwin explained this apparent adaptation by natural selection. If a species were not suited to the environment in which is appeared, its members would die. And, if several species were competing for a limited food supply, only those whose members were best able to collect and use this food would survive. But survivors would live and breed to produce new members of the species.
So, in Darwin’s theory, species do not adapt to changes in their environment. Changes occur at random and species that are successful, in a given environment, flourish: others perish.