Darwin clearly identified two major factors that contribute to speciation. The first of these is the isolation of populations followed by local adaptation by natural selection:
‘…isolation, by checking immigration and consequently competition, will give time for any new variety to be slowly improved; and this may sometimes be of importance in the production of new species.’
He also singled out migration to and isolation on islands as a means of promoting speciation:
‘If we… look at any small isolated area, such as an oceanic island, although the total number of the species inhabiting it, will be found to be small…of these species a very large proportion are endemic… Hence an oceanic island at first sight seems to have been highly favourable for the production of new species.’
The second factor Darwin discusses is the size of a species’ geographical range; he suggests that if a species covers a large range it is likely to encounter a number of different habitats or environments, especially if it is expanding its range. Natural selection will therefore favour local adaptation to these different environments and will hence promote speciation:
‘…the conditions of life are infinitely complex from the large number of already existing species; and if some of these many species become modified and improved, others will have to be improved in a corresponding degree… the course of modification will generally have been rapid on large areas’.
But how do these populations, once they begin to diverge, come to form separate species? Darwin did not answer this question clearly, and some scholars believe Darwin ruled out the possibility that natural selection has a role in separating species other than in their independent evolution, whilst some passages can be interpreted otherwise.
The answer to this problem is credited to Alfred Russel Wallace and is now known as the Wallace Effect. This idea was published in Wallace’s Darwinism in 1889, several years after Darwin had died. The Wallace Effect proposes that natural selection contributes to reproductive isolation by encouraging diverging populations to stop mating. Each population will have adaptations which increase its fitness in a local environment; matings between individuals of the two different populations will jumble up these adapatations with the result that the offspring cannot compete with individuals from either population; so the mixed offspring have decreased fitness. Now if an individual of one population were to breed with one from the other, they will have less successful young than if they mate within their own population. So natural selection acts to favour either behavioural or morphological mechanisms to prevent mixing, over time the two populations become reproductively isolated and form two species.