XXI.4 Important differences exist between speciation in species without sexual reproduction and in species with sexual reproduction
Speciation in species without sexual reproduction is somewhat simpler than in species with sexual reproduction (Cohan 2001). As soon as mutation occurs in an individual, changing the phenotypic traits of the individual, this line can lead to the evolution of a new species. Whether a new species is actually split off is finally decided by whether there is a suitable ecological niche in nature that the new species can occupy. If the change in the phenotypic traits is not sufficient to enable the carriers of the new trait to occupy a very different niche than that occupied in nature by the original species, its relative fitness will decide on the fate of the mutant. If the fitness of the mutant is substantially greater than that of the members of the original form, the descendants of the mutant will probably replace the original species and phyletic speciation will occur. If the fitness of the two forms does not differ much, the fate of the mutant will tend to be decided at random; in a population with size N, the mutant will be eliminated with a probability of (1 – 1/N); the mutant will force out the original species with a probability of 1/N. If the mutant has substantially lower fitness than the original, unmutated form, its line will understandably be eliminated from the population and no speciation will occur. If the mutants are capable of occupying a new niche, then the members of the original and new forms will compete to only a limited degree and both forms, now both species, will be capable of coexisting in nature for a long time.
The mechanisms of speciation in sexually reproducing species are more complicated. It is not sufficient to accumulate enough phenotypic differences between the parent and daughter species that would enable sufficient differentiation of their ecological niches. For the branching off of a new species it is simultaneously necessary for a sufficient reproductive barrier to be formed between it and the parent species, to prevent gene flow between the gene pools of the two species and blurring of the phenotype and thus the ecological differences between their members. Moreover, in order for at least phyletic speciation to occur, it is necessary that the new mutation have a similarly favorable effect on the fitness in combination with most of the alleles that occur in the gene pool of the given species and not only in combination with the alleles carried by the mutated individual.s