Reproductive isolation barriers external
- The environment of most species of organisms has a more or less heterogeneous character in space and time. The boundaries between the individual types of environment are usually sharply defined and can constitute important barriers preventing free movement of the members of a particular species from one side of the barrier to the other, or can at least retard this movement. This limits ecological and genetic interactions between the individual parts of the population, which can lead to phenotype and genetic differentiation of their members and to subsequent speciation.
Geographic isolation was discussed in Chap. XXI.3. Temporal isolation can occur, for example in species with a multi-year life cycle. In this case, the individual temporal cohorts of individuals that hatch and reproduce in individual years can be strictly separated. As a consequence, the adult members of the different cohorts never meet under normal conditions and thus gene flow cannot occur between the cohorts. If the spatial or temporal separation of populations lasts sufficiently long, this will most probably lead to allopatric speciation - the formation of internal reproductive isolation mechanisms that are capable of preventing crossing between the members of the two populations even after the external spatial or temporal barriers disappear. If the conditions of the environment at places occupied by the original and new populations differ substantially, then selection participates in the evolution of the species, in addition to evolutionary drives and genetic drift. This substantially accelerates the phenotype and genetic diversification of the populations. Genetic diversification also leads to faster establishment of reproductive barriers. For example, when trout were exposed to divergent selection in two different environments, a reproductive barrier developed within 13 generations (Hendry et al. 2000). Similarly, reproductive barriers evolved in lines of drosophila bred under different conditions, while these barriers were not formed between lines bred under identical conditions (Schluter 2001). Similar results were also obtained in studies of natural populations. For example, it was found for sticklebacks (Gasterosteus) that reproductive barriers are much stronger between allopatric species living in different types of environment, specifically between benetic species (bottom-dwelling species) and limnetic species (living in open water) than between allopatric species living in similar environments (Orr & Smith 1998).