Pre-zygotic isolation
– Most economical mechanism in terms of natural selection
– Avoids wasting resources on weak or non-viable descendants
– Includes physiological or systemic barriers to fertilization
– Prevents potentially fertile individuals from meeting
– Types of barriers include different habitats, physical barriers, and differences in sexual maturity or flowering
– Central Valley in California prevents salamander populations from interacting
– Salamander gene pools become mutated over generations
– Ecological or habitat differences impede meeting of potential fish pairs
– Reproductive isolation due to differences in mating season in toad species
– Certain plant species are reproductively isolated due to different flowering times and habitats
Behavioral isolation
– Different mating rituals create reproductive barriers
– Songs of birds and insects are specific to their own species
– Mating dances, songs, and grooming are examples of courtship behavior
– Even minimal morphological differences can prevent mating
– Pheromones play a role in sexual isolation of insect species
Mechanical isolation
– Flowers of angiosperm species attract specific pollinator species
– Diversity in form, color, fragrance, and nectar to attract pollinators
– Specific pollinators ensure reproductive isolation
– Structural differences in reproductive organs prevent mating
– Mechanisms ensure pollination by compatible species
– Insects’ rigid carapaces act as a lock and key mechanism for mating between species
– Evolution leads to complex and divergent genital organ characteristics, causing mechanical isolation
– Anatomically different genital organs can be functionally compatible in some cases
– Mechanical isolation also occurs in plants through pollination syndrome
– Pollination syndrome ensures pollen transfer within the same species, preventing cross-pollination
Post-zygotic isolation
– Mechanisms that prevent successful inter-population crossing after fertilization
– Zygote mortality and reduced viability of hybrids can occur in both plants and animals
– Differentiation of embryo development genes leads to non-viability of hybrids in frogs
– Genes from cell nucleus and cytoplasmic organelles contribute to non-viability in mosquitoes
– Endosperm development failure and abortion observed in interploidal and certain same-ploidy crosses in angiosperms
– Hybrids may have normal viability but deficient reproduction or sterility
– Sterility results from gene interaction, chromosomal imbalances, or nucleus-cytoplasmic interactions
– Mules and hinnies are sterile hybrids due to chromosomal differences between horses and donkeys
– Interspecific hybrids in angiosperms often exhibit sterility
Coevolution and other factors
– Reproductive isolation acts as a barrier between species in coevolution with pollinators
– Genital organ compatibility affects successful coupling between mating pairs
– Léon Dufour first signaled the relationship between reproductive isolation and genital organ form
– Co-specifics (males and females of the same species) can mate due to complementary structures
– Other factors besides anatomical differences determine the form of genital organs
– Species are isolated from each other by habitat, timing of mating season, and behavior during mating.
– Hybridization between species is impeded by sterile hybrid males.
– Descendants of backcrosses between hybrid females and parent species are weak and non-viable.
– These mechanisms restrict genetic interchange between species in the wild.
– Only a few fertile females have been found among thousands of analyzed species. Source: https://en.wikipedia.org/wiki/Reproductive_isolation
