These enzymes are most likely to be compatible in their function if they come from the same species. If they are from different species, they may not integrate well with other loci in the organism and the hybrid individual may die. The probability of a hybrid being more unfit has to do with how many loci in each semispecies are homozygous for different alleles. Hybrid breakdown occurs when many of these loci are fixed for different alleles in the two subspecies and the production of offspring by the hybrids is not possible because and F1 F1 produces too many incompatible gene by gene combinations. To understand the origin of species differences it is necessary to consider how genetic differences arise between species.
Geographic subdivision decreases gene flow.
Depending on the population size, genetic drift can play a major role in promoting genetic differentiation. If population size remains low for a long period of time it is possible for genetic differences to accumulate between geographic areas by the random process of genetic drift. Selection can promote speciation because genetic changes can rapidly accumulate in one area where selection favors certain behaviors, physiology, or morphology relative to another area where alternative alleles are favored by natural selection. Finally, the amount of gene flow which is usually described in terms of the migration rate is a key parameter that influences the rapidity of speciation. Models of speciation based on geographical subdivision.