A. Both theories describe the rates of speciation. For Gradualism, changes in species is slow and gradual, occurring in small periodic changes in the gene pool, whereas for Punctuated Equilibrium, evolution occurs in spurts of relatively rapid change with long periods of non-change.
B. New species arise through a process called speciation. In speciation, an ancestral species splits into two or more descendant species that are genetically different from one another and can no longer interbreed.
C. Allopatric speciation occurs when two species are living in separate environments and therefore there is no gene flow between the populations. ... Sympatric speciation occurs when two species are living in the same environment.
D. Reproductive isolation is a mechanism that keeps species from mating with others. Prezygotic isolation prevents the fertilization of eggs while postzygotic isolation prevents the formation of fertile offspring. ... Postzygotic mechanisms include hybrid inviability, hybrid sterility and hybrid "breakdown."
