Since the Pleistocene ice age, deserts have been gradually forming in the southwestern United States. As the original lakes and rivers of this area shrank into isolated streams, ponds, and springs, the fishes living in them developed a strong potential for .(A) speciation.(B) hybridization.(C) mechanical incompatibility.(D) temporal isolation.(E) hybrid inviability.

(A) speciation.

Explanation:

Shrinking of water bodies and development of deserts since the Pleistocene ice age produced new vacant niches that could be inhabited by organisms such as fishes. Production of new isolated streams, ponds, and springs allowed the fishes to occupy these new water bodies. Accumulation of genetic variations to become adapted to their new habitat resulted in the evolution of several new species of fishes with many different genetic traits. This represents speciation due to geographical isolation.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".