Question

speciation occurs via geographic isolation. once two or more populations are isolated by a geographic barrier, such as a stream, the level of genetic divergence among the populations due to mechanisms such as . stabilizes decreases increases sympatric allopatric natural selection gene flow

Explanation:

1. The first blank: Speciation that occurs via geographic isolation is called allopatric speciation. Sympatric speciation occurs without geographic isolation, so the first blank should be "allopatric".
2. The second blank: When populations are geographically isolated, gene flow between them is reduced or stopped. Over time, genetic divergence (differences in genetic makeup) among the populations increases because different selective pressures, mutations, and genetic drift can act on each isolated population. So the second blank is "increases".
3. The third blank: Mechanisms that contribute to genetic divergence in isolated populations include natural selection (different environments lead to different traits being favored), genetic drift (random changes in allele frequencies in small populations), and mutations. Among the options, "natural selection" is a key mechanism here. Gene flow would decrease (not increase divergence), so the third blank is "natural selection".

Answer:

First blank: allopatric
Second blank: increases
Third blank: natural selection