Final answer:
Lack of gene flow can lead to allopatric speciation, where separated populations evolve into distinct species, or decrease genetic diversity, increasing extinction risk.
Step-by-step explanation:
Two contrasting ways that lack of gene flow can be an important evolutionary force include promoting genetic divergence and speciation, and increasing the risk of extinction. One way is through the process of allopatric speciation, where geographical separation prevents gene flow, leading to divergent evolution as separate populations adapt to their unique environments and accumulate genetic mutations independently.
This can result in the emergence of new species. Conversely, with no new genes entering a population, lack of gene flow can also lead to inbreeding and a decrease in genetic diversity, which can increase the vulnerability of a population to diseases, changing environmental conditions, and other survival threats, potentially leading to extinction.