What is a key reason populations are often not in equilibrium?

Populations are often not in equilibrium primarily due to small population sizes, which can amplify the effects of genetic drift. In smaller populations, random fluctuations in allele frequencies can occur simply due to chance rather than due to natural selection. This can lead to significant changes in the genetic makeup of the population over time, potentially allowing for the loss of genetic diversity or the fixation of harmful alleles.

In contrast, random mating, stable environmental conditions, and equal viability of all alleles would promote a more stable genetic structure and could support equilibrium within a larger population framework. However, the inherent instability that comes with small population sizes can disrupt equilibrium, leading to evolutionary changes and making small populations particularly vulnerable to extinction.