Question

Why is a population the smallest possible unit of evolution, even though natural selection tends to act on individuals, not populations?

Answer 1

The similar organisms are combinations of different genes and different gene types. A peccary population, therefore, can be considered as a "pool" of genes and gene types that reflect the genetic diversity of all the individuals within the group. This "pool-of-genes" is what changes when evolution takes place. Every population has a pool of gene variants that acts as the raw material for evolutionary change. Armed with the measuring stick of the Hardy-Weinberg principle and genetic equilibrium, it is possible to study real populations of organisms as they interact.Quantitative and qualitative observations help define what types of agents influence gene pools and what type of changes they bring about.Even under conditions where nothing is changing, there is a range of variation for any phenotypic trait in any population. This can be quantified and represented in graphical form a normal distribution.As pressures from predators or the environment begin to act on the range of variation of phenotypes within a population, several types of changes can be seen directional selection, stabilizing selection or disruptive selection.In small populations, other forces are at work. When a population is small, the presence or absence of a single individual can have a profound effect on the population gene pool. A sudden reduction in population size can also alter the remaining gene pool. This is the bottleneck effect.