During Selection the Mean Phenotype Changes?

Sometimes new species may develop when species that share common genes reproduce. A distribution in which most of the population occurs close to the mean. During Which selection, the mean phenotype changes. Certain traits can improve survival, causing alleles for those traits to increase in frequency.

When an individual moves from reproduces in its population its alleles are no longer part of that population's gene pool?

When an individual moves from / reproduces in its population, its alleles are no longer part of that population's gene pool. 2. When an individual moves into a new population, the genetic diversity of the new population increases / decreases.

What type of selection sees the phenotype decline if it becomes too common?

In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population.