Unit 7: Natural Selection

Evolution is a change in the genetic makeup (allele frequencies) of a population over generations. Individuals do not evolve; populations do.

Natural selection consistently changes allele frequencies in adaptive directions, but mutation, genetic drift, gene flow, and nonrandom mating also change allele frequencies without necessarily producing adaptation.

Populations produce more offspring than resources can support, so individuals compete; those whose heritable traits help them obtain resources survive and reproduce more, passing those traits on.

Individuals vary; much variation is heritable; more offspring are produced than can survive; and individuals with advantageous heritable traits tend to leave more offspring, so those traits accumulate.

Selection acts on phenotypes, but only traits with a genetic basis are passed to offspring, so only heritable variation shifts the next generation's allele frequencies.

No. Selection only acts on existing variation; new alleles arise randomly through mutation and recombination, not in response to need.