The reproductive contribution of a phenotype to subsequent
generations relative to the contributions of other phenotypes is called
Changes in absolute numbers of offspring are responsible for increases and decreases in the size
of a population, but only changes in the relative success of different phenotypes within a
population lead to changes in allele frequencies from one generation to another.
The fitness of a phenotype is determined by the average rates of survival and reproduction of
individuals with that phenotype.
Natural selection produces variable results
Natural selection can act on characters with quantitative variation in any one several different
o Stabilizing selection—favouring average individuals
o Directional selection –favouring individuals that vary in one direction from the mean
o Disruptive selection—favouring individuals that vary in opposite directions from the
mean of the population.
Sexual selection influences reproductive success
Sexual selection is a special type of natural selection that acts on characteristics that determine
the reproductive success.
Sexual selection may favour traits that enhance the bearer’s chances of reproduction, but
reduce its chances of survival.
o Such costly traits reliably demonstrate the quality of their possessors as mates b/c they
enable the choosing sex (usually females) to distinguish between genuinely fit
individuals and exaggerators.
How is Genetic Variation Maintained within Populations?
Neutral mutations may accumulate within populations
An allele that does not affect the fitness of an organism is called a neutral allele.
Neutral alleles are unaffected by natural selection.
Neutral alleles tend to accumulate in a population over time, providing it with considerable
Sexual recombination amplifies the number of possible genotypes
Sexual recombination generates an endless variety of genotypic combinations that increases the