Types of selection

Stabilising selection

‣ Occurs when environmental conditions are stable.

‣ Individuals with phenotypes closest to the mean are favoured.

‣ Over time, extremes of phenotype are lost.

‣ Mean phenotype stays the same.

‣ Standard deviation decreases

Stabilising selection: human birth weight

‣ Very small AND very large babies are less likely to survive (and so less likely to reproduce).

‣ So babies with either extreme of birth weight are less likely to pass on their alleles for extreme birth weight.

‣ Extreme mass at birth decreases in frequency over time.

‣ Babies with birth weights around the mean are selected for.

Directional selection

‣ Occurs when there is a change in the environment (i.e. a change in selection pressures).

‣ One of the extremes of phenotype has a selective advantage.

‣ Mean shifts in the direction of the individuals with the selective advantage.

Directional selection: antibiotic resistance

‣ Random mutation produces allele that gives one bacterium resistance to an antibiotic.

‣ If the population is exposed to the antibiotic (= selection pressure), any bacteria without the resistance allele are killed.

‣ Resistant bacteria survive and reproduce, passing on the resistance allele to offspring.

‣ After many generations, most of the organisms in the population have the antibiotic resistance allele.

Genetic bottleneck

‣ An event that causes a big reduction in a population’s size.

‣ Many alleles lost.

‣ Leads to a reduction in the gene pool as modern population descended from few survivors.

Founder effect

‣ When a few individuals of a species start a new population.

‣ Therefore there are only a small number of alleles in the initial gene

pool (low genetic diversity).

‣ If carried to the new population, alleles that were rare in the original

population can become much more common in the new population