Theme 4c Pt 1

A process that is independent of phenotype/alleles and is random. Sampling error Change in alleles frequencies due to chance in finite populations (small populations)

When the populations are small Less variance in a small population

There is not heterozygosity

If the allele is fixed over time, it is constant, so the other allele (a) is eliminated. A/A

The other alleles is fixed (a) and the (A) is gone forever

The allele gets neither fixed or eliminated since the population is big. It’s stays relatively the same frequency

Temporary reductions in population size that cause drift and reduce variation because only the alleles of the small remaining population is present. This causes genetic differences between populations.

The founder effect Bottleneck

When New populations are started from a small number of individuals leaving the larger group Depending on the alleles they take with them some can be fixed and some eliminated

Reduces genetic variation and causes populations divergence

Mating with relatives

Mating with non relatives (less closely related)

Individuals with similar geno/phenotypes mate with each other (rather than others) more than expected under a random mating pattern

Choosing to Mate with individuals that are more or less closely related than those from a random mating population

Small populations Mating system (mating with relatives, cousins) Choosing to Mate more frequently

No just genotype. Allele frequency’s are not changed

Reduced fitness, because rare deleterious alleles are more likely to combine in homozygotes. They lose heterozygosity and become homozygous recessive

Directional Stabilizing (aka balancing) Disruptive

Individuals of a certain extreme phenotype are favoured Individuals at one end have higher fitness and other end has lower fitness The trait mean shifts toward one end

Individuals with intermediate phenotype are favoured (grey moths) and extremes phenotype are selected against (black white moths) Variance decreases between generations, but the trait mean doesn’t change Heterozygote advantage

Sickle cell anemia, extreme SS phenotype had more extremes anemia but heterozygote AS have mild condition AA extreme get ver bad malaria

It is like stabilizing selection, selecting against one extreme for a period of time then eventually selecting against the other extreme Looks like directional selection in short term

Both extreme phenotypes are favoured and the intermediate is selected against Average individuals have lower fitness, but the trait mean doesn’t change

When the population is large Small advantages in fitness can lead to large changes over the long term