Evolution and Genetics

Outline the 4 postulates of natural selection.

Individuals within populations are variable in their traits.

Some of the variation between individuals is inherited.

In every generation some individuals are more successful at surviving and reproducing than others.

The survival and reproduction of individuals is non-random.Individuals with the most favorable variations who are better at surviving and reproducing are naturally selected.


Define genetic drift. Explain why the effects of genetic drift are generally more pronounced in a population with a small effective size compared to a population with a large effective size.

Genetic drift is random changes in the frequency of alleles within a population from one generation to the next. This is driven by chance alone. Genetic drift is more pronounced in a population with a small effective size because of random sampling error associated with transmission of genetic variation from one generation to the next. The genetic variation present in any one generation is only sample of that from the previous generation and therefore is subject to random sampling error.


What is the genetically effective population size? Explain why the genetically effective population size is often less than the census population size giving examples of the circumstances under which this might occur.

The genetically effective population size is a theoretical value that predicts the genetic properties of a population. It is based on the concept of an ideal population, which is one where every individual contributes equally to the next generation. The Ne can be less than Nc because of a disproportionate contribution by some individuals- in a population of 10000, only 1000 reproduce. This is due to biased sex ratio, variation in population size and reproductive success.


Distinguish between qualitative and quantitative traits. Describe a method that can be used to estimate the heritability of a quantitative trait.

A quantitative trait is a trait that shows a large number of possible phenotypes that typically must be described by a quantitative measurement or count. A qualitative trait is a trait that exhibits only a few phenotypes. Heritability can be estimated by elimination of variance components, correlation between relatives and parent-offspring regression or deduced from response to selection.


Name the 5 variance components that contributed to the phenotypic variance of a quantitative trait and define each of these components.

VA Is additive genetic variance. Variance due to the effects of additive alleles.

VD is variance due to the effects of dominant alleles.

VI is genic interaction variance due to the interaction between alleles at different genes.

VE is the environmental impact on genetic variance

VG is the interactions between the genetic and environmental impact.


Distinguish between the 3 modes of selection operating on continuous phenotypic traits in natural populations.

Directional selection- selection in favor of or against a particular trait. Fitness consistently increases or decreases with the value of a trait. This changes the average value of a trait in a population.

Stabilizing selection- Selection for an intermediate characteristic of a continuous trait. Average value of the trait does not change.

Disruptive selection-maintains 2 extremes, can lead to a population splitting. Average value of a trait does not change.





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