Chapter 15, Problem 30A

Summary Introduction

To compare and contrast:

Genetic drift and natural selection as mechanisms of evolution.

Introduction:

Charles Darwin in his theory of natural selection proposed that natural selection could modify a population to produce new species. Any change in the allelic frequencies in a population that is due to chance is called genetic drift.

Answer to Problem 30A

Genetic drift and natural selection are mechanisms of evolution as they both bring about change in allele frequency. In genetic drift allele frequency changes by chance but in natural selection allele frequency changes by differential reproductive success.

Bottleneck and founder effect are two extreme examples of genetic drift. Natural selection acts on an organism’s phenotype and changes allelic frequencies. It alters phenotypes through stabilizing selection, directional selection and disruptive selection.

Genetic drift is a more random process but natural selection is not a random process.

Explanation of Solution

Genetic drift- In large populations we know that there are enough alleles that “drift” to ensure that the allelic frequency of the entire population remain constant from one generation to another. In smaller populations, the genetic drift is more pronounced and there is high chance of losing an allele.

Founder effect: Genetic drift may occur when a small population separates from a big population and settles in a new location. Because the small population is a random subset of the original population it carries a random subset of the population’s genes. Uncommon genes may become common in the new population and the offspring will carry those alleles. This can result in large genetic variations in the separated populations.

Bottleneck: This occurs when a population declines to a very low number and then rebounds. The gene pool of the rebound population is genetically similar to the population at its lowest level. Hence it has reduced diversity.

Natural selection- There are four principles of the theory of natural selection:

1. Individual in a population show differences called variations.
2. Variations can be inherited; i.e they can be passed down from parent to offspring.
3. Organisms produce more offspring than can survive on available resources.
4. Variations that increase reproductive success will have a greater chance of being passed on than those that do not increase reproductive success.

In this way new species can be introduced into nature. Natural selection can introduce new species into the environment by promoting favorable traits. It selects the individuals that are best adapted for survival and reproduction.

Stabilizing selection- It is the most common forms of natural selection. It occurs to eliminate the extreme expressions of a trait when the average expression leads to higher fitness.

Directional selection- If an extreme version of a trait makes an organism more fit, directional selection might occur. This form of selection increases the expression of the extreme version of a trait in a population.

Disruptive selection- It is a process that splits a population into two groups. It tends to remove individuals with average traits but retains those expressing extreme traits at both ends of a continuum.