4.2.2 Natural Selection and Microevolution

What is variation?

There is always genetic and phenotypic variation between individuals within a healthy population. This is due to different combinations of alleles that have risen from random mating, independent assortment and recombination during gamete formation and mutations. 

What is a selective pressure?

The conditions or factors that affect allele frequency in a population. Selection pressures can be positive or negative.

What is a positive selection pressure

• Selection that favours a heritable trait

• Allele frequency for a particular gene increases, therefore is advantageous in an organisms current environment

• Selective advantage (survive to reproduce to pass on trait)

What is a negative selection pressure

• Selection disfavours a heritable trait

• Allele for a particular gene decreases, therefore is disadvantageous in an organisms current environment

Examples of selective pressures include:

• Natural environmental pressures (e.g. predation, competition for food)

• Artificial pressures (e.g. by humans due to selective breeding)

What are the two driving forces of evolution?

Selection pressures and mutations

What is Natural selection?

The influence of environmental pressures on allele frequency of a population.

Note that : Environmental selection pressures affect the survivability (viability) and reproduction (fecundity) of an organism by removing individuals through death or reducing reproduction rates.

What are ‘allele frequencies’?

The proportion of a specific allele among all allele for that gene in a population.

How is microevolutionary change analysed/ determined?

Through calculations/change of allele frequencies over time

What is the allele frequency formula? 

Allele frequency ‘A’ = (Number of ‘A’ alleles in a population) / (Total number of alleles for that gene) × 100%

What are 3 main types of phenotypic selection?

• Stabilising

• Directional

• Disruptive

What is stabilising selection?

Selection favours intermediate traits in the phenotypic range and acts against extreme traits. Allele frequencies remain stable. (heterozygotes likely to increase)

What is directional selection?

Selection favours a particular phenotype and skews the allele frequency and phenotype in one direction. Allele frequency for the selected traits will increase.

• Genetic diversity may increase as extreme phenotypes are favoured

• Homozygotes will likely increase

• Intermediate traits could be lost

What is disruptive selection? 

Selection favours extremes in the phenotype range. 

List the key causes of microevolution

• Germ line mutations

• Gene flow

• Genetic drift

What are germline mutations

Changes in DNA of a reproductive cell (sperm or egg) that is passes onto the offspring

How and why do germline mutations cause microevolution?

• Creating new alleles, increases variation (A mutation might change a gene to make a new version (allele), which can result in a new trait)

• Introducing beneficial traits (If a mutation gives an organisms a survival advantage, it may become more common through natural selection

• Neutral or harmful effects (mutations can be neutral or harmful but they still contribute to the genetic pool and their effects might change under different environmental conditions)

What is Genetic flow 

Genetic flow is the movement of alleles (genes) from one population to another through migration.

How and why does genetic flow cause microevolution?

• Introduction of new alleles  (when individuals from a different population migrate and breed, they bring new genetic material into the population)

• Changes in allele frequencies (shifts the proportion of alleles in the gene pool)

• Increased genetic diversity (More variation means the population may have better chance of surviving environmental changes 

What is genetic drift

Genetic drift is a random change in allele frequencies in a population due to a chance event, not natural selection.

How and why does genetic drift cause microevolution?

• Random survival (sometime individuals with certain alleles survive and reproduce by chance, not because their better adapted)

• Alleles lost or fixed

• Microevolution