Selection and Evolution

Describe the difference between continuous and discontinuous variation

Describe the difference between continuous and discontinuous variation

Continuous variation - shown by a characteristic that can have any value over a range e.g. height

Discontinuous variation - shown by a characteristic that can only take certain values e.g. eye colour or blood group

Explain the genetic basis of continuous variation

Characteristics that show continuous variation often controlled by many genes

The alleles at each gene locus have a small effect, all of which are added together to control a characteristic

Explain the genetic basis of discontinuous variation

Typically characteristics that show discontinuous variation are controlled by one or few genes

The alleles present at these gene loci have a large effect on the characteristic

State the phenotype arises

The phenotype results from the interactions between the genotype and the environment

Why is the t-test used?

The t-test is used to determine whether there is a statistically significant difference between the means of two data sets that show normal distribution

Why is genetic variation important?

Variation in a population increases the likelihood that some individuals have a phenotype which is better suited to the environment, particularly as environmental changes occur

Explain why natural selection occurs

Natural selection occurs because populations have the capacity to produce many offspring and increase their numbers exponentially. The individuals of the population compete for resources; those who are better adapted will survive, reproduce and pass on their alleles to next generation

Describe stabilising selection

In stabilising selection, the extremes of a characteristics are selected against

This does eliminate some genetic variation conditions are constant for a long time

Give an example of stabilising selection.

Human birth weights - babies born too far below or above the optimum weight may not survive infancy. They therefore cannot reproduce and pass on their alleles. The extremes are selected against.

Describe and give an example of directional selection

When environmental conditions change, individuals with a characteristic away from the mean will be better suited to the new environment, and this extreme is selected for e.g. antibiotic resistance in bacteria

What is disruptive selection?

The extremes of a characteristic are favoured and the mean is selected against. This can result in two subpopulations with different phenotypes, and is important for evolutionary change

How does selection affect allele frequency in a population?

Environmental factors exerting the forces of selection change the allele frequency of a population by increasing advantageous alleles.

Selection does not affect the probability of new mutant alleles arising, it affects the frequency of the alleles already in the population.

What is genetic drift?

A change in allele frequency in a small population due to chance

Describe the founder effect

The founder effect is a type of genetic drift.

It occurs when a small subpopulation is isolated from the larger parent population. This small population has less genetic variation and may not the same proportion of alleles as there was in the original population.

What is the bottleneck effect?

Population numbers are significantly reduced in one generation, which reduces genetic variation in the next generations.

Meaning the next generations are less likely to survive environmental changes

State the Hardy-Weinberg equation

p² + 2pq + q² = 1

p is the frequency of the dominant allele

q is the frequency of the recessive allele

State the conditions that have to be satisfied in the Hardy-Weinberg principle

• The proportion of dominant and recessive alleles remain the same over generations

• No mutations arise

• There is no selection

• The population must be large

• There must be no flow of alleles in or out of the population

• Random mating

What is selective breeding?

The process by which humans artificially select organisms with desirable characteristics and breed them to produce offspring with desirable phenotypes

Give exmaples of selective bredding

• Introducing disease resistance into wheat and rise varieties to limit loss of crops by disease

• Improving the milk yield of cattle

• In maize, inbreeding and hybridisation to produce uniform vigorous maize crops

Outline the theory of evolution

Evolution is the process by which new species arise from pre-existing species over time due to changes to the gene pools between generations

Name one wat of investigation evolutionary relationships between species

Reading the DNA sequences of species

The more similar the sequences, the more closely related the speices

Describe how subpopulations of a species can be separated

• Geographically

• Ecologically

• Behaviourally

What is allopatric speciation?

Speciation that occurs due to geographical separation

The two isolated populations may be exposed to different environments where there are different selection pressures and this would result in changes in allele frequencies and eventually two new species emerge

Explain how sympatric speciation may occur

Within the same geographical region subpopulations can become reproductively separated

The different environmental conditions in each situation exert selective forces, therefore over time the populations become a separate species