Populations (A-level only)

Question 1

Population

Answer 1

All the individuals of a species living within a specific area. Individuals in the same species can interbreed.

Question 2

Species

Answer 2

A species is a group of individual organisms that interbreed and produce fertile, viable offspring.

According to this definition, one species is distinguished from another when matings between individuals from each species do not produce fertile offspring.

Question 3

Multiple populations

Answer 3

Multiple populations may live in the same specific area.

E.g. a forest includes populations of flowering plants, pine trees, insects and microorganisms.

There can also be multiple populations of the same species.

E.g. there are populations of pine trees throughout the Northern hemisphere.

Question 4

Gene pool

Answer 4

The gene pool is the sum of all the alleles in a population.

Every individual has a different combination of alleles in their chromosomes.

Populations of a species share a gene pool.

Question 5

Allele frequency

Answer 5

Allele frequency is the rate at which a specific allele appears within a population.

If an allele is particularly advantageous for survival, an individual that possesses it will be more likely to reproduce than an individual that does not.

This means the allele is more likely to be passed onto its offspring.

Over time, this allele will likely increase in frequency.

Question 6

Hardy-Weinberg principles

Answer 6

The Hardy-Weinberg principle predicts that allele frequencies will not change from generation to generation.

The allele frequencies do not change because a population’s allele and genotype frequencies are inherently stable.

Question 7

Hardy-Weinberg assumptions

Answer 7

The Hardy-Weinberg principle only works if you make some assumptions about the population you are studying.

The assumptions are:
No mutations, migration or emigration.
No selective pressure for or against a specific genotype.
Population size is infinite.

In real populations, these assumptions are not normally met but making assumptions is still useful because it provides a model we can compare to real population changes.

Question 8

p + q = 1

Answer 8

p + q = 1

For every gene, there are two alleles present in an individual (except some genes on the X chromosome).

If you know the frequency of one of these alleles, the equation p + q = 1 can be used to calculate the frequency of the other allele.

p = frequency of one allele.
q = frequency of other allele.

Question 9

p2 + 2pq + q2 = 1

Answer 9

p2 + 2pq + q2 = 1

The Hardy-Weinberg is equation: p2 + 2pq + q2 = 1

p2 = pp = homozygous dominant.

q2 = qq = homozygous recessive.

2pq = heterozygous.

The components of the equation allows us to calculate the number of individuals in a population that have each genotype.

Question 10

Predicting genotypes

Answer 10

By observing the phenotypes in a population, you can identify how many individuals carry the homozygous recessive allele genotype (q2).

Only individuals who are homozygous recessive show the recessive phenotype.

Individuals with the dominant phenotype could be homozygous dominant or heterozygous.

This number can be used in the Hardy-Weinberg equation to allow the frequencies other two genotypes to be estimated.