7.3 Evolution may lead to speciation

Question 1

Individuals within a population of a species may show a wide range of variation in phenotype. What is this due to?

Answer 1

Individuals within a population of a species may show a wide range
of variation in phenotype. This is due to genetic and environmental
factors. The primary source of genetic variation is mutation.
Meiosis and the random fertilisation of gametes during sexual
reproduction produce further genetic variation.

Question 2

What causes natural selection?

Answer 2

Predation, disease and competition for the means of survival result
in differential survival and reproduction, ie natural selection.

Those organisms with phenotypes providing selective advantages
are likely to produce more offspring, and so pass on their favourable
alleles to the next generation.

This differential reproductive success will increase the frequency of the favourable alleles within a gene pool.

NOTE THE WORDING: ‘likely to produce more offspring’ NOT ‘more likely to produce offspring’.

Question 3

Phenotype

Answer 3

The phenotype is the expression of this genotype and its
interaction with the environment. Individuals within a population of a species may show a wide range of variation in phenotype.

Question 4

Stabilising Selection

Answer 4

When a phenotype exists as a continuous range (lowest to highest), stabilising selection decreases the frequency of alleles which result in ‘extreme’ (lowest and highest) phenotypes whilst favouring those in the middle of the range. The allele frequencies remains constant, as alleles that result in change are selected against.

Question 5

Directional selection

Answer 5

When a phenotype exists as a continuous range (lowest to highest), directional selection decreases the frequency of alleles which result in one ‘extreme’ (either the lowest or the highest) whilst favouring those at the other end of the range. The allele frequency is changing in one direction.

Question 6

Disruptive selection

Answer 6

When a phenotype exists as a continuous range (lowest to highest), disruptive selection increases the frequency of alleles which result in ‘extreme’ (lowest and highest) phenotypes while those in the middle of the range are selected against, often because there is intraspecific competition between organisms with the most common phenotype (ie in the middle of the range). Disruptive selection enables sympatric speciation.

Question 7

Evolution

Answer 7

Evolution is a change in the allele frequencies in a population.

Question 8

Speciation

Answer 8

Reproductive separation of two populations can result in the
accumulation of difference in their gene pools. New species arise
when these genetic differences lead to an inability of members of
the populations to interbreed and produce fertile offspring. In this
way, new species arise from existing species.

Question 9

Allopatric speciation

Answer 9

Arises when one population becomes geographically separated into two reproductively isolated populations in two different habitats.

Question 10

Sympatric speciation

Answer 10

Arises when reproductive isolation occurs in one population while it occupies one habitat. Isolation could be:

TEMPORAL - eg time of day that courtship behaviour occurs

BEHAVIOURAL - eg mating preferences for one type or another

ECOLOGICAL - one population begins adapting to two separate niches in the ecosystem

(Other isolating mechanisms exist)

Question 11

Genetic drift

Answer 11

Genetic drift is a random change in allele frequencies in a population.

Genetic drift is not driven by natural selection, but by chance events. It is really only likely to occur in very small populations, because each individual organism in a small population possesses a bigger proportion of the total gene pool.

Genetic drift can lead to reduced genetic variation as alleles become lost, or may result in an allele becoming ‘fixed’ (which means the only allele for a gene in the whole population).

Evolution is a change in the allele frequencies in a population, so genetic drift results in evolution without adaptation.

Two small population scenarios:

Bottleneck effect: a population recovers after a drastic reduction in population size - the recovering population will have reduced genetic variation.

Founder effect: a small group breaks off to colonize a new area, carrying only a fraction of the original genetic variation.