Evolution may lead to speciation

Question 1

Give genetic factors that cause
phenotypic variation within a species

Answer 1

1. Mutation of alleles.
2. Random fertilisation by gametes.
3. Random assortment of genetic
   material during meiosis

Question 2

Other than genetic factors, why else may
phenotype vary within a species?

Answer 2

Environmental influences

Question 3

Why does natural selection occur?

Answer 3

1. Predation
2. Disease
3. Competition

Question 4

How does natural selection cause a
change in a population’s gene pool over
generations?

Answer 4

Organisms with advantageous
characteristics are more likely to survive
and pass their favourable alleles to
offspring. Frequency of unfavourable
alleles decreases

Question 5

What is stabilising selection?

Answer 5

Occurs when environmental conditions
stay the same. Individuals closest to the
mean are favoured, and any new
characteristics are selected against.
Results in low diversity

Question 6

What is directional selection?

Answer 6

Occurs when environmental conditions
change. Individuals with phenotypes suited to
the new conditions will survive and pass on
their genes. Over time the mean of the
population will move towards these
characteristics

Question 7

What is disruptive selection?

Answer 7

The opposite of stabilising selection, in
that both extremes of the normal
distribution are favoured over the mean.
Over time, the population becomes
phenotypically divided

Question 8

Define speciation

Answer 8

Where a population is split and isolated, there
are different selective pressures on the two
groups. If the genetic makeup changes to the
extent the two groups can not longer
interbreed, they have become separate
species.

Question 9

What is meant by allopatric speciation?

Answer 9

Speciation resulting from a physical
barrier e.g. river, mountain range. The
environments occupied by the two
groups are different, and therefore
different alleles are favoured

Question 10

What is meant by sympatric speciation?

Answer 10

Speciation resulting from a non-physical
barrier e.g. a mutation that no longer allows
two organisms to produce fertile offspring.
Any changes in anatomy or behaviour may
also prevent breeding

Question 11

Define genetic drift

Answer 11

A change in a population’s allele
frequencies that occurs due to chance
rather than selective pressures. In other
words, it is caused by ‘sampling error’
during reproduction

Question 12

Why does genetic drift affect small
populations more than large ones?

Answer 12

The gene pool is smaller, so there are
less alleles available and any change in
frequency becomes pronounced very
quickly