7.3 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 of 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 generation

Answer 4

• Organisms with advantageous characteristics are more likely to survive and pass on 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 characteristic 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 opposites 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 no longer interbreed, they have become separate species

Question 9

What is meant by allopatric speciation?

Answer 9

• Speciation resulting in 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. 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 and any change in frequency becomes pronounced very quickly