Evolution may lead to speciation

Question 1

Why might individuals within a population of a species show a wide range of variation in phenotype?

Answer 1

• Genetic factors:
• Primary source: mutations
• Gene mutation; change in base sequence of DNA 🡪 a new allele
• Meiosis:
• Crossing over between homologous chromosomes
• Independent segregation of homologous chromosomes
• Random fertilisation of gametes during sexual reproduction
• Environmental factors:
• Example: climate, food, lifestyle – apply these to the specific exam
  question

Question 2

Define the term evolution.

Answer 2

• Evolution: change in allele frequencies in a population
  over time

Question 3

Describe the process of evolution.

Answer 3

Evolution occurs through the process of Natural selection:
1. Variation within a population due to mutations
2. Selection pressure e.g. predation, disease, competition 🡪 struggle for survival
3. Some organisms have phenotypes providing selective advantages (due to favourable allele(s))
4. These organisms are more likely to survive and reproduce, producing more offspring and passing on their favourable allele(s) to the next generation i.e. differential levels of survival and reproductive success
E.g. those able to hide from / escape predators, compete for light / prey / mates, or resist a disease
5. Allele frequencies in gene pool change over many generations
(time)

Question 4

Explain the effects of Directional selection on a
population.

Answer 4

Directional selection
- Selective pressures / environment favours individuals with a favourable combination of alleles one direction from the mean
- Mean shifts in direction of
favourable allele /phenotype
Example: Pepper moth. Originally a
higher population of white moths. Change in environment – pollution –resulted in a higher population of
black moths

Question 5

Explain the effects of Stabilising selection on a
population. Give an example.

Answer 5

• Selective pressures favour the mean / act against the two extremes of a characteristic
• Individuals with extreme phenotypes less likely to survive 🡪 standard deviation smaller over time
• Mean stays the same
• Example: Clutch size in birds. Robins lay eggs. If they lay too many eggs, chicks become malnourished and die. If they lay not enough eggs, they may all die and not hatch.

Question 6

Explain the effects of Disruptive selection on a
population. Why is it the most important type of
selection for evolution?

Answer 6

• Selection against the mean
• Population becomes
  phenotypically divided –
  favours both extremes of
  phenotypes
• Most important type of
  selection for evolutionary
  change as could result in two
  separate species

Question 7

Explain how speciation occurs.

Answer 7

Formation of a new species from an existing species…
- Reproductive separation of two populations (of the same
species) can result in the accumulation of differences 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

Question 8

Explain how allopatric speciation occurs.

Answer 8

• Geographical isolation
• = Separate gene pools; no interbreeding / gene flow (between populations)
• Mutations create genetic variation in each population
• Different selection pressures (e.g. predation, disease, competition) act
  on each population
• Leading to natural selection of different favourable alleles /characteristics
• Differential survival and reproductive success
• Leads to change of allele frequencies within gene pools (favourable
  allele increases) over a long time
• Members of different populations can’t interbreed to produce fertile
  offspring = new species arises from existing species = speciation

Question 9

Explain how sympatric speciation occurs.

Answer 9

• Populations aren’t geographically isolated / population in the same
  area
• Genetic variation within the population due to mutations
• Resulting in a mechanism that makes individuals reproductively isolated (gene flow is restricted), for example…
• Gamete incompatibility
• Temporal – different breeding/mating seasons
• Behavioural – different courtship behaviour preventing mating
• Mechanical – incompatible genitalia
• Different selection pressures operate
• Leads to change of allele frequencies within gene pools / divergence of gene pools
• Members of different populations can’t interbreed to produce fertile offspring = new species arises from existing species = speciation

Question 10

What is Genetic drift and why is it important in
small populations?

Answer 10

• Genetic drift: mechanism of evolution in which allele frequencies
  of a population change over generations due to chance
• Strongest effects in small populations as chance has a greater influence
• Unlike natural selection, genetic drift doesn’t take into account how favourable or harmful an allele is
• Genetic drift has major effects when a population is sharply reduced in size (bottleneck effect) or when a small, new colony forms from a main population (founder effect)