Evolution and Speciation

Question 1

Define the term variation.

Answer 1

The difference between organisms.

Question 2

What are the reasons individuals in a population of a species may show a wide variation in phenotype?

Answer 2

1. Mutations.
2. Crossing over between homologous chromosomes during meiosis.
3. Independent segregation of homologous chromosomes during meiosis.
4. Random fertilisation.
5. The environment.

Question 3

Define what evolution is.

Answer 3

1. The change in allele frequency over time in a population.
2. Occurring through the process of natural selection.

Question 4

Explain the principles of natural selection.

Answer 4

1. Random gene mutations can result in new alleles of a gene.
2. The presence of a selection pressure means that organisms that have the allele that codes for a certain characteristic have a selective advantage.
3. These organisms are more likely to survive.
4. These organisms are more likely to reproduce and pass that allele on to their offspring.
5. Over many generations, the frequency of that allele increases.

Question 5

What is stabilising selection?

Answer 5

1. Organisms that have alleles that code for average phenotypes are more likely to survive and reproduce.
2. The frequency for alleles that code for average phenotypes increases, and the frequency of alleles that code for extreme phenotypes decreases.
3. The range in phenotypes decreases.

Question 6

What is directional selection?

Answer 6

1. Organisms that have alleles that code for an extreme phenotype are more likely to survive, reproduce and pass that allele on.
2. The frequency of the alleles that code for this extreme phenotype increases, and those coding for other phenotypes decrease.

Question 7

What is disruptive selection?

Answer 7

1. Organisms that have alleles that code for either extreme variation of a trait are more likely to survive and reproduce.
2. The frequency of the alleles that code for both extremes increases, and the frequency of alleles that code for average phenotypes decreases.
3. This can lead to speciation.

Question 8

Describe what speciation is.

Answer 8

1. Reproductive separation of two populations of the same species.
2. Leads to an accumulation of differences in their gene pools.
3. New species arise when the members of the two populations are no longer able to interbreed and produce fertile offspring.

Question 9

Describe what allopatric speciation is.

Answer 9

1. A population is split due to geographical isolation.
2. This leads to reproductive isolation
3. This separates the gene pools and prevents gene flow between the populations.
4. Random mutations can cause genetic variation within each population.
5. Different selection pressures act on each population, and this means different advantageous alleles are passed on in each population.
6. So the allele frequencies within each gene pool change over many generations.
7. Eventually, the members of the two different populations can’t interbreed and produce fertile offspring.

Question 10

What is sympatric speciation?

Answer 10

1. The population is not geographically isolated.
2. Mutations can lead to reproductive isolation.
3. This separates the gene pools and prevents gene flow within one population.
4. Different selection pressures act on each population.
5. So, different advantageous alleles are selected for in each population.
6. So allele frequencies within each gene pool change over many generations.
7. Eventually, different populations can’t interbreed and produce fertile offspring.

Question 11

What factors can lead to reproductive isolation?

Answer 11

1. Geographical isolation.
2. Gamete incompatibility.
3. Different courtship behaviours prevent mating.
4. Different breeding seasons.
5. Changes to body shape or reproductive organs.

Question 12

What is genetic drift?

Answer 12

A mechanism of evolution where allele frequencies in a population change over time due to chance.

Question 13

Describe the process of genetic drift and how it leads to evolution.

Answer 13

1. Some alleles are passed onto offspring more by chance.
2. The population has to be small and have no interbreeding with other populations.
3. This is so the gene pool is smaller and change has a greater influence on the gene pool.
4. This decreases genetic diversity and means that some alleles increase in frequency and others are lost.