3.7.3 Evolution may lead to speciation

Question 1

Explain why individuals within a population of a species may show a wide range of variation in phenotype

Answer 1

Variation in phenotype is due to genetic and environmental factors. Genetic variation arises primarily from mutation which introduces new alleles. Further genetic variation is produced by meiosis through independent segregation and crossing over and by the random fertilisation of gametes. Environmental factors such as diet and climate influence phenotypic expression.

Question 2

Explain why genetic drift is important only in small populations

Answer 2

In small populations genetic drift has a greater effect because allele frequencies change due to chance rather than selection. Random events can cause certain alleles to increase or decrease in frequency or even be lost entirely. In large populations the effects of genetic drift are reduced as the impact of chance events is diluted.

Question 3

Explain how natural selection and isolation may result in change in the allele and phenotype frequency

Answer 3

A population becomes geographically or reproductively isolated preventing gene flow. Different selection pressures act on each population leading to differential survival and reproduction.

Question 4

Explain how natural selection leads to the formation of a new species

Answer 4

Individuals with advantageous alleles survive and reproduce passing on these alleles. Over generations allele frequencies change leading to the accumulation of genetic differences. If differences become great enough that individuals can no longer interbreed to produce fertile offspring speciation has occurred.

Question 5

Explain how evolutionary change over a long period of time has resulted in a great diversity of species.

Answer 5

Mutation introduces new alleles and genetic variation. Natural selection favours individuals with advantageous alleles who survive and reproduce passing on these alleles. Over time differential reproductive success changes allele frequencies in a population. Reproductive isolation leads to speciation as genetic differences accumulate. Repeated speciation events over millions of years lead to a great diversity of species.

Question 6

What is the primary source of genetic variation?

Answer 6

Mutation introduces new alleles into a population.

Question 7

How does meiosis contribute to genetic variation?

Answer 7

Meiosis increases genetic variation through independent segregation and crossing over.

Question 8

How does random fertilisation contribute to genetic variation?

Answer 8

Random fertilisation of gametes creates new allele combinations in offspring.

Question 9

What factors result in differential survival and reproduction?

Answer 9

Predation, disease, and competition for resources result in natural selection.

Question 10

How does natural selection affect allele frequencies?

Answer 10

Individuals with advantageous alleles survive and reproduce, increasing allele frequency in the population.

Question 11

What are the effects of stabilising selection?

Answer 11

Stabilising selection favours the mean phenotype, reducing variation and maintaining the status quo.

Question 12

What are the effects of directional selection?

Answer 12

Directional selection favours an extreme phenotype, shifting allele frequencies over time.

Question 13

What are the effects of disruptive selection?

Answer 13

Disruptive selection favours extreme phenotypes, leading to increased variation and potential speciation.

Question 14

Define evolution in terms of allele frequencies.

Answer 14

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

Question 15

What is reproductive isolation?

Answer 15

Reproductive isolation prevents gene flow between populations, leading to speciation.

Question 16

How does reproductive separation lead to new species?

Answer 16

Genetic differences accumulate until individuals can no longer interbreed to produce fertile offspring.

Question 17

What is allopatric speciation?

Answer 17

Allopatric speciation occurs when populations are geographically isolated, preventing gene flow.

Question 18

What is sympatric speciation?

Answer 18

Sympatric speciation occurs without geographical isolation due to factors like behavioural or temporal differences.

Question 19

Why is genetic drift more significant in small populations?

Answer 19

In small populations, allele frequencies change due to chance rather than selection, increasing genetic drift’s impact.

Question 20

One hypothesis for LP in humans suggests that the selective pressure was
related to some human populations farming cattle as a source of milk.

Describe how farming cattle as a source of milk could have led to an
increase in LP.

Answer 20

1. LP due to mutation
   OR
   Allele due to mutation;

Reject mutation caused by drinking milk.
Reject (LP) gene

2. Milk provides named nutrient;

Accept any correct named nutrient e.g. glucose,
galactose, protein
Ignore ‘sugar’ ‘lactose’ as named nutrient

3. Individuals with LP more likely to survive and reproduce

Question 21

Describe allopatric speciation (5)

Answer 21

1.      Geographical isolation;

2.      Reproductive separation/isolation OR No gene flow
OR
Gene pools remain separate;

Accept no interbreeding but must be a separate idea from mp5 which relates to definition of a species. Reject no inbreeding.

3.      Different selection pressures;

4.      Variation due to mutations;

5.      Different allele/s passed on/selected
OR
Change in frequency of allele/s;

6.      Eventually different species cannot (inter)breed to produce fertile offspring;

Question 22

Why does speciation take a long time?

Answer 22

1. Initially one/few animals with favourable mutation/allele;
2. Individuals with (favourable) mutation/allele will have more offspring;
3. Takes many generations for (favourable) mutation/allele to become the most common allele (of this gene);

Question 23

Natural Selection in Resistant Strains (5)

Answer 23

1. Some individuals in population naturally resistant/not killed by pesticide/antibiotic;
2. Due to mutation;
3. These survive when pesticide/antibiotic applied/non-resistant ones are killed;
4. To reproduce and pass on allele/gene (for resistance);
5. Increase in frequency of allele for resistance;

Question 24

Describe sympatric speciation (4)

Answer 24

1.      Not geographically isolated;

2.   mutation   causes reproductive isolation

3. Gene pools kept separate/no gene flow;

4.      Different allele/s passed on / selected
OR
Change in frequency of allele/s

5.      Cannot breed/mate to produce fertile offspring;