7.3 Evolution

Question 1

1. What is evolution?

Answer 1

A change in allele frequencies in a population

Question 2

evolution to take place, there must be variation within the original population. What are the main sources of variation?

Answer 2

• Genetic variation from:
o New mutations
o Meiosis (crossing over and independent segregation)
o Sexual reproduction (the random fertilisation of gametes)
• Variation caused by the environment e.g. food availability affecting growth, access to light (for plants), temperature (which can control the sex of an organism) injuries which have been picked up during a lifetime

Question 3

3. What are the two ‘forces’ which can cause allele frequency to change?

Answer 3

Natural selection and genetic drift

Question 4

4. What is natural selection?

Does it have a bigger effect on allele frequency in large or small populations?

Answer 4

When individuals with certain phenotypes (are better adapted to their environment), so they are more likely to survive and reproduce and pass on their alleles to their offspring

Question 5

5. What is a ‘selection pressure’?

Answer 5

A feature of the abiotic or biotic environment which makes some individuals more likely to survive than others

Question 6

6. What are the four key forces which create selection pressures and therefore cause natural selection?

Answer 6

Predation
Disease
Competition for resources needed for survival
Competition for mates

Question 7

7. How does natural selection occur and what is the consequence?

Answer 7

The best adapted individuals (the ones with a selective advantage) are more likely to survive and reproduce and pass on their advantageous alleles to the next generation

Question 8

8. What is genetic drift? Does it have a bigger effect on allele frequency in large or small populations? Why?

Answer 8

Change in the frequency of an allele or genotype because of chance.

It has a bigger effect in small populations because ‘chance’ plays a bigger role in small populations.

Question 9

1. What is directional selection?

Answer 9

Selection that favours a phenotype that is away from the mean of the population range.
An extreme phenotype has a selective advantage.
This changes the characteristics of the population over time.

Question 10

What effect does directional selection have on

(i) the mean characteristic of the population?

Answer 10

(i) the mean characteristic of the population? The mean changes (either increases or decreases)

Question 11

(ii) The standard deviation of the characteristics of the population th

Answer 11

(ii) The standard deviation of the characteristics of the population the standard deviation stays the same

Question 12

2. What is stabilising selection?

Answer 12

When the average phenotype has a selective advantage over the extreme phenotypes and is selected for

Question 13

What effect does stabilising selection have on
(i) the mean characteristic of the population?
The mean stays the same
(ii) The standard deviation of the characteristics of the population
The standard deviation decreases

Answer 13

I

Question 14

3. What is disruptive selection?

Answer 14

Selection that favours both the extreme phenotypes and selects against the individuals with average phenotypes. This changes the characteristics of the population over time.

Question 15

What effect does stabilising selection have on
(iii) The standard deviation of the characteristics of the population
The standard deviation increases

Answer 15

Draw too

Question 16

1. What is meant by artificial selection? Give a few examples of when this might be carried out.

Answer 16

Artificial selection is when humans select which organisms breed. Humans select the organisms which have characteristics that are useful for the humans.
This results in a change in allele frequency of the population over time.

Examples:
Artificial selection is usually carried out on plants or animals which are used in food production. E.g.
- Strawberries that are big, red and juicy
- Sweetcorn that has many large grains of corn
- Cows that are big and have a lot of muscle so produce a lot of meat
- Dairy cows that produce a high volume of milk

Artificial selection has also been used to create different breeds of animal pets such as dogs. The dogs are selected for specific characteristics, e.g. fur colour, temperament, size etc

Question 17

2. Describe the process of artificial selection

Answer 17

• Humans select the individuals with the desired allele(s) which makes the individual ….. (always link to the context of the question and state what characteristic the desired allele codes for)
• These individuals are allowed to reproduce
• So they pass on their desirable alleles (more than other individuals)
• Therefore, the frequency of the desirable alleles increases in the population

Question 18

3. How is artificial selection different to natural selection?

Answer 18

In natural selection, it is the best adapted individuals with the advantageous alleles that give them a selective advantage which survive and reproduce

But in artificial selection is it the individuals with alleles that give desirable characteristics for humans are selected to reproduce

Sometimes artificial selection selects individuals who would not actually survive very well in the wild! E.g. pug dogs have great difficulty in breathing so they are very unlikely to survive and reproduce in the wild. But they are SELECTED by humans for their short noses which people think are cute.
(Mrs M thinks it’s very cruel! And Mr Tal agrees, it is an abomination to do that to Mother Nature. To take her proud wolf and turn it into this monstrosity. Have we not done enough harm already?)

Question 19

9. What is the definition of a species

Answer 19

A group of organisms with similar characteristics who can interbreed to produce fertile offspring

Question 20

10. What does ‘speciation’ mean?

Answer 20

The development a new species

Question 21

11. What is meant by geographic isolation?

Answer 21

When one population is divided into two populations which are geographically separated from each other, so that the individuals in each population can no longer meet each other and interact

Question 22

12. Why is geographic isolation key to the development of new species?

Answer 22

Because it prevents the individuals meeting and cross breeding and thus it prevents all gene flow between the populations

Question 23

13. What is allopatric speciation?

Answer 23

When new species evolve from a single ancestral species as a result of a physical separation between individuals of the original population.

Question 24

14. What is sympatric speciation?

Answer 24

When a new species evolves from a single ancestral species within the same geographic location.

Question 25

15. In the table below describe how each type of speciation occurs. Each box should contain just one point.

Answer 25

Allopatric speciation Sympatric speciation • This occurs when a population is separated into two due to geographic isolation • This occurs in the same habitat without geographic isolation • There is existing variation within the two populations due to previous mutations • There is existing variation within the population due to previous mutations • The geographic isolation prevents gene flow between the two populations • The mutations cause e.g. different courtship behaviour / assortative mating / different breeding seasons / using different habitats within an area etc • So there is no gene flow between individuals • So the two gene pools are kept separate • So the two gene pools are kept separate • New and different variations are introduced into the two populations by different random mutations [link to the context of the question] • New and different variations are introduced into the two populations by different random mutations [link to the context of the question] • The two populations experience different selective pressures • The two populations might experience different selective pressures • The individuals within each population with a selective advantage are more likely to survive and reproduce and pass on their favourable alleles to the next generation [always make this sentence relevant to the specific context of the question] • The individuals within each population with a selective advantage are more likely to survive and reproduce and pass on their favourable alleles to the next generation [always make this sentence relevant to the specific context of the question] • (and genetic drift might occur) • (and genetic drift might occur) • This leads to different changes in allele frequency in each population • This leads to different changes in allele frequency in each population • So disruptive natural selection occurs • The changes build up over many generations so that eventually individuals from the two populations cannot interbreed to produce fertile offspring • The changes build up over many generations so that eventually individuals from the two populations cannot interbreed to produce fertile offspring • The two populations become reproductively isolated • The two populations become reproductively isolated