20: Natural Selection

Question 1

What is the process that leads to evolution?
(Paper 1)

Answer 1

• natural selection

Question 2

What is Evolution?
(Paper 1)

Answer 2

• the change in the frequency of alleles within a population over many generations

Question 3

Allele frequencies change due to _______ ______ _____, some phenotypes are more likely to survive and reproduce than others

(Paper 1)

Answer 3

• different reproductive success

Question 4

Give 3 causes of genetic variation:
Give 4 examples of selection pressures:
What do selection pressures lead to?

(Paper 1)

Answer 4

• meiosis, mutation random fusion of gametes
• predation, disease, competition and changes in environment
• some phenotypes containing a selective advantage (advantageous allele)

Question 5

Describe the process of Natural Selection:
(5 marker)

(Paper 1)

Answer 5

1. there is variation in a population
2. this is due to random mutations giving rise to new alleles of a gene
3. organisms with the advantageous allele, would have a phenotype that gives them a selective advantage in their environment
4. organisms with the advantageous allele are more likely to survive to reproduce, or at least to have more offspring, than others
5. these organisms more likely to pass on advantageous alleles to the next generation than others
6. this causes an increase in the frequency of the advantageous allele in that population over many generations

Question 6

What are the 3 types of Natural Selection?
(Paper 1)

Answer 6

1. directional
2. stabilising
3. disruptive

Question 7

Describe Directional Selection:
(Paper 1)

Answer 7

• occurs when the environment change
• directional selection acts against one of the extreme phenotypes and selects the other

Question 8

Describe Stabilising Selection:
(Paper 1)

Answer 8

• occurs in an unchanging environment
• this type of selection acts against both the extremes in a range of phenotypes, so it favours the middle/intermediate phenotypes and acts to prevent change

Question 9

Describe Disruptive Selection:
(Paper 2)

Answer 9

• occurs in a fluctuating environment, where predation the middle range
• individuals with both extremes of a phenotype have a selective advantage over those in the middle, so both are selected, and those in the middle are selected against

Question 10

Which is the least common type of Selection?
(Paper 2)

Answer 10

• disruptive

Question 11

What is Speciation?

Answer 11

• if different populations of the same species face different selection pressures this can lead to the formation of a new species

Question 12

Define Species:

Answer 12

• a group of organisms with similar characteristics that can interbreed to produce fertile offspring

Question 13

What are the 2 types of Speciation?

Answer 13

allopatric, sympatric

Question 14

What is Allopatric Speciation?

Answer 14

1. occurs when 2 populations of the same species become geographically separated by a physical barrier e.g mountain, bodies of water
2. this leads to the 2 populations unable to interbreed, leading to genetic differences increasing through selection due to the different environmental conditions, so different selection pressures
3. eventually 2 populations unable to interbreed

Question 15

Describe the Allopatric Speciation 5 marker:

Answer 15

1. parts of the population become geographically isolated
2. no interbreeding between 2 populations
3. different environments lead to different selection pressures
4. different beneficial mutations occurs leading to formation of different advantageous alleles
5. in each population organisms with advantageous phenotypes would be more likely to survive and reproduce than the others
6. allele frequencies of each population change over many generations
7. eventually genotypes become so different that they can’t produce fertile offspring
8. now the 2 populations are classified as new species

Question 16

Describe Sympatric Selection:

Answer 16

• 2 populations may still inhabit same area, but still evolve into 2 different species
• this can occur due to random mutations leading to differences that prevent individuals with mutation from breeding fertile offspring with those that do not carry the mutation
• this is known as reproductive isolation

Question 17

Describe 3 causes of Sympatric Selection:

Answer 17

• Seasonal/Temporal Isolation: members of the 2 populations reproduce at different times of the year
• Mechanical Isolation: anatomical differences may prevent mating occurring
  e.g physically impossible for penis to enter vagina in mammals
• Behavioral Isolation: members of 2 population have different courtship patterns

Question 18

Evolution can also occur due to ____ ____
Describe how:

Answer 18

• genetic drift
• there is chance rather than environmental factors dictating which alleles are passed on
• this leads to differences in allele frequencies in 2 populations
• this could lead to reproductive isolation and speciation
• the smaller the population the bigger impact these chance changes in alleles have