Topic 7B: Populations and Evolution

Question 1

Definition: Species

Answer 1

A group of similar organisms that can reproduce to give fertile offspring.

Question 2

Definition: Population

Answer 2

A group of organisms of the same species living in a particular area at a particular time - giving them the potential to interbreed.

Question 3

Definition: Gene pool

Answer 3

The complete range of alleles present in a population.

Question 4

Definition: Allele frequency

Answer 4

How often an allele occurs in a population.

Question 5

What are the conditions when using the Hardy-Weinberg Principle?

Answer 5

• Large population.
• No immigration or emigration.
• No natural selection.
• Mating must be random.

Question 6

What is the Hardy-Weinberg Principle?

Answer 6

A mathematical model that predicts the frequency of alleles in a population won’t change from one generation to the next.

Question 7

What are the Hardy-Weinberg equations?

Answer 7

p + q = 1

p = frequency of dominant allele
q = frequency of recessive allele

p² + 2pq + q² = 1

p² = frequency of homozygous dominant genotype
2pq = frequency of heterozygous genotype.
q² = frequency of homozygous recessive genotype / frequency of recessive phenotype.

Question 8

Definition: Variation

Answer 8

The differences that exist between individuals due to mutations or genetic variation introduced in meiosis, leading to a population showing a wide range of different phenotypes.

Question 9

Definition: Evolution

Answer 9

When the frequency of an allele in a population changes over time.

Question 10

How does evolution occur?

Answer 10

• Natural Selection

* Genetic drift

Question 11

Evolution: Natural selection

Answer 11

1) Variation between organisms of the same species means some individuals are better adapted to selection pressures than others.
2) These individuals are more likely to survive and reproduce, passing on their genes.
3) This increases the frequency of these beneficial alleles in the gene pool of the next generation.

Question 12

Types of Natural Selection: Stabilising selection

Answer 12

• Individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce.
• It occurs in an unchanging environment and reduces the range of possible phenotypes.

Question 13

Types of natural selection: Directional selection

Answer 13

• Individuals with alleles for characteristics towards one end of the range are more likely to survive and reproduce.
• Usually due to an environmental change.

Question 14

Types of natural selection: Disruptive selection

Answer 14

• Individuals with alleles for characteristics towards both extremes of the range are more likely to survive and reproduce.
• Occurs when the environment favours more than one phenotype.

Question 15

What is speciation?

Answer 15

• The development of a new species from an existing species due to populations becoming reproductively isolated.
• This changes the allele frequency, causing a change in phenotype meaning the two populations can no longer interbreed to produce fertile offspring.

Question 16

Types of speciation: Allopatric speciation

Answer 16

1) The population becomes geographically separated by a physical barrier.
2) Each new population experiences different selection pressures in their different environments.
3) Natural selection occurs in each population causing the phenotypic frequency of each population to change.
4) Eventually, individuals from each population change so much that they cannot breed to produce fertile offspring, and so the two groups are now different species.

Question 17

Types of speciation: Sympatric speciation

Answer 17

1) A random mutation occurs within a population.
2) The individuals carrying this mutation are unable to breed with individuals not carrying the mutation.
3) Therefore, the individuals are reproductively isolated, and now a different species.

Question 18

Mechanisms of reproductive isolation: Seasonal changes

Answer 18

Individuals develop different flowering or mating seasons, or become sexually active at different times of the year. Therefore, they are unable to breed together.

Question 19

Mechanisms of reproductive isolation: Mechanical changes

Answer 19

Changes in the size and shape of genitalia can prevent successful mating. This prevents the individuals from breeding.

Question 20

Mechanisms of reproductive isolation: Behavioural changes

Answer 20

A group of individuals may develop undesirable courtship rituals. Therefore, they are less likely to breed with other individuals of the same species.

Question 21

Evolution: Genetic drift

Answer 21

When chance dictates which alleles are passed in to offspring.

1) Individuals within a population show variation in their genotypes.
2) By chance, the allele for one phenotype is passed on to more offspring than others.
3) Therefore, the number of individuals with that allele increases.
4) If by chance, this allele is passed on more frequently again and again, evolution occurs.