8.3 Gene pools

Question 1

Define population

Answer 1

All the organisms of a particular species living in the same habitat.

Question 2

What are the three types of selection?

Answer 2

• directional
• stabilising
• disruptive

Question 3

What is stabilising selection?

+ example

Answer 3

Stabilizing selection is a form of natural selection wherein individuals with moderate or average phenotypes are more fit (more likely to survive and reproduce). This form of selection picks against phenotypic extremes (e.g. the largest and the smallest are less fit).
eg: tigers having medium length tails

Question 4

What are the results of stabilizing selection?

Answer 4

• any new characteristics are selected against
• results in low diversity

Question 5

What is disruptive selection?

+ example

Answer 5

The opposite of stabilising selection in that both extremes of the normal distribution are favoured over the mean.
example: squirrel tails

Question 6

What is the result of disruptive selection?

Answer 6

Over time the new population becomes phenotypically divided and a new new species may develop.

Question 7

Define genetic drift

Answer 7

A change in a population’s allele frequencies that occurs due to chance rather than selective pressures.

caused by ‘sampling error’ during reproduction

Question 8

What are some selection pressures?

Answer 8

• predation
• disease
• environmental changes
• competition

Question 9

What is meant by the bottleneck effect?

Answer 9

Where a catastrophic event dramatically reduces the size of a population thereby decreasing the variety of the alleles in the gene pool and causing large changes in allele frequency.

Question 10

Examples of events causing bottleneck effect

Answer 10

famines, earthquakes, floods, fires, disease, and droughts; or human activities such as genocide, speciocide,

Question 11

What is meant by the founder effect?

Answer 11

When a small number of individuals becomes isolated , forming a new population with a limited gene pool with allele frequencies not reflective of the original population.

Question 12

What is the Hardy-Weinberg principle?

Answer 12

Allows us to estimate the frequency of alleles in a population as well as if allele frequency is changing over time.

Question 13

Give the assumptions made by the Hardy-Weinbery principle

Answer 13

• no mutations occur to create new alleles
• no migration in or out of the pop
• no selection pressures, so alleles are all equally passed on to the next generation
• random mating
• large population

Question 14

Explain the Hardy-Weinberg equation for calculating allele frequency

Answer 14

frequencies must add up to 1
p= (powerful) dominant allele
q = (quiet) recessive allele
The equation is therefore p+q=1

Question 15

Explain the Hardy-Weinberg equation for calculating genotype frequency

Answer 15

The frequencies of each genotype must add up to 1
p2 is the frequency of homozygous dominant
2pq is the frequency of heterozygous
q2 is the frequency of homozygous recessive
The equation is therefore p2 + 2qp+ q2 =1

Question 16

Compare and contrast stabilising selection with disruptive selection

Answer 16

• both change the frequency of alleles
• both select against (less well adapted) phenotypes
• disruptive selection selects against mean whereas stabillising selection selects against extreme phenotypes
• dispruptive selection leads to two distinct populations but stabilising selection maintains one population