17.2. Natural Selection

Question 1

Natural Selection

Answer 1

The increased chance of survival and reproduction of organisms with particular phenotypes, because they are better adapted to their environment than those other phenotypes

Question 2

When does Natural Selection occur?

Answer 2

occurs as populations have the capacity to produce many offspring that compete for resources; in the ‘struggle for existence’ individuals that are best adapted are most likely to survive to breed and pass on their alleles to the next generation

Question 3

Biotic Selection Pressures

Answer 3

caused by other living organisms such as through predation competition for food, or infection by pathogens

Question 4

Abiotic Selection Pressures

Answer 4

caused by non-living components of the
environment such as water supply or nutrient levels in
the soil.

Question 5

Fitness

Answer 5

the capacity of an organism to survive and transmit its genotype to its offspring.

Question 6

Stabilising Selection

Answer 6

a type of natural selection in which the population mean stabilizes on a particular non-extreme trait value.

Question 7

Example of stabilising selection

Answer 7

• Robins lay 4 eggs
• larger clutches may result in malnourished chicks
• while smaller clutches may result in no viable offspring

Question 8

Directional Selection

Answer 8

a type of natural selection in which the phenotype (the observable characteristics) of the species tends toward one extreme rather the mean phenotype or the opposite extreme phenotype

Question 9

Examples of directional selection

Answer 9

• Light-coloured peppered moths are better camouflaged against a pristine environment
• Dark-coloured peppered moths are better camouflaged against a sooty environment
• During industrial revolution in England, colour shifted to dark

Question 10

Disruptive Selection

Answer 10

Extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups

Question 11

Examples of disruptive selection

Answer 11

• For squirrels, short tails help keep predators from catching them on the ground
• With long tails, they are good for balance when climbing up trees
• Medium tails are selected against

Question 12

Genetic Drift

Answer 12

change in allele frequency that occurs by chance, because only some of the organisms of each generation reproduce.
- most noticeable when a small number of individuals are separated from the rest of a large population.

Question 13

Founder effect

Answer 13

the reduced genetic diversity which results when a population is descended from a small number of colonizing ancestors.

Question 14

Bottleneck effect

Answer 14

a population bottleneck or genetic bottleneck is a sharp reduction in the size of a population due to environmental events or human activities

Question 15

Which selection may result in evolution?

Answer 15

Directional Selection

Question 16

Harvey Weinberg Principle

Answer 16

Used to predict genotype proportions in the population

Question 17

Harvey Weinberg Principle assumptions

Answer 17

It assumes a stable community, where:

1) There are no mutations, so no new alleles are introduced / lost
2) There is no immigration / emigration, so no new alleles are introduced / lost
3) There is no selection, so no alleles are favoured or eliminated
4) Mating is random, so alleles are mixed randomly
5) The population is large, so there are no genetic bottlenecks

Question 18

Equation 1

Answer 18

p + q = 1
where:
p is the dominant allele
q is the recessive allele

Question 19

Harvey Weinberg Principle assumptions

Answer 19

It assumes a stable community, where:

1) There are no mutations, so no new alleles are introduced / lost
2) There is no immigration / emigration, so no new alleles are introduced / lost
3) There is no selection, so no alleles are favoured or eliminated
4) Mating is random, so alleles are mixed randomly
5) The population is large, so there are no genetic bottlenecks

Question 20

Harvey Weinberg Equation 1

Answer 20

p + q = 1
where:
p is the dominant allele
q is the recessive allele

Question 21

Harvey Weinberg Equation 2

Answer 21

p^2 + pq + q^2 = 1
where:
p^2 = frequency of dominant genotype
2pq = heterozygous genotype
q^2 = frequency of recessive genotype