T8.3 - Gene Pools

Question 1

Define population

Answer 1

All of the individuals of a particular species living in the same place

Question 2

Define gene pool.

Answer 2

All of the alleles present in a population.

Question 3

State five processes that can change allele frequencies in a population

Answer 3

Mutation, migration, sexual reproduction, genetic drift, natural selection

Question 4

Give examples of natural selection pressures

Answer 4

Predation, competition, disease, changes in environmental conditions e.g. temperature.

Question 5

Explain how selection pressures change allele frequencies in a population.

Answer 5

Individuals with characteristics best adapted to the environment will have an increased chance of survival.
These individuals are more likely to reproduce and pass on their advantageous alleles to the next generation, while non-advantageous alleles die out.

Question 6

Explain how a new characteristic (phenotype) evolves in a population.

Answer 6

Organisms show genetic variation in the characteristics between individuals of a species.
Selection pressures exist e.g. predation, competition, disease.
Individuals with characteristics best adapted to the environment will have an increased chance of survival.
These individuals are more likely to reproduce and pass on their advantageous alleles to the next generation.
This increases frequency of the advantageous allele (and the characteristic it controls) in the population, resulting in change in genotype over generations

Question 7

Describe stabilising selection.

Answer 7

The most common phenotype is the best adapted.
Natural selection eliminates the most extreme phenotypes.
Resulting in a reduction in variation in the population.

Question 8

Describe an example of stabilising selection in humans

Answer 8

Selection against extremes in human birth weights results in most babies being between 3-4 kg at birth.
Babies with low birth weights have poor organ development, higher susceptibility to disease, therefore higher mortality rates.
Childbirth complications can lead to mortality of large babies (and mothers).

Question 9

How could modern medical intervention be affecting human birth weights?

Answer 9

Modern medical intervention could be reducing selection pressure on human birth weight by increasing the survival of small and large babies.
(For example, forceps, vacuum cups, C-sections reducing mortality of large babies.) This means that alleles coding for larger babies are successfully passed on and so overtime these alleles could increase in frequency in the population.
Advancements in neonatal care e.g. incubators, blood transfusions, feeding tubes, bilirubin lights, surgery, mean lower mortality rates of small / premature babies.
This means that the frequency of alleles resulting in increased risk of premature babies could increase, resulting in more mothers giving birth to small babies).

Question 10

Describe directional selection.

Answer 10

Natural selection reduces variation at one extreme of the range while favoring variants at the other end.
The resulting bell shaped curve shifts in the direction of selection.
Results in a change in one phenotype to another which is more advantageous to the environment.

Question 11

Describe disruptive selection.

Answer 11

Natural selection favors phenotypes at both extremes of a phenotypic range over intermediate variants.
Over time, the population becomes phenotypically divided (the bell shaped curve acquires two peaks i.e. becomes bimodal).
May result in a new species - speciation

Question 12

Explain why individuals that are heterozygous for a gene (Aa) may be more common in the population than would be predicted using
Hardy-Weinberg laws.

Answer 12

Heterozygote advantage - the heterozygous condition can have higher fitness than either homozygote (AA or aa).
Example: Individuals heterozygous for the sickle cell mutation have higher fitness than either homozygous conditions.

Question 13

Explain why heterozygote advantage of the sickle cell mutation is apparent in regions where malaria is endemic.

Answer 13

Malaria is transmitted by certain species of mosquito which spreads the intracellular protozoan parasite, Plasmodium.
Individuals with a normal phenotype are very susceptible to malaria.
Heterozygotes are much less so because sickle cells have low potassium levels and the parasites infecting these cells die.
Sickle cell homozygotes die from sickle cell disease.

Question 14

What is genetic drift?

Answer 14

Random changes in allele frequencies in a population that occur due to chance (no selection pressure acting on alleles).

Question 15

What size of population is affected most by genetic drift, and what can happen to allele frequencies in this kind of population?

Answer 15

Very small; alleles may become fixed (allele frequency = 1.0), or lost (0.0).

Question 16

What is a population bottleneck?

Answer 16

A chance (unselective) event that drastically reduces the size of a population.
This can be natural disasters, e.g. heavy predation/disease/seasonal climate change/floods, or human activity e.g. overhunting, habitat destruction, genocides.

Question 17

What are the consequences of a bottleneck event?

Answer 17

Small surviving population
> affected by genetic drift.
> alleles may be fixed or lost
> reducing gene pool
> loss of genetic diversity.
> inbreeding depression

Question 18

What is inbreeding depression?

Answer 18

The reduction in fitness of a population due to inbreeding, as this increases the chances of harmful recessive alleles being passed on
and increasing their frequency in the next generation.

Question 19

What is the founder Effect?

Answer 19

Occurs when a new population is formed by a few original founders which carry only a small fraction of the total genetic variation of the
parental population.

Question 20

Explain the Founder Effect.

Answer 20

Small number of individuals migrate away or become isolated from their original population;
due to limited mobility (their dispersal is often dependent on prevailing winds e.g. butterflies and other insects, reptiles, and small birds);
This founder population contains a small, non-representative sample of alleles from the parent population’s gene pool;
The founder population may evolve in a different direction than the parent population, due to different selection pressures or random
mutations creating new alleles;
The small founder population has reduced genetic diversity as it is affected by inbreeding and genetic drift

Question 21

Why is genetic diversity of the founder population lower than the parent population?

Answer 21

Founder population is much smaller, so effects of genetic drift are more pronounced, and may result in fixed/lost alleles

Question 22

How are bottlenecks different to the founder effect?

Answer 22

In Bottlenecks individuals are killed, reducing the choice of mates, in the founder effect individuals are ecologically/geographically separated

Question 23

Explain how inbreeding leads to genetic defects / higher chances of genetic disorders

Answer 23

Closely related individuals mate with each other;
which leads to inbreeding depression;
as increases chances of inheriting harmful alleles / homozygous recessive genotype

Question 24

State what the Hardy-Weinberg equation is used for

Answer 24

To monitor changes in the allele frequencies in a population, to measure and study evolutionary changes.

Question 25

What does the Hardy-Weinberg Equilibrium state?

Answer 25

A population’s allele and genotype frequencies are unless there is some type of evolutionary force acting upon them.

Question 26

State the Hardy-Weinberg equation

Answer 26

p2 + 2pq + q2 = 1

Question 27

What does p2 (squared) represent

Answer 27

the frequency of the homozygous dominant genotype

Question 28

What does q2 (squared) represent

Answer 28

the frequency of the homozygous recessive genotype

Question 29

What does 2pq represent?

Answer 29

the frequency of the heterozygous genotype

Question 30

For a populatin in genetic equilibrium, p + q = 1.0. What does p and q represent?

Answer 30

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

Question 31

What are the conditions for the Hardy-Weinberg equation?

Answer 31

No mutations
Random mating
Large population
No migration into or out of the population
No selection pressure