Biology - Chapter 18: Populations and Evolution

Gene pool

All the alleles of all the genes of all the individuals in a population at a given time

Allelic frequency

The number of times an allele occurs within the gene pool

Population

A group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed

What is the Hardy-Weinberg principle?

A mathematical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population

What is the assumption being made when using the Hardy-Weinberg principle?

The proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next

5 conditions needed in order for the Hardy-Weinberg principle to work

1. No mutations arise

2. Population is isolated (no flow of genes into / out of the population)

3. No selection (equal chance alleles passed on to the next generation)

4. Mating within the population is random

5. Population is large

2 equations of the Hardy-Weinberg principle for 2 alleles, A and B

Let P(A) = p and P(B) = q
p + q = 1

p² + 2pq + q² = 1

2 causes for variation

Genetic factors and environmental factors

3 causes for variation due to genetic factors

1. Mutations (produces new alleles)

2. Meiosis (produces new combinations of alleles)

3. Random fertilisation of gametes (produces new combinations of alleles)

What is the primary source of genetic variation?

Mutation

Is it easy to tell whether variation is due to genetic or environmental factors?

No - the effects of each factor are often hard to distinguish

Selection pressures

The environmental factors that limit the population of a species

Give 3 examples of selection pressures

Predation, disease and competition

Describe what happens during natural selection (5)

1. Random mutation leads to variation

2. Organisms with advantageous allele more able to survive and reproduce (link to question topic if needed)

3. They produce more offspring and pass on the advantageous allele to their offspring

4. This causes the frequency of that allele to increase over time

5. This is an example of directional selection

Why do some species have high reproductive rates?

It compensates for high death rates from the selection pressures, ensuring a sufficiently large population survives to breed and produce the next generation

3 main types of selection

Stabilising selection, Directional selection and Disruptive selection

What does stabilising selection do?

It preserves the average phenotype of a population by favouring average individuals

What does directional selection do?

It changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population

What does disruptive selection do?

It favours individuals with extreme phenotypes rather than those with phenotypes around the mean of the population

What is the least common form of selection?

Disruptive selection

Which type of selection is the most important when bringing about evolutionary change?

Disruptive selection (as it favours two extremes so two separate species may develop)

Polymorphism

When some species of organisms have two or more forms that are genetically distinct but exist within the same interbreeding population

Evolution by natural selection

A change in the allelic frequencies within a population

Speciation

The evolution of new species from existing ones

How do new species form?

As a result of reproductive separation followed by genetic change due to natural selection

Adaptive radiation

The rapid evolution of multiple species from a common ancestor

Genetic drift

The change in allelic frequencies in a population from generation to generation as a result of chance events

Why is genetic drift important only in small populations?

There are relatively few members in the population, so reduced genetic diversity - small number of alleles. As small, unequal chance of each allele being passed on - those that are will quickly affect the whole population as their frequency would be high

2 types of speciation

Allopatric speciation and sympatric speciation

Allopatric speciation

A type of speciation that occurs after two populations become geographically separated

What might cause geographical separation?

Any physical barrier between the two populations, e.g. oceans, rivers, mountain ranges, deserts

Sympatric speciation

A type of speciation that occurs within a population in the same area, leading to them becoming reproductively separated

7 isolating mechanisms

1. Geographical

2. Ecological

3. Temporal

4. Behavioural

5. Mechanical

6. Gametic

7. Hybrid sterility

Describe the geographical isolating mechanism

Populations separated due to physical barriers

Describe the ecological isolating mechanism

Populations inhabit different habitats in the same area

Describe the temporal isolating mechanism

Populations have breeding seasons that do not coincide

Describe the gametic isolating mechanism

Gametes from each population are prevented from meeting due to genetic or biochemical incompatibility

Describe the hybrid sterility isolating mechanism

Hybrids formed from the fusion of gametes from different species cannot produce viable gametes, so are sterile

Describe the behavioural isolating mechanism

Populations have different courtship behaviours as a result of mutations

Describe the mechanical isolating mechanism

Populations have anatomical differences that prevent mating from occurring

In what size population does genetic drift take place?

Small populations

Describe how sympatric speciation occurs (6)

1. The populations are in the same area (not geographically isolated)

2. Mutation leads to a factor (link to question)

3. Causes reproductive isolation and the gene pools are kept separate

4. Different alleles are passed on so there are changes to the allele frequencies

5. This is an example of disruptive selection

6. This means the different species cannot breed to produce fertile offspring

Why might there be a high frequency of an allele in an isolated population? (2)

1. Because the population is isolated, it results in inbreeding

2. The allele is inherited from a common ancestor

Describe how allopatric speciation takes place (9)

1. Geographical isolation

2. Variation due to mutations

3. Leads to reproductive isolation

4. Mutation leads to a factor (link to question)

5. Different places have different selection pressures

6. Survival of each group is related to the factor

7. Adapted organisms more likely to survive and breed, pass alleles onto offspring

8. This causes a change in the allele frequency

9. Eventually, different populations cannot interbreed to produce fertile offspring

Why is speciation less frequent between organism that live in the same environment?

They have the same selefction pressures

Why might the observed frequencies of some alleles be different from the expected frequencies? (4)

1. Selection

2. High rate of mutation

3. Immigration / emigration

4. No random mating