Evolution may lead to speciation

Question 1

Why might individuals within a population of a species show a wide range of variation in phenotype?

Answer 1

Genetic factors, the primary source of which is mutations e.g. gene mutations; change in base sequence of DNA which produces a new allele

During meiosis e.g. crossing over between homologous chromosomes or during independent segregation of homologous chromosomes

Random fertilisation of gametes during sexual reproduction

Environmental factors e.g. climate, food, lifestyle

Question 2

Process of natural selection

Answer 2

Evolution occurs by natural selection

1. Variation occurs within a population due to mutation, and so individuals of the same species vary since they have different alleles
2. Various selection pressures e.g. predation, disease, competition create a struggle for survival
3. Some organisms have phenotypes providing selective advantages due to favourable alleles and differential levels of survival and reproductive success
4. These organisms are more likely to survive and reproduce, producing more offspring and passing on their favourable alleles to the next generation e.g. those able to hide from/escape predators, compete for light/prey/mates, or resist a disease
5. Allele frequencies in gene pool therefore change over many generations

Question 3

What is evolution

Answer 3

Evolution is the change in allele frequencies in a population over time

Question 4

The effects of directional selection

Answer 4

Selective pressures/environment favours individuals with a favourable combination of alleles one direction from the mean

This could be in response to an environmental change

Mean shifts in direction of favourable allele/phenotype

e.g. the pepper moth. Originally, there was a higher population of white moths. However, the change in environment due to pollution resulted in a higher population of black moths

Question 5

The effects of stabilising selection

Answer 5

Selective pressures favour the mean/act against the two extremes of a characteristic

It occurs when the environment isn’t changing and it reduces the range of possible phenotypes

Individuals with extreme phenotypes less likely to survive and the standard deviation decreases over time

Mean stays the same

e.g. clutch size in birds. Robins lay eggs. If they lay too many eggs, chicks become malnourished and die. If they do not lay enough eggs, they may all die and not hatch.

Question 6

The effects of disruptive selection

Answer 6

Selection against the mean

Population becomes phenotypically divided and favours both extremes (at either end of the range) of phenotypes

Opposite of stabilising selection

Occurs when the environment favours more than one phenotype

Most important type of selection for evolutionary change as it could result in two separate species

e.g. range of beak sizes in a population, where birds with large beaks are specialised to eat large seeds and birds with small beaks are specialised to eat small seeds. Medium beak birds may not be able to eat small or large sized seeds effectively

Question 7

What is speciation

Answer 7

Speciation is the formation of a new species from an existing species

Reproductive separation of two populations (of the same species) can result in changes in allele frequency and the accumulation of differences in their gene pools

New species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring

Question 8

Allopatric speciation

Answer 8

Populations that are geographically separated will experience slightly different conditions

So there will be separate gene pools; no interbreeding and gene flow between populations. Mutations create genetic variation in each population whilst different selection pressures (e.g. predation, disease, competition) act on each population

This leads to the natural selection of different favourable alleles/characteristics, causing changes in phenotype frequencies

Differential survival and reproductive success leads to change of allele frequencies within gene pools (favourable allele increases) over a long time

Members of different populations can no longer interbreed to produce fertile offspring, therefore a new species arises from existing species

Question 9

Sympatric speciation

Answer 9

Populations aren’t geographically isolated and populations reside in the same area

Genetic variation within the population occurs due to mutations

Resulting in a mechanism that makes individuals reproductively isolated (gene flow is restricted)

Examples of this are gamete incompatibility, temporal (different breeding/mating seasons), behavioural (different courtship behaviour preventing mating) and mechanical (incompatible genitalia)

Different selection pressures operate and lead to the change of allele frequencies within gene pools/divergence of gene pools

Members of different populations can no longer interbreed to produce fertile offspring and a new species arises from existing species

Question 10

Genetic drift and its importance in small populations

Answer 10

Genetic drift is the mechanism of evolution by which allele frequencies of a population change over generations due to chance

It has greater effects within small populations where chance has a greater influence

Unlike natural selection, genetic drift doesn’t take into account how favourable or harmful an allele is

Genetic drift has major effects when a population is sharply reduced in size (bottleneck effect) or when a small, new colony forms from a main population (founder effect)