Evolution may lead to speciation (A-level only)

Question 1

Genetic variation

Answer 1

There is huge variation between individuals in the same species.

E.g. A golden retriever looks completely different from a pug but they are both part of the species Canis familiaris.

This variation is caused by differences in the genetic code.

This is called genetic variation.

Genetic variation happens when individuals in the same species have different alleles.

Alleles are different versions of the same gene.

Question 2

Environmental variation

Answer 2

Environmental factors can also contribute to phenotypic variation.

The genotype of an individual interacts with the environment.

E.g. If an individual has the alleles for being tall but is fed a nutrient-poor diet, they will not grow as tall as an individual with the same alleles and a more nutrient-rich diet.

Question 3

Causes of genetic variation

Answer 3

Mutation
Recombination
Independent segregation
Random fertilisation

Question 4

Mutation

Answer 4

Mutation is the main source of genetic variation.

Mutations are changes in the base sequence.

Changes may cause the structure of proteins encoded by DNA to be altered.

Some mutations are harmful (e.g. the mutation that causes disease), some are beneficial and some have no effect on survival.

Question 5

Recombination

Answer 5

Meiosis is the process that produces haploid gametes from diploid cells.

During meiosis, the genes on homologous chromosomes are reshuffled through a process called recombination or crossing over.

Recombination produces new combinations of genes.

This increases genetic variation.

Question 6

Independent segregation

Answer 6

Meiosis also involves the independent segregation of chromosomes into haploid cells.

The chromosomes in a pair of homologous chromosomes are randomly separated and this creates genetic variation.

Question 7

Random fertilisation

Answer 7

During sexual reproduction, the gametes of two unrelated individuals are fused to produce a diploid individual.

This process is called random fertilisation.

Random fertilisation produces genetic variation because pairs of chromosomes from two separate individuals are combined.

Question 8

Whether an individual is capable of surviving or not is influenced by the following factors:

Answer 8

Predation
Disease
Competition

Question 9

Predation

Answer 9

Predation can decrease the probability of survival.

If an individual is exposed to high predation, they are more likely to be eaten by a predator.

If an individual is better at avoiding predators, they are more likely to survive and reproduce.

The genes that allow an individual to avoid predation are more likely to be passed onto offspring.

Question 10

Disease

Answer 10

Disease can decrease the probability of survival.

If an individual is better at combatting disease or avoiding infection, they are more likely to survive and reproduce.

The genes that allow an individual to avoid disease are more likely to be passed onto offspring.

Question 11

Competition

Answer 11

Competition can decrease the probability of survival.

Competition can exist between species (interspecific) or within species (intraspecific).

If an individual is better at outcompeting other individuals, they are more likely to survive and reproduce.

The genes that allow an individual to outcompete are more likely to be passed onto offspring.

Question 12

Natural selection

Answer 12

Natural selection is the process where the frequency of alleles in a population changes over time.

Natural selection is a process that gives rise to evolution.

Question 13

Selective advantage

Answer 13

Genetic variation exists between individuals in a population.

Some individuals will be more likely to survive (e.g. by being better at fighting disease) than others.

Individuals that are better at surviving than others have a selective advantage.

Question 14

Producing offspring

Answer 14

Individuals with a selective advantage are more likely to survive to reproduce than others.

This means that the genes of an individual with a selective advantage are more likely to be passed onto offspring than the genes of an individual without a selective advantage.

Question 15

Increasing allele frequencies

Answer 15

Individuals with a selective advantage are more likely to pass on their beneficial alleles than other individuals.

The next generation is more likely to have alleles that provide a selective advantage than alleles that do not.

This generation is also more likely to survive to reproduce and pass on their genes.

This causes the alleles that provide a selective advantage to increase in frequency in the population.

Question 16

Evolution

Answer 16

The process of natural selection gives rise to evolution.

Evolution is defined as a change in allele frequencies over time.

E.g. The evolution of humans is the change in allele frequencies that has taken place over millions of years.

This change has been driven by natural selection.

Question 17

Stabilising selection

Answer 17

In stabilising selection, natural selection favours an average phenotype.

The selection pressures in stabilising selection select against the extreme phenotypes.

Stabilising selection takes place in environments that do NOT change.

Question 18

Selection pressure

Answer 18

Natural selection acts on an individual by imposing a selection pressure.

The selection pressure is an external factor that influences the reproductive success of an individual.

E.g. Disease acts as a selection pressure that increases the reproductive success for individuals who are immune to the disease.

Question 19

Directional selection

Answer 19

In directional selection, natural selection favours one extreme phenotype.

The selection pressures in directional selection select against all other phenotypes.

Directional selection takes place after an environment has experienced a change.

Question 20

Disruptive selection

Answer 20

In disruptive selection, natural selection favours the two extremes of a phenotype.

The selection pressures in disruptive selection select against the average phenotype.

Disruptive selection takes place in an environment that favours more than one phenotype.

Disruptive selection can eventually lead to the production of two new species.

Question 21

Multiple male forms

Answer 21

Some populations of animals have multiple male forms.
Large, dominant alpha males obtain mates by brute force.

Small males can sneak in for secret copulations with the females in an alpha male’s territory.

The two phenotypes both have a selective advantage.

Natural selection selects for both phenotypes.

Medium-sized males are selected against because they are not big enough to obtain mates using size but they aren’t small enough for “sneaking”.

Question 22

Reproductive isolation

Answer 22

If two populations are prevented from interbreeding, differences begin to accumulate in the two gene pools.

Gene flow is the movement of alleles between a population.

When two populations are reproductively isolated, the gene flow of the populations is restricted.

The two populations are said to be reproductively isolated.

Question 23

Accumilation of differences

Answer 23

If the gene pools are reproductively isolated for an extended period of time, they will eventually accumulate enough genetic differences that the two populations can no longer interbreed.

If individuals from the two populations can no longer interbreed to produce fertile offspring, the two populations are considered separate species.

Question 24

Speciation

Answer 24

Speciation is when two (or more) populations have been reproductively isolated to produce two (or more) separate species.

This is how the thousands of species that exist today have been produced.

Question 25

Allopatric speciation

Answer 25

Allopatric speciation is a type of speciation where two (or more) populations are geographically isolated from each other.

Question 26

Geographical isolation

Answer 26

Populations of a species can split into two or more populations.

This may be caused by environmental changes (e.g. emergence of a river) or random events (e.g. a storm causing a population of birds to be carried to another island).

When two populations are physically separated, they are geographically isolated.

Question 27

Selection pressures

Answer 27

Populations in different environments will be exposed to different environmental conditions (e.g. climate, food availability).

The different environments will place different selection pressures on the populations.

The different selection pressures will drive natural selection in different directions.

Question 28

Allele frequencies

Answer 28

The different selection pressures on the populations cause the allele frequencies in the two populations to change.

If the populations continue to be geographically isolated for a long time, eventually the allele frequencies will change so much that if the populations are brought back together they can no longer interbreed.

Question 29

Sympatric speciation

Answer 29

Sympatric speciation is a type of speciation where two (or more) populations are not geographically isolated from each other.

Question 30

Reproductive isolation

Answer 30

In sympatric speciation, a population of species is reproductively isolated without geographical separation.

Reproductive isolation may arise where a mutation emerges in a population that prevents some members of the population from interbreeding with others.

Question 31

Reproductive isolation mutations

Answer 31

Sympatric speciation could be caused by mutations that influence many different factors:

Polyploidy - where a cell or organism has an extra set, or sets, of chromosomes.

Polyploidy organisms are reproductively isolated from diploid organisms.

Mutations may influence the flowering times or mating times of individuals.

Mutations may alter the reproductive organs.

Mutations may alter mating behaviours.

Question 32

Genetic drift

Answer 32

Speciation can also be driven by genetic drift.

Genetic drift is the random change in allele frequencies in a population.

Question 33

Random changes in alleles

Answer 33

By chance, some alleles are passed onto offspring and some are not.

This causes random changes in the allele frequencies in the population.

This is called genetic drift.

Question 34

Speciation in Genetic drift

Answer 34

When two populations are reproductively isolated, they are exposed to different selection pressures. T

hese selection pressures drive the evolution of the two populations in different directions.

This is called disruptive selection.

Genetic drift also contributes to the speciation by causing random changes in the allele frequencies of the two populations.

Because the two populations can no longer interbreed, the gene pools will become increasingly different.

This causes speciation.

Question 35

Genetic drift on smaller populations

Answer 35

Genetic drift tends to have a larger influence on evolutionary change in smaller populations.

This is because the gene pool (the sum of all the genes of a population) is smaller.

If the gene pool is smaller, any change in allele frequencies has a larger impact on the overall gene pool.