Populations and evolution

Question 1

What is a population?

Answer 1

A group of organisms of the same species that occupies a particular space at a particular time and can potentially interbreed.
Any species exists as one or more populations.

Question 2

What is the gene pool?

Answer 2

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

Question 3

What is allelic frequency?

Answer 3

The number of times an allele occurs within the gene pool.

Question 4

What does the Hardy-Weinberg principle assume?

Answer 4

No mutations arise.
The population is isolated (no flow of alleles into or out the population).
There is no selection (equal chance the allele is passed to the next generation).
The population is large.
Mating within the population is random.

Question 5

What is the Hardy-Weinberg equation?

Answer 5

p + q = 1.0
p^2 + 2pq + q^2 = 1.0

Question 6

How does genetic variation arise?

Answer 6

Mutations
Meiosis
Random fertilisation of gametes

Question 7

How does mutations cause genetic variation?

Answer 7

These sudden changes to genes and chromosomes may be passed on to the next generation.

Question 8

How does meiosis cause genetic variation?

Answer 8

This form of nuclear division produces new combinations of alleles before they are passed into the gametes.

Question 9

How does random fertilisation of gametes cause genetic variation?

Answer 9

In sexual reproduction this produces new combinations of alleles and the offspring are therefore different from parents.
Which gametes fuse with which is a random process, further increasing variety.

Question 10

What is variation due largely to environmental influences?

Answer 10

The environment influences affect the way the organism’s genes are expressed.
Influences include climatic conditions (temperature, rainfall, sunlight) soil conditions, pH and food availability.

Question 11

What is an example of environmental influence?

Answer 11

A buttercup plant may be genetically determined to grow tall, but if the seed is germinated in poor light or low soil nitrate, the plant may not grow properly and be short.

Question 12

What are selection pressures?

Answer 12

The environmental conditions that limit the population of a species.
This includes predation, disease and competition.
They vary from time to time.
They determine the frequency of alleles within the gene pool.

Question 13

What does the process of evolution by natural selection depend on?

Answer 13

Organisms produce more offspring than can be supported by the available supply of food, light, space.
There is genetic variety within the populations of all species.
A variety of phenotypes that selection operates against.

Question 14

What is the link between over-production and natural selection?

Answer 14

When there are too many offspring for the available resources, there is intraspecific competition amongst individuals for the limited resources.
The greater the numbers, the greater the competition and the more individuals die in the struggle to survive.
Those in a population more suited to prevailing conditions are more likely to survive, and breed, and so pass on the favourable allele combinations, which will therefore be a different allele frequency to the previous generation.

Question 15

What are examples of better adaptation to conditions?

Answer 15

Better ability to hide from or escape predators.
Better ability to obtain light or catch prey.
Better ability to resist disease or find a mate.

Question 16

Why is variation important in natural selection?

Answer 16

Populations showing little individual genetic variation are more vulnerable to new diseases and climate changes.
A genetically different population means some will have the combination of genes needed to survive in almost any new circumstances.
It is important that a species is capable of adapting to changes resulting from the evolution of other species.

Question 17

What is the role of variation in natural selection?

Answer 17

The larger a population is, and the more genetically varied, the greater the chance of individuals having the combination of alleles leading to a phenotype advantageous in the struggle for survival.
These individuals will breed and pass their allele combinations on.

Question 18

What is the definition of stabilising selection?

Answer 18

This preserves the average phenotype of a population by favouring average individuals, selection against the extreme phenotypes.

Question 19

What is the definition of directional selection?

Answer 19

This changes the phenotypes of a population by favouring the phenotypes that vary in one direction from the mean of the population, selection for one extreme phenotype.

Question 20

What is the defintion of disruptive selection?

Answer 20

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

Question 21

What is stabilising selection?

Answer 21

It tends to occur where the environmental conditions are constant over long periods of time.

Question 22

What is an example of not needing stabilising selection?

Answer 22

Fur length in a species - in years when the temperatures are hotter than usual, shorter length will be advantageous as they can lose body heat rapidly. In colder the opposite applies, as they are better insulated.
Therefore, if the environment fluctuates from year to year, both extremes will survive because each will have some years where it thrives at the expense of the other.

Question 23

What is an example of stabilising selection?

Answer 23

If the temperature remains constant in the middle, the extremes will never be at an advantage.
They will therefore be selected against in favour of the average.
The mean will remain the same, but there will be fewer individuals at the extremes.

Question 24

What is directional selection?

Answer 24

If the environmental conditions change, so will the optimum value for survival.
Some individuals, either to the left or right of the mean, will possess allelic combinations with the new optimum phenotypic character.
So there will be a selection pressure favouring this allele combination.

Question 25

What is disruptive selection?

Answer 25

This occurs when an environmental factor, e.g. temperature, takes two or more distinctive forms.
If the temperature alternated between low in winter and high in summer, this could ultimately lead to two separate species of the mammal - one with long fur active in winter, and one with short and active in summer.

Question 26

What is an example of disruptive selection?

Answer 26

The Coho salmon where large and small males have a selective advantage over medium males.
The small males are able to sneak up to females in spawning grounds.
The large males are fierce competitors.
This leaves the medium at a disadvantage.

Question 27

What is polymorphism?

Answer 27

When species have two or more distinct forms, that are genetically distinct, but exist within the same interbreeding population.

Question 28

What is natural selection of the peppered moth?

Answer 28

It existed in light colour, until a black variety arose due to a mutation.
By then, most walls, buildings, trees were blackened by the soot from industrial development. Soot killed the lichens that covered trees and walls.
The black form was less conspicuous than the light form against the trees.
The light form was then eaten more frequently by birds, so black moths became more predominant.

Question 29

How does selection affect allelic frequencies?

Answer 29

Environmental changes affect the probability of an allele being passed on in a population and hence the number of times it occurs in the gene pool.
It only affects the frequency of a mutant allele already present in the gene pool, not the probability of it arising.

Question 30

What is speciation?

Answer 30

The evolution of new species from existing ones.

Question 31

What is a species?

Answer 31

A group of individuals that have common ancestry and so share the same genes but different alleles, and are capable of breeding with one another to produce fertile offspring.
Members of a species are reproductively separated from other species.

Question 32

What is reproductive separation?

Answer 32

Reproductive separation followed by genetic change due to natural selection.
Although species tend to breed only with those in the population, they are capable of breeding with others.

Question 33

How can reproductive separation form a new species?

Answer 33

If a population becomes separated from other populations and undergoes different mutations, it will become genetically different.
Each of the populations will experience different selection pressures because of the different environments.
Natural selection will lead to changes in the allelic frequencies of each population.

Question 34

What is adaptive radiation?

Answer 34

The different phenotypes produced will be subject to selection pressure that will lead to each population becoming adapted to its local environment.
This means that even if the populations were no longer physically separated, they would be unable to interbreed successfully.
Each population would now be a different species, each with its own gene pool.

Question 35

What is genetic drift?

Answer 35

In small populations, the relatively few members possess a smaller variety of alleles than the members of large populations.
There is less genetic diversity.
Alleles that are passed on will quickly affect the whole population as their frequency is high.
This makes it more likely the population will develop into a separate species.

Question 36

What is allopatric speciation?

Answer 36

Speciation where two populations become geographically separated.
This can occur by a physical barrier between two populations that prevents them interbreeding.
E.g. oceans, rivers, mountains, and deserts.

Question 37

Why does allopatric speciation occur?

Answer 37

If environmental conditions either side of the barrier vary, then natural selection will influence the populations differently and each will evolve leading to adaptations.
This could take hundreds of generations, but ultimately leads to reproductive separation and the formation of a separate species.

Question 38

What is sympatric speciation?

Answer 38

Speciation that results within in a population in the same area leading to them becoming reproductively separated.
This could be ecological, temporal, behavioural, mechanical, gametic, hybrid sterility.

Question 39

What is ecological variation?

Answer 39

When populations inhabit different habitats within the same area and so individuals rarely meet.

Question 40

What is temporal variation?

Answer 40

The breeding seasons of each population do not coincide and so they do not interbreed.

Question 41

What is behavioural variation?

Answer 41

Mating is often preceded by courtship which is stimulated by the colour or markings of the opposite sex, the call or particular actions of a mate.
Any mutations that cause variations in these patterns may prevent mating.
E.g. different types of fireflies have different blinking patterns.

Question 42

What is mechanical variation?

Answer 42

Anatomical differences may prevent mating occuring.
e.g. different beetle species have different reproductive organs.

Question 43

What is gametic variation?

Answer 43

The gametes may be prevented from meeting due to genetic or biochemical incompatibility.
e.g. some pollen grains don’t germinate when they land on a stigma of different genetic makeup.

Question 44

What is hybrid sterility?

Answer 44

Hybrids formed from the fusion of gametes of different species often cannot produce viable gametes.
A horse (64) and donkey (62) results in a mule with 63 chromosomes.
It is impossible for these to pair up properly in meiosis and so the gametes are not viable.