Populations and evolution

Question 1

Define gene pool

Answer 1

The gene pool is all of the alleles of all the genes of all the individuals in a population at a given time

Question 2

Define allelic frequency

Answer 2

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

Question 3

What is the Hardy-Weinberg principle

Answer 3

• The Hardy-Weinberg principle provides a mathematical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population.
• The principle makes the assumption that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next.

Question 4

What five conditions must be met for the Hardy-Weinberg principle to work

Answer 4

• No mutations arise
• The population is isolated: there is no flow of alleles into or out of the population.
• There is no selection: all alleles are equally likely to be passed onto the next generation.
• The population is large
• Mating within the population is random.

Question 5

What are the two equations that make up the Hardy-Weinberg principle

Answer 5

1) p+q=1
2) p^2 + 2pq + q^2= 1

Question 6

What do each of the variables in the Hardy Weinberg equations refer to

Answer 6

• q= frequency of recessive allele
• q^2 = frequency of recessive phenotype (homozygous recessive)
• p= the frequency of the dominant allele in a population
• p^2 = the frequency of homozygous dominant genotype in the population
• 2pq= frequency of heterozygous genotype in the population.

Question 7

What is the primary source of genetic variation

Answer 7

Mutation

Question 8

List the three things that genetic variation arises as a result of

Answer 8

1) Mutations
2) Meiosis
3) Random fertilisation of gametes

Question 9

Explain how mutations lead to genetic variation

Answer 9

Mutations are random changes to the DNA sequence which cause a change to the gene and therefore the phenotype it codes for

Question 10

Explain how meiosis causes genetic variation

Answer 10

• Meiosis produces new combinations of alleles before they are passed into the gametes, all of which are different.
• The randomness of meiosis and processes such as independent segregation and crossing over during meiosis also increase genetic variation.

Question 11

Describe how random fertilisation of gametes causes genetic variation

Answer 11

Which gamete fuses with which at fertilisation is a random process which further adds to the variety of offspring two parents can produce.

Question 12

What are selection pressures

Answer 12

The environmental factors that limit the population of a species

Question 13

Name three key selection pressures

Answer 13

• Predation
• Disease
• Competition

Question 14

List three key factors that lead to the process of evolution by natural selection

Answer 14

• Organisms produce more offspring than can be supported by the available resources (supply of food/light/space)
• There is genetic variety within the populations of all species.
• There is a variety of phenotypes that selection operates against

Question 15

What is intraspecific competition

Answer 15

Competition between individuals of the same species

Question 16

Explain the role of over-production of offspring in natural selection

Answer 16

• All species have the potential to increase their numbers exponentially
• In reality this never happens which means that the death rate of every species must be high.
• High reproductive rates have evolved in many species to ensure a sufficiently large population survives to breed and produce the next generation.
• This means that organisms produce too many offspring for the available resources.
• This leads to natural selection because there is intraspecific competition between individuals for these resources.
• The individuals with phenotypes best suited to the environment will be more likely to survive.
• (Hence reproduce, pass their genetics on).

Question 17

Why is genetic variation important in a population (why do organisms not evolve to be exactly the same)

Answer 17

Because genetic variation increases the chance that if an environmental change were to occur, some organisms would have the phenotypes to survive.

Question 18

List the three main types of selection

Answer 18

1) Stabilising selection
2) Directional selection
3) Disruptive selection

Question 19

What is stabilising selection

Answer 19

Stabilising selection preserves the average phenotypes by selecting against extreme phenotypes and selecting towards the mean phenotype.

Question 20

What is directional selection

Answer 20

Directional selection changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population- it selects for one extreme and against the mean and the other extreme

Question 21

What is disruptive selection

Answer 21

Disruptive selection favours individuals with extreme phenotypes at both sides of the mean and selects against those individuals with the mean phenotype.

Question 22

What does stabilising selection do (in terms of phenotypes) and when does it occur

Answer 22

• Stabilising selection tends to eliminate the extremes of the phenotype range within a population and with it the capacity for evolutionary change.
• It tends to occur where the environmental conditions are constant over long periods of change

Question 23

Explain what disruptive selection is

Answer 23

• Disruptive selection is the opposite of stabilising selection.
• Disruptive selection favours extreme phenotypes at the expense of the intermediate phenotypes.
• Although it is the least common form of selection, it is the most important in bringing about evolutionary change.
• Disruptive section occurs when an environmental factor takes two or more distinct forms.

Question 24

What is speciation

Answer 24

The evolution of a new species from existing ones

Question 25

Define species

Answer 25

A group of organisms that are capable of breeding to produce living, fertile offspring.

Question 26

How do members of two different species differ

Answer 26

Members of different species are reproductively separated from each other

Question 27

What are the two forms of speciation

Answer 27

Allopatric speciation and sympatric speciation.

Question 28

Summarise the way in which new species are formed

Answer 28

Through reproductive separation followed by genetic change due to natural selection

Question 29

What is adaptive radiation

Answer 29

• Adaptive radiation refers to the adaptation (via genetic mutation) of an organism which enables it to successfully spread, or radiate, into other environments.
• Adaptive radiation leads to speciation and is only used to describe living organisms.
• Adaptive radiation can be opportunistic or forced through changes to natural habitats.

Question 30

Describe how speciation occurs most commonly

Answer 30

• A population becomes separated from other populations and undergoes different mutations causing it to be genetically different.
• Each of the populations will experience different selection pressures because the environment of each will be slightly different.
• Natural selection will then lead to changes in the allelic frequency of each population.
• The different phenotypes each combination of alleles produces will be subject to selection pressure that will lead to each population becoming adapted to its local environment (adaptive radiation).
• As a result of these genetic differences the populations would be unable to interbreed successfully- so speciation has occurred.

Question 31

Explain how genetic drift leads to speciation

Answer 31

• Small populations have a smaller variety of alleles in their gene pool: they have a lower genetic diversity.
• As these few individuals breed, the genetic diversity of the population is restricted to those few alleles in the original population.
• As there are only a small number of alleles, there is not an equal chance of each being passed on.
• Those that are passed on will quickly affect the whole population as their frequency is high.
• Any mutation to one of these alleles that is selectively favoured will also more quickly affect the whole population because it’s frequency will be high.
• The effects of genetic drift will be greater and the population will change relatively rapidly, making it more likely to develop into a separate species.

Question 32

Why does genetic drift not have a big affect in large populations

Answer 32

In large populations the effects of genetic drift are likely to be less, and development into a new species is likely to be slower.

Question 33

What is allopatric speciation

Answer 33

Allopatric speciation describes the form of speciation where two populations become geographically separated.

Question 34

What is sympatric speciation

Answer 34

Sympatric speciation describes the form of speciation that results in individuals within a population in the same area becoming reproductively separated.

Question 35

What are the seven forms of isolating mechanisms that may separate populations

Answer 35

1) Geographical
2) Ecological
3) Temporal
4) Behavioural
5) Mechanical
6) Gametic
7) Hybrid sterility

Question 36

What does a population being geographically isolated mean

Answer 36

Populations are isolated by physical barriers such as oceans etc

Question 37

What does populations being ecologically separated mean

Answer 37

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

Question 38

What does populations being temporally separated mean

Answer 38

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

Question 39

What does populations being behaviourally separated mean

Answer 39

• Mating is often preceded by courtship, which is stimulated by the colour or marking of the opposite sex, the call or particular actions of a mate.
• Any mutations which cause variations in these patterns may prevent mating.

Question 40

What does populations being mechanically separated mean

Answer 40

Anatomical differences are present that prevent mating

Question 41

Why does populations being gametically separated mean

Answer 41

The gametes may be prevented from meeting due to genetic or biochemical incompatibility.

Question 42

What is hybrid sterility

Answer 42

Hybrids formed from the fusion of gametes from different species are often sterile because they cannot produce viable gametes.