Evolution

Question 1

What is the Hardy-Weinberg principle used for?

Answer 1

To estimate the frequency of alleles of a particular gene

Question 2

What is the definition of population?

Answer 2

All of the individuals of one species in a habitat

Question 3

What is the definition of gene pool?

Answer 3

The complete range of alleles present in a population

Question 4

What is the definition of allele frequency?

Answer 4

How often an allele occurs in a population (either a frequency 0-1, or percentage 0-100)

Question 5

What are the Hardy-Weinberg assumptions?

Answer 5

-The population is large (eliminates sampling errors)
-Mating occurs randomly (no bias)
-There is no selective advantage for any genotype
-There is no mutation or migration (could alter allele frequencies)

Question 6

What is the Hardy-Weinberg equation?

Answer 6

p2 + 2pq + q2 = 1

Question 7

What do the letters represent in the Hardy-Weinberg equation?

Answer 7

p = frequency of dominant allele (A)
q = frequency of recessive allele (a)
p2 = frequency of genotype AA
q2 = frequency of genotype aa
2pq = frequency of genotype Aa

Question 8

What are the genetic factors that may influence variation? Briefly explain each factor.

Answer 8

-Mutations (create new alleles)
-Meiosis (new combinations of alleles through independent segregation and crossing over of chromosomes)
-Random fertilisation of gametes (combination of gametes from 2 different individuals during sexual reproduction)

Question 9

What are the 3 causes of variation?

Answer 9

-Genetic factors
-Environmental factors
-Combination of factors

Question 10

Why is it difficult to draw conclusions about the causes of variation?

Answer 10

Because it is very hard to distinguish between the effects of the many genetic and environmental influences that combine to produce differences between individuals.

Question 11

What are selection pressures?

Answer 11

Any environmental change that affects an organism’s chance of survival and limits the population

Question 12

What is the definition of evolution? What are the 2 causes?

Answer 12

A change in allele frequency in a population over time, caused by either natural selection or genetic drift

Question 13

What is the role of over-production of offspring in natural selection?

Answer 13

High reproduction rates have evolved in many species to ensure a large enough population survives to breed and produce the next generation. This compensates for high death rates from predation, competition, disease etc.

Question 14

Explain the role of variation in natural selection.

Answer 14

The larger a population is and the more genetically varied it is, the more likely the individuals are to have the advantageous alleles which are better adapted for their environment. These individuals are then more likely to reproduce and pass on their advantageous alleles to offspring. Over many generations, the advantageous alleles will become more frequent in the population, so the population evolves and the allele frequency increases.

Populations showing little genetic variation are often vulnerable to new diseases and climate changes, and are therefore more likely to be unsuccessful.

Question 15

What are the 3 types of selection?

Answer 15

-Stabilising
-Directional
-Disruptive

Question 16

What phenotype does stabilising selection favour?

Answer 16

It selects for the average phenotype, and against extreme phenotypes

Question 17

What phenotype does directional selection favour?

Answer 17

It selects for the extreme phenotype, and against the average phenotypes

Question 18

What phenotype does disruptive selection favour?

Answer 18

It selects for multiple extreme phenotypes, and against the average phenotype

Question 19

Describe the effect of stabilising selection on evolution

Answer 19

-The extreme phenotypes are eliminated.
-It tends to occur when the environmental conditions are constant for a long period, so organisms with the extreme phenotypes will never be at an advantage.
-The mean will stay the same, but there will be fewer individuals at either extreme.
-E.g. fur length in a mammalian species.

Question 20

Describe the effect of directional selection on evolution

Answer 20

-Normal distribution curve formed
-It tends to occur when environmental conditions change suddenly, and certain individuals have the phenotype to suit this, so survive and reach their optimum.
-Therefore the mean moves either left or right of its normal position.
-E.g. antibiotic resistance in bacteria.

Question 21

Describe the effect of disruptive selection on evolution

Answer 21

-The average phenotype will be eliminated.
-It tends to occur when an environmental factor alternates (e.g. temp) between 2 or more forms.

Question 22

How does selection affect allelic frequencies?

Answer 22

Environmental changes affect which selection occurs, so therefore affects the probability of an allele being passed on in a population, so hence how many times it occurs in the gene pool.

Question 23

What is speciation?

Answer 23

The evolution of a new species from existing ones

Question 24

What is a species?

Answer 24

A group of individuals that can breed together to produce fertile offspring

Question 25

How are new species formed?

Answer 25

Reproductive separation will occur, followed by a genetic change due to natural selection. For example, one population may become separated due to a natural disaster etc, and will undergo different mutations, becoming genetically different from other populations. Different selection pressures will be experienced between each population due to changes in environment. Over time, natural selection will lead to different allele frequencies. Due to this, they may not be able to breed successfully anymore, so they form separate species.

Question 26

What is meant by genetic drift?

Answer 26

The random change in in the allele frequency of a small population

Question 27

Describe the process of genetic drift

Answer 27

Not all individuals reproduce, so some alleles disappear instead of being passed on, causing some phenotypes to become rarer and others to become more common.

Question 28

Why does genetic drift drive evolution in small populations?

Answer 28

Because chance events have a greater influence because they have lower genetic diversity

Question 29

What are genetic bottlenecks?

Answer 29

Sudden reductions in population size caused by natural catastrophes

Question 30

Why is natural selection the main driving force for evolution in large populations?

Answer 30

Because chance events tend to even out, as the population is more genetically diverse

Question 31

What is meant by the founder effect?

Answer 31

Small colonies with a low genetic diversity

Question 32

What is allopatric speciation?

Answer 32

Speciation caused by geographical isolation

Question 33

What is sympatric speciation?

Answer 33

Speciation without geographical isolation

Question 34

Describe the process of allopatric speciation

Answer 34

-Different alleles arise in each population by random mutation.
-Different selection pressures drive evolution in different directions.
-Over many generations, genetic drift alters the allele frequencies in one or both populations by chance.
-Allopatric speciation has occurred if, once reunited, the 2 populations cannot interbreed successfully (they’ve become reproductively isolated).

Question 35

What does reproductively isolated mean?

Answer 35

The species cannot interbreed successfully

Question 36

What are the causes of reproductive isolation?

Answer 36

-Behavioural (different courtship patterns, so don’t mate)
-Ecological (different habitats, so don’t meet)
-Temporal (active at different parts of day or breed at different times of year)
-Mechanical (reproductive parts don’t fit)
-Hybrid inviability (mating occurs, offspring don’t survive)
-Hybrid sterility (mating occurs but offspring are sterile)

Question 37

Describe the process of sympatric speciation

Answer 37

-Happens mostly in plants when sterile hybrid plants undergo chromosome doubling to form a new fertile species. This species cannot breed with either of its parent plant species, so is reproductively isolated despite living in the same habitat.