Variation and Evolution

Question 1

What are the 3 main factors causing variation between individuals?

Answer 1

• Differences in genotype - genetic factors
• Different epigenetic modifications - but same genotype
• Differences in environment - environmental factors

Question 2

Explain how the environment could lead to an epigenetic change

Answer 2

An environmental factor eg diet alters DNA methylation or histone modification. This then leads to changes in the expression of genes

Question 3

Non-heritable variation (environmental) cannot be passed to offspring unless __________ _______ occurs

Answer 3

Epigenetic change

Question 4

How may genetic (heritable) variation be increased as a result of sexual reproduction?

Answer 4

• Crossing over between homologous chromosomes during prophase I in meiosis
• Independent assortment of chromosomes during metaphase I of meiosis
• Independent assortment of chromatids during metaphase II of meiosis
• Mixing of two different parental genotypes at fertilisation

Question 5

What is selection pressure?

Answer 5

An environmental factor that can alter the allele frequencies of the alleles present at a particular gene locus in a population

Question 6

What are selective agencies?

Answer 6

Factors that exert selection pressures eg climate, human impact, supply of food, breeding sites

Question 7

What selection pressure led to the increase in dark form moths during the Industrial Revolution?

Answer 7

Soot from the factories led to tree bark becoming darker - dark form moths were camouflaged which reduced predation

Question 8

If a dominant allele produces a phenotype which gives a selective disadvantage, what will happen to the frequency of this dominant allele in the gene pool?

Answer 8

The frequency of the allele will decrease and it may disappear from the gene pool as any organism with it will have the disadvantageous phenotype so is less likely to breed and pass on the allele

Question 9

If a recessive allele produces a phenotype that gives a selective disadvantage, what will happen to the frequency of this allele in the gene pool?

Answer 9

Organisms who are heterozygous will not have the disadvantageous phenotype so will survive and breed and pass on the allele. This means the recessive allele will stay at a low frequency in the population. Only homozygous recessive have the disadvantageous phenotype.

Question 10

Frequency of dominant allele + frequency of recessive allele = _____

Answer 10

1

Question 11

In a population of 65 organisms, 25 are homozygous dominant and 30 are heterozygous. What is the frequency of the dominant allele? What is the frequency of the recessive allele?

Answer 11

Total number of alleles = 65x2 = 130
Number of dominant alleles = (25x2) + 30 = 80
Dominant allele frequency = 80/130 = 0.62
Recessive allele frequency = 1 - 0.62 = 0.38

Question 12

The Hardy-Weinberg principle states that the frequencies of dominant and recessive alleles and genotypes will remain constant from one generation to the next, if certain conditions remain true. What are these conditions?

Answer 12

• A large population (100+ individuals)
• No selection for or against any phenotype
• Random mating throughout the population
• No mutations
• The population is isolated ie no immigration and emigration

Question 13

State the Hardy-Weinberg equation and what each letter represents

Answer 13

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

p = frequency of the dominant allele (A)
q = frequency of the recessive allele (a)
p + q = 1

p^2 = frequency of homozygous dominant (AA)
2pq = frequency of heterozygous (Aa)
q^2 = frequency of homozygous recessive (aa)

Question 14

What is evolution?

Answer 14

The change in the average phenotype of a population over time

Question 15

What is natural selection?

Answer 15

The theory that explains how existing species have arisen through modification of ancestral species. It encourages the transmission of favourable alleles and hinders the transmission of unfavourable alleles.

Question 16

Describe the process of natural selection

Answer 16

• In any population there is variation due to mutation
• Population numbers remain roughly the same, despite overproduction of offspring
• Competition (intraspecific) means there’s a struggle for survival
• The fittest have a selective advantage due to selection pressures
• They survive, interbreed and pass on alleles that give their offspring a selective advantage
• This process repeats over many generations and increases the frequency of the advantageous allele in the populations gene pool

Question 17

What is directional selection?

Answer 17

If the environment changes then natural selection may favour one extreme of the phenotypes resulting in a directional change in the allele frequency in a population

Question 18

What is stabilising selection?

Answer 18

If the environment is stable then extreme phenotypes tend to be eliminated as they do not confer any selective advantage. This prevents change and stabilises a population.

Question 19

Explain when you would carry out a t-test

Answer 19

To test of there’s a significant difference between the means of 2 samples of interval/continuous data when data is normally distributed

Question 20

Explain when you would carry out a chi-squared test

Answer 20

To test if there’s a significant difference between the observed and expected data in set categories/discontinuous data

Question 21

Define a species

Answer 21

A group of phenotypically similar organisms that can interbreed to produce fertile offspring

Question 22

What is speciation?

Answer 22

When populations of a species become isolated and new species can form

Question 23

What can speciation occur due to?

Answer 23

• Genetic drift - changing allele frequencies by chance
• Founder effect - disproportionate allele frequencies in small populations
• Natural selection

Question 24

What are the 2 main types of speciation?

Answer 24

• Allopatric speciation - geographical isolation
• Sympatric speciation - reproductive isolation

Question 25

In a species of birds, blue feathers are dominant to purple feathers. In a population of 120 birds, 15 have purple feathers. A disease kills 45 of the birds in the population, including all those with purple feathers. Explain why this is an example of genetic drift rather than natural selection.

Answer 25

Allele frequency in the gene pool was altered due to chance

Question 26

What is the founder effect (an example of genetic drift)?

Answer 26

When a new area is colonised by individuals from a population, those individuals may have a gene pool which, due to chance, has different allele frequencies than the original gene pool

Question 27

Madagascar is an island off the coast of Africa. Many of the species there are unique (only found on the island). Suggest how the founder effect may have led to these unique species arising

Answer 27

A small group of organisms (founder population) from mainland Africa colonised Madagascar. By chance the allele frequencies in their gene pool differed from the mainland gene pool. There are different selection pressures on the island. Over time these allele frequencies became so different that new unique species formed. This is an example of allopatric speciation.

Question 28

What is allopatric speciation?

Answer 28

When there is a physical barrier isolating individuals, such as a river or mountain range that splits a population into 2 separate demes, preventing interbreeding and the flow of genes.

Question 29

Describe the process of allopatric speciation

Answer 29

1. Large population with common gene pool 2. Population is separated into 2 demes by a physical barrier, preventing the flow of genes (breeding) 3. Mutations and different selection pressures on each deme alter the gene pool 4. If the barrier is removed, the gene pools will be so different that interbreeding will not be successful

Question 30

What is sympatric speciation?

Answer 30

When there is a reproductive barrier isolating individuals

Question 31

What are the 4 types of sympatric speciation?

Answer 31

- Behavioural isolation - Morphological isolation - Gametic isolation - Seasonal isolation

Question 32

What is behavioural isolation?

Answer 32

In animals with elaborate courtship behaviour, the steps in the ‘display’ of one group of organisms may fail to attract the necessary response in a potential partner from another group of organisms

Question 33

What is morphological isolation?

Answer 33

The body parts of organisms may not be compatible enough for them to mate. This is seen in insects where the rigid exoskeletons mean that the genitalia of males and females must be complementary

Question 34

What is gametic isolation?

Answer 34

There are barriers preventing gametes of different species fusing. Eg the stigma will only produce a sugary secretion for pollen germination if the pollen is compatible (same species). Spermatozoa can often only survive in an oviduct from the same species.

Question 35

What is seasonal isolation?

Answer 35

If reproductive organs of different groups mature at different times of year, the groups are unable to interbreed. This can occur due to differences in mating seasons or differences in flowering times. Eg the toad Bufo americanus mates in early summer whilst B. fowleri mates in late summer so the 2 species remain isolated

Question 36

As the demes become new species, what can arise?

Answer 36

- Hybrid inviability - Hybrid sterility

Question 37

What is hybrid inviability?

Answer 37

Fertilisation may occur but incompatibility between genes of the parents prevents the development of an embryo. Hybrid embryos formed from sheep and goats die in early stages of development.

Question 38

What is hybrid sterility?

Answer 38

In some cases, the embryo can survive when individuals of different species breed. Eg a horse can breed with a donkey to produce a hybrid mule. If the horse has 64 chromosomes and the donkey has 62 chromosomes, the offspring have 63 chromosomes. The hybrid offspring are unable to produce gametes and are sterile.