Natural selection

Question 1

Definition of phenotype.

Answer 1

The observable characteristics of an organism, resulting from its genotype and interaction with the environment.

Question 2

Definition of gene.

Answer 2

A length of DNA which codes for a protein or polypeptide.

Question 3

Definition of genotype.

Answer 3

The genetic constitution of an organism.

Question 4

Definition of homozygous.

Answer 4

The two alleles of a gene are the same.

Question 5

Definition of heterozygous.

Answer 5

The two alleles of a gene are different.

Question 6

Definition of allele.

Answer 6

Different forms of a gene.

Question 7

Definition of recessive.

Answer 7

The allele which is only expressed in the phenotype, in absence of a dominant allele.

Question 8

Definition of dominant.

Answer 8

The allele is always expressed in the phenotype, even if a different allele/heterozygous for the same gene is present.

Question 9

Definition of locus.

Answer 9

The position of a gene on a chromosome.

Question 10

Definition of genetic diversity.

Answer 10

Number of different alleles of the same gene.

Question 11

Definition of allele frequency.

Answer 11

Number of different times a specific allele appears in a population.

Question 12

Definition of gene pool.

Answer 12

Total number of alleles in a particular population.

Question 13

Explain why only 2 alleles can be carried for each gene.

Answer 13

2 chromosomes- maternal and paternal. Each chromosome has the same genes but different alleles- homologous pairs.

Question 14

Definition of evolution.

Answer 14

Observing the change in a species over time.

Question 15

Definition of natural selection.

Answer 15

Environmental (abiotic and biotic) factors in the environment around the species that causes the change.

Question 16

What are the two types of factors that cause variation?

Answer 16

1. Genetics.
2. Enviromental.

Question 17

What are the sources of genetic variation?

Answer 17

1. Mutations, which can produce different alleles of genes.
2. Meiosis, independent assortment and crossing over.
3. Random fertilisation of gametes during sexual reproduction to create new allele cominations.

Question 18

What are the sources of environmental variation?

Answer 18

1. Epigenetics, environment can influence the way an organisms’ genes are expressed.
2. Biological factors, suc as predators (biotic).
3. Non-biological factors, such as sunlight (abiotic).

Question 19

What are the two types of variation?

Answer 19

1. Continuous.
2. Discontinuous.

Question 20

Describe continuous variation (6 points).

Answer 20

1. Normal distribution curve showing variation around a mean.
2. Quantitative data.
3. Controlled by many genes.
4. Many intermediates, no discrete groups.
5. Affected by environment.
6. Examples: body mass, height.

Question 21

Describe discontinuous data (6 points).

Answer 21

1. Bar chart.
2. Qulaitative data.
3. Controlled by gene 1 or 2 genes.
4. Discrete groups, no intermediates.
5. Not affected by enviroment.
6. Examples: blood type, eye colour.

Question 22

Definition of selection.

Answer 22

Process that results in organisms with the best adaptations in the population surviving to reproduce and pass on favourable alleles to the next generation.

Question 23

Definition of selection pressure.

Answer 23

Fcator which impacts an organisms’ ability to survive.

Question 24

Definition of reproductive success.

Answer 24

The ability of an organism to pass on its genes to the next generation.

Question 25

Definition of reproductive isolation.

Answer 25

The inability of a species to breed successfully with related spcies due to geographical, behavioural, physiological or genetic barries or differences.

Question 26

Describe the process of natural selection (9 points).

Answer 26

1. New allele arises due to random mutation.
2. Leads to variation in a population.
3. Populations may be geographically isolated so no interbreeding.
4. Different selection pressures: food, predators or disease?
5. Competition for resources- intraspecific competition.
6. New alleles give a selective advantage so organisms survive and reproduces.
7. Pass on advatageous alleles to the next generation.
8. Frequency of allele in population increases.
9. Frequency of the phenotype increases.

Question 27

What are the 3 main selection pressures?

Answer 27

1. Competition for food.
2. Predators.
3. Diseases.

Question 28

What are the three types of selection?

Answer 28

1. Stabilising.
2. Directional.
3. Disruptive.

Question 29

Describe the benefit of organisms having an advantageous allele.

Answer 29

Those organisms with phenotypes/alleles providing a selective advantage are more likely to survive and reproduce to pass on their favourable alleles to the next generation. This causes the gene pool to decrease because the favourable alleles are increasing and the less favourable alleles decreasing, there is less allele variation.

Question 30

Definition of stabilising selection.

Answer 30

Organisms with phenotypes in the middle are more favourable than the two extremes within a population. Usually when selection pressures stay constant.

Question 31

Definition of directional selection.

Answer 31

Organisms with phenotypes of one extreme are more favourable within a population. Usually when selection pressure changes.

Question 32

Definition disruptive selection.

Answer 32

Organisms with phenotypes at both extremes are kore favourable within a population. Usually when selection pressure changes leading to speciation.

Question 33

Give an example of stabilising selection. Describe and explain this example.

Answer 33

Mass of babies at birth can be small, large or average. Stabilising selection favours the phenotypes closest to the mean value of a trait. Babies that tend to have extreme birth weights have higher mortality rates.

Question 34

Explain the impact of stabilising selection on the gene pool of the human population.

Answer 34

Mutation leads to variation in population. Bbay weight alleles at extreme phenotypes (babies that are too light or too heavy). Less likely to survive to reproduce to pass on alleles to offspring. Descreases allele frequency. Descrease in gene pool.

Question 34

Explain the impact of stabilising selection on the gene pool of the human population.

Answer 34

Mutation leads to variation in population. Bbay weight alleles at extreme phenotypes (babies that are too light or too heavy). Less likely to survive to reproduce to pass on alleles to offspring. Descreases allele frequency. Descrease in gene pool.

Question 35

Give an example of directional selection. Describe and explain this example.

Answer 35

Cheetahs are the fastest animals on land. Individuals that are the fastest are more likely to catch their prey. Directional selection favours individuals with an extreme form of a trait and selects against phenotypes at the other extreme.

Question 36

Explain the impact of directional selection on the gene pool of the cheetah population.

Answer 36

Mutation leads to variation in the population. Cheetahs with faster speeds are more likely to survive. More likely to reproduce to pass on advantageous allele to offspring. Increase in allele frequency within the population. Gene pool moves towards extreme phenotype away from the mean.

Question 37

Give an example of disruptive selection. Describe and explain this example.

Answer 37

Birds’ beaks can either be big or small depending on the food source they have. Birds that have beaks that are in the middle cannot really eat anything as their beaks won’t be adapted to the food source.

Question 38

Explain the impact of disruptive selection on the gene pool of the bird population.

Answer 38

Mutation leads to variation in the population. Birds with different shaped beaks are more likely to survive. More likely to reproduce and pass on advantageous alleles to offspring. Allele frequency splits towards the extreme phenotypes which leads to speciation.

Question 39

Describe and explain antibiotic resistance in bacteria (6 points).

Answer 39

1. New allele arises due to random mutation in a bacterium, leading to variation.
2. This gives the bacteria a selective advantage if antibiotic in the environment.
3. These bacteria survived and reproduced by binary fission to pass on advantageous allele to the next generation. Non-resistant bacteria die.
4. Resistant bacterium increased in population.
5. Increases in allele frequency in the population.
6. Example of directional selection.

Question 40

Definition of speciation.

Answer 40

The evolution of 2 or more species from 1 original species due to reproductive isolation resulting in genetic changes/changes in allele frequency through natural selection.

Question 41

What are the two mechanisms of speciation?

Answer 41

1. Allopatric speciation.
2. Sympatric speciation.

Question 42

What is allopatric speciation?

Answer 42

Organisms divided by a physical/geographical barrier with different selection pressures either side of the barrier resulting in different advantageous alleles.
As the two groups become separated and reproductively isolated, the result is that the gene flow is reduced. Each group experiences a different selection pressure as the environment they live in is different. Over time, the frequency of alleles changes through natural selection and the two parts of the population can no longer interbreed and become spearate species.

Question 43

What is sympatric speciation?

Answer 43

Organisms not physically/geographically divided, organisms live in the same environment, change in behaviour/phenotypes resulting in different advantageous alleles.
This is the idea that new species evolve from a single ancestral species when inhabiting the same geographic region.

Question 44

Give an example of sympatric speciation.

Answer 44

A chromosomal error during cell division which leads to reproductive isolation (members of a population can no longer interbreed forming two separate populations). Other reasons include mutations that mean courtship behaviour isn’t recognised and anatomical changes that mean mating cannot physically occur.

Question 45

What are the two types of sypatric speciation? Describe both of these.

Answer 45

1. Pre-zygotic isolation: These stop two individuals coming together to mate.
2. Post-zygotic isolation: This is when the zygote is prevented from further development after it has formed.

Question 46

Definition of micro-habitat

Answer 46

The same habitat but spaces within the habitat experience different abiotic and biotic factors.

Question 47

Describe and explain genetic drift.

Answer 47

A change in allele frequencies in the gene pool of a population due to purely chance events and not selection pressures. Due to the random nature of gamete production and fertilisation, certain alleles may increase in the population due to chance.

Question 48

What are the three similarities between genetic drift and natural selection?

Answer 48

1. Change in allele frequency.
2. Change in phenotype frequency.
3. Change in genetic drift/gene pool.

Question 49

What are the two differences between natural selection and genetic drift?

Answer 49

1. Natural selection requires a selection pressure, genetic drift doesn’t require a selection pressure.
2. Natural selection is not random, genetic drift is random.

Question 50

Explain why genetic drift is more important in small populations?

Answer 50

The effect of genetic drift is more prominent within small populations because chance has a greater influence, whereas in larger populations the random fluctuations even out across the whole population.

Question 51

What is genetic diversity?

Answer 51

This links to evolution by natural slection, speciation and classification. How closely two species are related can be determined by considering the genetic diversity.

Question 52

How can genetic diversity within or between species be made?

Answer 52

By comparing:
1. Base sequence of DNA.
2. Base sequence of mRNA.
3. Amino acid sequence made in protein synthesis.

Question 53

Suggest why scientists use DNA, mRNA or amino acid sequences to identify relationships between living organisms.

Answer 53

Genetic code is universal so shared between all organisms. Mutations are passed on to the offspring.

Question 54

Explain how DNA, mRNA or amino acid sequences can be used to identify relationships between living organisms.

Answer 54

DNA/mRNA: complementary base pairing. Sequences that are similar will be able to pair. Sequence with more variation will not be able to pair.

Question 55

Describe the methos for DNA hybridisation (6 points).

Answer 55

1. Extract DNA from 2 species and remove non-coding regions as this is where the variation occurs in the DNA sequences.
2. Heat DNA to break hydrogen bonds and separate the strands.
3. Mix DNA from both species in a tube.
4. Cool to allow base pairs to form between strands. DNA and mRNA with complementary sequences will base pair, non-complementary sequences will not.
5. Warm slowly to measure the amount of single-stranded DNA every 2 degrees.
6. Determine the temperature at which 50% of the DNA has separated into single strands.

Question 56

Suggest what is shown about the relationship between 2 species the higher the temperature to reach the 50% separation mark.

Answer 56

The 2 species are closely related as the higher the temperature, the more hydrogen bonds to break (more kinetic energy), so more complementary base pairings.

Question 57

If the temperature to reach the 50% separation is higher, suggest what this shows about when speciation happened between these 2 species.

Answer 57

The 2 species are closely related so not as many mutations. Higher temperature means more complementary base pairings, so not as amny mutations. Speciation must have happened more recently so more recent common ancestor.

Question 58

If the temperature to reach th 50% mark is lower, suggest what this means about the gene pool between the 2 species.

Answer 58

The 2 species are no closely related, so the gene pool of these 2 species differs incredibly from one another. Lower temperature means less complementary base pairing so more mutations/variation.