17. Selection and Evolution

Question 1

How are existing alleles reshuffled in populations, to increase variation?

Answer 1

• Independent assortment
• Crossing over
• Random mating + fertilisation

All these methods of genetic variation produce phenotypic variation, as offspring have different combinations of alleles to their parents.

Question 2

How are new alleles created?

Answer 2

Mutations - new alleles are often recessive and don’t show up until generations later. Those in somatic cells do not affect the offspring, but mutations in ovaries + testes/anthers can pass on (cell -> gametes -> zygote -> mitosis -> all cells in organism).

Question 3

What is discontinuous variation?

Answer 3

Qualitative - distinct categories with no intermediates (eg. blood groups).

Question 4

What is continuous variation?

Answer 4

Quantitative - small differences between classes, hard to distinguish the range between two extremes.

Question 5

How are qualitative differences inherited?

Answer 5

• Different alleles at a single locus have large effects on the phenotype.
• Different genes have different effects on the phenotype.
• Eg. sickle cell anaemia, haemophilia

Question 6

How are quantitative differences inherited?

Answer 6

• Different alleles at a single locus have small effects on the phenotype - different genes have same and cumulative effects.
• A large number of genes can have a combined effect on a phenotypic trait (polygenes).

Question 7

How is phenotypic variation decreased?

Answer 7

Dominance and gene interaction.

Question 8

How can the effects of continuous variation be seen in the inheritance of height?

Answer 8

Eg. unlinked A/a and B/b - dominant alleles contribute 2x, recessive contribute x to the height.
- Crossbreeding AaBb (6x) gives all 16 possibilities - heights fall on a normal distribution curve.

Question 9

How does genetic variation increase continuous variation?

Answer 9

Polygenes / multiple alleles / different chromosomes / crossing over (even if linked) / environmental factors all cause the number of discrete classes to increase and the differences between classes to decrease.

Question 10

How does the environment affect phenotype?

Answer 10

May allow full genetic potential to be reached / may stunt it. For example, organisms may have access to better food/light intensity/soil etc..

Question 11

Outline the effects of the environment on the phenotypes of Siamese cats.

Answer 11

Development of dark fur in extremities - allele works at low temperatures. If an area is plucked and kept cold, dark fur will grow here.

Question 12

Describe the experiment with purebred varieties of corn.

Answer 12

Black Mexican x Tom Thumb (different cob lengths).

• Parents homozygous at many loci -> offspring genetically identical to each other
• Differences amongst F1 caused by environment
• Second generation caused by a mix of both`

Question 13

Define ‘natural selection’.

Answer 13

Effect of selection pressures on the frequency of alleles in a population - high frequency of alleles conferring advantage and low frequency of those conferring disadvantage.

Question 14

Define ‘selection pressure’.

Answer 14

Factors which increase the chances of some alleles being passed / not passed to the next generation.

Question 15

Define ‘ fitness’.

Answer 15

Capacity of an organism to survive and transmit its genotype to offspring - extent of adaptation to an environment. Beneficial in the struggle for existence.

Question 16

How do organisms with high reproductive potential not grow exponentially?

Answer 16

As population increases, factors reduce growth - they may be biotic or abiotic.
For example, high rabbit numbers decrease food supply, increase predator populations, increase spread of disease (eg. myxomatosis - transmitted via flea), and increase overcrowding.
If pressure is high enough, population size decreases and only increases when numbers are low enough (oscillation).

Question 17

Define ‘biotic factor’.

Answer 17

Caused by other living organisms via predation, food competition or pathogenic infection.

Question 18

Define ‘abiotic factor’.

Answer 18

Caused by non-living components eg. water supply, soil nutrients.

Question 19

State the general theory of evolution.

Answer 19

Organisms have changed over time. Natural selection gives some alleles better survival chances than others - changes in allele frequency result in better adaptation to environment.

Question 20

Define ‘stabilising selection’.

Answer 20

Keeps things the way they are - if characteristics show wide variation, selection pressures act against the two extremes to yield a narrower range and the characteristic centred about the mean.

Question 21

Define ‘directional selection’.

Answer 21

Changes allele frequencies when a new environmental factor / allele is introduced - results in change in a particular direction (one extreme favoured, one isn’t).

Question 22

Define ‘disruptive selection’.

Answer 22

Conditions favour both extremes - maintains polymorphism (different phenotypes) in a population.

Question 23

Describe evolution due to a new environmental factor.

Answer 23

Eg. rabbits - an ice age favours the white fur allele over the agouti allele, so white rabbits have a selective advantage and are more likely to survive and reproduce.

Question 24

Describe evolution due to a new allele.

Answer 24

Most mutations produce harmful features due to their randomness, but others may be neutral/positive.
- eg. better camouflage than agouti = selective advantage, more likely to survive and reproduce - eventually all rabbits will have the new allele.

Question 25

Define ‘antibiotics’.

Answer 25

Chemicals produced by living organisms (mostly fungi), which inhibit/kill bacteria but do not harm human tissue.

Question 26

Outline how antibiotic resistance works.

Answer 26

Bacteria have a single loop of DNA and therefore only one copy of each gene, so an allele directly affects phenotype. Resistant bacteria survive and reproduce quickly, and can also pass alleles to other bacteria via plasmids.
Using ABX changes environmental factors, exerting selection pressures on bacteria and increasing the likelihood of evolving resistance.

Question 27

Describe industrial melanism with the example of Biston Betularia.

Answer 27

Peppered moth, relies on camouflage during the day (selection pressure = predation by birds).

• Normal = speckled (c), new = melanic (C).
• C more common in industrial areas - lichen (grey/green/brown) sensitive to SO2, tree bark darker.
• As air pollution decrease, selective advantage favours c.

Mutations were probably always occurring (not caused by pollution), but C rare due to selection pressure. Changes in environment affect likelihood of allele survival, not creation.

Question 28

How do organisms with different Hb genotypes differ in terms of selective advantage?

Answer 28

• HbS HbS: disadvantage - no malaria but sickle cell anaemia
• HbA HbA: disadvantage - Plasmodium protoctist enters RBCs and multiplies
• HbA HbS: advantage - 1/3 of Plasmodium and no sickle cell anaemia (both alleles exist in populations where malaria is a selection pressure).

Effects of non-random processes on allele frequencies in populations, interaction of two strong selection pressures maintains two alleles within certain populations.

Question 29

Define ‘genetic drift’.

Answer 29

Random process - change in allele frequency because only some organisms of each generation reproduce.

• small number separated from larger population
• founder effect: further drift in the smaller population alters frequency more, resulting in evolution in a different direction (occurs in recently isolated, small population).

Question 30

State the formula for the Hardy-Weinberg principle.

Answer 30

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

• 1 = whole population, p = dominant allele, q = recessive (decimals of whole population)
• p + q = 1 (A and a)
• chance of AA = p^2
• chance of Aa = 2pq (A can be from mum or from dad)
• chance of aa = q^2

Predicted ratios compared with next gen’s actual ratios (chi-squared test) - significant difference indicates directional selection.

Question 31

When doesn’t the Hardy-Weinberg principle apply?

Answer 31

• Population is small
• High selection pressure against one genotype
• Migration carrying one allele in/out
• Non-random mating

Question 32

Define ‘artificial selection’.

Answer 32

Humans purposefully apply selection pressures to populations. Achieved by selective breeding.

Question 33

What are the disadvantages of artificial selection in dairy cattle?

Answer 33

• Animals are large and take time to reach maturity
• Gestation period is longer
• Number of offspring is smaller
• Bulls cannot be assessed for milk production (sex-limited), so female offspring need to be tested (progeny testing).

Question 34

Why do selective breeders need to consider the whole genotype?

Answer 34

Background genes (all alleles adapting an organism to a particular environment) exist too - breeding parents from the same environment produces an organism with the same adaptations.

Question 35

How have crops been bred for desirable qualities?

Answer 35

Wheat short stems - high yield, easy to harvest, high resistance to wind.
Seeds screened for disease resistance, climate resilience, efficient use of N fertilisers (eg. avoidance of head blight from Fusarium).

Question 36

How does dwarfism work in wheat?

Answer 36

Mutant Rht gene alleles which code for DELLAs (reduce effect of GA). DELLAs produce more / more active transcription inhibitors.
Tom Thumb allele - plants have no GA receptors.

Question 37

Define ‘inbreeding depression’ (maize).

Answer 37

Inbreeding (crossing plants with similar genotypes) results in smaller and weaker offspring, increasing every generation.
Homozygous plants are less vigorous than hetero - outbreeding gives heterozygous plants (higher yield, healthier).

Question 38

How can breeders achieve both heterozygosity and uniformity?

Answer 38

Outbreeding randomly increases variation, not good commercially. Inbreeding -> homozygous, then crossing different homozygous varieties produces F1 with all the same genotype. There are many different homozygous varieties, so crossing them produces different hybrids (hybridisation)

Question 39

What are the first two observations, and first deduction, in the Darwin-Wallace theory of evolution by natural selection?

Answer 39

• Organisms produce more offspring than needed to replace the parents
• Natural populations tend to remain stable in size over long periods of time

There is competition for survival (struggle for existence).

Question 40

What are the third observation and the second deduction?

Answer 40

• There is variation amongst individuals of a given species.

Natural selection and survival of the fittest occur - variants with a selective advantage are selected for by the natural conditions at the time (now, variants replaced by alleles).

Question 41

Define ‘speciation’.

Answer 41

The process by which new species are produced.

Question 42

Define ‘species’.

Answer 42

Group of organisms with similar morphological/physiological/biochemical/behavioural features, which can interbreed to produce fertile offspring and are reproductively isolated from other species.

Question 43

Define the types of features in a species.

Answer 43

Morphological = structural features
Physiological = way the body works
Biochemical = base sequences in DNA, a.a. sequences in proteins
Behavioural = way the organism acts in response to stimuli

Question 44

What makes donkeys and horses different species?

Answer 44

They can interbreed with each other to produce mules, but these are infertile.

Question 45

How can we find out whether organisms are different species?

Answer 45

• Testing fertility of offspring (doesn’t always work - dead, fossils, same sex, won’t breed in captivity, immature, asexual reproduction).
• Comparing morphological, physiological, biochemical and behavioural features (time-consuming).

Question 46

How are new species produced?

Answer 46

Group of interbreeding organisms produces offspring that cannot interbreed successfully with the first group - they undergo reproductive isolation.

Question 47

Define ‘prezygotic isolation’.

Answer 47

Individuals don’t recognise each other as mates / do not respond to mating behaviour.
Physically unable to mate / pollen and stigma incompatible / male and female gametes cannot fuse / temporal / ecological mating locations.

Question 48

Define ‘postzygotic isolation’.

Answer 48

Failure of cell division in zygote.

Non-viable offspring produced / viable but sterile offspring produced (wasteful of energy and resources).

Question 49

Define ‘allopatric speciation’.

Answer 49

Two populations separated from each other via geographical isolation.

Question 50

How does allopatric speciation work?

Answer 50

A geographical barrier arises, preventing populations from mixing.
Isolated group interbreeds under different selection pressures, so different alleles are selected for.
Features become so different that the populations cannot successfully interbreed.

Question 51

Define ‘sympatric speciation’ and ‘polyploidy’.

Answer 51

Occurs through polyploidy (more than two complete sets of chromosomes in a cell).
Autopolyploid = sets of chromosomes all from the same species
Allopolyploid = sets of chromosomes from closely related species (easier to pair up, same go with same, but cannot breed with parental species).

Question 52

How does polyploidy work?

Answer 52

Meiosis goes wrong, two gametes fuse -> tetraploid zygote (sterile).

• All four chromosomes try to pair un meiosis I - difficult to divide into cells with complete sets.
• Asexual reproduction may occur as chromosomes do not need to pair up.

Tetraploid produces diploid gametes.

• These fuse with normal gametes -> triploid (sterile).
• Therefore the original diploid and tetraploid cannot interbreed successfully.

Question 53

Describe the creation of the new S. anglia species.

Answer 53

Original = maritima, hybridised with alterniflora -> townsendii.

• new = diploid - sterile because chromosomes cannot pair up
• new species - cannot reproduce with parents
• reproduces asexually via rhizomes (long underground stems).

Faulty division in townsendii -> tetraploid (two from maritima, two from alterniflora).
- allotetraploid = anglica (fertile).

Question 54

Outline the method of comparing amino acid sequences to find similarities between species.

Answer 54

Small changes in a.a. sequence keep the overall protein structure and function the same (essential parts not affected).
Number of differences = closeness of species.

Question 55

Describe the method of comparing amino acid sequences to find similarities between species.

Answer 55

Cytochrome c (electron transport chain) - similar in different species.

• humans, mice, rats = all 104 amino acids
• mice and rats = sequences identical
• humans = 9 amino acids different (substitutions of amino acids with same R group type).

Mice and rats are closely related.

Question 56

Outline the method of comparing nucleotide sequences to find similarities between species.

Answer 56

Mitochondrial DNA (mtDNA) inherited through female line (zygote contains mitochondria from ovum).
Circular DNA, no crossing over - all changes are due to mutation.
Faster mutation than nuclear DNA, no histone proteins, OP can produce forms of oxygen that act as mutagens.

Question 57

What is the evidence for the origin of Homo sapiens in Africa?

Answer 57

Differences in mtDNA between populations.
Mitochondrial Eve (150-200000 years ago in Africa) = common ancestor.
Molecular clock hypothesis = constant rate of mutation over time. More differences = longer ago common ancestor (calibrated by comparing species whose date of speciation can be estimated from fossil evidence).

Question 58

How can extinctions be caused naturally?

Answer 58

Change in climate, increased competition from better-adapted species, sudden natural change (eg. asteroid).

Question 59

How have extinctions been caused by humans?

Answer 59

Habitat loss - many species are adapted for a particular habitat with a range of environmental conditions, humans change this by: draining wetlands, deforestation, pollution.
Overconsumption (killing too many animals for sport, food).
Eg. rhinos - low political support for conservation, high demand for rhino products, criminal groups targeting rhinos.