Population genetics, variation and evolution

Question 1

What is variation?

Answer 1

Physical, physiological, biochemical, genetical and behavioural differences between organisms (either same species or different species?

Question 2

What is intraspecific variation?

Answer 2

Variation between members of the same species.

Question 3

What is interspecific variation?

Answer 3

Variation between members of different species.

Question 4

What are the differences between continuous and discontinuous variation?

Answer 4

• Continuous variation is quantitative (measured) whereas discontinuous variation is qualitative (described).
• Continuous variation consists of data with measurements from one extreme to another whereas discontinuous variation consists of data that fall into distinct categories.
• Continuous variation has many intermediates that grade into each other whereas discontinuous variation has discrete data whereby features either fall into one or another category.
• The environment usually plays a large role in determining continuous variation whereas it has a little role in discontinuous variation.
• Continuous variation can be plotted as frequency histograms whereby a normal (bell-shaped) distribution curve is formed whereas discontinuous variation is represented by bar charts.

Question 5

What are the genetics responsible for discontinuous variation?

Answer 5

• Characteristics that display discontinuous variations are usually those controlled by one gene.
• Different alleles of at same loci have huge effects on phenotype.
• If multiple genes are involved, they often interact in epistatic ways as opposed to additive ways, or each has big distinct influence over phenotype.

Question 6

What are the genetics responsible for continuous variation?

Answer 6

• Characteristics that display continuous variation are usually controlled by more than 1 gene.
• They are often polygenic (controlled by polygenes). These genes are located on different chromosomes and are inherited separately. Their effects combine to give phenotype.
• Each gene and alleles of genes have very small effect on phenotype.
• Effects of multiple genes are additive (add to each other).

Question 7

How can the environment influence variation?

Answer 7

• Environmental factors such as available of nutrients, temperature, climate and interactions with other organisms can all limit the expression of certain genes.
• If a vital factor a gene requires for expression is missing from environment, it will not be expressed fully and its potential will not be fully realised.
• Genes usually code for polypeptides. If no amino acids are available due to organism being malnourished, polypeptides will not be produced.
• Environmental influences are usually most prominent in polygenic characteristics as more genes are involved and can potentially be affected by environmental factors.

Question 8

What are examples of environmental influences over variation?

Answer 8

• Intelligence is partially determined by genetics and genes inherited by parents. However, without stimulating learning environment and good nutrition to ensure brain and nervous system develops correctly, intelligence may not be fully expressed.
• A plant may have genes for tall growth, but if it doesn’t obtain enough nutrients, water, sunlight… It will not have enough energy, proteins in order to grow and fully realise its growth potential. The expression of its growth genes are limited.

Question 9

Why is (genetic) variation important for evolution?

Answer 9

• Variation ensures that individuals in the population of a species are slightly different from each other and have different characteristics determined by different alleles.
• When there’s change in environment/human activity, some characteristics are better adapted to new environment than others.
• Individuals with these characteristics survive to reproduce and pass on advantageous alleles to offspring.
• Individuals without these characteristics die.
• This results in the gradual change of characteristics possessed by population of species over time, which forms basis of evolution.

Question 10

What are some examples of discontinuous variation?

Answer 10

Humans:
- Lobed ears.
- Tongue rolling.
- Blood groups.
Plants:
- Flower colour.

Question 11

What are some examples of continuous variation?

Answer 11

Humans:
- Height.
- Body mass.
Plants:
- Height.
- Number of flowers/fruit produced.
Microorganisms:
- Diameter.
- Metabolite production.
- Lifetime.

Question 12

What does the Hardy-Weinberg principle state?

Answer 12

Allele frequency in a population will remain constant from one generation to next so long as specific conditions are met. Hence there will be no evolution.

Question 13

What are the 5 conditions that need to be met according to Hardy-Weinberg principle in order for evolution not to take place?

Answer 13

1. No migration.
2. No natural selection.
3. No mutations.
4. Large population size.
5. Random mating.

Question 14

What is the equation to determine allele frequency in population?

Answer 14

p + q = 1
Where:
p = frequency of dominant allele.
q = frequency of recessive allele.

Question 15

What is the equation to determine phenotype frequency in population?

Answer 15

p² + 2pq + 2q² = 1
Where:
p = frequency of dominant allele.
q = frequency of recessive allele.
p² = frequency of homozygous dominant phenotype.
2pq = frequency of heterozygous phenotype.
q² = frequency pf homozygous recessive phenotype.

Question 16

What is evolution?

Answer 16

The change in allele frequencies present in gene pool of population over time.

Question 17

What is a gene pool?

Answer 17

The collective of all alleles of all genes of all individuals in a population at a given time.

Question 18

What causes changes in a gene pool (and evolution)?

Answer 18

• New alleles/genes acquired through chance mutations and immigration.
• Disadvantageous genes/alleles are lost through natural selection.
• Loss of genes/alleles through emigration.
• Chance lost of genes through genetic drift.

Question 19

How does natural selection take place?

Answer 19

1. Variation amongst members of the population of a species due to presence of many different alleles.
2. Chance mutations allow for completely new characteristics and alleles to develop.
3. Selection pressure applied to the population, e.g. change in climate.
4. Organisms with better adapted characteristics are more likely to survive, while those less well adapted die.
5. Organisms that survive are able to pass on their advantageous alleles to offspring.
6. The next generation population will have a higher proportion of advantageous alleles in gene pool.
7. Offsprings with advantageous alleles are more likely to survive and reproduce.
8. Cycle repeats so that proportion of advantageous alleles in population also increases after each. generation.

Question 20

What is stabilising selection?

Answer 20

• Form of selection that occurs in stable environment where there’s very little change.
• Median of population (i.e. most common characteristics) favoured and selected for by natural selection as these are the best adapted to current environment.
• These characteristics become more common as the extremes become less common, as they are less well adapted.
• Range of phenotypes reduce.

Question 21

What is directional selection?

Answer 21

• Form of selection that occurs when there is a sudden and significant change in environment.
• Individuals with more extreme characteristics are selected for by natural selection they are better adapted to new environment than what is currently most common characteristics.
• Median shifts in either direction depending on selection pressures as extreme population increases in size (their alleles also increasing in frequency) and current dominant population decreasing in size (their alleles decreasing in frequency).
• This type of selection is an evolutionary force of natural selection as it leads to change in population allele frequencies and thus evolution.

Question 22

What is genetic drift?

Answer 22

When the allele frequency of a population changes over time purely due to chance.

Question 23

How does genetic drift take place?

Answer 23

1. There’s variation in population due to individuals having different genotypes (alleles).
2. When 2 individuals reproduce, chance determines which alleles are past on to their offspring.
3. E.g. if Aa x Aa occurs, AA, Aa and aa are possible offspring genotypes.
4. If by chance, offspring inherits aa genotype, then A allele lost from gene pool.
5. If this occurs for many offsprings, and population is small, then allele frequency is drastically changed in next generation.
6. E.g. if frequency of allele ‘a’ in next generation greater, then the generation after that are even more likely to inherit ‘a’ allele.
7. Over generations, this would result in frequency of ‘a’ allele increasing and A allele decreasing, which is evolution.
8. In extreme circumstances, A may be completely lost from gene pool.

Question 24

Why does genetic drift have more prominent effects over small populations?

Answer 24

• In smaller populations, changes in allele frequencies due to chance in each offspring has larger effect on allele frequencies of overall population.
• In larger populations, small changes in allele frequencies for each offspring has very small effect on overall population allele frequencies, meaning genetic drift is unlikely to have big effect.

Question 25

What mechanisms cause variation?

Answer 25

• Meiosis.
• Fertilisation.
• Environment.
• Chance mutations.

Question 26

What are the differences between natural selection and genetic drift?

Answer 26

• Natural selection depends on the environment determining which alleles are more frequently passed from generation to generation (selective pressure) and the direction of evolution.
• Genetic drift depends on chance determining which alleles are more frequently passed from generation to generation, so direction of evolution is also randomly determined.

Question 27

What are the causes for a small population in nature?

Answer 27

• When any population experiences a natural disaster and a genetic bottleneck occurs, whereby a random sample of the population survives.
• This random sample may by chance, contain higher frequencies of some alleles others, increasing chance of these alleles being passed on.
• By genetic drift, these alleles become increasingly common from one generation to the next and species evolves.

Question 28

What is speciation?

Answer 28

Evolutionary process whereby new biological species arise from the same common ancestor.

Question 29

What is sympatric speciation?

Answer 29

When speciation occurs between organisms living in the same geographic location.

Question 30

What is allopatric speciation?

Answer 30

When speciation occurs due to geographic isolation of different populations.

Question 31

What is reproductive isolation?

Answer 31

When groups of organisms are unable to breed freely with each other by some mechanisms (e.g. physical barrier, incompatible physiology…).

Question 32

How does speciation occur?

Answer 32

1. 2 populations of the same species are reproductively isolated, preventing them from breeding with each other.
2. The allele frequencies in their gene pools will change over time in different directions, by different selection pressures (natural selection) or chance (genetic drift).
3. These changes in gene pool don’t affect each other as the gene pools cannot mix.
4. The 2 populations gradually evolve in different directions until individuals from each are so different that they can no longer interbreed to produce fertile offsprings.
5. Speciation has occurred whereby 2 species have formed from one common ancestral species.

Question 33

What is geographical isolation?

Answer 33

• When 2 or more populations occupy environments separated by a physical barrier such as a river or mountain which prevents them from mixing and breeding together.
• E.g. Darwin’s finches.

Question 34

What is temporal isolation?

Answer 34

• When 2 or more species or populations live within the same area but are reproductively active at different times of the year so cannot interbreed.
• E.g. American frog species.

Question 35

What is mechanical isolation?

Answer 35

• When the reproductive structures between different groups of organisms are incompatible, preventing them from interbreeding.
• E.g. A great dane and chihuahua.

Question 36

What is behavioural isolation?

Answer 36

When different groups of organisms evolve different courtship displays/behaviours so that they cannot attract mate from different groups, so cannot interbreed.

Question 37

What is a biological species?

Answer 37

A group of similar organisms that can interbreed and produce fertile offsprings.

Question 38

What are the problems with biological species?

Answer 38

• Only classifies species that produce sexually.
• Does not take into account the fact that males and females of some species look very different from each other.
• Some species, like ants, contains different members with different roles that look different from each other.
• Some species hare extinct so cannot be observed.

Question 39

What is a phylogenic species?

Answer 39

A group of organisms that have similar morphology (shape), physiology, biochemistry, embryology and behaviour, as well as occupy same niche.

Question 40

How are phylogenetic species determined?

Answer 40

• Comparative genome mapping is used to compare genomes (or common genes) of different organisms. If there are enough similarities, then they are probably members of the same species.
• Members of different species have more distinct differences in their genome.
• The more similar the genome of 2 species are, the closer they are related to each other.

Question 41

What is a monophyletic group?

Answer 41

Group that includes an ancestral species and all of its subsequent descendants.

Question 42

What is a clade?

Answer 42

Group of species who are more closely related to each other than any other species, stemming from a common ancestor but including all descendants (monophyletic).

Question 43

How is the cladistic classification system different from the taxonomic?

Answer 43

• Focuses on evolutionary relationship as opposed to physical similarities.
• Requires objective analysis techniques (such as genome sequencing) as opposed to subjective judgment.
• Computers are used to process data from genome sequencing and determine which individuals are more closely related to each other by plotting cladogram; instead of humans comparing features.
• Makes no difference between extinct and extant species as monophyletic system used.

Question 44

What is a paraphyletic group?

Answer 44

Group that contains the most recent common ancestors of a number of closely related species, but does not include all of the descendants of that ancestor. For example, the group of prokaryotes ins paraphyletic includes bacteria and archaea, and their recent common ancestor, but doesn’t include eukaryotes which share same common ancestor.

Question 45

What is artificial selection?

Answer 45

Evolution of a species as a consequence of selective breeding from humans, in order to achieve desired set of characteristics for agriculture.

Question 46

What are the differences between natural and artificial selection?

Answer 46

• Agent: Humans (e.g. farmers, breeders…) in artificial, as opposed to the environment in natural.
• Features selected for: Features decided by genotype that are determined by the breeder to be commercially valuable (not necessarily beneficial to animal) in artificial as opposed those that adapt an organism better to its environment at a specific time in natural.
• Number of features involved: A small number of specific commercially beneficial characteristics in artificial as opposed to many that all have small benefit to survival in natural.
• Speed: Fast and predictable results in artificial as opposed to slow and unpredictable in natural.

Question 47

What are the similarities between natural and artificial selection?

Answer 47

• They both result in change in allele frequencies in gene pool, resulting in evolution.
• They both require variation in the population.
• They both select for characteristics determined by genetics (inheritable) as opposed to environmental.

Question 48

How were modern dairy cows produced by artificial selection?

Answer 48

1. Milk yield of female cows were measured and compared. The ones with the highest are selected.
2. Progeny testing used to determine which bulls produced daughters with high milk yields.
3. Small number of bulls selected and kept (due to availability of artificial insemination).
4. Highest yielding female cows (elites) are treated with hormones so that they ovulate more.
5. The eggs are collected and are fertilised in vitro with semen from bulls.
6. Fertilised embryos can be tested to determine their sex (only females produce milk) and only females selected.
7. Selected embryos can be cloned or implanted directly into womb of surrogate mother.
8. This process allows many high milk yielding females to be produced quickly from a relatively small number of parents.
9. Process repeated for generation, with milk yield increasing each generation.

Question 49

What are the desired features for modern bread wheat?

Answer 49

• Resistance to fungal and other types of infections.
• High protein content.
• Stiff and shorter stems to prevent lodging (stems bending due to rain/wind).
• High grain yield.

Question 50

What is polypolidy?

Answer 50

Organisms who’s cells contain more than 2 sets of chromosomes.

Question 51

How was modern day bread wheat (Triticum aestivum) produced by artificial selection?

Answer 51

1. Einkorn wheat (e.g. T. urartu) (genome = AA, 2n = 14) was crossed with wild grass (e.g. (genome = BB, 2n = 14) to produce sterile hybrid species (genome = AB, 2n = 14).
2. Chromosome mutation occurs whereby every chromosome of AB hybrid is copied and becomes part of genome to produce fertile hybrid (genome = AABB, 4n [tetrapliod] = 28), wild emmer wheat.
3. Wild emmer crossed with wild goat grass (genome = DD, 2n = 14) to produce second sterile hybrid (genome = ABD, 3n [triploid] = 21).
4. Chromosome mutation occurs in ABD hybrid to produce fertile hybrid (genome = AABBDD, 6n [hexaploid] = 42).
5. Further selection and cross breeding amongst hexaploid hybrids to produce desired characteristics found in modern wheat.

Question 52

Why were hybrids sterile and how does chromosome mutation and polyploidy restore their fertility?

Answer 52

• Hybrids were sterile as their genomes contained sets of completely different chromosomes, meaning that homologous pairs do not exist and meiosis fails. They cannot produce any gametes, so are sterile.
• Chromosome mutation and polyploidy ensures that each set of chromosome is duplicated, restoring homologous pairs of chromosomes. This allows meiosis to occur and for hybrids to produce gametes, restoring their fertility.

Question 53

What are the benefits of polyploidy?

Answer 53

Polyploidy offsprings have multiple sets of chromosomes that code for different characteristics from different species. This produces hybrid vigour whereby the hybrid possesses beneficial characteristics from all of the species that were crossed to create it.