Gene pools and speciations

Question 1

What is a gene pool and population?

Answer 1

• Total of alleles (variations of genes) for all genes present in an interbreeding population
• Population: individuals of the same species living in the same place and time

Question 2

What determines a large gene pool? What is allele frequency?

Answer 2

• High amounts of genetic diversity, increasing the chances of biological fitness and survival
• Allele frequency: measures how common an allele is in a population
• The depth of the gene pool is measured by the no. of alleles and their relative frequencies

Question 3

What needs to change with time in a population for evolution and why?

Answer 3

• For evolution to occur, the allele frequencies need to change within the gene pool of the population
• These changes in heritable characteristics are needed across generations

Question 4

What are 5 processes that affect the allele frequencies in a gene pool?

Answer 4

• Mutations: random change in genetic composition, due to changes in DNA base sequences
• Gene flow: movement of alleles into or out of a population (immigration, emigration)
• Reproduction: new gene combinations, alter allele frequencies
• Genetic drift: as result of random event (external)
• Natural selection: environmental pressures

Question 5

What is selective pressure? What are the three types of selection?

Answer 5

• Environmental factors affect the rates of survival of certain phenotypes
• Directional, stabilizing and disruptive selection

Question 6

Explain stabilising selection.

Answer 6

• Intermediate phenotypes are favoured
• The extreme phenotypes are not suitable
• Occurs when environmental conditions are stable, lack of competition
• Phenotypic distribution clusters in centre
• E.g. length of a tail (intermediate length favoured)

Question 7

Explain directional selection.

Answer 7

• When one extreme phenotype is favoured and the other is rejected
• Causes phenotypic distribution to shift to one direction (towards beneficial extreme)
• Gradual changes in environmental conditions
• E.g. antibiotic resistance of bacteria

Question 8

Explain disruptive selection.

Answer 8

• When both phenotypic extremes are favoured, intermediate is rejected
• Phenotypic distribution deviates in the centre, bimodal spread
• Fluctuating environmental conditions, favours two different phenotypes
• Continued separation of population lead to speciation

Question 9

What is reproductive isolation?

Answer 9

• The failure of individuals from two populations to mate and produce fertile offspring, results in the reduction or elimination of gene flow between the populations

Question 10

How does reproductive isolation occur?

Answer 10

• When barriers prevent two populations from interbreeding, keeping the gene pools separate which can lead to speciation

Question 11

What are three ways a population can become reproductively isolated?

Answer 11

• Geographical isolation
• Behavioural isolation
• Temporal isolation

Question 12

Explain geographical isolation.

Answer 12

• Occurs when two populations occupy different habitats within a common region or due to a physical barrier
• E.g. lions and tigers could interbreed by occupy different habitats

Question 13

Explain behavioural isolation.

Answer 13

• Occurs when genetically-influenced differences in behaviour reduce or prevent mating between two parts of a population
• E.g. production of a mating song in male crickets
• These behavioural differences can be bird dances, mating sites or mating songs

Question 14

Explain temporal isolation.

Answer 14

• When two populations differ in their periods of activity or reproductive cycles
• E.g. leopard frogs and wood frogs reach sexual maturity at different times in the spring, don’t interbreed

Question 15

What is speciation and when does it occur?

Answer 15

• Evolutionary process that results in the formation of a new species from pre-existing species
• Occurs when reproductive isolating mechanisms prevent two breeding organisms from producing fertile offspring

Question 16

What are the two types of speciation?

Answer 16

• Allopatric speciation (geographical)
• Sympatric speciation (reproductive)

Question 17

What is allopatric speciation?

Answer 17

• When a new species develops as a result of a part of a population becoming geographically isolated from others
• The populations evolve separately and reach a degree of genetic divergence, can no longer interbreed

Question 18

What is sympatric speciation?

Answer 18

• When a different species develops in the same geographic area due to behavioural, temporal or other forms of speciation.
• Isolation due genetic abnormalities, chromosomal error

Question 19

What is polyploidy? When will speciation occur?

Answer 19

• The presence of more than two complete sets of chromosomes in a cell, (diploid instead of haploid)
• This is a non-disjunction, meiotic cell fails to undergo cytokinesis, chromosome number will double
• Offspring has additional sets of chromosomes (polyploidy)
• Speciation will occur when the polyploid offspring is fertile but cannot interbreed with the original parent population

Question 20

What happens when a triploid and diploid individual reproduce?

Answer 20

• The offspring will also be triploid and usually sterile (uneven number of chromosomes)

Question 21

Why are polyploid crops desirable to farmers?

Answer 21

• Occurs by self-pollination and asexual reproduction, more common in plants
• It allows the production of seedless fruits, since they are infertile
• AND the crops typically grow larger and demonstrate improved disease resistance
• E.g. seedless watermelons are triploid

Question 22

What is hybrid vigour?

Answer 22

• When the hybrid exhibits traits that are more desirable than either parent
• Often case in polyploidy

Question 23

Explain the speciation in allium.

Answer 23

• Allium is a genus involving monocotyledon flowering plants e.g. onions, garlic, chives, leeks
• In these species polyploidy occured, resulted in reproductively isolated populations

Question 24

What are two paces of speciation?

Answer 24

• Gradually (divergence of isolated populations) or punctuated equilibrium

Question 25

Explain gradualism.

Answer 25

• Speciation occurs uniformly, by gradual transformations of whole lineages
• Continuous process, big changes result from many small continuous changes
• E.g. immigration, introduction of new gene pool
• Generally adaptations for better survival too

Question 26

Explain punctuated equilibrium.

Answer 26

• Species remain stable for long periods before undergoing abrupt and rapid change (rapid burst of phenotypic change)
• Periodic process, big changes occur suddenly, followed by long periods of no change
• Often lack of transitional fossil evidence

Question 27

What is genetic drift?

Answer 27

• Change in the composition of a gene pool as as result of chance or random events
• Occur faster in smaller populations, chance events will have a bigger impact on the gene pool
• Unstable allele frequencies, low genetic drift

Question 28

What are allele frequencies?

Answer 28

• Percentage or proportion how frequently an allele from a specific gene locus occurs in a population

Question 29

How do you compare the allele frequencies of geographically isolated populations?

Answer 29

• Two types: ancestral and derived alleles
• Identify regions where the ancestral allele is the only one present
• Identify in how many regions the derived allele is present
• The population with the greatest percentage of the derived allele

Question 30

What is genotype frequency?

Answer 30

• Number of individuals with a given genotype as proportion of the entire population

Question 31

What are the equations for allele frequency and genotype frequency?

Answer 31

Allele frequency: p+q=1
Genotype frequency: p2+2pq+q2=1