18: Populations and evolution

Question 1

Define gene pool

Answer 1

All alleles of all genes of all the individuals in a population at a given time

Question 2

Define allelic frequency

Answer 2

Number of times an allele occurs within the gene pool

Question 3

What does the Hardy-Weinberg principle assume?

Answer 3

Proportion of dominant and recessive alleles of every gene remains constant between generations
This means: No mutations arise
Isolated population
No natural selection
Large population
Mating within the population is random

Question 4

What are the Hardy-Weinberg equations?

Answer 4

Frequency of allele A = p
Frequency of allele a = q
p + q = 1
p^2 + 2pq + q^2 = 1

Question 5

What is variation in phenotypes usually caused by?

Answer 5

Both genetic and environmental factor

Question 6

How does genetic variation occur?

Answer 6

Mutations - sudden changes to genes which could be passed on. Main source of variation
Meiosis - produces new form of alleles before passed into gametes (independent assortment and crossing over)
Random fertilisation - random which gamete fuses with another meaning there is an increased variety

Question 7

How does genetic and environmental influences affect the phenotype?

Answer 7

Genetics set the limits but the environment determines where within those limits the final phenotype is found

Question 8

How does the environment affect genes?

Answer 8

Environment influences the way the organism’s genes are expressed

Question 9

What does polygenes when describing a phenotype describe?

Answer 9

One phenotype that is determined by many genes

Question 10

Define a phenotype

Answer 10

The physical expression of genes which results from both the environment and the genotype of the organism

Question 11

Define selection pressures

Answer 11

Environmental factors that limit the population of a species
Determine the frequency of all alleles within the gene pool
E.g. Predation, disease and competition

Question 12

What factors does natural selection depend upon?

Answer 12

Organisms produce more offspring than can be supported by available supplies (food,light,space)
Genetic variety within the populations of all species
Variety of phenotypes that selection operates against

Question 13

Why is variation important in natural selection?

Answer 13

Greater variety raises chance one or more individuals will have the combination of alleles which lead to a beneficial phenotype
Accelerates natural selection

Question 14

Define stabilising selection

Answer 14

Preserves phenotypes around the mean and selects against the extreme phenotype

Question 15

Define directional selection

Answer 15

Changes the phenotypes of the population by favouring phenotypes in one direction, selects one extreme phenotype

Question 16

Define disruptive selection

Answer 16

Favours individuals with both sides of the extreme phenotype rather than the mean

Question 17

When does stabilising selection occur?

Answer 17

When the environment conditions are constant over very long periods of time

Question 18

When does directional selection occur?

Answer 18

When the environment conditions change in one direction

Question 19

When does disruptive selection occur?

Answer 19

Occurs when an environmental factor takes two distinct forms, or due to a barrier causing two environments

Question 20

Define speciation

Answer 20

The evolution of new species from existing ones

Question 21

Define a species

Answer 21

A group of individuals that have a common ancestry and can breed to produce fertile offspring

Question 22

What is adaptive radiation?

Answer 22

Different phenotypes due to different selection pressures in different environments

Question 23

What is genetic drift?

Answer 23

Allele frequencies of a population change over generations due to chance
More prominent in small populations as lower genetic diversity

Question 24

What are the types of speciation?

Answer 24

Allopatric speciation

Sympatric speciation

Question 25

Define allopatric speciation

Answer 25

Speciation which forms due to members of a species becoming geographically separated
Different locations have different conditions causing natural selection

Question 26

Define sympatric speciation

Answer 26

Speciation which occurs within a population in the same area

Question 27

What are the isolating mechanisms?

Answer 27

Geographical
Ecological
Temporal
Behavioural
Mechanical
Gametic
Hybrid sterility

Question 28

What occurs in geographical isolation?

Answer 28

Populations are isolated by physical barriers such as oceans, mountains etc

Question 29

What occurs in ecological isolation?

Answer 29

Populations inhabit different habitats within the same area so individuals rarely meet

Question 30

What occurs in temporal isolation?

Answer 30

Breeding seasons of each population do not coincide and cannot interbreed

Question 31

What occurs in behavioural isolation?

Answer 31

Courtship or other mating prerequisites are changed by variation meaning mating does not occur

Question 32

What occurs in mechanical isolation?

Answer 32

Anatomical differences may prevent mating

E.g. May be physically possible for the penis to enter the females vagina

Question 33

What occurs in gametic isolation?

Answer 33

Gametes may be prevented from meeting due to genetic or biochemical incompatibility

Question 34

What occurs in hybrid sterility

Answer 34

Hybrids are the offspring of the parents of two different species often are sterile as cannot produce viable gametes

Question 35

Define genotype

Answer 35

The genetic constitution of an organism

Question 36

Why can protein structure be used to investigate evolutionary relationships between different species?

Answer 36

Closer the amino acid sequence the closer the relationship

Protein structure related to the DNA base sequence

Question 37

Why does comparing the base sequence of genes provides more evolutionary information than
comparing the structure of proteins?

Answer 37

More bases in the base sequence than amino acids in amino acid sequence
Due to introns being present