18.1 - 18.5 Populations and evolution

Question 1

Define population

Answer 1

A group of organisms of the same species that occupy the same space and can interbreed

Question 2

Define gene pool

Answer 2

All of the alleles of all genes of all the individuals in a population

Question 3

Define allelic frequency

Answer 3

Number of times an allele occurs

Question 4

Give an overview of cystic fibrosis

Answer 4

• Thicker than normal mucus produced in the lungs
• Recessive disease

Question 5

Define the Hardy-Weinberg principle

Answer 5

States that the proportion of dominant and recessive alleles of a gene remains the same generation to generation.

Question 6

What is the shorter Hardy-Weinberg equation?

Answer 6

p + q = 1

p = frequency of dominant allele
q = frequency of recessive allele

Question 7

What is the longer Hardy-Weinberg equation?

Answer 7

p2 + 2pq + q2 = 1

p2 = frequency of homozygous dominant individuals
q2 = frequency of homozygous recessive individuals
2pq = frequency of heterozygous individuals

Question 8

Causes of variation

Answer 8

Can be due to genetic and/or environmental factors

If it is mostly due to genetics the resulting phenotypes will fit into distinct groups i.e blood groups, inherited diseases

If the environment has an impact it will result in more continuous phenotypes i.e height, weight

Question 9

Describe discontinuous variation

Answer 9

• Distinct categories with no overlap
• Controlled by a small number of genes
• Largely unaffected by the environment

Question 10

What are the causes of genetic variation?

Answer 10

1. Meiosis - Produces new combinations of alleles (crossing over, independent segregation)
2. Mutations - Random changes to the genetic material. Most common reason for variation.
3. Random fusion of gametes - Produces new combinations of alleles

Question 11

Describe continuous variation

Answer 11

• No categories to place individuals
• Controlled by large number of genes
• Significantly affected by the environment

Question 12

How might the environment impact gene expression?

Answer 12

• Weather (light intensity, temperature, rainfall, wind speed)
• Nutrient availability
• pH
• Food availability

Question 13

Define polygenic characteristics

Answer 13

A characteristic which is controlled by multiple genes

More likely to be influenced by the environment

Question 14

What factors does natural selection depend on?

Answer 14

1. Organisms produce more offspring than can be supported
2. Genetic variation in a population
3. Selection pressures against certain phenotypes

Question 15

Describe natural selection

Answer 15

1. Within any population there will be a gene pool containing a wide variety of alleles
2. Random mutation of an allele may result in a new allele of a gene
3. In a certain environment this allele may give its possessor an advantage against selection pressure
4. These individuals are more likely to obtain resources and survive long enough to reproduce
5. Individuals that successfully reproduce will pass on their alleles
6. New individuals are in turn more likely to survive and reproduce
7. Over many generations allele frequency will increase
8. Name type of selection

Question 16

Define selection

Answer 16

A process in which organisms that are better adapted to the environment survive and breed while less well adapted organisms do not

Question 17

Define selection pressure

Answer 17

Any environmental factor that alters the frequency of alleles in a population

Question 18

Define stabilising selection

Answer 18

Eliminates the extremes of phenotype in both direction. Tends to occur when the environment remains constant over extended periods.

Question 19

Define directional selection

Answer 19

Favours one phenotypic extreme. Tends to occur when the environment changes overtime.

Question 20

Define disruptive selection

Answer 20

Favour the extremes of phenotypes in both directions. Tends to occur in a changing environment.

Question 21

Define differential reproductive success

Answer 21

Those organisms best adapted to a given environment will be most likely to survive to reproductive age and have offspring of their own

Question 22

Define speciation

Answer 22

Evolution of a new species from an existing one. Has lead to greater diversity.

Question 23

Define the two types of speciation

Answer 23

1. Allopatric - Geographically isolated populations
2. Sympatric - Reproductively isolated populations

Question 24

Describe speciation

Answer 24

1. Variation in an interbreeding population. Variety of alleles due to mutations.
2. Population geographically or reproductively isolated which prevents interbreeding. No gene flow between the two populations.
3. Variations due to mutations in each population
4. Environmental conditions faces by each population are different, different selection pressures
5. Natural selection occurs in both populations. Different alleles are more beneficial for survival.
6. Different changes in allele frequency in two populations
7. Become genetically different from each other which causes further reproductive separation
8. Formation of a new species - no longer able to interbreed to produce fertile offspring

Question 25

List and describe the different mechanisms of isolation

Answer 25

1. Geographical - Oceans, mountain ranges etc
2. Ecological - Different habitats within the same area
3. Temporal - Different breeding seasons
4. Behavioural - Different courtship behaviours
5. Mechanical - Anatomical differences that make mating impossible
6. Gametic - Gametes prevented from meeting due to genetic or biochemical incompatability
7. Hybrid sterility - Hybrids are often sterile because they cannot produce viable gametes

Question 26

Define genetic drift

Answer 26

A method of changing the frequency of an allele by chance where some specific individuals reproduce more than others. This may result in loss of beneficial alleles or increase in harmful alleles as it is not linked to natural selection.

Question 27

What are the conditions for the Hardy-Weinberg principle to apply?

Answer 27

• The population is isolated and large
• No mutations occur
• No selection pressures for any particular allele
• Mating within the population is random