Chapter 18 - Populations and evolution

Question 1

Define species

Answer 1

Species – a group of organisms with similar characteristics that can reproduce to produce fertile offspring (exist as one or more populations)

Question 2

Define population

Answer 2

Population – a group of organisms of the same species, occupying a particular space at a particular time

Question 3

Define allele frequency

Answer 3

Allele frequency – the number of times an allele occurs within a population

Question 4

Define gene pool

Answer 4

Gene pool – all the alleles of all the genes in a population

Question 5

What is the hardy Weinberg principle used to calculate

Answer 5

• Allele frequency
• Genotype and phenotype frequency, in a population

Question 6

What assumptions are needed for the hardy Weinberg principle to work

Answer 6

Assumes allele frequency will not change from generation to generation

This is because:
* No mutations arise
* No selection
* Mating is random
* Population is isolated
* Population is large

Question 7

What are the two equations used in the hardy Weinberg principle

Answer 7

p = frequency of dominant allele in a population
q = frequency of recessive allele in a population

1. p + q = 1.0 (used to calculate allele frequency)
2. p2 + 2pq + q2 = 1.0 (used to calculate phenotype / genotype frequency)

p2 and q2 is squared

Question 8

What are the genotype frequencies represented by in the Hardy Weinberg equations

Answer 8

Genotype frequencies:
p2 = homozygous dominant
2pq = heterozygous
q2 = homozygous recessive

Question 9

What are the phenotype frequencies represented by in the Hardy Weinberg equations

Answer 9

Phenotype frequencies:
p2 + 2pq = dominant phenotype
q2 = recessive phenotype

Question 10

table of frequencies from Hardy Weinberg equations

Answer 10

Question 11

What causes variation in phenotype

Answer 11

• Genes
• environment
• both

Question 12

example of only genetics in phenotypic variation

Answer 12

Example: blood group
Usually controlled by a single gene
Environment has very little influence
Creates discrete groups – discontinuous variation
Shown as bar chart

Mutations are the primary source of genetic variation in all organisms.
Meiosis and random fertilisation during sexual reproduction produce further genetic variation.

Question 13

example of only environmental factors in phenotypic variation

Answer 13

Only scars and tattoos

Question 14

example of both genes and environmental factors in phenotypic variation

Answer 14

Example: height, weight
Usually a combination of genetic and environmental factors
Usually controlled by many genes (polygenes)
Creates a continuum of variation – continuous variation
Shown as normal distribution curve

Question 15

example natural selection answer

Answer 15

• Random mutations can result in new alleles of a gene
• Selection pressure = (any abiotic factor such as predation, competition or disease)
• Individuals with the advantageous allele survive, reproduce and pass on the advantageous allele to the next generation
• Over many generations, the new allele increases in frequency in the population

Question 16

What is directional selection

Answer 16

• Phenotypes falling one side of the mean are selected for.
• Increased frequency of allele to one side of the mean.
  e.g Antibiotic resistance,fur legnth

Question 17

diagram for directional selection

Answer 17

Question 18

What is stabilising selection

Answer 18

• Phenotypes around the mean being selected for.
• Stabilising selection occurs when environmental conditions are constant over a period of time.
• A species no longer needs the range of phenotypes to survive

Question 19

diagram for stabilising selection

Answer 19

Question 20

What is disruptive selection

Answer 20

• Occurs when extreme environmental conditions occur.
• Extreme phenotypes are favoured.
• They survive and reproduce.
• Over time, it may lead to two sub-populations or separate species arising (speciation) when they each favour differing conditions.

Question 21

diagram for disruptive selection

Answer 21

Question 22

define speciation

Answer 22

Speciation - Evolution of a new species from an existing one

Question 23

Define genetic drift

Answer 23

Genetic drift - A change in allele frequency that takes place randomly, by chance, especially in small populations

Question 24

Define reproductive isolation

Answer 24

Reproductive isolation - Inability to reproduce to produce fertile offspring

Question 25

define allopatric speciation

Answer 25

Allopatric speciation - Formation of a new species from different populations in different areas

Question 26

define sympatric speciation

Answer 26

Sympatric speciation - Formation of a new species from a population living in the same area, without geographical isolation

Question 27

How does speciation generally occur

Answer 27

* Populations become reproductively isolated * Mutations occur in each population * Changes in allele frequency occur in each population by natural selection due to differing selection pressures * Genetic differences accumulate in each gene pool * Over time, new species arise as individuals of the separated populations are now unable to interbreed and produce fertile offspring. * This means there is restricted gene flow

Question 28

How does allopatric speciation occur

Answer 28

* When two populations are geographically isolated by a physical barrier (examples = rivers, mountain ranges, deserts) and there is restricted gene flow * Variation due to mutations occurs in each population * Different environmental conditions/selection pressures for each population * Selection for advantageous alleles leads to differential reproductive success in each population * Over time, a change in allele frequency occurs in each population * Leads to reproductive isolation and formation of two species (separate populations cannot interbreed to produce fertile offspring)

Question 29

How does sympatric speciation occur

Answer 29

* Occurs in the same area (not geographically isolated) * Mutations lead to reproductive isolation and therefore restricted gene flow between gene pools * Different mutations occur and selection pressures operate * Differential reproductive success leads to a change in allele frequency occurs * Eventually they cannot breed to produce fertile offspring Can be an example of disruptive selection

Question 30

What are the 5 types of isolating mechanism

Answer 30

* temporal − different breeding seasons, feeding times * ecological – different niches, habitats, feeding areas * behavioural − different courtship displays * mechanical − mismatch of reproductive parts * gamete incompatibility − genetic/biochemical incompatibility e.g. sperm killed in female’s reproductive tract

Question 31

What is genetic drift

Answer 31

* Genetic drift - a change in allele frequency that takes place randomly * Magnified in smaller populations * Sometimes if a random event occurs, alleles may be completely lost from a population’s gene pool