Topic 7B - Populations and Evolution

Question 1

Define ‘species’.

Answer 1

A group of similar organisms that can reproduce to give fertile offspring

Question 2

Define ‘gene pool’

Answer 2

The complete range of alleles present in a population

Question 3

Define ‘allele frequency’

Answer 3

How often an allele occurs in a population (usually given as a %)

Question 4

What is the ‘Hardy-Weinberg’ principle

Answer 4

A mathematical model that predicts that allele frequencies wont change from 1 generation to the next.

Question 5

What 2 conditions must take place in order to apply the hardy-weinberg principle?

Answer 5

1. It has to be a large population where there’s no immigration, emigration, mutations or natural selection.
2. There needs to be random mating - all possible genotypes can breed with all others.

Question 6

What is the Hardy Weinberg equation USED FOR?

Answer 6

1. To calculate frequency of particular alleles, genotypes, and phenotypes within populations
2. To test whether or not the Hardy-Weinberg principle applies to particular alleles in particular populations i.e to test whether selection or any other factors are influencing allele frequencies - if frequencies do change between generations in a large population then there is an influence of some kind.

Question 7

Give the equation that will help you predict allele frequencies?

Answer 7

If a gene has 2 alleles, you can figure out the frequency of 1 of the alleles of the gene, if you know the frequency of the other allele, using this equation:

p+q = 1

p= frequency of 1 allele, usually the dominant one
q= frequency of other allele, usually the recessive
* the total frequency of all possible alleles for a characteristic in a certain population is 1 so the frequency of individual alleles must add up to 1

Question 8

Give the hardy weinberg equation that will help you predict genotype and phenotype frequency?

Answer 8

You can figure out the frequency of 1 genotype if you the know the frequencies of the others using

p^2 + 2pq + q^2 = 1

p^2 = frequency of homozygous dominant genotype
2pq = frequency of heterozygous genotype
q^2 = frequency of homozygous recessive genotype
* the total frequency of all possible alleles for a characteristic in a certain population is 1 so the frequency of individual alleles must add up to 1.

Question 9

Do practice questions on the hardy-weinberg principle

Answer 9

page 176-177

Question 10

Define variation?

Answer 10

The differences that exist between individuals

Question 11

Does genetic variation exist between species?

Answer 11

Yes, even though they have the same genes. they have different alleles

Question 12

What are the main 3 factors that causes genetic variation?

Answer 12

1. Mutations
2. Meiosis (through crossing over and independent segregation)
3. Random fertilisation of gametes

Question 13

Define evolution

Answer 13

The change in allele frequencies over time

Question 14

How does natural selection work?

Answer 14

1. Individuals with an allele that increases their chance of survival are more likely to survive, reproduce and pass on their genes
2. This means that a greater proportion of next generation inherits beneficial allele
3. They, in turn, are more likely to survive, reproduce and pass on genes
4. Frequency of beneficient allele increases from generation to generation
5. Over generations, this leads to evolution as advantageous alleles become more common in pop

Question 15

What is a selection pressure?

Answer 15

Anything that affects an organisms chance of survival and reproduction e.g. competition, disease and predation

Question 16

Define intraspecific variation

Answer 16

Variation within a species

Question 17

Define stabilising selection

Answer 17

Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce. It occurs when the environment isn’t changing, and it reduces the range of possible phenotypes

Question 18

Define directional selection

Answer 18

Where individuals with alleles for a single extreme characteristic are more likely to survive and reproduce. It occurs when there is a change in the environment.

Question 19

State the 3 types of selection

Answer 19

1. Stabilising selection
2. Directional selection
3. Disruptive selection

Question 20

Define disruptive selection

example on page 179

Answer 20

Where individuals with allele for extreme characteristics at either end of the range are more likely to survive and reproduce. It’s the opposite of stabilising selection as characteristics towards the middle of the range are lost. It occurs when the environment favours more than 1 phenotype

Question 21

What is speciation?

Answer 21

Speciation is the development of a new species from an existing species

Question 22

When does speciation occur?

Answer 22

When populations of the same species become reproductively isolated - changes in allele frequency causes changes in phenotype, which mean they can no longer interbreed to produce fertile offspring
- Reproductive isolation can happen due to geographical isolation: when a physical barrier e.g. flood/earthquake, separates individuals from main species OR due to sympiatric speciation (no physical barrier)

Question 23

How can geographical isolation lead to allopatric speciation?

diagram

Answer 23

ALLOPATRIC SPECIATION

1. Population of 1 species exist
2. Physical barriers stop interbreeding between pops.
3. Pops adapt to new environment (due to directional selection, mutations and genetic drift)
4. Allele and phenotype frequency change, leading to reproductive isolation and the development of new species.

Question 24

Describe how sympiatric speciation works?

Answer 24

1. Most eukaryotic organisms are diploid, with 2 sets of homologous pairs. Sometimes, mutations occur that increase number of chromosome (polyploidy)
2. Individuals with different numbers of chromosomes can’t reproduce sexually to give off fertile offspring, so polyploid organism will be reproductively isolated.
3. If polyploid reproduces asexually successfully, a new species could develop.

Question 25

Name the 3 categories of allele and phenotype changes that lead to reproductive isolation

Answer 25

1. Seasonal
2. Mechanical
3. Behavioural

Question 26

Describe the ‘seasonal’ category.

Answer 26

Seasonal: Individuals from same population develop different flowering or mating seasons, or become sexually active at different times of the year.

Question 27

Describe the mechanical category.

Answer 27

Mechanical: Changes in genitalia prevent successful mating.

Question 28

Describe the behavioural category.

Answer 28

Behavioural: A group of individuals develop courtship rituals that aren’t attractive to the main population.

Question 29

What 2 factors can cause evolution

Answer 29

1. Natural selection

2. Genetic drift

Question 30

What is genetic drift?

Answer 30

This is when chance, rather than selection pressures, dictates which individuals survive, breed, and pass on their allele.

1. Individuals within pop. show variation in genotypes.
2. By chance, allele for 1 genotype is passed on to offspring more often than others
3. So frequency of this allele increases, which can lead to reproductive isolation and speciation.

Question 31

Does natural selection and genetic drift work together to separately to drive evolution?

Answer 31

They work alongside each other to drive evolution, but 1 process may drive evolution more than the other depending on pop. size.
- Evolution by genetic drift usually has greater effect in smaller pops where chance has a greater influence. In larger populations, chance variations are too insignificant to count towards evolution.

Question 32

What has led to the diversity of life today?

Answer 32

Speciation and evolutionary change over millions of years