7B: Populations & evolution

Question 1

What is the definition of a species?

Answer 1

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

Question 2

What is a population?

Answer 2

A group of organisms of the same species living in a particular area at a particular time

Question 3

Define evolution

Answer 3

Change in allele frequency over time

Question 4

Define gene pool

Answer 4

The complete range of alleles for all genes in all the organisms in a population

Question 5

Define allele frequency

Answer 5

The proportion of a particular type of allele in a population

Question 6

What are the 5 conditions that must be met for the Hardy-Weinberg principle?

Answer 6

• No mutations
• No movement in/out of the population
• No selection
• Random mating
• Large population

Question 7

What does the Hardy-Weinberg principle predict?

Answer 7

It predicts that the frequencies of alleles in a population won’t change from one generation to the next (this is only true under certain conditions)

Question 8

What is the Hardy-Weinberg equation?

Answer 8

p + q = 1

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

Question 9

In Hardy-Weinberg what is 2pq?

Answer 9

2pq = frequency of heterozygous genotype

Question 10

In Hardy-Weinberg what is p²?

Answer 10

p² = frequency of homozygous dominant genotype

Question 11

In Hardy-Weinberg what is q²?

Answer 11

q² = frequency of homozygous recessive genotype

Question 12

Brown fur coloring is dominant to skewbald fur coloring in horses. If you have 168 brown horses in a population of 200 horses. What is the predicted frequency of homozygous dominants?

Answer 12

36%

Question 13

Brown fur coloring is dominant to skewbald fur coloring in horses. If you have 168 brown horses in a population of 200 horses. What is the predicted frequency of heterozygotes?

Answer 13

48%

Question 14

Brown fur coloring is dominant to skewbald fur coloring in horses. If you have 168 brown horses in a population of 200 horses. What is the predicted frequency of homozygous recessive?

Answer 14

16%

Question 15

The frequency of cystic fibrosis (ff) in the UK is currently around 1 birth in every 2500. From this information estimate the percentage of people in the UK that are cystic fibrosis carriers.

Answer 15

To do this you need to find 2pq.

1. ff = q² = 1 / 2500 = 0.0004
   so, q = 0.02
2. p = 1 - q
   p = 1 - 0.02 = 0.98
3. 2pq = 2 x 0.02 x 0.98 = 0.039
   The frequency of Ff is 0.039, so the % in the UK population is 3.9%

Question 16

What is variation?

Answer 16

The differences that exist between individuals

Question 17

What does variation within a species result in?

Answer 17

A wide range of different phenotypes

Question 18

What is the main source of genetic variation?

Answer 18

Mutations (when changes in the DNA base sequence lead to the production of new alleles)

Question 19

What are 2 other ways that genetic variation can happen?

Answer 19

1. During meiosis (through crossing over and independent segregation)
2. Random fertilisation

Question 20

What is the definition of evolution?

Answer 20

Changes in the allele frequency of a population over time

Question 21

Name 3 selection pressures that create a struggle for survival

Answer 21

Predation, disease and competition

Question 22

Why does a population show a wide range of phenotypes?

Answer 22

Due to genetic and environmental variation

Question 23

Describe natural selection

Answer 23

• Selection pressures create a struggle for survival.
• Organisms with phenotypes providing selective advantages are likely to produce more offspring and pass on their favourable alleles to the next generation.
• The effect of this differential reproductive success on the allele frequencies within a gene pool

Question 24

What are 3 types of natural selection?

Answer 24

Stabilising, directional and disruptive (they affect allele frequency in different ways)

Question 25

Describe stabilising selection

Answer 25

Where individuals with alleles for average phenotypes are more likely to survive and reproduce

• Occurs when environment is NOT changing
• Reduces the range of possible characteristics
• Results in a narrow bell curve

Question 26

Describe directional selection

Answer 26

Where individuals with alleles for one extreme phenotype are more likely to survive and reproduce

• Could be a response to an environmental change
• Results in the bell curve shifting in the direction of selection

Question 27

Describe disruptive selection

Answer 27

Where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce

• Occurs when the environment favours more than ones phenotype
• Is the opposite of stabilising selection because characteristics towards the middle of the range are lost

Question 28

What is speciation?

Answer 28

The development of a new species from an existing species

Question 29

When does speciation occur?

Answer 29

It occurs when populations of the same species become reproductively isolated

Question 30

What is reproductive isolation and how does it occur?

Answer 30

When members of the same species can no longer interbreed to produce fertile offspring. It occurs due to changes in allele frequency causing changes in phenotype.

Question 31

What is geographical isolation?

Answer 31

When a physical barrier, e.g a flood or an earthquake, divides a population of a species, causing some individuals to become separated from the main population

Question 32

Describe sympatric speciation

Answer 32

• Does NOT require geographical isolation

- Random mutations can occur in a population causing some to be reproductively isolated

Question 33

Describe allopatric speciation

Answer 33

• Requires geographical isolation, so populations will experience different selection pressures + conditions
• This leads to different changes in allele frequencies, which will cause differences to accumulate in the gene pools and will eventually cause changes in phenotypic frequencies
  = reproductive isolation

Question 34

Name and describe 3 changes that can cause reproductive isolation

Answer 34

Seasonal - individuals of the same pop. develop different flowering/mating seasons or become sexually active at different times of the year

Mechanical - changes in genitalia prevent successful mating

Behavioural - a group of individuals develop courtship rituals that aren’t attractive to the main population

Question 35

What is evolution by genetic drift and in what type of population does it normally occur?

Answer 35

When chance, rather than environmental factors, dictates which individuals survive, breed and pass on their alleles. In smaller populations

Question 36

Describe how evolution by genetic drift occurs

Answer 36

• Individuals within a pop. show variation in their genotypes (e.g. A and B)
• By chance, the allele for one genotype (B) is passed on to the offspring more often than others
• So the number of individuals with the allele increases
• Changes in allele frequency in two isolated populations could eventually lead to reproductive isolation and speciation

Question 37

Evolution by genetic drift usually has a _____ effect in _____ populations where _____ has a greater influence.

Answer 37

Evolution by genetic drift usually has a GREATER effect in SMALLER populations where CHANCE has a greater influence.

Question 38

What is the diversity of life on earth a result of?

Answer 38

Speciation and evolutionary change over millions of years