Topic 7 B: Populations and Evolution

Question 1

Species definition?

Answer 1

Group of organisms, with common ancestor, that can reproduce to give fertile offspring.

Question 2

Population definition?

Answer 2

Group of organisms of the same species living in a particular area at a particular time and can potentially interbreed.

Question 3

Gene pool definition?

Answer 3

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

Question 4

Allele frequency definition?

Answer 4

How often an allele occurs in a population / gene pool. (usually decimals or percentage)

Question 5

Allele frequency formula?

Answer 5

p + q = 1
p = relative frequency of dominant allele
q = relative frequency of recessive allele

Question 6

What does the Hardy-Weinberg principle predict?

Answer 6

The frequency / proportion of alleles in a population wont change from one generation to the next, provided that certain conditions are met.

Question 7

Hardy-Weinberg formula?

Answer 7

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

p^2 = proportion of homozygous dominant
2pq = proportion of heterozygous
q^2 = proportion of homozygous recessive

Question 8

Hardy-Weinberg assumptions?

Answer 8

1. no mutations occur to create new alleles
2. no movement of alleles into or out of the population by migration
3. population is large
4. no selection
5. mating is random

Question 9

Variation definition?

Answer 9

The differences that exist between individuals.

Question 10

Natural selection overall definition?

Answer 10

Process whereby an allele becomes common in a population because it codes for characteristics that make an organism more likely to survive, reproduce and pass on its genes to offspring.

Question 11

Genetic factors which cause variation?

Answer 11

1. random fertilisation = gametes carrying different alleles will join together at random.
2. meiosis = type of nuclear division that creates gametes and causes alleles to be assorted into gametes at random.
3. mutations = could create a new allele, which then could be passed on to the next gen.

Question 12

How is variation caused by genetic factors displayed on a graph and why?

Answer 12

• bar chart
• organisms fit into a few distinct forms and there are no intermediates e.g., blood group / eye colour

Question 13

How can environmental factors cause genetic variation? Give example.

Answer 13

• e.g., food, climate, or lifestyle.
• e.g., identical twins with identical alleles are different heights and body masses due to diet.

Question 14

How is variation caused by environmental factors displayed on a graph and why?

Answer 14

• line graph
• organisms may have intermediate phenotypes e.g., weight

Question 15

Exam question: State 3 ways in which genetic variation can be increased in sexually reproducing organisms?

Answer 15

Mutations
Meiosis
Random fusion of gametes

Question 16

Exam question: State how genetic variation is increased in asexually reproducing organisms?

Answer 16

Mutations only

Question 17

Why don’t populations increase alot in nature?

Answer 17

Because when there are too many offspring, there wouldn’t be enough resources for them all due to intraspecific competition.

Question 18

Sources of competition?

Answer 18

Food, light, space, territory, and mates.

Question 19

Not all members in population survive, why?

Answer 19

Due to predation, disease, or competition for resources.
These are known as selection pressures - create struggle for survival.

Question 20

Selective advantage definition?

Answer 20

Organisms with variations that make them more likely to survive.

Question 21

Overproduction definition?

Answer 21

When organisms produce more offspring than the environment can support, so there aren’t enough resources for all offspring so some die.

Question 22

GCSE recap of natural selection process?

Answer 22

1. Variation - exists due to the genes inherited from sexual reproduction / mutations.
2. Competition - selection pressure arises which forces organisms to compete for survival.
3. Survival of the fittest - only those with advantageous alleles can outcompete competitors.
4. Reproduction - these individuals survive and reproduce, passing on their advantageous alleles to their offspring. Overtime, the frequency of adv. allele increases in gene pool.

Question 23

Limitations of natural selection?

Answer 23

It isnt favourable for the fittest organisms to produce identical offspring because environments / conditions change over time, so new selection pressures arise.
Having large populations with wide range of genetically different individuals means that some will have the combination of genes needed to survive almost any new set of circumstances.

Question 24

Exam question: State 4 factors that lead to differential survival and reproduction?

Answer 24

Predation
Competition for food/water/shelter
Diseases
Natural disasters

Question 25

Evolution definition?

Answer 25

The frequency of an allele in a population changing over time. (due to genetic drift or natural selection)

Question 26

Speciation definition?

Answer 26

Evolution of a new species from an existing one. (due to a phenotype changing so much over long period of time).

Question 27

What is an isolation mechanism?

Answer 27

Some kind of barrier that prevents gene flow between 2 populations of species so that they become reproductively isolated.

Question 28

Allopatric speciation definition?

Answer 28

When populations are seperated by geographical barriers.
(e.g., mountain range and lakes)

Question 29

Sympatric speciation definition?

Answer 29

When populations become reproductively isolated whilst being in the same area.

Question 30

Examples of sympatric speciation?

Answer 30

1. mechanical
2. hybrid sterility
3. ecological
4. behavioural
5. temporal
6. gametic

Question 31

Mechanical barrier?

Answer 31

Anatomical differences may prevent mating e.g., physically impossible for penis to enter vagina in mammals.

Question 32

Hybrid sterility barrier?

Answer 32

When 2 different species reproduce a hybrid which is often sterile as they cant produce viable gametes e.g., cross between horse and donkey, the resultant mule has 63 chromosomes so its impossible for them to pair up equally in meiosis so sterile gametes formed.

Question 33

Ecological barrier?

Answer 33

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

Question 34

Behavioural barrier?

Answer 34

Mating often preceded by courtship, which is stimulated by the coloured markings of the opposite sex, the call or particular actions of a mate. Any mutations which cause variations in these patterns may prevent mating.

Question 35

Temporal barrier?

Answer 35

The breeding seasons of each population don’t coincide and so they don’t interbreed.

Question 36

Gametic barrier?

Answer 36

Gametes may be prevented from meeting due to genetic / biochemical incomaptability. E.g., some pollen grains fail to germinate / grow when they land on stigma of different genetic makeup.

Question 37

Genetic drift definition?

Answer 37

Allele frequencies of a population change over generations due to chance.

Question 38

Genetic drift process?

Answer 38

1. individuals within population show variations in genotypes.
2. by chance, allele for one genotype is passed onto more offspring than other, so number of individuals with the allele increase.
3. if by chance the same allele is passed on more often again and again, it can lead to evolution as the allele becomes more common in the population.

Question 39

What type of population size does genetic drift affect most?

Answer 39

Small populations

Question 40

What is disruptive selection?

Answer 40

When selection pressures favour two extremes, creating two modal values. (e.g., beak size)

Question 41

What is stabilising selection?

Answer 41

Selection pressure favouring middle trait, increasing number of individuals at the modal value. (e.g., baby size)

Question 42

What is directional selection?

Answer 42

Selection pressure favours one extreme and moves modal value in this direction. (e.g., giraffes with long necks)

Question 43

Exam question : How is antibiotic resistance an example of directional selection?

Answer 43

Bacteria population has genetic variation due to mutations
Some individuals have the resistance alleles some don’t
A change in the environment occurs which is an antibiotic is taken
Individuals with the antibiotic resistance alleles survive
The bacteria that survive reproduce and pass on the favourable alleles
The frequency of the antibiotic resistance alleles increases in the gene pool

Question 44

Exam question : Why is human birth weight an example of stabilising selection?

Answer 44

Humans are born with a range of weights and the extremes of these ranges are undesirable
Very small:
High surface area to volume ratio so loose heat quickly so rely on respiratory systems that then face to much pressure and causes fatalities
Very big:
Too big to leave the pelvis so cause birth complications

Question 45

Natural selection vague mark scheme?

Answer 45

1. genetic variation due to mutation
2. selection pressure e.g., temp change
3. some individuals have selective advantage
4. advantageous allele allows them to survive, breed and reproduce
5. pass on advantageous allele to offspring
6. allele frequency of advantageous allele increases in gene pool

Question 46

Speciation vague mark scheme (sympatric speciation)

Answer 46

1. mutation occurs causing genetic variation
2. identify type of speciation and define (sympatric)
3. reproductively isolated
4. type of barrier (e.g., temporal)
5. selection pressure identified
6. selective advantage identified and advantageous allele
7. identify type of selection (e.g., stabilising)
8. eventually when they recombine, too different and cant breed to produce fertile offspring

Question 47

Speciation vague mark scheme (allopatric)

Answer 47

1. identify allopatric speciation and explain how there are geographical barriers
2. mutations occur
3. selection pressure
4. selective advantage and advantageous allele
5. breed and pass onto offspring in their own populations
6. identify type of selection
7. when they recombine, cant interbreed to form fertile offspring