AQA A2 Biology B18 Population And Evolution

Banner 1

C18.1 Population genetics:AQA A2 Biology B18 Population And Evolution Kerboodle Answers Page No. 186  

 

1

The number of times an allele occurs within the gene pool is referred to as the allelic frequency.

2 The Hardy-Weinberg principle provides a mathematical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population. The principle makes the assumption that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next.

3 • No mutations arise.

• The population is isolated, that is, there is no flow of alleles into or out of the population.
• There is no selection, that is, all alleles are equally likely to be passed to the next generation.
• The population is large.
• Mating within the population is random.

4

p² + 2pq + q² = 1.0

 

p + q = 1.0

 

1.0 – p = q 

 

1.0 – 0.942 = 0.136

 

q = 0.058

 

Frequency of heterozygous genotype = 2pq

 

2 x 0.942 x 0.058 = 0.109

 

As a percentage of the population- 10.9% 

 

5

p = frequency of the dominant allele in the population

q = frequency of the recessive allele in the population

p² = percentage of homozygous dominant individuals

q² = percentage of homozygous recessive individuals

2pq = percentage of heterozygous individuals

P² + 2pq + q² = 1

p + q = 1

Determining the frequency of the recessive genotype (q²) 26 / 416= 0.0625

q² = 0.0625

q = .25 frequency of recessive allele

p + 0.25 = 1

p = 0.75 frequency of dominant allele

p²= 0.5625

p² + 2pq + q² = 1

(0.75)² + 2(0.75) (0.25) + (0.25)² = 1

0.5625 + 0.375 + 0.0625 = 1

0.5625 freq. of genotype x 416 people = 234 homozygous dominant people

 

18.2 Variation in phenotype AQA A2 Biology B18 Population And Evolution Kerboodle Answers : Page No. 188

 

 

1 Sexually reproducing organisms, increase variation by all three methods. Where variation is very largely the result of genetic factors organisms fit into a few distinct forms and there are no intermediate types.

1 Mutations. These sudden changes to genes and chromosomes may, or may not, be passed on to the next generation. Mutations are a main source of variation.

2 Meiosis. This special form of nuclear division produces new combinations of alleles before they are passed into the gametes, all of which are therefore different.

3 Random fertilisation of gametes. In sexual reproduction this produces new combinations of alleles and the offspring are therefore different from parents. Which gamete fuses with which at fertilisation is a random process further adding to the variety of offspring two parents can produce.

2 Organisms raised through asexual reproduction are the exact copies of parents, showing the same characters like those of their parents. These organisms identical to their parents and other offsprings are called clones.

These suffer no or very little variations. Usually the variations are environmental due to environmental factors like light, temperature, humidity, water scarcity or food availability for asexually produced organisms.

Variation may also occur because of Mitotic recombination.

3 a Environmental.

b Genetic.

c Genetic.

d Environmental.

e Environmental.

18.3 Natural selection AQA A2 Biology B18 Population And Evolution Kerboodle Answers : Page No. 190

 

 

1 A gene pool is the total number of all the alleles of all the genes of all the individuals within a particular population at a given time.

2

The process of evolution by means of natural selection depends upon a number of factors. These include:

* Organisms produce more offspring than can be supported by the available supply of food, light, space, etc.

* There is genetic variety within the populations of all species.

* A variety of phenotypes that selection operates against.

3

Sickle cell anemia is a genetic disease that slightly alters the structure of haemoglobin, the oxygen carrying protein in red blood cells. This modified haemoglobin, when not carrying oxygen, tends to clump with other deoxygenated haemoglobin, deforming red blood cells and causing blood to clot throughout the body. The disease is prevalent in Africa where incidence of malaria is high because sickle cell anemia imparts some resistance to malaria.

 

Those with sickle cell anemia are resistant to the parasite that causes malaria.

 

A small genetic change in hemoglobin both causes sickle cell anemia and protects against malaria. Hemoglobin is the protein in red blood cells that carries oxygen to the tissues. Deoxygenated sickle hemoglobin distorts the blood cells into a “sickle” shape that obstructs circulation and in turn damages tissues; the resulting anemia shortens life expectancy to about 45. Conversely the weakened sickle blood cell is a poor host for the malaria parasite as the cell leaks needed nutrients and is selectively eliminated by the liver. This selective advantage of sickle cell concentrates its occurrence to regions of high malaria incidence.

 

18.4 Effects of different forms of selection on evolution AQA A2 Biology B18 Population And Evolution Kerboodle Answers : Page No. 195

a Stabilising

b Stabilising

c Directional

d Disruptive

e Disruptive

f Directional

g Stabilising

 

2

The peppered math {Biston betuiaria’} existed almost entirely in its natural light form until the middle of the nineteenth century. Around this time a melanic (black) variety arose as the result of a mutation. These mutant moths had undoubtedly occurred before (one existed in a collection made before 1819) but they were highly conspicuous against the light background of the lichen-covered trees and rocks on which they normally rest.

As a result, the black mutants were subjected to greater predation from insect-eating birds, far example, robins and hedge sparrows, than were the better camouflaged, light forms.

When, in 1848, a melanic form of the peppered moth was captured in Manchester, most buildings, walls and trees were blackened by the soot of 50 years of industrial development.

 

The sulfur dioxide in smoke emissions killed the lichens that formerly covered trees and walls. Against this black background the melanic form was less, not more, conspicuous than the light form. As a result, the light form was eaten by birds more frequently than the melanic form and, by 1895, 98% of Manchester’s population of the math was of the melanic type. This is an example of selective predation by birds favouring individuals that lie at one extreme or the other of a range of different colour types.

18.5 Isolation and speciation AQA A2 Biology B18 Population And Evolution Kerboodle Answers : Page No. 199

 

1 A species is a group of individuals that have a common ancestry and so share the same genes but different alleles and are capable of breeding with one another to product fertile offspring. In other words, members of a species are reproductively separated from other species.

2 Speciation is the evolution of new species from existing ones.

It is through the process of speciation that evolutionary change has taken place over millions of years. This has resulted in great diversity of forms amongst organisms, past and present.

3 Geographic isolation refers to the separation of members of a population by a physical barrier, such as a mountain or body of water, which disrupts the gene flow between them and begins the process of speciation. Geographic isolation of a species occurs as a result of physical changes in its natural environment. Right from the formation of mountain ranges and canyons to the formation or destruction of land bridges, all these physical events are considered causes of geographic isolation.

4 The best example of speciation resulting from geographic isolation will be that of the Darwin’s finches (subfamily Geospizinae), also known as the Galápagos finches, found on the Galápagos Islands. These species are found on different islands of this archipelago. A closer look at the 15 odd species, and you realize that each of them is perfectly adapted to its environment. A striking feature, for instance, is their beak, which has evolved to complement their diet. More importantly, their beaks play a crucial role when it comes to choosing mates.

5

Allopatric speciation occurs when populations of a species are separated by a physical barrier – this could be a river for animals that cannot swim, for example. Separation of the populations means that there is very low or no gene flow between them – the proportion of different genotypes in each population is therefore able to change independently of the other (there’s no mixing up of genes between the two populations). Over time, these changes may be so drastic that the populations become unable or unwilling to breed with each other, and could therefore be described as a pair of species.

 

 

Sympatric speciation occurs without a physical barrier to gene flow. This is more common in plant species – plants can mutate in a way which results in them producing offspring with double or even quadruple the number of chromosomes they normally do. The sex cells (sperm and eggs) produced by these individuals cannot fuse with sex cells from a “normal” plant – the plants with unusually high numbers of chromosomes therefore become isolated genetically from the “normal” plants, even though they may be growing right next to each other. This genetic isolation results in the two types of plants developing into species due to lack of gene flow and independent changes in the genotypes of plant populations.

 

Banner 3

Practice Questions: Chapter 18: Page No. 200-201

 

 

1 (a)

The frequency/proportion of alleles (of a particular gene).

Will stay constant from one generation to the next/over generations / no genetic change over time.

Providing no mutation/no selection/population large/population genetically isolated/mating at random/no migration.

(b)

White/deaf cats unlikely to survive/selected against.

Will not pass on allele (for deafness/white fur) (to next generation)/will reduce frequency of allele.

 

(c) In Paris/London frequencies (of these alleles) add up to more than 1.

 

(d)

 

44(.22)

p/frequency of H determined as 0.67 and q/frequency of h as 0.33

0.44(22);

2 (a)
Births per thousand/given number of the population and per year/given period of time;

(b)

(i) 1. Females have higher life expectancies;

2. (ii)
   1. Females tend to outlive males linked to reason e.g.

Male risk of CVD

More males smoke/drink to excess

Males involved in fighting / war.

2. Medical care/vaccination programmes better in UK/infectious disease common in Sudan;
3. More food/better diet in UK.
4. Food preservation/sanitation/clean water supply better in UK.

3

 

 

(a)
1. Draw grid over (map of) area;

2. Select squares/coordinates at random;

(b)

1. No emigration/immigration;
2. No losses to predation;
3. Marking does not affect survival;
4. Birth rate and death rate equal;
5. (In this case) all belong to one population;

 

(c)
1. Only glows brightly with UV, so doesn’t make insects more visible;

2. So doesn’t affect/increase predation;

(d)
10 130;

Tolerance of ± 1

4

(a)

Small surface area to volume ratio / more fat.

Lose less heat (to the environment) / for insulation.

When they are sitting on eggs.

(b) (i)
The further north/higher the latitude, the higher the percentage (of white snow geese);

(ii)
Snow lying longer/melts slower further north/at greater latitudes.

White geese better camouflaged (further north).

Predation linked to survival/reproductive success.

(c) Snow melts earlier/snow melts further north / less snow;

White geese decreasing as less well camouflaged/at disadvantage/blue geese increasing as better camouflaged/at an advantage.

(d) (i)
Stabilising.

(iiFew geese survive at the extremes/most survive from the middle of the range.

5
(a)
1. No interbreeding / gene pools are separate / geographical isolation.

2. Mutation.
3. Different selection pressures / different foods/niches/habitats.
4. Adapted organisms survive and breed / differential reproductive success.
5. Change/increase in allele frequency/frequencies.

(b)

Similar/same environmental/abiotic/biotic factors / similar/same selection pressures/ no isolation / gene flow can occur (within a species).

Banner 4

DISCLAIMER

Disclaimer: I have tried by level best to provide the answers and video explanations to the best of my knowledge. All the answers and notes are written by me and if there is any similarity in the content then it is purely coincidental. But this is not an alternative to the textbook. You should cover the specification or the textbook thoroughly. This is the quick revision to help you cover the gist of everything. In case you spot any errors then do let us know and we will rectify it. References: BBC Bitesize AQA GCSE Science Kerboodle textbook Wikipedia Wikimedia Commons Join Our Free Facebook Group : Get A* in GCSE and A LEVEL Science and Maths by Mahima Laroyia: https://www.facebook.com/groups/expertguidance.co.uk/ For Free Tips, advice and Maths and Science Help