17.2 Natural & Artificial Selection

Artificial insemination (AI) is one method used in assisted reproduction programmes for large mammals.

For many years, horse breeders have collected semen samples from male horses and used these to inseminate female horses. Success rates have been good. However, due to the sample containing live sperm cells, the process needed to be carried out quickly.

New technologies exist to allow horse semen to be frozen in small plastic straws. Semen samples can now be stored for many years and transported all over the world.

• Each 0.5 cm³ straw can hold 7.5 × 10⁷ sperm cells.
• A typical sample of horse semen contains 7.5 × 10⁹ sperm cells.

 [1]

To inseminate one female horse, 5.0 × 10⁸ sperm cells are needed.

Calculate the minimum number of straws needed to carry out this process.

[1]

The semen from a horse is analysed by technicians to determine its quality before the horse is accepted onto the assisted reproduction programme.

Technicians use microscopy to look at the appearance and motility of the sperm cells and to estimate the sperm count.

Fig.1 shows the surface view of a haemocytometer used to estimate the number of sperm cells in a sample.

Each number represents a sperm cell which was counted in the sample.

Each star (*) represents a sperm cell not counted in the sample.

The depth of the haemocytometer is 0.1mm.

Fig. 1

Suggest how this apparatus could be used to estimate the number of sperm cells per cm³ of semen and describe how the technician decided which sperm cells to include in the count.

To ensure that the sperm cells remain viable after they have been frozen, a solution is added to the sample before freezing the semen in the straws.

The solution contains:

• a sugar
• a buffer to maintain pH
• antibiotics.

Explain why it is necessary for the solution to contain these three substances.

Researchers have found evidence of natural selection in humans.

• Originally, in human populations it was only babies and children that needed to digest the milk sugar, lactose. The gene coding for the enzyme lactase (LCT gene) was switched off before adulthood.
• Today, in many populations, some adult individuals have lactose intolerance, which means they cannot digest lactose. Lactose intolerance leads to side-effects such as abdominal pain after eating food containing lactose.
• A mutation has been identified that keeps the LCT gene switched on. An adult who has this mutation is able to digest lactose. This is called lactose persistence.
• Lactose persistence increased in populations in Europe several thousand years ago.
• The increase in lactose persistence in Europe coincided with an increase in farming of cows for milk.

(i)

Natural selection has caused this increase in lactose persistence.

State the type of selection that has caused this increase.

(ii)

Explain why there was selection for lactose persistence in humans several thousand years ago.

Lactose intolerance and lactose persistence were investigated in a test population in Europe.

The mutation which causes lactose persistence is in a regulatory gene (T/t).

• People with lactose intolerance have the genotype tt.
• People with lactose persistence have the genotypes TT and Tt.
• 166 people were tested for their genotype.
• 58 people were found to have lactose intolerance.

(i)

The Hardy–Weinberg principle can be used to calculate allele, genotype and phenotype frequencies in populations.

The Hardy–Weinberg equations are shown in Fig.1:

Fig. 1

Calculate the frequency of allele T.

Show your working.

[3]

(ii)

When the calculated phenotype frequencies were compared to those in the general population in Europe, it was found that the percentage of people with lactose intolerance in this test population was much higher than in the general population.

Suggest two reasons why the percentage of people with lactose intolerance was much higher in the test population than in the general population.

In eukaryotes, gene expression is controlled by transcription factors, coded for by regulatory genes.

(i)

Outline ways in which transcription factors carry out their role.

(ii)

It is estimated that 2% of human DNA consists of genes coding for proteins (structural genes). Of the remaining 98%, some of the DNA consists of regulatory genes and control sequences that together control gene expression.

State one type of control sequence found in human DNA.

(iii)

A study of human evolution identified the location of mutations that result in a change in human phenotype. The study found most examples of mutations had occurred in regulatory genes, not structural genes.

Suggest and explain why most changes in human phenotype are due to mutations in regulatory genes.

The mango tilapia (Sarotherodon galilaeus) is a species of fish found in rivers and lakes across Africa and Eurasia. Scientists monitored the numbers of mango tilapia in a lake over a period of 10 years. Fig. 1 shows the results of this study.

Fig. 1

The number of different alleles present in the mango tilapia population shown in Fig. 1 changed between 2000 and 2004.

Name the precise biological process or event that has affected the number of alleles in the population of mango tilapia represented in Fig. 1

Explain the effect that the process identified in part a) would have on the tilapia population.

The population of tilapia decreased from 2400 individuals in 2003 to 52 individuals in 2005.

Calculate the percentage decrease in the tilapia population over this time period.

A new species of predatory fish is introduced into the lake which preys on mango tilapia. The biologists monitoring the tilapia population noticed that the predatory fish mainly catches tilapia of medium body size.

Explain the effect that this would have on the body size of mango tilapia over time.

Klebsiella pneumonia are bacteria that commonly occur in the intestines of humans. In the intestines, K. pneumonia cause no harm but once they spread to other parts of the body, such as the lungs and brain, they can cause serious bacterial infections.

Carbapenems are a class of powerful antibiotic that are often prescribed to treat serious bacterial infections, such as those caused by K. pneumonia. In recent years there have been an increase in cases of K. pneumonia infections where the bacteria are resistant to carbapenems.

Fig. 1 shows the increase in antibiotic resistance shown by K. pneumonia in two European countries.

Fig. 1

Describe the trend in antibiotic resistance observed in Fig. 1 for both countries.

Greece has one of the highest uses of antibiotics outside of hospitals in Europe.

Explain how such widespread use of antibiotics can lead to the development of antibiotic resistance in bacteria such as K. pneumonia.

K. pneumonia are resistant to several different types of antibiotics and they are able to pass this resistance on to other species of bacteria.

Outline the mechanism by which K. pneumonia would be able to transfer antibiotic resistance to other species of bacteria.

In tomato plants, the gene that codes for stem colour has two alleles. Allele B codes for a purple stem, while allele b codes for a green stem. A farmer growing tomatoes in a greenhouse noticed that about 20 % of the tomatoes had green stems.

The Hardy-Weinberg equation can be used to determine the allele frequency in a population

p + q = 1

In this equation, p represents the frequency of the dominant allele and q represents the frequency of the recessive allele.

Calculate the allele frequencies in this population.

The farmer decided to only allow the green stemmed tomato plants to reproduce with one another.

Explain why the Hardy-Weinberg principle would not apply to the population that results from the farmer's intervention.

Explain why the homozygous recessive individuals in the tomato population are the only individuals for whom genotype can be accurately identified from observation alone.

Use the following Hardy-Weinberg equation to calculate the proportion of heterozygous tomato plants that were present in the original population in part a).

p² + 2pq + q² = 1

Show your working and give your answer to one decimal place.

Artificial selection has been carried out for thousands of years. An example of this is the Dorper breed of sheep; a fast-growing sheep with a high fertility rate that is well adapted to survive in the arid regions of South Africa.  The Dorper was developed from the following two breeds:

• Dorset Horn sheep; a British breed with up to two lambing seasons per year, but not very tolerant of heat or arid conditions
• Blackhead Persian sheep; a Somalian breed with a fast growth rate and high tolerance to heat

Explain how artificial selection resulted in the development of Dorper sheep.

Suggest two other characteristics that breeders may select for when carrying out an artificial selection in sheep.

Artificial selection can result in inbreeding which can have a negative impact on a species.

Describe the negative impact that inbreeding can have on a species.

Fig. 1 shows a red deer, Cervus elaphus. Red deer feed on a wide range of plants.

Fig. 1

The number of red deer in the UK increased from 135 000 in 1960 to 360 000 in 2010.

Environmental factors affect the population size of red deer so that numbers do not continue to increase.

Suggest environmental factors that may prevent further increases in the size of red deer populations.

The body mass of red deer shows wide variation within a population. This is shown in Fig. 2.

Fig. 2

A selection pressure acted consistently over many years against red deer of low body mass in a population.

Sketch a curve on Fig. 3 to show the pattern of variation of body mass in this red deer population after this time.

Fig. 3

Name the type of force of natural selection that is acting on this population.

[1]

A selection pressure acted consistently over many years against red deer of medium body mass in a population.

Sketch a curve on Fig. 7.4 to show the pattern of variation of body mass in this red deer population after this time.

Fig. 4

[1]

Name the type of force of natural selection that is acting on this population.

 [1]