Why is genetic diversity needed for natural selection?

Selection can only act if individuals differ in their alleles and characteristics.

State the four steps of natural selection.

Random mutation gives a new allele; the allele benefits its possessor, giving greater reproductive success; it is inherited by the next generation; its frequency increases over generations.

What is directional selection?

Selection favouring one extreme of a characteristic, so the mean moves, e.g. antibiotic resistance in bacteria.

What is stabilising selection?

Selection favouring the mean and acting against both extremes, so the range narrows, e.g. human birth weight.

Name three types of adaptation.

Anatomical, physiological and behavioural.

What is an anatomical adaptation?

A structural feature, such as beak depth, that helps survival and reproduction.

A change in allele frequency in a population over many generations.

Why do bacteria in a patient become resistant to an antibiotic?

Bacteria with a resistance allele survive and reproduce, passing on the allele, so it becomes more frequent.

Why is the lid of the Petri dish taped but not completely sealed?

To prevent contamination while still letting oxygen in, so harmful anaerobic pathogens do not grow.

Why is the dish incubated at about 25 °C?

So that human pathogens, which grow best at body temperature, are not cultured.

What is a zone of inhibition?

The clear area around an antimicrobial disc where bacterial growth is prevented.

Why is a control disc used in the antimicrobial practical?

It shows that the clear zone is caused by the substance and not by the disc or water.

Natural selection and adaptationAQA A-Level Biology: Flashcards

Card 1 of 140 of 14 known

Question

What is genetic diversity?

Tap or press Space to reveal

Tap card or press Space to flip

See all 14 cards

What is genetic diversity?: The number of different alleles of genes in a population.
Why is genetic diversity needed for natural selection?: Selection can only act if individuals differ in their alleles and characteristics.
How do new alleles arise?: By random mutation of the DNA.
State the four steps of natural selection.: Random mutation gives a new allele; the allele benefits its possessor, giving greater reproductive success; it is inherited by the next generation; its frequency increases over generations.
What is directional selection?: Selection favouring one extreme of a characteristic, so the mean moves, e.g. antibiotic resistance in bacteria.
What is stabilising selection?: Selection favouring the mean and acting against both extremes, so the range narrows, e.g. human birth weight.
Name three types of adaptation.: Anatomical, physiological and behavioural.
What is an anatomical adaptation?: A structural feature, such as beak depth, that helps survival and reproduction.
What is evolution?: A change in allele frequency in a population over many generations.
Why do bacteria in a patient become resistant to an antibiotic?: Bacteria with a resistance allele survive and reproduce, passing on the allele, so it becomes more frequent.
Why is the lid of the Petri dish taped but not completely sealed?: To prevent contamination while still letting oxygen in, so harmful anaerobic pathogens do not grow.
Why is the dish incubated at about 25 °C?: So that human pathogens, which grow best at body temperature, are not cultured.
What is a zone of inhibition?: The clear area around an antimicrobial disc where bacterial growth is prevented.
Why is a control disc used in the antimicrobial practical?: It shows that the clear zone is caused by the substance and not by the disc or water.

Exam questions on Natural selection and adaptation

A hospital ward has a patient infected with a bacterium. When the patient is first treated with an antibiotic, almost all the bacteria are killed. A few bacteria carry an allele for an enzyme that breaks down the antibiotic. Over several weeks of repeated treatment, most of the bacteria in the patient are found to carry this allele.
Explain why the proportion of bacteria carrying the allele increased during the treatment.2 marks
A study of a large number of human births recorded the birth weight of each baby and whether the baby survived the first month. Mortality was lowest for babies with a birth weight of about 3.5 kg. It was 1.2% at 3.5 kg, but 18% for babies of 1.5 kg, and also much higher for babies far heavier than 3.5 kg.
Explain how selection keeps the mean birth weight of humans about the same over many generations.2 marks
A student investigates the effect of three antibiotics on a bacterium. She spreads a culture of the bacterium over the surface of nutrient agar in a Petri dish. She then places four sterile paper discs on the agar: one soaked in antibiotic A, one in B, one in C, and a control disc soaked in sterile water. After incubation, the diameters of the clear zones around the discs are: A 12 mm, B 20 mm, C 0 mm and the control 0 mm.
Describe three precautions the student should take to reduce the risk of contamination and of culturing pathogens.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).