OxfordAQA International GCSE Biology: Variation, natural selection and species (9201) – Revision Notes

Short recall notes for sections 3.6.1 and 3.6.2 a to c of the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams in May/June 2018 onwards. The qualification is untiered. For full explanations and worked examples, use the study guide. Then try the practice questions, the course hub and the printable checklist. Unsure where to start? Take a free diagnostic.

3.6.1 Causes of variation

Cause	What it means	Examples
Genetic	Different characteristics from mutation or reproduction	Blood group, eye colour, flower colour controlled by alleles
Environmental	Different characteristics caused by the organism’s environment (acquired characteristics)	Scar, suntan, plant height with and without mineral ions
Both	Genes set the potential, the environment affects how far it is reached	Human height, body mass

Reproduction and variation

• Sexual reproduction: gametes fuse, genetic information from two parents is mixed, offspring vary.
• Asexual reproduction: one parent, no fusion of gametes, no mixing, no genetic variation. Offspring are clones.
• Mutation: a change in a gene that makes a new allele. Random. Original source of new alleles.

Continuous and discontinuous (section heading). Continuous: a full range of values (height, leaf length). Discontinuous: separate groups (blood group).

Method: telling genes from environment

1. Look for organisms with identical genes (clones, cuttings).
2. Keep one factor different between groups, for example nitrate.
3. A difference between groups of clones is environmental.
4. A difference between individuals in the same conditions suggests genes.

Small reminder. Cuttings from one plant (means 19.0 cm with nitrate and 12.0 cm without): the 7.0 cm difference is environmental, because the genes are identical.

3.6.2 a Theories of evolution

	Natural selection	Lamarck
Starting point	Variation already present, from genes	Changes during an individual’s lifetime
Mechanism	Best-suited individuals survive and breed	Acquired changes are inherited
What is inherited	Genes (alleles)	Acquired characteristics
Today	Accepted theory	In the vast majority of cases this inheritance cannot occur

• Acquired characteristics (scar, tan, trained muscle) are not carried in the gametes, so they are not normally inherited.
• Lamarck-style wording to avoid: “the bird stretched its legs so its chicks had longer legs”.

3.6.2 b Natural selection

Three statements from the specification

1. Individuals in a species show a wide range of variation because of differences in their genes.
2. Individuals with characteristics most suited to the environment are more likely to survive to breed successfully.
3. The genes that enabled them to survive are passed on to the next generation.

Chain to write out

variation (alleles) then a selection pressure (competition, predation, disease or environmental change) then survival and breeding of the best-suited then alleles passed on then allele proportion rises over many generations.

Selection pressures are factors that affect survival: predators, food supply, disease, climate change.

Key distinctions

Say this	Not this
The population changes	The individual evolves
The alleles are passed on	The characteristic is passed on because it was needed
Variation was already there	The organism changed to fit
Survive to breed	Survive (alone)

Time scales

• Evolution is usually slow: it takes many generations.
• Count generations, not years. A bacterium dividing every 30 minutes passes through 40 generations in 20 hours; a species with a 25-year generation time needs 1000 years for 40 generations.
• Life has existed on Earth for billions of years, long enough for all present species to arise from a simple single-celled common ancestor.
• Species that cannot respond to environmental change are at risk of becoming extinct.

Method: working with selection data

1. Survival of a class = survivors / starting number x 100.
2. Compare classes and quote values.
3. State the characteristic that survived best.
4. Link to the explanation: if the characteristic is genetic, its alleles become more common.

Small reminder. Out of 240 birds, 88 survive a drought: 88 / 240 x 100 = 36.7%.

3.6.2 c New species

Stage	What happens
Isolation	Two populations become separated, for example geographically
Genetic variation	Each population has a wide range of alleles; mutation adds new ones
Natural selection	In each population, the alleles that help survival are selected
Speciation	Populations become so different that successful interbreeding leading to fertile offspring is no longer possible

• Learn the four stages in this order.
• The test for separate species is fertile offspring. A mule is usually sterile, so horses and donkeys are different species even though they can mate.
• Different environments select different alleles, so isolated populations drift apart in characteristics.

Model answers to learn

Explain how natural selection could make hairy leaves more common in a wild plant (a made-up scenario). Leaves vary in hairiness because of different alleles [variation]. Insects that feed on leaves find hairy leaves harder to eat, so hairy plants are damaged less [selection pressure]. Hairy plants are more likely to survive to produce seeds [survive to breed]. They pass on the alleles for hairy leaves in their seeds [genes passed on]. Over many generations the proportion of plants with the allele rises.

Describe how two populations of one beetle species could become two species. A new river course separates the populations (isolation). Each has a wide range of alleles, and mutation adds more (genetic variation). The two habitats differ, so different alleles are selected in each (natural selection). After many generations, beetles from the two sides cannot breed to give fertile offspring (speciation).

Compare a Lamarck explanation with a natural selection explanation for a weightlifter whose child is also strong. Lamarck: muscle built by training is passed to the child. Natural selection and genetics: training is an acquired characteristic and is not inherited; a child may be strong because of inherited alleles, and their own training.

Reading data in selection questions

If the question says	Do this
Table of survivors by class	Convert to percentages; compare classes
Rising proportion over generations	Quote start and end values; give the change
Identical clones, different treatments	State the cause is environmental
Two populations, different habitats	Link each habitat to the alleles favoured

Must-know distinctions

Pair	Difference
Genetic and environmental variation	Inherited alleles versus conditions during development
Mutation and selection	Mutation creates alleles at random; selection changes how common they are
Natural selection and Lamarck	Selection acts on existing genetic variation; Lamarck needs acquired change to be inherited
Isolation and speciation	Isolation is the start (a barrier); speciation is the end (no fertile offspring)

Quick self-test

1. State the two kinds of cause of variation named in the specification.
2. Why do offspring of asexual reproduction show no genetic variation?
3. Give one example of an acquired characteristic.
4. What is a mutation?
5. State the main idea of Lamarck’s theory.
6. Give one reason Lamarck’s idea is not accepted.
7. A population of snails has shells that vary in thickness because of different alleles. Thick-shelled snails are eaten less by birds. State what happens to the alleles for thick shells over many generations.
8. A species has a generation time of 20 years. How many generations occur in 1 million years?
9. Name the four stages in the formation of a new species, in order.
10. How do scientists decide whether two populations are now separate species?
11. Two groups of identical clone seedlings have mean heights of 31.0 cm and 23.0 cm. What is the difference, and what causes it?

Answers

1. Genetic causes and environmental causes.
2. There is one parent and no mixing of genetic information, so the offspring are clones.
3. Any one of: a scar, a suntan, muscle gained by training, a plant stunted by lack of mineral ions.
4. A change in a gene that produces a new allele.
5. Changes that occur in an organism during its lifetime can be inherited.
6. Acquired changes do not alter the alleles in the gametes; in the vast majority of cases this type of inheritance cannot occur.
7. Thick-shelled snails survive and breed more; they pass on the alleles; the alleles become more common.
8. 1 000 000 / 20 = 50 000 generations.
9. Isolation, genetic variation, natural selection, speciation.
10. Whether they can interbreed to produce fertile offspring.
11. 31.0 - 23.0 = 8.0 cm; the genes are identical, so the cause is environmental.

Where marks are usually lost

• Writing “the animal adapted” without saying that variation was there first.
• Saying a characteristic is passed on without naming genes or alleles.
• Missing out “survive to breed”: surviving is not enough if no young are produced.
• Stating that a mutation happens because the organism needs it.
• Leaving out isolation, or describing it with no barrier.
• Ending a speciation answer without “cannot produce fertile offspring”.
• Calling any difference between individuals genetic when the question describes clones or the same alleles.
• Quoting survival data without numbers, or without comparing the groups.
• Treating generations and years as the same thing in a time-scale answer.

Official syllabus

OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams in May/June 2018 onwards, published by OxfordAQA. Section 3.6 Variation and evolution: 3.6.1 and 3.6.2 a to c.