OxfordAQA International GCSE Biology: Variation, natural selection and the origin of species (9201)

This guide covers section 3.6 (Variation and evolution) of the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams in May/June 2018 onwards: 3.6.1 (causes of variation) and 3.6.2 parts a to c (natural selection). The qualification is untiered, and the specification says content from any part of it may be assessed in either paper. Use it with the course hub and the printable checklist.

Companion pages: revision notes and practice questions. To find your weak spots first, try the free diagnostics. For paper technique, see OxfordAQA IGCSE Biology exam preparation.

What this unit covers

Section	What you must be able to do
3.6.1	Explain that variation has genetic causes (mutation or reproduction) and environmental causes (acquired characteristics)
3.6.2 a	Name the theory of evolution by natural selection and compare it with Lamarck’s idea that changes during a lifetime are inherited
3.6.2 b	Describe natural selection step by step and show you understand the time scales involved
3.6.2 c	Explain how new species arise: isolation, genetic variation, natural selection, speciation

3.6.1 Causes of variation

Individuals of the same species are not identical. The specification lists two causes. Most real characteristics involve both.

Genetic variation

Genetic variation means different characteristics as a result of mutation or reproduction.

• Sexual reproduction mixes the genetic information of two parents. Each gamete carries a different mix of alleles, and any male gamete can fuse with any female gamete. Siblings therefore differ from each other and from both parents.
• Asexual reproduction involves one parent and no fusion of gametes, so there is no mixing and no genetic variation in the offspring. The offspring are clones.
• Mutation is a change in the genetic material (a gene) that creates a new allele. Mutations are random. They happen without any need for them. Most have no effect on the organism or are harmful, and a few give a characteristic that suits the environment. Mutation is the original source of every new allele. Some radiation and some chemicals increase the rate of mutation.

Environmental variation

Environmental variation means different characteristics caused by an organism’s environment. These are acquired characteristics: they develop during the lifetime of the individual. Examples are a scar, a suntan, the muscle of someone who trains, and a plant that is short because the soil lacks mineral ions. Light, temperature, water, mineral ions and diet are typical environmental factors.

A combination of both

Height in humans depends on the alleles inherited and on diet and health while growing. A plant with alleles for tall growth still grows short if it lacks water.

Continuous and discontinuous variation

The section heading names these two types. Continuous variation shows a full range of values with no gaps, such as height or leaf length. Discontinuous variation falls into separate groups, such as blood group. Data on variation is often collected from a sample, so a larger sample gives a more reliable picture.

Worked example 1: separating genes from environment

A gardener takes six cuttings from one parent plant. Cuttings are produced by asexual reproduction, so all six have the same genes as the parent. Group A has compost with added nitrate. Group B has compost without it. Heights after four weeks (cm):

Group	Heights	Total	Mean
A (nitrate added)	18, 20, 19, 21, 17, 19	114	19.0
B (no nitrate)	12, 11, 13, 12, 10, 14	72	12.0

1. Mean of A = 114 / 6 = 19.0 cm. Mean of B = 72 / 6 = 12.0 cm.
2. Difference between means = 19.0 - 12.0 = 7.0 cm.
3. Group A is taller by 7.0 / 12.0 x 100 = 58.3%.
4. All the plants are clones, so the 7.0 cm difference between groups cannot be genetic. It is environmental, caused by the nitrate. Plants in the same group also differ (17 to 21 cm), and since the genes are identical, even that smaller spread has an environmental cause, such as small differences in light or water.

3.6.2 a: Theories of evolution

Evolution means species change over many generations as the inherited characteristics in a population shift. All life today is descended from a universal common ancestor that was a simple single-celled organism.

The specification names two kinds of theory:

• The theory of evolution by natural selection. Organisms vary because of their genes, those best suited to the environment survive and breed, and they pass on the genes that helped them.
• Other theories, including Lamarck’s. These are based mainly on the idea that changes that occur in an organism during its lifetime can be inherited. For example, a wading bird that stretches its legs to keep out of deep water would, on this idea, have chicks with longer legs.

We now know that in the vast majority of cases this type of inheritance cannot occur. Characteristics are inherited through genes carried in the gametes. A stretched leg, a suntan or a scar is an acquired characteristic, and it does not alter the alleles in the gametes. In a well-known test of Lamarck’s idea, the tails of mice were cut off over many generations, and the young were still born with tails.

	Lamarck	Natural selection
What changes first?	The individual, during its life	The population has variation already
What is passed on?	The change acquired by the individual	Genes (alleles)
Role of the environment	Causes the useful change	Selects which individuals survive and breed
Status	Not supported in the vast majority of cases	The accepted explanation

3.6.2 b: Natural selection

The specification gives three linked statements:

1. Individuals within a species may 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 these individuals to survive are passed on to the next generation.

Written as an explanation, with a chain of cause and effect:

Method: explaining natural selection variation caused by different alleles, then competition or a change in the environment, then the best-suited individuals survive and breed, then they pass on those alleles, then the alleles become more common over many generations.

Two points gain credit on almost any question. Say that the variation is genetic, and say the alleles are passed on, not the characteristic itself. An individual does not change. The population changes, because the proportion of individuals carrying useful alleles rises.

Worked example 2: a drought on an island

On an island, 240 seed-eating birds are measured. A drought leaves mainly large, hard seeds. Only birds with deep beaks can crack them. The table shows how many birds in each beak-depth class lived through the drought (this is a made-up data set).

Beak depth / mm	Birds before	Survivors	Survival
6 to 7	60	6	10.0%
8 to 9	100	30	30.0%
10 to 11	60	36	60.0%
12 to 13	20	16	80.0%
Total	240	88	36.7%

1. Survival in a class = survivors / birds before x 100. For 6 to 7 mm: 6 / 60 x 100 = 10.0%. The other classes give 30.0%, 60.0% and 80.0%.
2. Overall survival = 88 / 240 x 100 = 36.7%.
3. The birds with the two deepest beak classes were 80 of 240 (33.3%) before the drought but 52 of 88 (59.1%) of the survivors.
4. Using the middle of each class (6.5, 8.5, 10.5 and 12.5 mm) the mean beak depth rises from 8.8 mm before to 9.9 mm among the survivors.
5. Explanation. Beak depth varies because of different alleles. In the drought, deep-beaked birds could eat the hard seeds, so more survived to breed. They passed on the alleles for deep beaks. If beak depth is inherited, the next generation has a higher proportion of deep beaks.

Step 5 depends on the word if. The survivors only pass the change on when the characteristic is genetic. Survival data alone does not show this.

Time scales

The specification expects you to understand the time scales involved. Evolution by natural selection is usually slow, because a change in the proportion of an allele builds up over many generations. A population of an organism with a long life cycle needs very many years. Organisms that reproduce quickly pass through many generations in a short time.

• Life on Earth has existed for billions of years. This is long enough for single-celled organisms to give rise to the huge range of species alive today.
• The number of generations matters more than the number of years. A bacterium that divides every 30 minutes passes through 40 generations in 40 x 30 min = 1200 min = 20 hours. A species with a generation time of 25 years needs 40 x 25 = 1000 years for the same 40 generations.
• In 1 million years, a species with a 20-year generation time goes through 1 000 000 / 20 = 50 000 generations.

3.6.2 c: How new species arise

The specification gives four stages, in order:

1. Isolation. Two populations of a species become separated, for example geographically by a rising sea, a mountain range or a river that changes course. Individuals can no longer breed across the barrier.
2. Genetic variation. Each population has a wide range of alleles that control their characteristics. Mutation produces new alleles separately in each population.
3. Natural selection. In each population the alleles that control characteristics helping survival are selected. If the two environments differ, different alleles are selected in each.
4. Speciation. The populations become so different that successful interbreeding leading to fertile offspring is no longer possible (specification wording). They are now separate species.

Worked example 3: lizards on two hills

A species of lizard lives on a plain. Over thousands of years the plain turns to desert, leaving one population on a cool, damp hill and another on a hot, dry hill. Lizards cannot cross the desert.

• Isolation: the desert separates the two populations, so they stop interbreeding.
• Genetic variation: within each population there are lizards with different alleles for skin pattern, size and tolerance of heat. Random mutations occur separately on each hill.
• Natural selection: on the hot hill, lizards with alleles for heat tolerance survive and breed more often, so those alleles become more common. Different alleles are favoured on the damp hill.
• Speciation: after many generations the two populations differ so much that, if brought together, they cannot breed to produce fertile offspring. They are two species.

Fertile offspring are the test. Two kinds of animal may mate and still be separate species if their young are sterile. The mule, the offspring of a mare and a male donkey, is normally sterile, so horses and donkeys are separate species.

Common errors

• Saying organisms change on purpose to suit a new habitat. Variation exists first, then selection acts.
• Saying that an individual evolves. Populations evolve.
• Describing a characteristic as passed on without naming genes or alleles.
• Calling a mutation a response to the environment. Mutations are random.
• Using Lamarck’s idea (stretching, use and disuse) as if it were the accepted explanation.
• Leaving out isolation in a speciation answer, or ending at “they are different” without saying they can no longer produce fertile offspring.
• Treating a scar, a tan or a short, nitrate-starved plant as genetic.

Next steps

Work through the revision notes, then test yourself with the practice questions. Tick off sections on the checklist.

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.