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Topic 18 · 18.1–18.3

Variation and selection

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe variation; distinguish continuous and discontinuous variation and their causes; investigate examples.
  • Describe mutation, how new alleles form, and what increases the mutation rate; gene mutation and sources of variation EXTENDED.
  • Describe adaptive features; explain those of hydrophytes and xerophytes EXTENDED.
  • Describe natural selection and selective breeding; adaptation, antibiotic resistance and natural versus artificial selection EXTENDED.

📚The biology

Variation

Variation is the differences between individuals of the same species.

Left: a histogram of the heights of 100 students in 5 centimetre classes from 150 to 190 centimetres, rising to a peak at 165 to 170 and falling again, with touching bars. Right: a bar chart of the percentage of people in blood groups O, A, B and AB, with separate bars.
Continuous variation gives a histogram; discontinuous variation a bar chart. (Illustrative data.)
continuous variation: a range of phenotypes between two extremes (e.g. body length, body mass); caused by both genes and the environment
discontinuous variation: a limited number of phenotypes with no intermediates (e.g. ABO blood groups, seed shape and seed colour in peas); usually caused by genes only

A mutation is a genetic change. Mutation is the way in which new alleles are formed. Ionising radiation (X-rays, gamma rays, UV) and some chemicals (such as those in tobacco smoke) increase the rate of mutation.

EXTENDED A gene mutation is a random change in the base sequence of DNA. Sources of genetic variation in populations: mutation, meiosis, random mating and random fertilisation.

Adaptive features

An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment — for example, a camel’s wide feet and fat store, or a polar bear’s thick fur and white colour.

Left, a xerophyte, a cactus in dry sand: spines instead of leaves, a thick waxy stem that stores water with few sunken stomata, and long wide-spreading roots. Right, a hydrophyte, a water lily: large flat leaves float on the water to catch light, with stomata on the upper surface, and air spaces in the stems and leaves help them float.
EXTENDED Xerophytes reduce water loss; hydrophytes get enough light and air.

Selection

Natural selection involves:

  1. genetic variation within populations;
  2. production of many offspring;
  3. a struggle for survival, including competition for resources;
  4. a greater chance of reproduction by individuals that are better adapted to the environment than others;
  5. these individuals passing on their alleles to the next generation.
Four panels of moths on dark tree bark. First, a population with more pale moths than dark ones. Second, birds have eaten many of the pale moths, which are easy to see. Third, the dark moths survive and reproduce, so the next generation has more dark moths. Fourth, after many generations most moths are dark.
The peppered moth: the environment does the selecting.

EXTENDED Adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations. The development of antibiotic-resistant bacteria is an example (topic 15).

Selective breeding: humans select individuals with desirable features, cross them to produce the next generation, and select offspring showing the desirable features — repeated over many generations to improve crop plants and domesticated animals.

A flow chart of four steps: choose parents with the desired feature, for example high milk yield; cross them; select the offspring that show the feature best; breed these and repeat for many generations.
Selective breeding is artificial selection.

EXTENDED Natural and artificial selection compared: in natural selection the environment selects features that help survival; in artificial selection humans choose features useful to them, which may not help the organism survive in the wild. Artificial selection usually produces change faster.

✏️Worked example

(a) Classify each as continuous or discontinuous variation: hand span; ability to roll the tongue; mass of apples on a tree. [3] (b) Explain why the heights of identical twins raised apart may differ. [2] (c) EXTENDED Hares living in snowy regions have white fur in winter. Explain how this feature could have become common through natural selection. [4]

(a) Hand span: continuous. Tongue rolling: discontinuous. Mass of apples: continuous.

(b) They have the same genes, so the difference is due to the environment (e.g. diet), which affects continuous variation such as height.

(c) There was variation in fur colour, caused by mutation. Hares with whiter fur were better camouflaged, so fewer were caught by predators; they survived and reproduced more, passing on the alleles for white fur; over many generations these alleles became more common in the population.

Check it. Every natural selection answer should contain: variation → selection pressure → better adapted survive and reproduce → pass on alleles → over many generations.
“The hares turned white to camouflage themselves.” Individuals do not change their genes to suit the environment; the variation already existed, and the environment selected it.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which is an example of discontinuous variation? A: height. B: body mass. C: ABO blood group. D: leaf length.
C.
2. (Practical.) Describe how to collect and present data on the variation in hand span in your class. [3]
Measure each hand span with a ruler, to the nearest mm; group the measurements into equal class intervals; draw a histogram (bars touching) of number of students against hand span.
3. (Theory.) State two factors that increase the rate of mutation. [2]
Ionising radiation (e.g. X-rays, gamma rays); some chemicals (e.g. in tobacco smoke).
4. (Theory.) Define an adaptive feature, and give one example from an animal. [2]
An inherited feature that helps an organism survive and reproduce in its environment. E.g. thick fur in a polar bear, for insulation.
5. (Theory.) Describe how a farmer could use selective breeding to produce wheat with a higher yield. [3]
Choose parent plants with high yields; cross them; select the offspring with the highest yields and breed them; repeat over many generations.
6. (Theory.) EXTENDED Explain two adaptations of a cactus that reduce water loss. [4]
Leaves reduced to spines: much smaller surface area for transpiration. Thick waxy cuticle: waterproof. (Also: few stomata, often sunken; stomata open at night.)
7. (Theory.) EXTENDED Name four sources of genetic variation in a population. [2]
Mutation; meiosis; random mating; random fertilisation.
8. (Theory.) EXTENDED Give two differences between natural selection and artificial selection. [2]
In natural selection the environment selects, in artificial selection humans select; natural selection favours features that help survival, artificial features that humans want; artificial selection is usually faster.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • PhET “Natural Selection” — simulate bunnies, predators and mutations
  • Natural History Museum — the peppered moth story