Variation and Selection
Variation, mutation, adaptive features, natural selection and selective breeding.
Variation
Variation is the differences between individuals of the same species. Some features show a range of values, and others fall into a small number of separate groups.
| Continuous variation | Discontinuous variation | |
|---|---|---|
| Phenotypes | A range between two extremes | A limited number, with no intermediates |
| Examples | Body length, body mass | ABO blood groups, seed shape in peas, seed colour in peas |
Worked example (classifying variation): State whether seed shape in peas shows continuous or discontinuous variation. Give a reason. Step 1. Pea seeds are either round or wrinkled. Step 2. There are a limited number of phenotypes with no intermediates, so it is discontinuous variation. Answer: discontinuous.
Worked example (classifying variation): State whether body mass in a class of students shows continuous or discontinuous variation. Step 1. Body mass can take any value between a lowest and a highest mass. Step 2. This is a range of phenotypes between two extremes, so it is continuous variation. Answer: continuous.
Causes of Variation
Discontinuous variation is usually caused by genes only. Continuous variation is caused by both genes and the environment.
| Type | Cause | Example |
|---|---|---|
| Discontinuous | Genes only (usually) | ABO blood group is set by alleles. Diet does not change it. |
| Continuous | Genes and the environment | Body mass depends on the alleles inherited and on the food eaten. |
Investigating variation
To investigate variation, measure a feature in many individuals of one species, then record the results. For a continuous feature, such as leaf length, group the measurements into ranges. For a discontinuous feature, such as seed colour, count how many individuals fall into each group.
| Leaf length (mm, nearest mm) | Number of leaves |
|---|---|
| 40–49 | 3 |
| 50–59 | 9 |
| 60–69 | 14 |
| 70–79 | 8 |
| 80–89 | 2 |
Example results (illustrative data): lengths of 36 leaves from one plant species. The lengths are spread across every class from 40 to 89 mm, with most near the middle and no separate groups. Leaf length can take any value between the two extremes, so it shows continuous variation.
Worked example (explaining causes): Explain why body mass shows continuous variation. Step 1. Body mass is affected by genes, because alleles influence growth. Step 2. It is also affected by the environment, such as the amount of food eaten, so individuals differ in body mass because they have different alleles and different amounts of food. Answer: genes and the environment.
Mutation
Mutation is a genetic change. It is the way in which new alleles are formed. Ionising radiation and some chemicals increase the rate of mutation.
Worked example (causes of mutation): State two things that increase the rate of mutation. Step 1. Ionising radiation increases the rate of mutation. Step 2. Some chemicals also increase the rate of mutation. Answer: ionising radiation; some chemicals.
ExtendedGene mutation and variation
A gene mutation is a random change in the base sequence of DNA. Four processes are sources of genetic variation in populations: mutation, meiosis, random mating and random fertilisation.
| Source | How it gives variation |
|---|---|
| Mutation | Forms new alleles |
| Meiosis | Makes gametes with different combinations of alleles |
| Random mating | Any male may mate with any female; mates are not chosen for particular alleles |
| Random fertilisation | Any male gamete may fuse with any female gamete, by chance |
Adaptive Features
An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment. To interpret an image or other information about a species, pick out each feature and say how it helps the organism.
| Feature | How it helps survival and reproduction |
|---|---|
| Streamlined shape | Less water resistance, so the fish can swim fast to catch prey or escape |
| Tail fin | Pushes the fish forward |
| Other fins | Steer the fish and keep it balanced |
Worked example (interpreting an image): Use Fig 18.14 to describe one adaptive feature of the fish. Step 1. Choose a feature shown: the streamlined shape. Step 2. Say how it helps: it reduces water resistance, so the fish can swim fast to catch food or escape predators. Answer: feature + how it helps survival.
Worked example (using information about a species): A mammal lives in snowy regions. It has thick fur and white fur in winter. Describe two adaptive features of the mammal. Step 1. Thick fur: it keeps the animal warm in the cold. Step 2. White fur: it matches the snow, so predators find it harder to see and it is more likely to survive and reproduce. Answer: thick fur; white fur.
Adaptive features can be shapes, colours, body coverings or behaviours. They are inherited because the alleles for them are passed to offspring.
Hydrophytes and Xerophytes
ExtendedHydrophytes
A hydrophyte lives in water. Its adaptive features help it to float and to obtain gases, and it lives where water supply is not a problem.
| Feature | Explanation |
|---|---|
| Air spaces in stem and leaves | Keep the plant afloat and let gases move through it |
| Stomata on upper surface | The lower surface is in water, and the upper surface is in air |
| Small roots | Water is absorbed over the whole surface, so large roots are not needed |
ExtendedXerophytes
A xerophyte lives where water is scarce. Its adaptive features reduce water loss or store water.
| Feature | Explanation |
|---|---|
| Thick waxy cuticle | Reduces evaporation from the surface |
| Spines or small leaves | Smaller surface area, so less water is lost |
| Thick stem | Stores water |
| Widespread, shallow roots | Absorb water quickly after rain |
Natural Selection
Natural selection follows a sequence. Each step links to the next.
1. There is genetic variation within the population.
2. Organisms produce many offspring.
3. There is a struggle for survival, including competition for resources.
4. Individuals better adapted to the environment have a greater chance of reproducing.
5. These individuals pass on their alleles to the next generation.
Worked example (describing natural selection): Describe how natural selection could increase the number of brown beetles in a population. Step 1. There is genetic variation: because of different alleles, some beetles are brown and some are green. More offspring are produced than can survive. Birds find green beetles easier to see on brown soil, so they eat more of them. Step 2. Brown beetles are better adapted, so more of them reproduce and pass on their alleles. The next generation has more brown beetles. Answer: better adapted individuals pass on alleles.
| Step in natural selection | In the beetle example |
|---|---|
| Genetic variation within the population | Beetles are brown or green |
| Many offspring | More beetles are born than can survive |
| Struggle for survival | Birds eat beetles; beetles compete for food |
| Better adapted reproduce more | Brown beetles survive and breed |
| Alleles passed on | The allele for brown becomes more common |
Adaptation and Resistance
ExtendedAdaptation
Adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations. It happens to populations, not to single organisms within their lifetime.
ExtendedAntibiotic resistant bacteria
The development of strains of antibiotic resistant bacteria is an example of natural selection.
1. A random mutation produces an allele for resistance in a few bacteria.
2. The antibiotic kills the non-resistant bacteria.
3. The resistant bacteria survive, reproduce and pass on the allele.
4. The population is now mostly resistant bacteria.
Worked example (antibiotic resistance): Explain how a strain of antibiotic resistant bacteria can develop in a patient taking an antibiotic. Step 1. A mutation produced a resistant allele in a few bacteria. The antibiotic kills non-resistant bacteria, but the resistant bacteria survive. Step 2. The survivors reproduce and pass on the resistant allele, so the population becomes resistant. Answer: natural selection of resistant bacteria.
Selective Breeding
Selective breeding, also called artificial selection, is carried out by humans. It involves selecting individuals with desirable features, crossing them to produce the next generation, and selecting the offspring that show the desirable features. Repeated over many generations, it improves crop plants and domesticated animals.
| Context | Desirable feature |
|---|---|
| Crop plant such as wheat | High yield, disease resistance |
| Domesticated animal such as a cow | High milk yield |
Worked example (applying selective breeding): A farmer wants wheat plants with a high yield. Outline how selective breeding could be used. Step 1. Select wheat plants with a high yield and cross them together. Step 2. Select the offspring with the highest yield and cross them together. Repeat this over many generations. Answer: select, cross, select; repeat.
ExtendedNatural and artificial selection
| Natural selection | Artificial selection | |
|---|---|---|
| What selects | Environmental factors, such as predators, disease and climate; no one chooses | Humans |
| Features favoured | Features that increase the chance of surviving and reproducing | Those that humans find desirable |
Exam advice
Common mistakes
Model answer
Recall checklist
- Define variation; compare continuous and discontinuous.
- State the causes of each kind of variation.
- Describe mutation and what increases its rate.
- Define an adaptive feature.
- Describe the five steps of natural selection.
- Describe selective breeding.
- Explain hydrophyte and xerophyte features (Extended)
- Explain antibiotic resistance (Extended)
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