Variation and selection
Contents: 6 sections
Variation
Variation is the differences between individuals of the same species.
Two kinds, and the distinction is usually asked from a graph.
Continuous variation gives a range of values with no distinct categories. Height, mass and leaf length are examples. It is usually controlled by several genes and strongly influenced by the environment. Plotted, it gives a smooth bell-shaped curve, and the data is drawn as a histogram or line graph.
Discontinuous variation gives a limited number of distinct categories with nothing in between. Blood group, tongue rolling, sex and seed shape are examples. It is usually controlled by a single gene and is unaffected by the environment. Plotted, it gives separate bars, and the data is drawn as a bar chart.
If the question shows a smooth curve, the answer is continuous; separate bars means discontinuous.
Variation has two sources: genetic, from the alleles inherited, and environmental, from conditions during life. A plant's height depends on both its alleles and how much light and water it got. Only the genetic part can be inherited, which is why an athlete's trained muscles are not passed on.
Mutation
A mutation is a genetic change.
Mutation is the only way genuinely new alleles arise, so it is the ultimate source of all variation. Meiosis and sexual reproduction shuffle existing alleles into new combinations, but they cannot invent one.
Most mutations are neutral or harmful. Occasionally one is beneficial in a particular environment, and that is the raw material natural selection works on.
The rate of mutation is increased by ionising radiation such as X-rays and gamma rays, and by chemical mutagens such as the tar in tobacco smoke.
Adaptive features
An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment.
The word inherited matters. A feature acquired during life is not adaptive in this sense.
Xerophytes are plants adapted to dry conditions. Their features all reduce water loss or store water: a thick waxy cuticle, few stomata and often sunken ones, leaves reduced to spines, a thick fleshy stem for storage, and long or widely spread roots.
Hydrophytes are plants adapted to living in water. Their features exploit an environment where water is abundant and support and gas exchange are the problems: large flat floating leaves, stomata on the upper surface only since the lower one is submerged, air spaces in the tissues for buoyancy, and very little supporting tissue because the water holds the plant up.
When explaining any adaptation, name the feature and then say what it achieves, since the mark is for the link and not for the feature.
Natural selection
The process, in the order examiners want:
- There is variation within a population, caused ultimately by mutation.
- There is competition for resources, and more offspring are produced than can survive.
- Individuals with features better suited to the environment are more likely to survive.
- Those survivors reproduce and pass on the alleles for those features.
- Over many generations the frequency of those alleles increases in the population.
Over long periods this changes the population, and that change is evolution, defined as the change over time in the adaptive features of a population.
Two errors to avoid. Organisms do not change themselves to suit the environment; the variation is already present and the environment selects among it. And it is the population that evolves, not the individual.
Antibiotic resistance in bacteria is the fastest-observed example and follows the five steps exactly. So does insecticide resistance in insects, and the change in peppered moth colouring as soot darkened tree trunks.
Selective breeding
Selective breeding is when humans choose which individuals reproduce, to develop desired features.
The process:
- Select the individuals with the desired characteristic.
- Breed them together.
- Select the offspring showing the characteristic most strongly.
- Repeat over many generations.
Uses include crops with higher yield or disease resistance, cattle producing more milk, and dogs bred for particular roles.
Comparing it with natural selection is a standard question:
| Natural selection | Selective breeding | |
|---|---|---|
| Who selects | the environment | humans |
| Selected for | survival and reproduction | features useful to people |
| Speed | usually slow | faster, deliberate |
| Result | organisms suited to their environment | organisms suited to human needs |
The mechanism is the same in both: differential reproduction of particular alleles. Only the agent doing the selecting differs.
The main disadvantage of selective breeding is reduced variation. Repeatedly breeding from a small number of similar individuals narrows the gene pool, so the population becomes vulnerable to a new disease or a change in conditions, and inherited defects can become common.
Check you have it
Question 1
The diagram shows some of the stages involved in natural selection in wild rabbits. Which statement is missing from box Z?
Answer: A.
The three boxes are natural selection in order. Variation exists within the population. Resources such as food, water and space are limited, so the rabbits compete for them. Those whose variations suit the conditions best survive and reproduce, passing their alleles on.
Competition is the step that turns variation into selection. Without a shortage of something, all the variants would survive equally and nothing would be selected.
Production of limited numbers of young (C) reverses a key point. Organisms produce more offspring than can survive, which is what makes the competition intense.
Passing on of alleles to the next generation (B) is what the third box already says, so it would leave the middle of the chain empty.
Selection by humans (D) is artificial selection, and the question is about a wild population where the environment does the selecting.
Question 2
New strains of the crop plant wheat can be produced by cross-breeding disease resistant plants with those that give high yields of grain.
What is this an example of?
Answer: B.
A breeder chooses which wheat plants to cross, picking disease resistant ones and high yielding ones, so that the offspring may combine both. The human choice of which individuals reproduce is exactly what artificial selection, or selective breeding, means.
Natural selection (A) uses the same mechanism with the environment choosing. Nobody decides; the individuals that happen to suit the conditions survive and reproduce.
Genetic engineering (C) means transferring a specific gene from one organism into another, often across the species barrier. Nothing is transferred here: both parents are wheat, and the breeder works only with variation already present in the species.
Asexual reproduction (D) involves one parent and produces clones, so it could never combine characteristics from two different plants.
Cross-breeding is the point: sexual reproduction shuffles the alleles, and the breeder then selects the offspring that inherited both desirable features.
Question 3
Chickens are birds that are farmed to produce eggs for human consumption.
A type of chicken has been bred to lay more eggs.
Which method would be used to produce this type of chicken?
Answer: D.
The farmer chooses the hens that lay the most eggs, breeds from them, and repeats the process generation after generation. The alleles for high egg production become more common, and the average yield of the flock rises. Deliberate human choice of which individuals reproduce is what selective breeding means.
Natural selection (C) uses the same mechanism with the environment doing the choosing. Laying more eggs is useful to a farmer rather than to a wild bird, so the environment would not select for it.
Asexual reproduction (A) is not available to birds, and it would produce clones rather than improve the flock.
Biotechnology (B) covers the use of microorganisms and enzymes in industry, such as making penicillin or insulin. It is not how a breed of chicken is developed.
The same method has produced high-yield crops, dairy cattle and every domestic dog breed.
What the syllabus asks for on this topicSyllabus points
Syllabus points
- Describe variation and distinguish between continuous and discontinuous variation.
- Describe mutation and state that it is the way new alleles arise.
- Describe adaptive features and explain them in terms of the environment.
- Explain the process of natural selection and how it leads to evolution.
- Describe selective breeding and compare it with natural selection.
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