Variation and selection

Cambridge IGCSE Biology 0610 · Core and Extended

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.

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:

1. There is variation within a population, caused ultimately by mutation.
2. There is competition for resources, and more offspring are produced than can survive.
3. Individuals with features better suited to the environment are more likely to survive.
4. Those survivors reproduce and pass on the alleles for those features.
5. 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:

1. Select the individuals with the desired characteristic.
2. Breed them together.
3. Select the offspring showing the characteristic most strongly.
4. 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:

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.