Cambridge IGCSE Biology 18 Variation and Selection Exam Questions

About Cambridge IGCSE Biology 18 Variation and Selection

Variation and selection explains why individuals differ and how populations change over time in Cambridge IGCSE Biology. Core candidates describe continuous and discontinuous variation, mutation as a source of new alleles, adaptive features, the process of natural selection and selective breeding. Extended candidates link discontinuous variation to genes only and continuous variation to genes and environment, explain gene mutation and its causes, describe the adaptations of xerophytes and hydrophytes, and use natural selection to explain antibiotic resistance. Examiners expect natural selection answers to follow a clear sequence of variation, competition, survival, reproduction and passing on of alleles, and often ask you to compare natural with artificial selection.

What you need to know

• 18.1 Variation Variation is the differences between individuals of the same species. Continuous variation gives a range of phenotypes between two extremes, such as height and mass; discontinuous variation gives a limited number of phenotypes with no intermediates, such as ABO blood groups. Plot continuous data as a histogram and discontinuous data as a bar chart.
• 18.1 Causes of variation Mutation is genetic change and is the way new alleles are formed. Ionising radiation and some chemicals increase the rate of mutation. Extended: discontinuous variation is usually caused by genes only; continuous variation is influenced by genes and the environment.
• 18.2 Adaptive features An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment. Extended: xerophytes such as cacti have thick waxy cuticles, sunken stomata, rolled leaves or spines and deep or extensive roots; hydrophytes have large air spaces and stomata on the upper surface of floating leaves.
• 18.3 Natural selection Individuals show variation; there is competition for resources and a struggle for survival; those with better adapted features are more likely to survive and reproduce; they pass on their alleles; over many generations the population becomes better adapted. This is how populations evolve.
• 18.3 Antibiotic resistance (Extended) Bacteria with a resistance mutation survive antibiotic treatment and reproduce, so the resistant allele becomes more common in the population.
• 18.3 Selective breeding Humans select individuals with desirable features, breed them together, select the best offspring and repeat over many generations. Examples include crop plants with high yield or disease resistance and cattle with high milk yield.

Common mistakes in 18 Variation and Selection

• Saying organisms change because they need to or try to adapt. Individuals do not change; variation already exists and better adapted individuals survive and reproduce.
• Saying the strongest survive. Say the best adapted individuals are more likely to survive.
• Forgetting to say that alleles are passed on to offspring. This step is needed for the population to change.
• Calling height discontinuous because it is measured in whole centimetres. Height is continuous because it has a range of intermediate values.
• Confusing selective breeding with genetic modification. Selective breeding chooses parents; genetic modification transfers genes directly.

Try three 18 Variation and Selection exam-style questions