18. Variation and selection

18.1 Variation

Core

• Define variation as differences between individuals of the same species 

• Distinguish between phenotypic variation and genetic variation 

• State that continuous variation results in a range of phenotypes between two extremes, e.g. height in humans 

• State that discontinuous variation results in a limited number of phenotypes with no intermediates, e.g. tongue rolling 

• Record and present the results of investigations into continuous and discontinuous variation 

• Define mutation as genetic change 

• State that mutation is the way in which new alleles are formed 

• State that ionising radiation and some chemicals increase the rate of mutation 

Supplement 

• State that phenotypic variation is caused by both genetic and environmental factors 

• State that discontinuous variation is mostly caused by genes alone, e.g. A, B, AB and O blood groups in humans 

• Define gene mutation as a change in the base sequence of DNA 

• Describe the symptoms of sickle cell anaemia 

• Explain how a change in the base sequence of the gene for haemoglobin results in abnormal haemoglobin and sickle-shaped red blood cells 

• Use genetic diagrams to show how sickle cell anaemia is inherited 

• State that people who are heterozygous (HbS HbA ) for the sickle cell allele have a resistance to malaria 

• Explain the distribution of the sickle cell allele in human populations with reference to the distribution of malaria.

18.2 Adaptive features

Core

• Define adaptive feature as an inherited feature that helps an organism to survive and reproduce in its environment 

• Interpret images or other information about a species to describe its adaptive features 

Supplement 

• Define adaptive feature as the inherited functional features of an organism that increase its fitness 

• Define fitness as the probability of an organism surviving and reproducing in the environment in which it is found 

• Explain the adaptive features of hydrophytes and xerophytes to their environments 

18.3 Selection

Core

• Describe natural selection with reference to: 

– variation within populations 

– production of many offspring 

– competition for resources 

– struggle for survival 

– reproduction by individuals that are better adapted to the environment than others 

– passing on of their alleles to the next generation 

• Describe selective breeding with reference to: 

– selection by humans of individuals with desirable features 

– crossing these individuals to produce the next generation 

– selection of offspring showing the desirable features 

Supplement 

• Describe evolution as the change in adaptive features of a population over time as the result of natural selection 

• Define the process of adaptation as the process, resulting from natural selection, by which populations become more suited to their environment over many generations 

• Describe the development of strains of antibiotic resistant bacteria as an example of evolution by natural selection 

• State the differences between natural and artificial selection 

• Outline how selective breeding by artificial selection is carried out over many generations to improve crop plants and domesticated animals