Selection and Evolution

Charles Darwin and Alfred Russel Wallace proposed a theory of natural selection to account for the evolution of species in 1858. A year later, Darwin published On the Origin of Species providing evidence for the way in which aspects of the environment act as agents of selection and determine which variants survive and which do not. The individuals best adapted to the prevailing conditions succeed in the ‘struggle for existence’. 

Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts.

Learning outcomes 

Candidates should be able to:

17.1 Variation

The variation that exists within a species is categorised as continuous and discontinuous. The environment has considerable influence on the expression of features that show continuous (or quantitative) variation.

a) describe the differences between continuous and discontinuous variation and explain the genetic basis of continuous (many, additive genes control a characteristic) and discontinuous variation (one or few genes control a characteristic) (examples from 16.2f may be used to illustrate discontinuous variation; height and mass may be used as examples of continuous variation) 

b) explain, with examples, how the environment may affect the phenotype of plants and animals 

c) use the t-test to compare the variation of two different populations (see Mathematical requirements) 

d) explain why genetic variation is important in selection

17.2 Natural and artificial selection

Populations of organisms have the potential to produce large numbers of offspring, yet their numbers remain fairly constant year after year. 

Humans use selective breeding (artificial selection) to improve features in ornamental plants, crop plants, domesticated animals and livestock.

a) explain that natural selection occurs as populations have the capacity to produce many offspring that compete for resources; in the ‘struggle for existence’ only the individuals that are best adapted survive to breed and pass on their alleles to the next generation 

b) explain, with examples, how environmental factors can act as stabilising, disruptive and directional forces of natural selection 

c) explain how selection, the founder effect and genetic drift may affect allele frequencies in populations 

d) use the Hardy–Weinberg principle to calculate allele, genotype and phenotype frequencies in populations and explain situations when this principle does not apply 

e) describe how selective breeding (artificial selection) has been used to improve the milk yield of dairy cattle 

f) outline the following examples of crop improvement by selective breeding: 
• the introduction of disease resistance to varieties of wheat and rice 
• the incorporation of mutant alleles for gibberellin synthesis into dwarf varieties so increasing yield by having a greater proportion of energy put into grain 
• inbreeding and hybridisation to produce vigorous, uniform varieties of maize

17.3 Evolution

Isolating mechanisms can lead to the accumulation of different genetic information in populations, potentially leading to new species. 

 Over prolonged periods of time, some species have remained virtually unchanged, others have changed significantly and many have become extinct.

a) state the general theory of evolution that organisms have changed over time 

b) discuss the molecular evidence that reveals similarities between closely related organisms with reference to mitochondrial DNA and protein sequence data 

c) explain how speciation may occur as a result of geographical separation (allopatric speciation), and ecological and behavioural separation (sympatric speciation) 

d) explain the role of pre-zygotic and post-zygotic isolating mechanisms in the evolution of new species 

e) explain why organisms become extinct, with reference to climate change, competition, habitat loss and killing by humans