Cambridge O Level Biology · Syllabus 5090 · Inheritance
Evolution
What is Evolution?
The change in the inherited features of a population over generations, brought about by processes such as natural selection acting on inherited variation. Evolution is a property of populations rather than of individuals: an individual organism never evolves, because the alleles it carries are fixed at fertilisation. What changes is the frequency with which particular alleles occur among the members of the population, and therefore how common the features those alleles produce become. Characteristics acquired during an organism's lifetime are not inherited and do not contribute to evolution.
This definition is part of the Inheritance chapter in Cambridge O Level Biology.
Evolution in context
Inheritance is the transmission of genetic information from one generation to the next. A gene is a length of DNA that codes for a protein; an allele is one of the alternative forms that gene can take. The order of the bases A, T, C and G along a strand of DNA sets the order of amino acids in a protein, which sets the protein’s shape, which sets what the protein can do — and that is how a molecule you cannot see decides a feature you can. Because gametes carry one allele of each gene, a cross between two known genotypes has a predictable outcome: Tt × Tt is expected to give 3 tall : 1 short, and Tt × tt is expected to give 1 tall : 1 short. Those are probabilities, not guarantees. Mutation makes new alleles at random, meiosis and fertilisation shuffle them into new combinations, and selection — by the environment in natural selection, by people in artificial selection — changes how common each allele becomes over generations. That change is evolution.
Common mistakes with Evolution
- “Individuals evolve.” Evolution is a change in the inherited features of a population over generations. An individual’s genotype is fixed at fertilisation. Write “The population evolved: the proportion of individuals with that feature increased.”
Questions students ask about Evolution
Can an individual organism evolve?
No. Evolution is the change in the inherited features of a population over generations. An individual’s genotype is fixed at fertilisation and does not change during its life. What changes during evolution is how common each allele is among the members of the population, and that can only happen as one generation is replaced by the next.