Chapter summary
Variation, selection and species, at a glance
Scan the key ideas, or hide the answers and try to recall them.
01
Supply the variation selection needs
Which processes make new alleles and which reshuffle existing ones?
Key idea and reminders
Mutation can create new alleles. Meiosis and fertilisation create new combinations. Heritable differences allow unequal reproductive success to change later generations.
- Mutation supplies new alleles.
- Meiosis and fertilisation reshuffle alleles.
- An evolutionary response requires heritable variation.
Keep in mind: Crossing over reshuffles DNA between homologous chromosomes. Mutation supplies new sequence variants; neither process is directed towards an organism's needs.
02
Explain why an environment favours a variant
How can predation or an antibiotic change reproductive success?
Key idea and reminders
An environmental factor is a selection pressure when inherited variants differ in their success under it. Survivors must reproduce for the change to reach future generations.
- State a specific selection pressure.
- Explain a specific advantage.
- Link survival to reproduction and inheritance.
Keep in mind: Selection favours resistant variants among a population. Mutations are not instruction-driven responses to the required survival outcome.
03
Read a change across generations
Does a higher allele frequency prove why it changed?
Key idea and reminders
Natural selection can change allele frequencies over generations. A frequency change documents population change, while a causal explanation also needs evidence of inherited differences and unequal reproductive success.
- Populations change across generations.
- Allele frequency counts copies, not visible individuals.
- Selection requires a causal link to reproductive success.
Keep in mind: Dominant phenotypes may include both homozygotes and heterozygotes. Allele frequency requires allele counts or sufficient genotype information.
04
Explain why harmful alleles can persist
Why does selection not remove every recessive harmful allele?
Key idea and reminders
A harmful recessive allele can be carried without its recessive phenotype in heterozygotes. Different conditions and heterozygote advantages can also maintain more than one allele.
- Recessive alleles can remain in heterozygotes.
- Heterozygote advantage is environment-dependent.
- Preserved variation is compatible with natural selection.
Keep in mind: Recessive describes a phenotype relationship in heterozygotes. It does not predict inevitable disappearance or low frequency.
05
Decide what makes a biological species
Is looking different enough to establish two species?
Key idea and reminders
A biological species is a group of natural populations whose members can actually or potentially interbreed and produce fertile offspring, while being reproductively isolated from other groups.
- Actual or potential natural interbreeding.
- Fertile offspring and gene exchange.
- Limited application to fossils and asexual organisms.
Keep in mind: Similar appearance does not establish the ability to interbreed and produce fertile offspring. Reproductive evidence is central to this concept.