What evolution means
In biology, evolution is change in the heritable characteristics of populations over generations. Individuals do not evolve during their lifetimes; populations do. Evolution is a fact supported by many lines of evidence, and the theory of natural selection explains much of how it happens.
Natural selection
Charles Darwin and Alfred Russel Wallace described the process. It follows from four observations:
- Individuals vary in their traits.
- Some of that variation is heritable.
- More offspring are produced than can survive.
- Individuals with traits that suit their environment tend to survive and reproduce more.
So over generations, helpful heritable traits become more common. Fitness in biology means reproductive success: how many offspring an individual leaves compared with others. It is not strength or size.
Sources of variation
New alleles come from mutation. Sexual reproduction reshuffles existing alleles (crossing over, independent assortment, random fertilisation).
Other mechanisms
- Genetic drift: random changes in allele frequencies. It is strongest in small populations.
- Gene flow: movement of alleles between populations through migration.
- Non-random mating, such as mate choice (sexual selection).
Evidence
- Fossils show change over time and transitional forms.
- Homologous structures have a common ancestry, though perhaps different functions: the human arm, whale flipper and bat wing share the same bone pattern. Analogous structures have similar functions but separate origins, such as bird and insect wings.
- DNA comparisons show related species have more similar sequences.
- Observed evolution, as in bacteria and finches.
Speciation
By one common definition, a species is a group of organisms that can interbreed and produce fertile offspring. New species often form when populations are separated, for example by a mountain range or an island (geographic isolation). Over time, different selection and drift cause them to diverge until they can no longer interbreed.
Worked example: antibiotic resistance
A bacterial population has a few cells with a mutation that makes them resistant to an antibiotic.
- Variation: most cells are susceptible, a few are resistant.
- Selection pressure: the antibiotic kills susceptible cells.
- Survival and reproduction: the resistant cells survive and divide.
- Result: the next generation is mostly resistant.
The antibiotic did not cause the mutation; it only selected for cells that already had it. This is why finishing a prescribed course, and not overusing antibiotics, matters.
Common mistakes
- Saying organisms evolve "because they need to".
- Treating "survival of the fittest" as survival of the strongest.
- Thinking that an individual can evolve.