Definition
The survival and reproduction of individuals with advantageous traits (over those without them) is called natural selection — over generations, this causes advantageous traits to become more common in a population.
Definition
Steps in the process of natural selection: (1) variation exists among individuals in a population, (2) some variations are advantageous for survival/reproduction in the current environment, (3) individuals with advantageous traits survive and reproduce more successfully than others, (4) these advantageous traits are passed on to offspring, becoming more common in the population over successive generations.
Example 1 — a new advantageous variation
A new variation appears suddenly in a population and helps survival. What happens next, according to natural selection?
- Individuals carrying this new advantageous variation are more likely to survive and reproduce than those without it.
- They pass the variation on to their offspring.
- Over successive generations, the variation becomes more common in the population, since its carriers consistently out-reproduce non-carriers.
Answer: The advantageous variation spreads through the population over generations
Definition
Darwin's observations of finches (on the Galápagos Islands during his voyage) showed that closely related finch species on different islands had beak shapes closely matched to the specific food sources available on each island — this proved that natural selection could shape a species' physical traits over time to suit its particular environment, and that closely related species could diverge from a shared ancestor under different environmental pressures.
Example 2 — the peppered moth
During the Industrial Revolution, peppered moth populations gradually darkened as pollution darkened tree bark. What is this an example of?
- Before pollution, lighter moths were camouflaged against light tree bark and survived better; darker moths stood out and were eaten more by predators.
- As pollution darkened the bark, darker moths became better camouflaged and survived/reproduced better instead, while lighter moths became more visible to predators.
- Over generations, the population's colour shifted towards the darker form.
Answer: Natural selection in action (a directly observed, real-world example)
Definition
Variation among individuals is essential for evolution because natural selection can only act on traits that already differ between individuals — without variation, there would be nothing for an environment to select for or against, and populations couldn't adapt or change over time.
Four main sources of variation: mutation (random changes in DNA sequence, creating entirely new alleles), genetic recombination (during meiosis, alleles from two parents are shuffled into new combinations in offspring), sexual reproduction (combining genetic material from two parents, multiplying the possible trait combinations), and gene flow/migration (new individuals moving into a population and interbreeding, introducing alleles not previously present there).
Definition
Genetic recombination produces variation among offspring because, during meiosis, chromosomes from each parent exchange segments and are shuffled independently before being passed into gametes — this means siblings from the same two parents still inherit different specific combinations of alleles, rather than being genetically identical.
Gene flow (migration) introduces new variation into a population when individuals from a different population (carrying different alleles) move in and interbreed with the existing population, adding alleles that weren't there before and increasing the population's overall genetic diversity.
Definition
Selective breeding (artificial selection) is when humans deliberately choose organisms with desirable traits to breed together, aiming to increase the frequency of those traits in future generations — e.g. farmers choosing their best-producing cows to breed, to improve milk yield in the herd over time.
Artificial selection is similar to natural selection because both work by selectively favouring certain traits' reproduction over others across generations, gradually shifting a population's characteristics — the key difference is simply who or what is doing the selecting: nature and environmental pressures in natural selection, versus deliberate human choice in artificial selection.
Definition
The fossil record provides evidence of evolution by preserving a chronological sequence of past life forms in rock layers, showing how organisms have gradually changed over vast timescales, and revealing transitional forms linking major groups.
Archaeopteryx fossils provided evidence that birds evolved from reptiles because Archaeopteryx shows a clear mix of reptilian features (like teeth and a long bony tail) alongside clearly bird-like features (like feathers and wings) — exactly the kind of transitional form expected if birds gradually evolved from a reptilian ancestor.
Fossils help scientists understand the history of life on Earth by letting them determine roughly when different groups of organisms first appeared, how they changed over time, and how different species are related to one another through shared ancestry.
Definition
Homologous structures are structures with a similar underlying structure/origin in different species (inherited from a shared ancestor), even though they may now serve different functions — e.g. the forelimb bones of a human arm, a whale flipper, and a bat wing all share the same basic bone arrangement, despite being used for very different purposes (grasping, swimming, flying).
Analogous structures, in contrast, serve a similar function in different species but don't share the same underlying structure or evolutionary origin — e.g. a bird's wing and an insect's wing both allow flight, but evolved completely independently from very different starting structures.
Which characteristics in whales support their ancestors having lived on land: whales retain small, vestigial pelvic and leg bones inside their bodies (with no functional legs), which only make sense as remnants inherited from four-legged, land-dwelling ancestors.
Definition
Vestigial structures are body parts that have become reduced and largely or entirely non-functional over evolutionary time, since they served a purpose in an ancestor but no longer do in the current species — they provide evidence of evolution because their reduced, functionless presence only makes sense as an inherited leftover from a functional ancestral structure, not as something that would be built from scratch for a species that doesn't need it.
The human appendix is considered a vestigial organ because it appears to serve little to no essential digestive function in modern humans, unlike the larger, more functional equivalent structure (the caecum) used by some plant-eating ancestral relatives for digesting tough plant material.
A vestigial structure found in flightless birds like ostriches is their small, non-functional wings — remnants inherited from flying ancestors, no longer needed or used for flight.
Definition
A species is a group of organisms capable of interbreeding and producing fertile offspring together — e.g. a horse and a donkey are different species because, although they can mate to produce a mule, mules are typically infertile.
Speciation is the process by which new species form, typically requiring reproductive isolation — some barrier that prevents two populations from interbreeding, allowing them to accumulate genetic differences independently until they can no longer interbreed even if reunited.
Definition
Two types of speciation: allopatric speciation (populations become geographically separated, e.g. by a river, mountain range, or ocean, preventing interbreeding) and sympatric speciation (new species form without geographic separation, through some other reproductive isolating mechanism while populations still share the same geographic area).
Sympatric speciation occurs when reproductive isolation happens through mechanisms other than physical separation — such as differences in breeding timing/behaviour, preference for different mates or habitats within the same area, or chromosomal changes that prevent successful interbreeding.
Example 3 — speciation from geographic separation
A population is split by a river; after many generations, the two separated populations evolve into different species. What type of speciation occurred?
- The river physically separated the population into two groups that could no longer interbreed.
- This is a geographic barrier causing reproductive isolation, letting each group accumulate independent genetic changes over time.
Answer: Allopatric speciation
Quick Recap
- ●Natural selection: variation advantageous traits survive/reproduce more passed on become more common (e.g. Darwin's finches, peppered moth)
- ●Variation sources: mutation, genetic recombination, sexual reproduction, gene flow/migration — variation is essential for selection to act on
- ●Artificial selection (selective breeding): same principle as natural selection, but humans choose the traits favoured
- ●Fossil record: shows life's change over time, transitional forms (e.g. Archaeopteryx: reptile bird features)
- ●Homologous structures (same origin, different function, e.g. limb bones) vs analogous structures (same function, different origin, e.g. bird/insect wings)
- ●Vestigial structures (reduced, functionless remnants, e.g. human appendix, ostrich wings, whale pelvic bones) are evidence of evolutionary ancestry
- ●Species: interbreed fertile offspring. Speciation: allopatric (geographic separation) vs sympatric (isolation without geographic separation)
