Evolution & Natural Selection
Evolution is the change in heritable traits in a population over time. Natural selection is the primary mechanism — organisms with advantageous traits survive and reproduce more.
Darwin's Four Principles
1. Variation — individuals in a population differ.
2. Heredity — traits pass from parent to offspring.
3. Overproduction — more offspring produced than can survive.
4. Differential survival — better-adapted individuals survive and reproduce more.
Natural Selection in Action
Classic example: Peppered moths in England. Before industrialization, light moths survived better on light trees. After soot darkened trees, dark moths had an advantage and increased in population.
Adaptations
An adaptation is any trait that increases fitness (ability to survive and reproduce). Examples: giraffe long neck (reaching leaves), polar bear white fur (camouflage on snow), cactus waxy skin (reducing water loss).
Evidence for Evolution
- Fossil record — transitional forms showing gradual change
- Comparative anatomy — homologous structures (human arm, whale flipper, bat wing)
- DNA evidence — related species share similar gene sequences
- Direct observation — antibiotic resistance in bacteria
How Natural Selection Actually Operates
The mechanism is easy to state and easy to misstate. Individuals do not evolve — populations do, as the proportions of traits shift across generations.
Antibiotic resistance, step by step
- A bacterial population already contains random variation. A few cells happen to carry a mutation that blunts a particular antibiotic.
- The antibiotic arrives and kills the susceptible majority.
- The resistant few survive and reproduce — and bacteria reproduce fast.
- Within days, the population is largely resistant.
No individual bacterium changed. The mutation existed beforehand; the environment simply decided which cells left descendants. This is evolution observable in a lab within a week.
Where Variation Comes From
Natural selection can only act on variation that already exists. Three sources supply it:
- Mutation: random changes in DNA, the ultimate source of every new allele.
- Sexual reproduction: shuffles existing alleles into fresh combinations each generation.
- Gene flow: individuals moving between populations carry alleles with them.
Other Mechanisms of Evolution
Natural selection is the most important mechanism but not the only one.
- Genetic drift: random change in allele frequencies, powerful in small populations where chance events matter more than fitness.
- Sexual selection: traits that improve mating success spread even at a survival cost — a peacock's tail is heavy and conspicuous.
- Artificial selection: humans choosing which organisms breed. Every dog breed and nearly every crop is a product of it, and Darwin used it as his opening argument.
How New Species Form
Speciation
A population becomes separated — a river shifts, a group colonizes an island. The two groups then face different conditions and accumulate different mutations. Given enough time, they diverge so far that they can no longer interbreed successfully. At that point they are separate species.
Darwin's finches show this: one ancestral species spread across the Galápagos, and beak shapes diverged to match the food available on each island.
Common Mistakes
- Reading "fittest" as strongest. Fitness means reproductive success — often achieved through camouflage, cooperation, or simply breeding early.
- Describing evolution as progress toward a goal. It has no direction; it tracks whatever conditions currently favour.
- Saying humans descended from modern apes. Humans and modern apes share a common ancestor that was neither.
- Calling a theory a guess. In science, a theory is an explanation supported by extensive evidence, not a hunch.
Summary
- Evolution is change in the heritable traits of a population over generations.
- Natural selection needs variation, heredity, overproduction, and differential survival.
- Mutation and sexual reproduction supply the variation selection acts on.
- Drift, sexual selection, and artificial selection also change populations.
- Fossils, anatomy, DNA, and direct observation all converge on the same conclusion.
FAQ
Does "survival of the fittest" mean strongest? No — "fittest" means best suited to the current environment, not physically strongest.
Is evolution a theory or fact? Both. It's a scientific theory (well-tested explanation) supported by overwhelming evidence.
How long does evolution take? It depends entirely on generation time. Bacteria evolve measurably in days; large mammals take many thousands of years for comparable change.
What are vestigial structures? Features that have lost their original function, such as the human tailbone or the tiny hip bones inside whales. They are evidence of ancestry.
Is evolution still happening in humans? Yes. Lactose tolerance in adults and resistance to certain diseases have both spread through human populations within the last several thousand years.
What is a homologous structure? A body part shared by different species because they inherited it from a common ancestor — the same bone arrangement appears in a human arm, a whale flipper, and a bat wing.
Quick Quiz
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