Evolution and Natural Selection
Grade 12 · Science · Study Guide
Understand the mechanisms driving evolutionary change, evidence for evolution, and how populations change over generations.
Evolution is the change in heritable characteristics of populations over successive generations. Charles Darwin and Alfred Russel Wallace independently developed the theory of natural selection: organisms with traits better suited to their environment survive and reproduce more successfully, passing those traits to offspring. Over many generations, this process can lead to significant changes in populations and, ultimately, to the formation of new species.
For natural selection to operate, three conditions must be met: there must be variation in a trait within the population, the trait must be heritable (passed from parents to offspring), and the trait must affect survival or reproduction (fitness). Not all evolution is driven by natural selection — genetic drift (random changes in allele frequencies, especially in small populations), gene flow (movement of alleles between populations), and mutation (the ultimate source of new variation) also drive evolutionary change. These are collectively part of the Modern Synthesis, which combines Darwinian natural selection with Mendelian genetics.
Evidence for evolution comes from multiple independent sources: the fossil record showing progressive changes over time; comparative anatomy (homologous structures suggesting common ancestry, vestigial structures with no current function); molecular biology (DNA sequence similarities between species); biogeography (distribution of species across continents); and direct observation of evolution in bacteria, viruses, and insects. Speciation — the formation of new species — can occur when populations become reproductively isolated (allopatric speciation due to geographic barriers, or sympatric speciation in the same geographic area).
| Mechanism | Description | Effect on genetic diversity |
|---|---|---|
| Natural selection | Differential survival/reproduction based on fitness | Decreases diversity for harmful alleles; increases for beneficial |
| Genetic drift | Random allele frequency changes | Decreases diversity (especially in small populations) |
| Gene flow | Allele movement between populations | Increases diversity in receiving population |
| Mutation | New heritable changes in DNA | Increases diversity; ultimate source of new alleles |
Key terms
- Natural selection
- Differential survival and reproduction of individuals based on heritable traits that affect fitness.
- Fitness
- An organism's reproductive success relative to others in the population; determined by how well adapted it is.
- Genetic drift
- Random changes in allele frequencies in a population, most significant in small populations.
- Gene flow
- Movement of alleles between populations through migration; tends to homogenize populations.
- Speciation
- The formation of a new species; occurs when populations become reproductively isolated.
- Homologous structures
- Anatomical features shared by different species due to common ancestry, even if they serve different functions.
- Hardy-Weinberg equilibrium
- A model describing allele and genotype frequencies in a non-evolving population; requires no mutation, selection, drift, or gene flow.
Flashcards
- Natural selection
- Organisms with favorable heritable traits survive and reproduce more; drives adaptation
- Fitness
- Relative reproductive success; well-adapted individuals leave more offspring
- Genetic drift
- Random allele frequency changes; strongest in small populations; includes founder effect and bottleneck
- Allopatric speciation
- New species forms when populations are separated by geographic barriers
- Hardy-Weinberg principle
- Allele frequencies stay constant in large, random-mating populations with no selection, mutation, or gene flow
- Vestigial structure
- Reduced/non-functional remnant of a structure that had a function in an ancestor; evidence of evolution
Practice questions
Which of the following is NOT a mechanism of evolutionary change?
- Natural selection
- Genetic drift
- Gene flow
- Acquired characteristics
Show answer
Answer: Acquired characteristics
Lamarck's now-discredited idea that traits acquired during an organism's lifetime can be passed to offspring is not a mechanism of evolution. Only heritable genetic changes can be passed on.
Natural selection acts on an organism's genotype directly, not its phenotype.
Show answer
Answer: False
Natural selection acts on phenotype (the observable traits). The underlying genotype is what gets passed to offspring, but selection can only 'see' the phenotype expressed in the environment.
The formation of a new species through geographic isolation is called ___ speciation.
Show answer
Answer: allopatric
Allopatric speciation occurs when a geographic barrier divides a population, preventing gene flow. Over time, the separated populations diverge genetically until they can no longer interbreed.
The Hardy-Weinberg equilibrium describes a population that is:
- Rapidly evolving
- Not evolving
- Experiencing strong selection
- Undergoing genetic drift
Show answer
Answer: Not evolving
Hardy-Weinberg equilibrium is a null model describing a hypothetical non-evolving population. Any deviation from predicted frequencies indicates evolution is occurring.
Homologous structures in different species are evidence of common ancestry.
Show answer
Answer: True
Homologous structures (like the arm of a human, flipper of a whale, and wing of a bat) share the same bone arrangement due to common ancestry, even though they have adapted for different functions.
Fun facts
- Bacteria evolve antibiotic resistance in real time — some strains of Staphylococcus aureus evolved resistance to methicillin within just a few years of the drug's introduction.
- The whale's front flipper contains the same bones as a human hand — clear evidence of common mammalian ancestry despite very different functions.
- Darwin spent 20+ years gathering evidence before publishing On the Origin of Species in 1859, only rushing publication when Wallace sent him an identical theory.
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