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Biology
Evolution
From Common Ancestors to Modern Species
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Evolution explains how modern species arose from earlier forms of life through changes in inherited traits across many generations. A branching tree of life shows that species are related by common ancestry rather than arranged in a simple ladder of progress. This idea matters because it connects fossils, DNA, anatomy, behavior, ecology, and medicine into one scientific framework. It helps explain both the diversity of life and the deep similarities shared by all organisms.
Key Facts
- Evolution is change in inherited traits of populations over generations.
- Common ancestry means two species share an ancestral population in the past.
- Natural selection occurs when heritable traits affect survival or reproduction.
- Allele frequency = number of copies of an allele / total copies of that gene in the population.
- Genetic drift has the strongest effect in small populations because chance events can shift allele frequencies quickly.
- A simple molecular clock estimate is time since divergence = genetic differences / mutation rate.
Vocabulary
- Evolution
- Evolution is the change in inherited traits of a population over many generations.
- Common ancestor
- A common ancestor is an earlier population from which two or more later species descended.
- Natural selection
- Natural selection is the process in which individuals with advantageous heritable traits leave more offspring on average.
- Speciation
- Speciation is the formation of a new species when populations become reproductively isolated and genetically distinct.
- Phylogenetic tree
- A phylogenetic tree is a branching diagram that shows hypothesized evolutionary relationships among organisms.
Common Mistakes to Avoid
- Thinking individuals evolve during their lifetimes. Evolution happens to populations across generations because allele frequencies change in the gene pool.
- Reading a phylogenetic tree as a ladder from simple to advanced. Branch tips represent modern or sampled groups, and no living species is the ancestor of another living species shown beside it.
- Assuming natural selection always makes organisms perfect. Selection works on existing variation and is limited by tradeoffs, environment, chance, and history.
- Confusing similarity with close relatedness without considering evidence. Similar traits can evolve independently through convergent evolution, so DNA and multiple traits are needed to infer relationships.
Practice Questions
- 1 In a population of 200 diploid beetles, 60 copies of allele A are found at one gene. What is the frequency of allele A in the population?
- 2 Two species differ at 18 DNA positions in a gene region. If mutations accumulate at an average rate of 3 differences per million years for that region, estimate the time since the species diverged.
- 3 Two animals both have streamlined bodies and fins, but one is a mammal and the other is a fish. Explain why these similarities do not necessarily mean they are each other's closest relatives.