Virginia Biology SOL evolution and classification: a complete overview of the evidence for evolution, natural selection, speciation, and classification for BIO.6 and BIO.7

What this part of Reporting Category 4 demands

Evolution and classification are the first half of the Virginia Biology SOL's Reporting Category 4: Classification, Evolution, and Ecology, covering standards BIO.6 and BIO.7. The thread is common ancestry: classification groups organisms by the features and genes they inherited, evolution explains how those features changed, and the evidence for evolution and the trees of classification are two views of the same history. The practices that matter most here are analyzing and interpreting data (reading trees, keys, and evidence), constructing explanations (why a population changed, how two species formed), and engaging with evidence.

This guide ties together the matching dot-point pages, each with its own practice questions: evidence for evolution, natural selection and adaptation, speciation and population change, and classification and phylogeny.

The evidence for evolution

Several independent lines of evidence support evolution, and their strength is that they agree. The fossil record, in layers of sedimentary rock with deeper layers older, gives a time-ordered sequence and transitional forms. Comparative anatomy shows homologous structures (the same bone plan from a shared ancestor, such as the forelimbs of humans, whales, and bats) and vestigial structures (reduced parts with little function); analogous structures (similar function, different structure) are the contrast that does not indicate close relationship. Comparative embryology reveals shared early-development features. Molecular evidence compares DNA and protein sequences, with closely related species more alike. Biogeography shows that where species live fits descent from common ancestors on separating landmasses. No single line is proof; the case is built from all of them together.

Natural selection and adaptation

Natural selection needs four ingredients: variation, overproduction and competition, differential survival and reproduction (fitness), and inheritance. The variation it acts on comes from mutation (the source of new alleles) and the shuffling of alleles in meiosis and sexual reproduction; selection does not create variation, it filters it. Individuals with traits that raise fitness (reproductive success) in their environment survive and reproduce more, so those heritable traits become more common, producing adaptation. The defining idea, and the most-tested misconception, is that populations evolve, individuals do not: a single organism does not change its genes during its life; the proportions of alleles in the population shift over generations.

Speciation

A species is a group that can interbreed and produce fertile offspring, so reproductive isolation is the bar for becoming separate species. Speciation usually begins with geographic isolation, when a physical barrier separates a population and the groups stop interbreeding. Apart, each group accumulates different mutations and is shaped by natural selection in its own environment, so their gene pools diverge. Eventually they become so different that they can no longer interbreed to make fertile offspring even if reunited; they are reproductively isolated and are now two species. Barriers can be prezygotic (preventing mating or fertilization) or postzygotic (offspring inviable or sterile, like the mule). Geographic isolation is a common cause; reproductive isolation is the outcome that completes speciation.

Classification and phylogeny

Modern classification (taxonomy) groups organisms by shared characteristics and molecular evidence, so the groups reflect evolutionary relationships. The three domains are Bacteria and Archaea (prokaryotes) and Eukarya (with a nucleus), which holds the kingdoms Protista, Fungi, Plantae, and Animalia. The hierarchy runs domain, kingdom, phylum, class, order, family, genus, species, and each species has a two-part binomial name (genus capitalized, species lowercase, both italicized). A dichotomous key identifies an organism through paired either/or choices. A phylogenetic tree or cladogram shows relationships, with each branch point a common ancestor: organisms that share a more recent branch point are more closely related, so judge relatedness by the shared branch point, not by how close the labels sit.

Check your knowledge

A mix of recall and reasoning questions covering BIO.6 and BIO.7. Attempt them under timed conditions, then check against the solutions.

1. Name three independent lines of evidence for evolution. (3 marks)
2. Explain the difference between homologous and analogous structures, and state which indicates common ancestry. (2 marks)
3. State the four conditions required for natural selection. (2 marks)
4. Why can natural selection only act on existing variation, and where does that variation come from? (2 marks)
5. Explain how geographic isolation can lead to the formation of a new species. (3 marks)
6. Define a species, and explain why a horse and a donkey are different species even though they can mate. (2 marks)
7. List the levels of classification from broadest to most specific. (2 marks)
8. On a cladogram, how do you tell which two species are most closely related? (2 marks)