How to Read This Evolution and Ecosystems Chapter

Evolution is cumulative change in the heritable characteristics of a population. That definition is deliberately about populations and generations. An individual may grow, acclimatize or learn during its lifetime, but it does not evolve genetically in response to need. Evolution occurs when alleles are inherited at different frequencies, introduced, lost or changed, so that the composition of a population's gene pool differs through time.

This chapter examines the same history from four directions. Classification gives names and organizes organisms into nested groups. Cladistics reconstructs branching ancestry from shared characters and molecular sequences. Selection and drift explain why allele frequencies change. Ecology supplies the environmental conditions, resources and interactions that make some variants more successful than others. Speciation connects these scales when diverging gene pools become reproductively isolated.

Fossils, DNA sequences, amino-acid sequences, observed phenotype changes and selective breeding are evidence that populations change and share ancestry. Natural selection is a mechanism that can explain a directional component of that change. Genetic drift is another mechanism, based on chance rather than differential adaptation. A cladogram is a testable hypothesis about relationships, not a photograph of evolution. Strong answers identify which claim a piece of evidence supports rather than treating every evolutionary term as interchangeable.

Ecological explanations require similar discipline. A habitat is the place where an organism lives; a niche is its mode of existence, including resources used, abiotic tolerances and interactions. An adaptation is an inherited feature that increases fitness in a particular environment. Fitness means contribution to future generations, not strength, longevity or perfection. Every adaptation has a context and may carry costs when conditions change.

Section 11.1 builds taxonomy, dichotomous keys and phylogenetic reasoning. Section 11.2 moves from variation to natural, artificial and sexual selection, then models gene pools, drift and Hardy–Weinberg equilibrium. Section 11.3 evaluates evidence for evolution and follows allopatric and sympatric routes to reproductive isolation. Section 11.4 links tolerance, sampling, competition, niche partitioning, adaptive radiation and biodiversity. The synthesis section returns to the distinctions most often tested in unfamiliar data.