Most useful takeaways
Domestic breeds show marked variability in form, color, and behavior compared with wild ancestors.
Artificial selection demonstrates that selection of small, heritable differences can produce major changes.
Correlation of growth and inheritance patterns mean selecting one trait often alters others.
Use and disuse, changed conditions, and crossing influence variability and improvement.
Use examples of selective breeding to illustrate how incremental inherited changes can accumulate into major differences over time.
Darwin surveys the wide range of variation produced in domesticated animals and plants, and how breeders select for desirable traits. He argues that human selection shows how significant changes can accumulate from small hereditary variations over generations.
Wild species exhibit individual differences, local varieties, and gradations between forms.
The distinction between species and varieties is often arbitrary and blurred by intermediates.
Geographical distribution and isolation contribute to divergence of varieties.
Natural variation supplies the raw material upon which selection acts.
Look for continuous variation and geographic patterns as evidence of populations undergoing evolutionary change.
Darwin examines variation among wild organisms, noting continuous variation, local races, and the difficulty of drawing sharp species boundaries. He emphasizes that natural varieties mirror domesticated variation and can be acted upon by natural selection.