Darwinism: MDCAT Biology notes
Darwinism MDCAT notes: Darwin's voyage, Galapagos finches, natural selection, descent with modification, and homologous, analogous and vestigial organs.
Charles Darwin and the voyage
Charles Darwin sailed on HMS Beagle (1831–1836). On the Galapagos Islands, off Ecuador, he noticed that the finches and other animals resembled species of the South American mainland, but differed from island to island. He concluded that a few ancestors had reached the islands from the mainland and then diversified. In 1859 he published On the Origin of Species by Means of Natural Selection. Alfred Wallace reached the same idea independently, and Malthus's essay on population influenced both men.
The theory of natural selection
- Overproduction: organisms produce more offspring than the environment can support.
- Struggle for existence: resources are limited, so individuals compete.
- Variation: individuals differ, and many differences are heritable. Variation is the raw material for selection.
- Survival of the fittest: individuals with favourable heritable traits survive and reproduce more. Survival therefore depends partly on the hereditary makeup of the individual.
- Natural selection: over successive generations favourable traits accumulate, populations become better adapted, and new species can form.
Darwin saw natural selection as the main force behind evolution, and he saw it as closely tied to adaptation. Evolution happens because selection gives some alleles a better chance of being passed on than others. Fitness is an individual's ability to pass its traits (phenotype and genes) to the next generation, measured by reproductive success.
- Natural selection acts only on heritable variation. It is local or regional, because it depends on the environment.
- Artificial selection is carried out by a breeder, so it is not natural selection. Darwin used it as an analogy.
- Descent with modification: all life shares a common ancestor. Descendants resemble their ancestors but change over time.
Evidence for evolution
| Organ type | Structure / origin | Function | Shows | Examples |
|---|---|---|---|---|
| Homologous | Same basic structure and common origin | Different | Divergent evolution | Forelimbs of human, bird, bat and whale |
| Analogous | Different origin and structure | Similar | Convergent evolution | Wings of bird and insect (butterfly, beetle); bird and flying lizard |
| Vestigial | Reduced remnants | Functional in ancestors, little or none now | Common descent | Human vermiform appendix, ear muscles, tailbone; whale pelvis and hind-leg bones |
- Fossil record: considered the strongest direct evidence. Vertebrate classes appear in the order fish → amphibians → reptiles → birds and mammals.
- Comparative embryology: all vertebrate embryos look alike in early stages, for example in having gill slits. In land vertebrates, the first gill pouch becomes the Eustachian tube.
- Molecular biology: proteins such as cytochrome c, a respiratory protein found in many organisms, differ less between closely related species.
Worked example
Question: Whale flippers and bat wings have the same bone pattern but do different jobs. What does this show?
Answer: They are homologous organs, which are evidence of divergent evolution from a common ancestor.
Common MDCAT traps
- Homologous organs share the same origin and structure; analogous organs share the same function.
- Bird wings and insect wings are analogous, not homologous.
- Galapagos species resemble the South American mainland, not North America.
- Use and disuse is Lamarck's idea, not Darwin's.
- Selection by a breeder is artificial selection.
Quick revision
- Darwin wrote Origin of Species in 1859.
- Natural selection is the main force of evolution.
- Homologous organs point to divergent evolution.
- Whale hind-leg bones are vestigial.
- Gill slits give rise to the Eustachian tube.