Darwin and natural selection
Darwin’s contribution, biological evolution by natural selection, and adaptive radiation.
The essentials
Charles Darwin made a sea voyage around the world on the ship H.M.S. Beagle and observed that existing living forms share similarities, to different degrees, with each other and with forms that lived millions of years ago, many of which are extinct. He concluded that there has been gradual evolution of life forms. Alfred Wallace, a naturalist who worked in the Malay Archipelago, reached similar conclusions independently.
Darwin's theory rests on two key concepts: branching descent and natural selection. Populations show variation in characteristics; some variations make individuals better able to survive and reproduce in their environment. Darwin used the term fitness to mean reproductive fitness: those better fit leave more progeny than others. Over generations, the better-fit variants are "selected by nature". Since resources are limited, there is a struggle for existence. Over many generations, such changes accumulate and new types of organisms appear.
Adaptive radiation is the evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats). Darwin observed small black birds, later called Darwin's finches, on the Galapagos Islands. From the original seed-eating ancestor, forms with altered beaks arose, enabling some to become insectivorous and others vegetarian. Australian marsupials also radiated from a common ancestral stock into many forms (such as the kangaroo, koala, wombat and Tasmanian wolf). When more than one adaptive radiation occurs in an isolated area, representing different habitats, it may be called convergent evolution; placental mammals in Australia show similarity with corresponding marsupials (for example, the placental wolf and the Tasmanian wolf).
Key points
- Lamarck had proposed that use and disuse of organs drives evolution (e.g. the giraffe's neck), an idea not supported today.
- Natural selection acts on heritable variation; acquired characters are not inherited.
- Darwin's variations were small and directional; de Vries proposed large sudden mutations.
Exam tips
Remember that "fitness" in exam answers means reproductive success, not physical strength.