Biological evolution and Darwin's reasoning

Evolution by natural selection, in the true sense, would have begun when cellular life forms with different metabolic abilities appeared.

The essence of Darwin's theory is natural selection. How fast new forms appear depends on the life cycle or life span.

Bacteria divide fast and become millions within hours. A colony A with built-in variation in using a feed component meets a changed medium; only the part B that can survive grows, outgrows the rest and appears as a new species within days. In fish or fowl the same would take millions of years.

Under the new conditions B is fitter than A, and nature picks the fitter. Because this fitness depends on characters that are inherited, selection and evolution need a genetic basis.

The ability to adapt is itself inherited. Fitness is what results when an organism can adapt and so gets selected.

Darwin's theory rests on two key ideas: branching descent and natural selection.

Before Darwin, the French naturalist Lamarck said evolution was driven by use and disuse of organs: giraffes stretching for high leaves lengthened their necks and passed on this acquired character. Nobody accepts this now.

Darwin may have been influenced by Thomas Malthus's work on populations. Natural selection rests on facts: resources are limited, population sizes stay stable apart from seasonal changes, members vary (no two are alike), and most variations are inherited.

Populations could grow exponentially if all reproduced to the maximum, yet real sizes are limited, so there is competition for resources. Darwin's insight: heritable variations that make resource use better let their bearers reproduce more, so over generations the population's characters change and new forms appear.