evolution

noun

(L. evolutio: unfolding, unrolling) Darwin's definition: descent with modification. The term has been variously used and abused since Darwin to include everything from the origin of man to the origin of life. The process by which living organisms have developed from earlier ancestral forms. Evidence of evolution comes indirectly from studies of fossil records, comparative anatomy, biochemistry and physiology, and directly by various experiments observing the behaviour and survival of organisms in their natural environments. Various theories of evolution have been proposed, but the most widely accepted mechanism is natural selection ( See Darwinism). Darwin's explanation of evolution remains the central theme of current evolutionary thinking. It has now been assimilated with modern genetics to form what is referred to as neo-Darwinism, in which evolution may be defined as the change in gene frequency in a population over time. In speciation, new species arise when the genetic difference between populations that have been subjected to different patterns of natural selection is sufficiently different to prevent interbreeding between the populations. Two main additional lines of thinking have recently been applied to the evolutionary debate. The first of these seeks to explain the major breaks in the fossil record, such as that at the end of the Cretaceous period which signalled the end of the dinosaurs, by suggesting that long periods of stability have been interrupted by relatively short bursts of rapid evolutionary change, and is referred to as punctuated equilibrium. The second reinforces the basic idea of natural selection, and sees the evolutionary process as being driven by the accumulation of random mutations which produce small but significant changes in an organism's phenotype; this has been tested by mathematical and computer models. Evolution is a major unifying concept in virtually all areas of biology. See ethology.