The word “evolution” in its broadest sense refers to change or growth that occurs in a particular order. Although this broad version of the term would include astronomical evolution and the evolution of computer design, this article focuses on the evolution of biological organisms. That use of the term dates back to the ancient Greeks, but today the word is more often used to refer to Darwin’s theory of evolution by natural selection. This theory is sometimes crudely referred to as the theory of “survival of the fittest.” It was proposed by Charles Darwin in On the Origin of Species in 1859 and, independently, by Alfred Wallace in 1858—although Wallace, unlike Darwin, said the human soul is not the product of evolution.
Greek and medieval references to “evolution” use it as a descriptive term for a state of nature, in which everything in nature has a certain order or purpose. This is a teleological view of nature. For example, Aristotle classified all living organisms hierarchically in his great scala naturae or Great Chain of Being, with plants at the bottom, moving through lesser animals, and on to humans at the pinnacle of creation, each becoming progressively more perfect in form. It was the medieval philosophers, such as Augustine, who began to incorporate teleological views of nature with religion: God is the designer of all creatures, and everything has a purpose and a place as ordained by Him.
In current times, to some, the terms “evolution” and “God” may look like unlikely bed fellows (see the discussion on teleology). This is due primarily to today’s rejection by biologists of a teleological view of evolution in favor of a more mechanistic one. The process of rejection is commonly considered to have begun with Descartes and to have culminated in Darwin’s theory of evolution by natural selection.
Fundamental to natural selection is the idea of change by common descent. This implies that all living organisms are related to each other; for any two species, if we look back far enough we will find that they are descended from a common ancestor. This is a radically different view than Aristotle’s Great Chain of Being, in which each species is formed individually with its own purpose and place in nature and where no species evolves into a new species. Evolution by natural selection is a purely mechanistic theory of change that does not appeal to any sense of purpose or a designer. There is no foresight or purpose in nature, and there is no implication that one species is more perfect than another. There is only change driven by selection pressures from the environment. Although the modern theory of biological evolution by natural selection is well accepted among professional biologists, there is still controversy about whether natural selection selects for fit genes or fit organisms or fit species.
Evolution by natural selection is a theory about the process of change. Although Darwin’s original theory did not specify that genes account for an organism’s heritable traits, that is now universally accepted among modern evolutionists. In a given population, natural selection occurs when genetically-based traits that promote survival in one’s environment are passed onto future generations and become more frequent in later generations. Organisms develop different survival and reproduction enhancing traits in response to their different environments (with abundance or shortage of food, presence or absence of predators, and so forth) and, given enough time and environmental changes, these small changes can accumulate to form a whole new species. Thus for Darwin there is no sharp distinction between a new variation and a new species. This theory accounts for the diversity of Earth’s organisms better than theological design theories or competing scientific theories such as Lamarck’s theory that an organism can pass on to its offspring characteristics that it acquired during its lifetime.
Evolution by natural selection works on three principles: variation (within a given generation there will be variation in traits, some that aid survival and reproduction and some that don’t, and some that have a genetic basis and some that don’t); competition (there will be limited resources that individuals must compete for, and traits that aid survival and reproduction will help in competition); and heritability (only traits that aid survival and reproduction and have a genetic basis can passed onto future generations).