According to your mainstream brand neo-Darwinian biologist, natural selection is the stuff of which evolution is made, the First Principle of life. There is nothing in the natural world which cannot be explained by random mutations within the genome and subsequent selection of the fittest form by the natural environment. Beyond the constraints set by the period of waiting for mutations to occur and external conditions, there are no limits to this system, and an organism forms from scratch to a furry crawling thing in a gradual process reliant on external factors. Neo-Darwinism is an attempt to reconcile two theories which are quite simply at odds with one another: Mendelian genetics, which claims that organisms do not change with time, and Darwinism, which claims that they do. This is usually done in a mathematical way, with natural selection as the linchpin of some tight equations. There can be no internal feedback from the body (phenotype) to the genes (genotype). There is no self-organising adaptive order: all emerges from the process of selection as a carefully articulated tree diagram, and adapts over eons.