A short summary, before going into the subject in a little more depth: The word 'evolution', in the context of biology, can mean one of two things: 1. The observed biolo…gical phenomenon of evolution, meaning changes in the frequencies of inherited traits in population gene pools between generations. This means that if you follow a single population, and sample the frequencies of inherited traits within that population at some interval exceeding the lifespan of a generation, you'll get different frequencies. The differences may include some new traits, or traits that have vanished entirely. 2. The word can also be used to refer to the theoretical model that tries to explain observations in biology and palaeontology in terms of what we know of the above phenomenon of evolution. The theoretical model is largely based on genetics and population dynamics. The observations it explains include: the matching nested hierarchies of genetics and zoology, the sequences of progressing traits we find in the fossil record, atavisms, the many transitional fossils found in the fossil record, and so on. There have been several theories to explain evolution. Some historical background: Naturalists had observed that there are a lot of species; there are over 300,000 species of beetles alone. There are also "related" species; for example, the Indian and African elephants are different species but are clearly related. The huge diversity of species could be explained if new species could derive from existing species. In the 18th century, a Swedish zoologist called Carl Linnaeus made an interesting observation. He observed that his attempts to classify types of animals by their traits invariably resulted in a pattern of nested hierarchies: sets, within sets, within sets, each set having the traits of the superset as well as its own unique traits. He was forced to conclude that he could not produce a consistent and coherent classification scheme in which the human species was not part of the taxon of apes. It is interesting to notice that, although he made this pivotal discovery in the history of evolutionary thought, Linnaeus was basically a creationist: he believed in special creation of life's species. Nevertheless, over the course of the following decades, it became clear that life did not always have the shapes it has now, and that this fact, as well as Linneaus' nested hierarchies, required some explanation. One of the earliest theories was by Larmark; and is, not unsurprisingly, called Larmarkian evolution. This was that offspring inherited developed characteristics; his example was that the giraffe's neck was extended by reaching for the highest leaves and that the offspring were born with longer necks because of that. Although this theory was largely discarded as no-one could point to a biological mechanism for offspring to inherit acquired characteristics, it has fairly recently come back as genes can be modified during the lifetime of a species and affect the offspring. Charles Darwin and Alfred Russel Wallace independently developed evolution by natural selection. Their theory had three key assumptions: .
Offspring inherit characteristics from their parents. .
More offspring are born than actually survive to breed. .
The offspring that survive are those which are better suited to their environment (Darwin used the expression the "fittest", meaning the best fit). From these three assumptions, it's easy to see that the offspring of offspring will inherit characteristics that make them more suited to the environment. Going back to elephants; the theory of natural selection would suggest that African and Indian elephants have a common ancestor that spread across Africa and India and evolved to match the different environments. Since Darwin and Wallace developed the hypothesis, it has been extended by careful examination of the fossil record and by the discovery of genetic theory. Before the theory of evolution we had no idea why we existed. So we came up with bogus answers like creationism. Now that we have a scientific theory based on empirical evidence that has stood up to very rigorous scrutiny, we no longer have to say "God did it". The theory of evolution explains facts about evolution. Just like the theory of gravitation explains facts about gravity. This can be said of all theories including kinetic theory, atomic theory, friction theory, electromagnetic theory, quantum theory, etc. Basically the theory of evolution states that organisms evolve because mutation occurs every time organisms reproduce, and the individuals with the best genes that suit the environment will survive. So mutation is what makes evolution possible by creating variation among individuals, and natural selection is the entity that picks who gets to survive. For example, my brother and I are different in many ways. If height was important to survival, my brother would have a better chance to have sex and reproducing while I die. If looks were important to survival and making babies then I would survive and he would die. But in modern society short people make babies, tall people make babies, pretty people make babies and ugly people make babies. Also smart people make babies and idiots make babies. So everybody makes babies and there is little to no natural selection which is the force that drives evolution. In other words, mutation occurs to people, but there is little to no natural selection now. That is why people don't evolve. But some disagree with this evolution. We call them creationists. Evolution is the process of the change in subsequent generations with different characteristics than that of ancestors, brought about by survival of the fittest, which means that organisms, either human or animal, that have superior characteristics that allow them to thrive and survive and procreate will continue their lives with probably some new idiosyncrasies becoming a part of them, or another description of evolution. Evolving entails developing and retaining certain new that enable the individual to maintain some superior qualities that will allow it to . ( Full Answer )