(1) Heredity: Darwin knew that characteristics tend to be inherited but he did not know the laws of genetics. These laws were not discovered until 1865 by Gregor Mendel. And because Mendel published his discovery in an obscure journal his findings were ignored until 1900 when they were rediscovered by other people.
(2) Variation: This simply means that two individuals of the same species are never exactly alike except in the case of identical twins. There is always variation. Any characteristic that you can measure, whether it be blood pressure in alligators or the length of puppy tails, will follow the classic bell-shaped probability distribution, where the farther away you move from the average, the less likely that value will occur.
(3) Excess Replication: Every species of plant or animal will reproduce excessively, and only a small percentage of offspring will live to pass on their genes to the next generation. The others must perish. Darwin was familiar with the work of Thomas Malthus who pointed out that an unchecked population will grow geometrically. Geometric growth is what we now call exponential.
Thomas Malthus was a political economist and it is because of him that economics is called the dismal science. He predicted starvation for England in the nineteenth century. This did not occur because of the industrial revolution. Of course, today England cannot feed itself, instead it gets by with food imports, which works alright for England but if every country tries to be a food-importing country, you can understand there is going to be problems. Today there is large-scale hunger in the world but I think the worst consequence of the population explosion is the enormous and irreversible environmental damage to the planet.
(4) Natural Selection: This was Darwin's great contribution. And I would say that unless you understand natural selection you are not an educated person, no matter what else you know.
Because of excess replication, most plants and animals are doomed to perish without producing progeny. And there is a difference between the individuals that survive and those that do not survive. The ones that survive may just be lucky, but the probability is that they are more fit to survive in some way. And that's natural selection: nature is constantly weeding out those individuals who are less fit for survival. A classic example is the giraffe; why is it so tall ?
Tallness is an obvious advantage in reaching leaves higher up on a tree. And so over many generations the taller animals had a better chance to survive and were therefore selected by nature to pass on their tallness genes to future generations.
So you can see that given the above 4 elements, evolution must inevitably occur because nature is constantly doing just what breeders do when they selectively breed a desired characteristic into a plant or animal. As Thomas Huxley said, "How extremely stupid not to have thought of it." Like many great ideas, it seems almost obvious after someone points it out.
But it's worth noting that it took 3000 years of civilization before someone did point it out.
Darwin's work is sometimes called the Darwinian Revolution because it revolutionized
human thinking more than any other scientific discovery. Understanding Darwinism gives
you an understanding of nature and also an understanding of man who is a part of nature.
Darwin also contributed the idea of sexual selection. This was introduced in Origin of Species and amplified in a later book, The Descent of Man and Selection in Relation to Sex, published in 1871.
Sexual selection is an important evolutionary concept because it explains so many things that cannot be explained with natural selection. Here, the selecting element is success in mating.
Obviously, mere survival does not insure that your genes are going to be transmitted to the next generation. You must also be successful at mating. Sexual selection explains such phenomena as the peacock's tail, which certainly has no survival value, quite the contrary, but it is important to attracting a peahen. And so over many generations peacocks with the more attractive tails were more successful at mating, and peacock tails became more and more elaborate. It's worth noting that in bird species where the female does the selecting, it is the male that is colorful, whereas in the human species where the male has done the selecting (this may not be true currently), it is the female that is the "fair sex."
Another way sexual selection works is through combat between rivals. With regard to mammals Darwin says, "The male appears to win the female much more through
the law of battle than through the display of his charms. This explains why the males are often larger and equipped with special weapons such as antlers, horns, or tusks, which are absent or diminished in females." This tendency of hostility toward a rival appears in human consciousness in the form of what we call jealousy.
Another example of sexual selection in humans is skin color. People come in different colors. The human race originated in Africa and I'm sure that at the beginning everyone was about the same color which was probably quite dark. But now we have fair-skinned people who are deficient in melanin. There is no survival value to the loss of this pigmentation. In fact, quite the opposite, fair skinned people are susceptible to ultra-violet skin damage which causes sunburn and skin cancer. But in Europe especially, people have fair skins because of sexual selection which has discriminated against darker skins over the millennia. (This analysis agrees with Darwin's view but is contrary to the current convention. However, I believe that Darwin and not current thinking is correct for reasons that I will not go into here.)
Much has been added to organic evolution since Darwin's day. For instance, the importance of mutations. These have a number of causes, one such is nuclear radiation. Mutations can produce sudden changes in only one generation. Of course 99% of mutations are unfavourable, but every ten thousand years or so a mutation will occur which is actually beneficial and the mutant will have a better chance of survival and pass on the favourable change to its offspring.
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