In the mid-19th century, two English naturalists, Charles Darwin (1809-82) and Alfred Russel Wallace (1823-1913) came up with some new ideas about the relationships among species, known as the theory of evolution. Darwin's book On the Origin of Species (1859) expressed the central idea that species change over long periods of time as a result of slight changes that distinguish an individual organism from others of its species. These changes were known as mutations and are caused by random variations in an organism's DNA. These variations, or mutations, can be transmitted to an organism's offspring.
Evolution consists of two major processes. The first is characterized by a gradual change in a single line of descent. For example, mosquitoes develop a resistance to pesticides over a period of around ten generations (less than two weeks). Eventually, the offspring of the mosquito population are not affected at all by the pesticide. They have gone from organisms sensitive to the chemicals into organisms resistant to it.
Another major evolutionary process is called speciation. This transforms one kind of organism into two or more new kinds of organisms. Back in the Triassic period (about 200 million years ago) a group of reptiles began to undergo slight transformations. Their normal reptilian cheek teeth changed into a set of complex molars and premolars, their flat scales became hairs, and the evolving animals began to care for their young and produce milk to feed them. Though these changes occurred neither simultaneously nor quickly, these reptiles were on their way to becoming mammals. Occasionally, speciation takes place in "bursts," when more evolution seems to occur than at other times. However, in most cases, the process takes tens of thousands or even millions of years.
Darwin also developed the principal of natural selection, in which organisms are constantly struggling to survive. Only the strongest and fittest will live. Darwin found that variations that help an organism to survive are more likely to be passed on to later generations than are variations that hurt the organism or have no effect on it. Take for instance the giraffe. It is widely believed that a genetic mutation that made some giraffes' necks longer enabled them to forage for food in higher branches where other animals could not reach. The animals with this advantage could feed better and live longer, and produce more offspring. Eventually, after many generations, the long-necked giraffes outnumbered the others and replaced them altogether. What was so amazing about Darwin's idea was that a species can change, or evolve, into a new species. This suggested that all species are related to one another by descent from the same ancestral life-forms, which had evolved into all the different species that exist today.
So what makes species begin to evolve? Sometimes their geographic location and surroundings stimulate change. Sometimes, a group of the same animals will find themselves suddenly split apart by geographic conditions. These populations will respond differently to their individual environments, and adapt through natural selection. These populations are now genetically different from each other, and these distinctions are reflected in characteristics such as structure or behavior. Taxonomists call varying groups of populations within a species geographic subspecies.
A famous example of geographic speciation is that of Darwin's finches on the Galapagos Islands. Within just this small area, ornithologists have isolated fourteen different species of the finch! What's important about this finding is that populations of a single, sparrow-like ancestor were separated from each other by geological boundaries. Each distinct population then evolved into an entirely new subspecies by adjusting to their particular environment. These birds are structurally similar, but each varies a little in shape, color, and size. In the words of Charles Darwin himself, " . . . seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends."
The genetic differences that appear through the centuries are very, very important. The more genetic diversity there is within and between populations of a particular species, the greater the chances that the species will survive if its environment changes. Pretend for just a moment that there was a far-reaching, rapid cooling of the climate. Populations that live in the north are already suited to the more rigorous weather conditions, could move south, and replace the previous southern populations that couldn't adapt to the weather changes. Or perhaps a hardier southern population would survive and take over. In the end, genetic variability is the critical resource of natural populations and all species. If living creatures are to survive, no single population can be lost, or else a piece of genetic diversity will be lost with them.