It was Darwin who first suggested that all life on Earth had a single origin, and from that beginning "endless forms most beautiful and most wonderful have been, and are being, evolved". Evidence from molecular biology in recent years has supported the idea that all life is related by common descent.
Strong evidence for common descent comes from vestigial structures. The useless wings of flightless beetles are sealed under fused wing covers. This can be simply explained by their descent from ancestral beetles which had wings that worked.
Rudimentary body parts, those that are smaller and simpler in structure than corresponding parts in ancestral species, are called vestigial organs. Those organs are functional in the ancestral species but are now either nonfunctional or re-adapted to a new function. Examples are the pelvic girdles of whales, halteres (hind wings) of flies, wings of flightless birds, and the leaves of some xerophytes ("e.g." cactus) and parasitic plants ("e.g." dodder).
However, vestigial structures may have their original function replaced with another. For example, the halteres in flies help balance the insect while in flight, and the wings of ostriches are used in mating rituals, and in aggressive display. The ear ossicles in mammals are former bones of the lower jaw.
In 1893, Robert Wiedersheim published a book on human anatomy and its relevance to man's evolutionary history. This book contained a list of 86 human organs that he considered vestigial. This list included examples such as the appendix and the 3rd molar teeth (wisdom teeth).
The strong grip of a baby is another example. It is a vestigial reflex, a remnant of the past when pre-human babies clung to their mothers' hair as the mothers swung through the trees. This is borne out by the babies' feet, which curl up when it is sitting down (primate babies grip with the feet as well). All primates except modern man have thick body hair to which an infant can cling, unlike modern humans. The grasp reflex allows the mother to escape danger by climbing a tree using both hands and feet.
Vestigial organs often have some selection against them. The original organs took resources, sometimes huge resources. If they no longer have a function, reducing their size improves fitness. And there is direct evidence of selection. Some cave crustacea reproduce more successfully with smaller eyes than do those with larger eyes. This may be because the nervous tissue dealing with sight now becomes available to handle other sensory input.
From the eighteenth century it was known that embryos of different species were much more similar than the adults. In particular, some parts of embryos reflect their evolutionary past. For example, the embryos of land vertebrates develop gill slits like fish embryos. Of course, this is only a temporary stage, which gives rise to many structures in the neck of reptiles, birds and mammals. The proto-gill slits are part of a complicated system of development: that is why they persisted.
Another example are the embryonic teeth of baleen whales. They are later lost. The baleen filter is developed from different tissue, called keratin. Early fossil baleen whales did actually have teeth as well as the baleen.
A good example is the barnacle. It took many centuries before natural historians discovered that barnacles were crustacea. Their adults look so unlike other crustacea, but their larvae are very similar to those of other crustacea.
Charles Darwin lived in a world where animal husbandry and domesticated crops were vitally important. In both cases farmers selected for breeding individuals with special properties, and prevented the breeding of individuals with less desirable characteristics. The eighteenth and early nineteenth century saw a growth in scientific agriculture, and artificial breeding was part of this.
Darwin discussed artificial selection as a model for natural selection in the 1859 first edition of his work "On the Origin of Species", in Chapter IV: Natural selection:
Nikolai Vavilov showed that rye, originally a weed, came to be a crop plant by unintentional selection. Rye is a tougher plant than wheat: it survives in harsher conditions. Having become a crop like the wheat, rye was able to become a crop plant in harsh areas, such as hills and mountains.
There is no real difference in the genetic processes underlying artificial and natural selection, and the concept of artificial selection was used by Charles Darwin as an illustration of the wider process of natural selection. There are practical differences. Experimental studies of artificial selection show that "the rate of evolution in selection experiments is at least two orders of magnitude (that is 100 times) greater than any rate seen in nature or the fossil record".
Some have thought that artificial selection could not produce new species. It now seems that it can.
New species have been created by domesticated animal husbandry, but the details are not known or not clear. For example, domestic sheep were created by hybridisation, and no longer produce viable offspring with "Ovis orientalis", one species from which they are descended. Domestic cattle, on the other hand, can be considered the same species as several varieties of wild ox, gaur, yak, etc., as they readily produce fertile offspring with them.
The best-documented new species came from laboratory experiments in the late 1980s. William Rice and G.W. Salt bred fruit flies, "Drosophila melanogaster," using a maze with three different choices of habitat such as light/dark and wet/dry. Each generation was put into the maze, and the groups of flies that came out of two of the eight exits were set apart to breed with each other in their respective groups.
After thirty-five generations, the two groups and their offspring were isolated reproductively because of their strong habitat preferences: they mated only within the areas they preferred, and so did not mate with flies that preferred the other areas.
Diane Dodd was also able to show how reproductive isolation can develop from mating preferences in "Drosophila pseudoobscura" fruit flies after only eight generations using different food types, starch and maltose.
Dodd's experiment has been easy for others to repeat. It has also been done with other fruit flies and foods.
Some biologists say that evolution has happened when a trait that is caused by genetics becomes more or less common in a group of organisms. Others call it evolution when new species appear.
Changes can happen quickly in the smaller, simpler organisms. For example, many bacteria that cause disease can no longer be killed with some of the antibiotic medicines. These medicines have only been in use about eighty years, and at first worked extremely well. The bacteria have evolved so that they are no longer affected by antibiotics anymore. The drugs killed off all the bacteria except a few which had some resistance. These few resistant bacteria produced the next generation.
The Colorado beetle is famous for its ability to resist pesticides. Over the last 50 years it has become resistant to 52 chemical compounds used in insecticides, including cyanide. This is natural selection speeded up by the artificial conditions. However, not every population is resistant to every chemical. The populations only become resistant to chemicals used in their area.
Although there were a number of natural historians in the 18th century who had some idea of evolution, the first well-formed ideas came in the 19th century. Three biologists are most important.