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So, while it's plain that evolution seems logical, it doesn't necessarily mean it's true. What matters more than logic is evidence. Is evolution supported by evidence? Evolution is one of the most well-supported theories accepted by modern science. More importantly, it makes predictions which are later shown to be true. If a theory can predict, that's powerful evidence that it's true.

    Let's look at some of the best evidence for evolution, and also look at some of the predictions that evolution makes.

    First, we have tens of thousands of fossils that clearly show bushy trees of descent, with one type of organism evolving into others (or several others) over time.  The fossils are found in the earth in reverse of the order in which they evolved, with the fossils of the species at the bases of their bushy family trees found deeper in the rock than fossils of the organisms that evolved from them. Dating of different layers of rocks with radioisotope dating unquestionably supports the idea that deeper rocks are older rocks, and that deeper fossils are older fossils.

    The information found in fossils and rock layers also makes predictions. In 2006, a team of researchers from the University of Chicago began looking for a fossil of an animal part-way between a fish and an amphibian. They searched rock layers known to have formed in areas of fresh water between 380 and 363 million years ago, when fish first started to evolve into amphibians. There, they discovered a fossil of a transitional species midway between fish and amphibians. This creature, which they named the Tiktaalik, is a world-class example of an animal acted on by selection pressures to evolve between two very different forms. Evolutionary theory perfectly predicted where, when, and what the scientists would find. Evolutionary theory frequently makes predictions like this.

    For centuries, scientists relied mostly on fossils as support for evolution. Fossils are useful, but not as accurate as working with molecules or living organisms. With the development of modern techniques, we can go beyond fossils for evolutionary support.

    Genes control the chemical makeup of things called proteins. Proteins are tiny globs composed of strings of compounds called amino acids. Genes code for the order of the amino acids in the strings, and the order of the amino acids makes the strings tangle together in a very specific way, making a very specific shape. The shape of the protein is what makes it useful. For example, a protein might have a shape that can hold two molecules together until they react. Over time, mutations in the genes cause changes in the amino acid sequences. In most proteins, most of the amino acids don't affect the shape. Changing them doesn't change the protein, and so evolution neither works against these mutations, nor helps them along. What you would expect, then, is for more closely-related organisms to have few differences between their amino acid sequences due to mutations, but for more distantly-related organisms to have lots of mutation differences in their amino acid sequences. The more time that's passed, after all, the more time there's been to have these mutations.

    This is exactly what you find – again, evolution makes a successful prediction. The cytochrome c protein causes an important step in the breakdown of food for energy. Almost any organism that breaks down food has a cytochrome c protein. Only about a third of the amino acids in cytochrome c keep it in its specific shape. The other two thirds can mutate fairly freely. Chimpanzees and humans, which evolutionary theory and fossils predict to be very closely related, have identical cytochrome c proteins. No visible mutations have happened since we split off from a common ancestor, because not enough time has passed. Humans and yeast, though, which are extraordinarily different creatures (yeast isn't even an animal) have over 50 differences between their cytochrome c amino acid sequences. All species of organisms show this pattern with mutations; the more distantly related, the more mutations you have. The molecular evidence is exactly what evolutionary theory predicts.

    So evolution can be conclusively shown to have happened. But it would be a lot more convincing if it could be shown to still be happening now. As it so happens, there are thousands of experiments showing this. Let's look at some of the latest and best.

    In 2006, a team of researchers from Harvard University introduced a lizard-eating species of predator onto a small Caribbean island. Understandably, the size of the population of the only species of local lizard dropped by half. Scientists found that over the first six months of the study, the surviving lizards had, on average, much longer legs. Then, in the next six months, average leg length radically decreased far beyond the average length before the predator was introduced. The evolution of the lizards stopped on a dime, reversed direction, and went even faster than before.

    This data shows some very interesting things. When the predator was first introduced, the lizards with shorter legs, who can't run on the ground as fast, were eaten more than those with longer legs. During this time, the lizards were reproducing, making a new generation of longer-legged lizards. Then, they started moving into the trees. Shorter legs and a lower center of gravity are a real help in trees, so the shorter-legged lizards severely outcompeted the longer-legged ones. While that was going on, this generation was breeding, making a new shorter-legged generation. This is a major, macroevolutionary change in a population in only a year, or (for these lizards) two generations. This experiment shows that serious evolution can occur in a very short time.

    Even bigger changes can be seen in bacteria, because they have such short generation times, and so can evolve in very little time. Since 1988, a professor at Michigan State University has been tracking twelve populations of the intestinal bacteria E. coli. He has kept them in containers with very little of the sugar glucose, but a great deal of the simple chemical citrate. One of the important characteristics of E. coli is that it is very good at eating glucose, but cannot eat citrate. So, there's a strong selection pressure towards something that will allow the E. coli bacteria to eat citrate, since any citrate-eating bacteria would flourish. Every five hundred generations, the researcher froze samples of the bacteria, so that he could go back later and track any changes. Though it took tens of thousands of generations, the E. coli bacteria eventually developed the ability to eat citrate. This took three independent mutations, which evolved at three different times. None of the mutations do very much on their own, but when working together, they enable the E. coli to thrive and do something it's never been able to do before. The new strain of E. coli proceeded to outcompete the rest of the strains. Because of the freezing, the researcher could actually track the evolution of the E. coli over time. This is, indisputably, mutations at random causing nonrandom reproduction.

    In London, the most common species of mosquito is known as Culex pipiens. In 1863, the world's first underground railway opened in London. It was the start of the now-famous London Underground. In 1940, the Germans began bombing London, and many Londoners took shelter in the railway tunnels, where they would be safe. While they slept, they were bitten by mosquitoes. Upon analysis, it was discovered that these mosquitoes were a new species. It seems as though some common Culex pipiens mosquitoes entered the Underground, which has a very different set of selection pressures from aboveground London, and started evolving. Between 1863 and 1940, in less than one hundred years, the pipiens mosquito evolved into a new species, called Culex molestus. The two species have different genes, do not tend to breed with one another, and have very different behaviors. While the pipiens mosquito bites birds in the night skies over London, molestus prefers mammals – mainly the rats that infest the tunnels and the humans that infest the stations. This is definitive proof of one species of organism evolving into another, and in at most 77 years.

    These three examples, using real organisms, show definitively that evolution not only happens, but is still happening to this day.  The best part is, they're repeatable. You could probably do the bacteria experiment in your garage. If you're in London, grab a mosquito in the Underground and one outside your flat; they even look different. You might have a little more trouble reproducing the lizard experiment, unless you happen to have a few Caribbean islands handy, but the folks who did the experiment are already planning to repeat it.

    All of these examples you've seen here are not only conclusive evidence for evolution, but they're just a small sample of the total data that's out there. Evolution is a fact, and this has been proven time and time again, to the point where it's possibly the best-supported scientific theory in history.

Previous page – Explanation: what evolution is and how it works
Next and final page – Arguments against evolution

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