As the preceding section explains, Darwin hypothesized that natural selection operating over a long period accumulates enough small changes in a population to make that population so different that it would be its own species, no longer able to interbreed with other populations of the species it had previously been a member of. Once again, Darwin turns out to have been right. Scientists have evidence that such small changes can have such large consequences over time.
What constitutes a species? The answer depends on the type of organisms you're talking about. Scientists have a reasonably good handle on what constitutes an animal species; determining what differentiates plant and micro-bial species (such as viruses and bacteria) is a bit slipperier. For animals, though, differentiating one species from another is fairly clear cut. A species is a group of organisms that can breed with one another but not with organisms in different species. In other words, reproductive isolation is the key to differentiating species.
Given the way evolution works (small changes over time produce enough changes to create a different species), researchers should be able to find all intermediate forms in nature. Ring species, species in which two populations of a particular species can't interbreed with each other (usually because of geographical distance) even though both can breed with other populations of that species, allow scientists to observe how the gradual changes can result in reproductive isolation. In addition to the ring species, scientists have numerous examples of intermediary species: those that have recently diverged and are very similar to one another yet unable to interbreed, and those that are in the process of diverging, in which case they've already differentiated to the point where reproduction is less successful or less common. You can find out more about these patterns in Chapter 8.
This book concerns itself with the evolution of organisms that are already present, not with the question — fascinating as it is — of where organisms came from in the first place. The question of how life arose on Earth really isn't a question for evolutionary biology. It's a question for chemistry, because in asking about the transition from nonliving to living systems, you must ask questions about the chemical environment that existed on Earth at the time when life appeared. Although no one has succeeded yet with an experiment that involves mixing a bunch of things in a beaker and waiting for something to crawl out, some very clever experiments have been conducted that show how complex biochemicals can arise spontaneously out of simple mixtures under conditions thought to be present on Earth more than 3 billion years ago.
Darwin imagined that such things might happen in a warm pond. Chemists Harold C. Urey (who won the Nobel Prize for discovering heavy water) and Stanley L. Miller actually made the pond. They combined water hydrogen, methane, and ammonia in a sterile glass system; heated the flask to produce a humid atmosphere; and then sent electrical shocks though the mixture to simulate lightning. They repeated this procedure for a week and then analyzed the contents of the flask. By using this simple procedure, they were able to produce DNA, RNA, amino acids, sugars, and lipids — all the building blocks of life from four very simple molecules.
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