i As a fact: Evolution is simply genetic changes occurring through time in a group of individuals (a population, a species, and so on). Scientists know that these changes occur. They can see the changes; measure them; and, in many instances, figure out when they happened.
i As a theory: Evolutionary theory seeks to explain what's responsible for the evolutionary process — in other words, what causes these changes. What scientists know today is that natural selection (Chapter 5) and genetic drift (Chapter 6) are two key forces driving these changes.
The second law of thermodynamics states that entropy — essentially, randomness — increases (or stays constant) in a closed, or isolated, system; it cannot decrease. In other words, left on their own, isolated systems become more uniform, not less. The differences smooth out until one common state exists. Think about a glass of ice water. After the ice goes in, the water gets a little colder, and then the ice melts: The entropy has increased in the glass. According to the second law of thermodynamics, the whole universe is doing the same thing: Increasing entropy is "smoothing out" the world. Rather than having hot regions and cold regions, for example, the world would have all its parts the same temperature.
So what does this law have to do with evolution? Diversification of life on Earth has involved very complex organisms evolving from simple forms that were present a few billion years ago — a fact that seems to fly in the face of the second law, because on Earth entropy is decreasing, not increasing. And there's the key. Earth is not a closed system. It gets loads of energy from the sun, and that energy is what powers the increase in complexity.
It's Been Proved Wrong (by Scientists!)
I love this one! This argument stems from the fact that in the hundred-plus years since Darwin's death, scientists have contributed to his original thoughts and refined their understanding of evolutionary events and principles. The spin you see in lots of articles, though, implies that a particular piece of research is at odds with what Darwin thought and, therefore, is proof that Darwin got things wrong.
The best example is the importance of random factors — genetic drift (see Chapter 6), which is one of the key insights modern evolutionary biologists have added to our current understanding of evolution. What scientists know today that they didn't know during Darwin's time is that random events, as well as natural selection, can be evolutionary forces; that random events can be evolutionarily important is an example of a major change in theory of evolution, but it doesn't negate in any way Darwin's theory of evolution by natural selection. It simply makes his ideas more broadly applicable.
It's Completely Random
How long would it take a million monkeys hammering away on a million typewriters to produce Moby-Dick? Who knows? (How long did it take one monkey hammering away on one typewriter to come up with the premise for "Who Wants to Marry a Millionaire?") The point? That a complex work — whether it's a Shakespearean sonnet or a book about evolution — can't possibly be the result of random processes.
The problem is that people who make this argument are confusing the fact that some of the evolutionary process of natural selection involves random events with the idea that the whole process is random. True, the mutations produced are random (that is, not directed toward a goal), but natural selection sorts through these mutations in a nonrandom fashion, selecting for those that increase fitness.
A major stumbling block that prevents many religious people from accepting what science has learned about the evolutionary process is the idea that evolution is connected to a random process, the one whereby DNA sequences are passed inexactly from one generation to the next — in other words, the process of mutation. Yet the very process of replicating the DNA is error prone. Scientists can measure the rate at which errors occur in DNA replication just as they can measure the rate of radioisotope decay, but whether an error occurs in one location or another is random.
The random aspect is unsettling for many people. Although we know that, given the amount of time available, the process of natural selection acting on randomly produced mutations is more than sufficient to generate our own species, this viewpoint is at odds with some people's view of humanity's place in God's universe. To reconcile the role of randomness with the religious beliefs that things happen for a reason or with purpose, some people suggest that nothing is truly random — that perhaps God set into motion the series of events that caused exactly the particular sequence of mutations that resulted in Homo sapiens. Maybe. But no way exists to scientifically measure whether God is or isn't directing these mutations. So these possibilities are outside the realm of science.
It Can't Result in Big Changes
According to this argument, some changes (namely, the small ones, a mutated nucleotide here or there) can be the result of the evolutionary process, but others (namely, the big ones) can't be. The key areas of dis-sention are
1 Speciation: The argument goes like this: Although evolution can lead to changes within a lineage, it can't lead to lineages splitting or spe-ciating. Au contraire. Gradual changes can lead to reproductive isolation (and the key characteristic differentiating one species from another is the inability to interbreed). The best examples for understanding specia-tion involve ring species, species where some but not all subpopulations can interbreed. The geographically adjacent populations are different enough from each other such that reproductive isolation occurs in some but not all cases. Moreover, we can select for the start of reproductive isolation in the laboratory. For more on when, how, and why speciation occurs, and for a more complete explanation of ring species, go to Chapter 8.
1 Evolution of new characteristics: Some folks insist that mutations can affect existing structures or traits but can't be responsible for new ones. Except that they can. As Chapter 15 explains, the process of gene duplication can result in multiple copies of a gene, and these copies can evolve along different trajectories. Changes in one copy that would have been deleterious in the absence of the other copy now are not deleterious (because you've got a spare copy) and are potentially advantageous. Through this process, the number of genes present in the organism can increase and diversify in function.
1 Big changes in physical characteristics: If I'm starting to sound like a broken record, it's because the same goes for big changes in body structures: Small changes can produce big results. See Chapter 14 for more details on the evolution of development.
In the period immediately after Darwin published On the Origin of Species, there was a lot of talk about missing links. If humans and apes were relatives, where was the fossil evidence? There wasn't a good answer back then, but fast forward to today and the answer is easy: in museums all over the world!
We've found a wealth of fossils, everything from recent relatives like Neanderthals, to more distant relations whose two-legged stance puts them clearly in our part of the tree of life but whose tiny brains suggest that we wonder about their lives a fair bit more than they probably did. Every few months a story appears in the news about some new fossil discovery. Modern paleontologists have gotten really good at finding these things! And this in spite of the fact that
^ Fossils generally are rare and hard to find. If, as scientists suspect, speciation occurs more often in small, isolated populations (refer to Chapter 8), finding such fossils would be much harder. But just because a certain fossil hasn't been found doesn't mean that it doesn't exist. Just that we need to keep looking — and look we do.
^ As scientists get better at fossil-hunting, they've found more and more fossils, some of which definitely qualify as transitional life forms.
Although scientists haven't found all missing links, they have found series of fossils that document transitions for many cases. Any of the following creatures discovered in the fossil record could be considered missing links, for example:
• The series of feathered dinosaurs leading up to flight Who knows what we'll see on tomorrow's news!
It Can't Account for Everything: Enter the Intelligent Designer
There are almost as many descriptions of intelligent design (ID) as there are proponents of the theory, and some even allow a limited role for evolution via natural selection. But all the versions of ID have one thing in common: the belief that some things in the biological world could not have come about without a "designer."
Proponents of ID argue that some structures (or systems, processes, or whatever) in the biological world clearly show that they were produced by an intelligent designer. This designer may be a divine entity but doesn't need to
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