Biological Evolution at a Glance

Evolution can be defined simply as change through time, and it can refer to anything that changes. Languages evolve; tastes evolve; cultures, art forms, and football offense strategy all evolve. This book isn't about evolution in general, though, but about biological evolution: the changes, over time, in organisms.

Biological evolution deals with a very specific type of change through time — changes in the frequencies of different genes — throughout an entire species, or within a single population of that species, from generation to generation. Evolutionary biologists — scientists who study evolution — just love that stuff. Their mission? To understand how evolution works (by figuring out what causes changes in gene frequencies) and what evolution does (by figuring out what sorts of things happen when gene frequencies change).

The following sections offer a general overview of how evolution works and what it does. Parts II and III delve into these topics in a great deal more detail.

Gene defined

Back in Charles Darwin's day, a gene was defined simply as the unit of heredity. People knew that specific traits, such as blue eyes or red hair, were passed from parent to child, but they didn't know exactly what a gene was or how the process worked. Today, we know a lot more:

¡^ We know about DNA (deoxyribonucleic acid), which is what gets passed from parent to offspring.

¡^ We know that DNA is a long molecule made up of a string of four subunits (four letters); that the order of these letters, commonly called the DNA sequence, stores genetic information; and that a gene is a particular sequence of a particular piece of an organism's DNA.

¡^ We've developed the chemical techniques that allow researchers to determine the exact sequence of an organism's DNA. As a result of this ability to work with DNA, scientists have a much better handle on the details of the evolutionary process.

What this means — and why it's important enough to include here — is that by being able to identify the DNA sequence of a particular gene, scientists can measure exactly what genetic changes occur across generations. Being able to measure things, especially things like DNA strands, gets evolutionary biologists all goose-pimply. (For more information about genes and DNA, head to Chapter 3.)

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What's the (gene) frequency, Kenneth?

Simply put, the frequency of a particular gene is how often it appears in a population. When researchers examine the DNA sequence at a particular location in a species' DNA in different individuals, they sometimes find that all the individuals have the same sequence. In this case, because only one gene (or one DNA sequence) exists at this location, its frequency is 100 percent. At other times, different sequences are present in different individuals. In this case, when more than one gene is present at this location, scientists speak of the frequencies of the different genes.

Suppose that you've discovered three different DNA sequences; call them genes A, B, and, C. If half the individuals you examine have gene A, one quarter have gene B, and one quarter have gene C, the frequencies of the three genes are 50 percent gene A, 25 percent gene B, and 25 percent gene C.

By identifying changes in the frequency of particular genes through the passing of generations, you can determine whether the organism has evolved. Using the example of genes A, B, and C from the preceding section, if you came back generations later to measure the frequency of these three genes again, and you found that the frequencies had changed, evolution has happened.

Here's an example: Suppose that you collect a bunch of a particular kind of bacteria and measure the frequency of the gene that makes the bacteria resistant to a new type of antibiotic. In your initial count, you find that the frequency of this gene is extremely low: Less than 1 percent of the bacteria have the gene that makes them antibiotic resistant. You come back in a few years. Your original bacteria are gone, but in their place are their great-great-great-great-etcetera grandkids, and you repeat the analysis. This time, you find that 30 percent of the bacteria have the antibiotic-resistant gene. Although you haven't actually witnessed evolution, you're looking at its result: the change in the frequency of particular genes over time. The antibiotic-resistant gene appeared in less than 1 percent of the original bacteria; it appears in 30 percent of the descendents. (Go to Chapter 17 for an in-depth discussion of the evolution of antibiotic resistance in bacteria.)

In a nutshell, biological evolution is simply a change in the frequency of one or more genes through time. Scientists collect this sort of data about the occurrence of evolution all the time — not only for bacteria, but also for all sorts of organisms, both simple and complex.

The timescales of evolution

Although the changes in gene frequencies happen gradually through time, the rate of evolution isn't constant. Gene frequencies can remain constant for long periods of time and then change in response to changes in the environment. The rate of change can increase or decrease, but the basic process — gene frequencies changing over time — continues. To differentiate between these time scales of the evolutionary process, scientists use the terms microevolution and macroevolution:

t Microevolution refers to the results of the evolutionary process over short time scales and small changes. An example is a bacterium in a laboratory beaker experiencing a mutation that creates a gene that confers higher growth and division rates relative to the other bacteria and beaker. Microevolution, because it happens on a time scale that we're able to observe, tends to be a bit easier for us to wrap our brains around than macroevolution.

t Macroevolution refers to the results of the evolutionary process typically among species (or above the species level; see Chapter 11) over long periods. Nothing is different about the process; nothing special is happening. Macroevolution simply refers to the larger changes researchers can observe when evolution has been going on for a longer time and involves processes such as extinction, which may have little to do with microevolution. Speciation, the process whereby one species gives rise to two, is an example of macroevolution. Speciation isn't all that complicated, and scientists are getting a pretty good idea about how it works; you can find out more in Chapter 8.

Other than the time frame, no difference exists between micro- and macroevolution. The process isn't any different from what scientists can observe in a test tube in the laboratory (an example of microevolution); there's just been a lot more of it.

Gene extremes: Mutation and extinction

Genes can go to extremes. At one extreme is the disappearing gene. Suppose that you measure the frequency of the three different genes at a particular site in a species' DNA and then return some years later to find that one of the genes is no longer present. That gene's frequency has dropped to zero. It's gone. It's extinct. When a gene goes extinct, the species that had the gene is still around, but at least at this particular location in its DNA, it's not as diverse.

At the other extreme, new genes can appear. The process by which the sequence of a parent's DNA is copied and passed on to the next generation is remarkably accurate. If it weren't, none of us would be here. But no process is perfect, and mistakes happen. These mistakes are called mutations, and they can result in a DNA sequence different from the original — in other words, a new, different gene. These new genes can affect the functioning of the organism in several ways:

i They can have no effect at all. Because there's a certain amount of redundancy in the code of the DNA sequence (go to Chapter 3 for the details), it's possible to change a letter here and there with no effect whatsoever. Even if the mutation does create a change, that change may not affect how the gene product functions. In both cases, the new genes don't have an impact — either positive or negative — on whether an organism survives.

i They can result in a change that's harmful to the organism. Most mutations that cause a change fall into this category. Even the simplest organisms are really quite complicated. If you change something randomly, most often the outcome is bad. Genes of this sort vanish as rapidly as they appear.

Occasionally, bad mutations — which typically are destined for a short run before becoming extinct — actually increase in frequency. Here's how it could happen: If a gene with negative effects is present in the same critter as a gene with positive effects, the frequency of the bad gene can increase as it rides the evolutionary coattails of the really great new gene. Suppose that two mutations occur simultaneously in different locations on an organism's DNA: one resulting in a gene that is slightly harmful and another resulting in a gene that is advantageous. The slightly harmful gene may increase in frequency simply because it's along for the ride.

i They can result in a change that's advantageous to the organism. This class of mutations is by far the rarest, but beneficial mutations do occur. These mutations, although rare, can increase in frequency. Ultimately, they're the source of all the variation upon which evolution by natural selection acts. (Skip to Chapter 4 for more detail about the role variation plays in evolution.)

All the different genes in all the organisms on earth started out as mutations that, though initially rare, ended up increasing in frequency. As the source of new genes, mutations are a key part of the evolutionary process. A gene can't increase or decrease in frequency until it first appears, and mutations are how that happens.

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