Evolution for AP Biology
Practice problems for these concepts can be found at: Evolution Review Questions for AP Biology
Definition of Evolution
How often have you heard executives report that "the idea evolved into a successful project" or popular science show narrators describe how a star "has been evolving for millions of years"? Evolution is no longer strictly a biological term since every academic field and nonacademic industry uses it. Such uses of the verb "evolve" reveal its meaning in its simplest form—to evolve means to change. For the AP Biology exam, however, you should remember the biological definition of evolution: descent with modification. Don't let the general uses of the word mislead you; a key part of this definition is descent, which can happen only when one group of organisms gives rise to another. When you see the word evolution, think of something that happens in populations, not in individuals.
More specifically, evolution describes change in allele frequencies in populations over time. When one generation of organisms (whether algae or giraffes or ferns) reproduces and creates the next, the frequencies of the alleles for the various genes represented in the population may be different from what they were in the parent generation. Frequencies can change so much that certain alleles are lost or others become fixed—all individuals have the same allele for that character. Over many generations, the species can change so much that it becomes quite different from the ancestral species, or a part of the population can branch off and become a new species (speciation). Why do we see this change in allele frequencies with time?
Allele frequencies may change because of random factors or by natural selection. Let's consider chance events first. Imagine a population of fish in a large pond that exhibits two alleles for fin length (short and long) and is isolated from other populations of the same species. One day a tornado kills 50 percent of the fish population. Completely by chance, most of the fish killed possess the long-fin allele, very few of these individuals are left in the population. In the next generation, there are many fewer fish with long fins because fewer long finned fish were left to reproduce; that allele is much more poorly represented in the pond than it was in the original parent generation before the catastrophe. This is an example of genetic drift: a change in allele frequencies that is due to chance events. When drift dramatically reduces population size, we call it a bottleneck.
Now imagine that the same pond becomes connected to another pond by a small stream. The two populations mix, and by chance, all the long-finned fish migrate to the other pond, and no long-finned fish migrate in. Again, which individuals migrated was random in this example; thus, there will be a change in the allele frequencies in the next generation. This is an example of gene flow, or the change in alleles frequencies as genes from one population are incorporated into another.
Gene flow (also more loosely known as migration when the individuals are actively relocating) is random with respect to which organisms succeed, but keep in mind that we could think of situations in which migration is not random. For example, if only the short-finned fish could fit in the stream connecting the two ponds, the alleles represented in the subsequent generation would not be random with respect to that allele. We also have not stated that the short-finned fish have an advantage by swimming to the other pond—if they did, this would be an example of natural selection, which we'll discuss below.
Finally, let's consider mutation, the third random event that can cause changes in allele frequencies. Mutation is always random with respect to which genes are affected, although the changes in allele frequencies that occur as a result of the mutation may not be. Let's say that a mutation occurs in the offspring of a fish in our hypothetical pond. The mutation creates a new allele. As a result, the allele frequencies in the offspring generation has changed, simply because we have added a new allele (remember that allele frequencies for a given gene always add up to one). As you can imagine, one mutation on its own does not have the potential to dramatically alter the allele frequencies in a population, unless this is a really small pond! But mutation is extremely important because it is the basis of the variation we see in the first place and it is a very strong force when it is paired with natural selection.
Remember that the first three factors act randomly with respect to the alleles in the population—which alleles increase and which decrease in frequency are determined by chance events, not because some alleles are inherently better than others are. We'll now turn to the fourth mode or process of evolution, natural selection, where the modification that occurs with descent is nonrandom.
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