5 Steps to a 5 AP Biology, 2014-2015 Edition

138 ❯ STEP 4. Review the Knowledge You Need to Score High

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 notbe 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 alwaysrandom 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 cre-

ates 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.

The four major modes of evolution are

1.Genetic drift

2.Gene flow (also called migration)

3.Mutation

4.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. We’ll now turn to

the fourth mode or process of evolution, natural selection, where the modification that

occurs with descent is nonrandom.

Natural Selection

Probably the biggest mistake people make when thinking about natural selection is think-

ing that it is synonymous with evolution. Natural selectionis only one process by which

evolution occurs (the others are discussed in the previous section). However, it is an impor-

tant process because it has been instrumental in shaping the natural world. Because of the

theory of natural selection, we can explain why organisms look and behave the way they do.

Natural selection is based on three conditions:

1.Variation:for natural selection to occur, a population must exhibit phenotypic variance—

in other words, differences must exist between individuals, even if they are slight.

2.Heritability:parents must be able to pass on the traits that are under natural selection.

If a trait cannot be inherited, it cannot be selected for or against.

3.Differential reproductive success:this sounds complicated, but it’s a simple concept.

Reproductive successmeasures how many offspring you produce that survive relative

to how many the other individuals in your population produce. The condition simply

states that there must be variation between parents in how many offspring they pro-

duce as a result of the different traits that the parents have.

KEY IDEA

KEY IDEA

BIG IDEA 1.A.1
Natural selection is
a major mechanism
of evolution.