Evolution, 4th Edition

60 CHAPTER 3

Among several slightly different definitions of natural selection used by biologists

today [12], we use this one: natural selection is any consistent difference in fitness among

different classes of biological entities. A simple way to think of fitness is as the number

of offspring an individual leaves in the next generation. Suppose, for example, that

in a species of annual plant, only 1 of every 1000 seeds survives to reproductive age,

and that those that survive produce an average of 3000 seeds. The average fitness of

that type of individual is 0.001 × 3000 = 3. The components of fitness are survival

and reproduction. Fitness is sometimes called reproductive success, which includes

survival because organisms do not reproduce when they are dead.

If evolution by natural selection is to occur, there must be a change in the popu-

lation across generations, and this requires that the phenotypic differences among

the entities be inherited. Thus, evolution by natural selection occurs if (1) there is

a correlation between an individual’s phenotype and its fitness, and (2) variation

in the phenotype is correlated between parents and their offspring. Suppose, for

example, that in an asexually reproducing annual plant, genotypes A and B differ

in a characteristic that affects their fitness (e.g., susceptibility versus resistance to

a herbicide), and that their average fitnesses are 3 and 4, respectively. If these val-

ues are constant from generation to generation, genotype B increases in number

much faster than A, and will make up the great majority of the population within a

few generations (FIGURE 3.7). We say that the frequency (proportion) of genotype

B has increased (and conversely, that the frequency of A has declined). In sexu-

ally reproducing organisms, fitness is more complicated. Males vary in survival

and reproduction. In particular, they vary in mating success, which Darwin called

sexual selection. (In some species, females also experience sexual selection.) In

sexual species, moreover, individuals’ genes replicate, but because of recombina-

tion their genotypes do not. So it can be useful to think about the fitness of a type

of gene (i.e., an allele), and consequently of selection among genes, even though

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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1 2 3

Generation

4 5 6 7

Frequency

Genotype B

Genotype A

1.0

0.75

0.5

0.25

0

(B)

1 2 3

Generation

4 5 6 7

Numbers of individuals

Genotype B

Genotype A

20,000

15,000

10,000

5000

0

FIGURE 3.7 Two genotypes of a plant are (A)

growing together. Genotype A has a fitness

of 3, while genotype B has a fitness of 4. Both

genotypes start with 10 individuals. (A) The

population size of genotype B grows much

more rapidly. (B) Plotting the frequencies of

the two genotypes shows that genotype B,

which starts at a frequency of 0.5, makes up

almost 90% of the population just 7 genera-

tions later.

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