Evolution, 4th Edition

106 CHAPTER 5

and crops was likely unconscious at first, but it is now practiced with sophisticated

statistical methods, and it results in spectacularly rapid evolution (FIGURE 5.3).

Evolution by Selection and inheritance

As we stressed in Chapter 3, selection results when there is a consistent relation

between a phenotype and fitness. Evolution by selection also requires a second

ingredient: inheritance. In the simplest terms,

If:

• there is a correlation between a phenotypic trait and the number of offspring
  that individuals leave to the next generation, and


• there is a correlation between the phenotype of a trait in parents and in their
  offspring,
  Then:


• the trait will evolve.
  The first condition means that selection on the trait is occurring, favoring indi-
  viduals with more extreme phenotypes than average. The second condition means
  that at least some of the phenotypic variation is inherited, causing the individuals
  with those more extreme phenotypes to pass on their traits to their offspring. The
  term “correlation” is used here in the statistical sense described in the Appendix.
  The logic behind this idea is shown graphically in FIGURE 5.4. Of Darwin’s
  many intellectual breakthroughs, this was his most brilliant. This chapter is about
  evolution that results from selection acting on individuals; Chapter 12 will explore
  how this concept applies to other entities.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_05.03.ai Date 11-02-2016

1957

34 g 42 g 44 g

316 g 632 g 1396 g

905 g 1808 g 4202 g

0 days

28 days

56 days

1978

Strain

2005

FIGURE 5.3 Comparison of three strains of

broiler chickens at different ages. In the first

strain, artificial selection was stopped in 1957,

in the second it stopped in 1978, and in the

third it stopped in 2005. Birds from the three

strains were grown under identical conditions.

The average weight of a 56-day-old chicken

increased more than four-fold as the result of

the 48 years of additional artificial selection

between 1957 and 2005. (From [46].)

05_EVOL4E_CH05.indd 106 3/23/17 9:01 AM