Evolution is descent with modification

In many parts of this book, when we refer to evolution, we will be referring to allele frequency change over time. But evolution can also be defined more broadly as descent with modification.

FIGURE 6.3 Descent with Modification Michael Bell and colleagues have analyzed thousands of 10-million-year-old fossils of the stickleback fish Gasterosteus doryssus. Their specimens are unique in that the lake bed in which they were found is so finely layered that the ages of the fossils can be determined to the nearest 250-year interval. (A) Representative G. doryssus fossils, showing how the pelvic bone became reduced over time; the scale bar for each fossil is 1 cm. (B) The average pelvic score at different times. Fossil pelvic bones were scored by size according to a scale that ranged from 3 (complete bone) to 0 (no bone). (B after M. A. Bell et al. 2006. Paleobiology 32: 562-577.) View larger image

Charles Darwin (1859) used the phrase “descent with modification” to summarize the evolutionary process in his book The Origin of Species. Darwin proposed that populations accumulate differences over time primarily by natural selection, the process by which individuals with certain genetically determined characteristics survive and reproduce more successfully than other individuals because of those characteristics. We've already seen several examples of selection at work in this chapter's Case Study.

How can natural selection explain the accumulation of differences between populations? Darwin argued that if two populations experience different environmental conditions, individuals with one set of characteristics may be favored by natural selection in one population, while individuals with a different set of characteristics may be favored in the other population (FIGURE 6.4). By favoring individuals with different heritable characteristics in different populations, natural selection can cause populations to diverge genetically from one another over time; that is, each population will accumulate more and more genetic differences. Thus, natural selection can be responsible for the modification part of “descent with modification.”

FIGURE 6.4 Natural Selection Can Result in Differences between Populations

Populations of rock pocket mice (Chaetodipus intermedius) that live on dark lava formations in Arizona and New Mexico have dark coats, while nearby populations that live on light-colored rocks have light coats. In each population, natural selection has favored individuals whose coat colors match their surroundings, making them less visible to predators. View larger image