10.11–10.14: Macroevolution gives rise to great diversity.

More than 400 Anolis lizard species exist. (Shown here: A blue-throated anole, Anolis chrysolepsis, from Ecuador.)
10.11: Macroevolution is evolution above the species level.

When water runs over rocks, it wears them away. The process is simple and slow, yet powerful enough to have created the Grand Canyon. To be sure, water running over rocks does not always make a Grand Canyon. Nonetheless, no additional physical processes are necessary. The process of evolution has a lot in common with a stream of water running over rocks: in the short term, it produces small changes in a population, yet the accumulation of these changes over the long term can be “canyonesque” (FIGURE 10-21). Let’s consider some examples.

Figure 10.21: From a trickle of water to the Grand Canyon.

The production of 200-ton dinosaurs from rabbit-size reptile ancestors. The diversification from a single species of flowering plant into more than 230,000 species. These large-scale examples, which are the products of evolutionary change involving the origins of entirely new groups of organisms, are referred to as macroevolution. These examples can be contrasted with phenomena that involve changes in allele frequencies in a population—referred to as microevolution—such as the increase in milk production in cows during the first half of the 20th century, or the gradual change in the average beak size of birds with changing patterns of rainfall (FIGURE 10-22).

Figure 10.22: The scale of evolution.

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These micro and macro events might seem like two very different processes, but they are not. Evolution, whether at the micro or macro level, is one thing only: a change in allele frequencies over time. In the short term, over one or a few generations, evolution can appear as a slight and gradual change within a species. But over a longer period of time, the accumulated effects, acting continuously and combined with reproductive isolation of populations, can lead to the dramatic phenomena described as macroevolution. Just as a trickle of water, given enough time, produced the Grand Canyon, so evolution has created the endless forms of diversity on earth. In a sense, microevolution is the process and macroevolution is the result.

TAKE-HOME MESSAGE 10.11

The process of evolution—changes in allele frequencies within a population—in conjunction with reproductive isolation is sufficient to produce speciation and the rich diversity of life on earth.

Microevolution refers to evolutionary changes within a species, while macroevolution refers to larger-scale evolutionary change that forms new species. Both are similar in that they involve adaptation to the environment, and both occur through an accumulation of allele changes over time. But what is the relationship between macroevolution and microevolution?