chapter 22

• On November 24, 1859, Charles Darwin published On the Origin of Species by Means of Natural Selection.
• Darwin’s book drew a cohesive picture of life by connecting what had once seemed a bewildering array of unrelated facts.
• Darwin made two major points in The Origin of Species:
  1. Today’s organisms descended from ancestral species that were different from modern species.
  2. Natural selection provided a mechanism for this evolutionary change.
  • The basic idea of natural selection is that a population can change over time if individuals that possess certain heritable traits leave more offspring than other individuals.
  • Natural selection results in evolutionary adaptation, an accumulation of inherited characteristics that increase the ability of an organism to survive and reproduce in its environment.
• Eventually, a population may accumulate enough change that it constitutes a new species.
• In modern terms, we can define evolution as a change over time in the genetic composition of a population.
  • Evolution also refers to the gradual appearance of all biological diversity.
• Evolution is such a fundamental concept that its study is relevant to biology at every level, from molecules to ecosystems.
  • Evolutionary perspectives continue to transform medicine, agriculture, biotechnology, and conservation biology.

The following questions refer to the evolutionary tree in Figure 22.2.

The horizontal axis of the cladogram depicted below is a timeline that extends from 100,000 years ago to the present; the vertical axis represents nothing in particular. The labeled branch points on the tree (V—Z) represent various common ancestors. Let's say that only since 50,000 years ago has there been enough variation between the lineages depicted here to separate them into distinct species, and only the tips of the lineages on this tree represent distinct species.

How many separate species, both extant and extinct, are depicted in this tree?

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A) two
B) three
C) four
D) five
E) six

According to this tree, what percent of the species seem to be extant (in other words, not extinct)?

A) 25%
B) 33%
C) 50%
D) 66%
E) 75%