Evidence For Evolution

Evidence in support of organic evolution is drawn from many areas, including similarities in the form and ecology of living organisms and the way they are related to each other today. Homologies (characteristics shared by different organisms) point to common ancestry. For example, members of the mustard family (Brassicaceae—2,500 species), which includes broccoli, cabbage, radish, and stocks, produce a pungent, watery juice that has the smell of sulfur when it breaks down. However, similarities are not always due to common ancestry. For example, a number of African spurges (species of Euphorbia) and American cacti have similar, succulent stems even though they are not closely related (Fig. 15.3). The plants do share similar arid habitats to which they have independently become adapted (an adaptation is a characteristic that makes an organism better suited to its environment). In other words, plants of very different ancestry have adapted in similar ways to common environmental conditions in different parts of the world. This type of evolution is called convergent evolution.

Other evidence comes from the structure and relationships of proteins, DNA, and other molecules, and the common use of ATP by living organisms. Cytochrome c oxidase, for example, is an enzyme that occurs universally in all living organisms, which suggests it appeared very early, probably in a single organism, and that it has been successively inherited by the myriad of organisms in existence today.

Fossils, which are remnants of previously living forms (Fig. 15.4), provide compelling evidence for descent with modification. The simplest fossils are generally found in the oldest geological strata, while more complex forms tend to be found in younger strata.

1. Our understanding of the term species has been modified and refined since Darwin's time, but, as discussed in Chapter 16, most biologists today think of a species in general terms of a population of individuals of similar form and structure with a common ancestry, capable of interbreeding freely with one another in nature but not generally interbreeding with individuals of other dissimilar populations. A shorter definition is simply to refer to a species as a group of organisms with a common gene pool. Note that species is like the word sheep in that it is spelled and pronounced the same in either singular or plural usage. There is no such thing as a plant or animal "specie."

Evolution

Convergent Evolution Plants

Figure 15.3 Convergent evolution. Plants of different ancestry that adapt to similar habitats may evolve similar life forms. The barrel cactus on the left (Jasminocereus howellii) and the barrel spurge on the right (Euphorbia obesa) are completely unrelated but have evolved similar forms in adaptation to arid habitats.

Figure 15.3 Convergent evolution. Plants of different ancestry that adapt to similar habitats may evolve similar life forms. The barrel cactus on the left (Jasminocereus howellii) and the barrel spurge on the right (Euphorbia obesa) are completely unrelated but have evolved similar forms in adaptation to arid habitats.

Derwent Reservoir
Figure 15.4 A fossil fern.

Still further evidence is drawn from the geographical distribution of organisms. Many groups are confined to a single continent or island. In some instances where similar organisms occur on more than one land mass, there is evidence that the land masses concerned were once linked, which would have permitted terrestrial migration. Other conclusions are drawn from the physiology and chemistry of the organisms.

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