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Authors: Jerry A. Coyne
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species showed a net increase in segment number, the changes among different species were not only uncorrelated, but sometimes went in opposite directions during the same period.
Unfortunately, we have no idea what selective pressures drove the evolutionary changes in these plankton and trilobites. It is always easier to document evolution in the fossil record than to understand what caused it, for although fossils are preserved, their environments are not. What we can say is that there was evolution, it was gradual, and it varied in both pace and direction.
Marine plankton give evidence for the splitting of lineages as well as evolution within a lineage. Figure 7 shows an ancestral plankton species dividing into two descendants, distinguishable by both size and shape. Interestingly, the new species, Eucyrtidium matuyamai, first evolved in an area to the north of the area from where these cores were taken, and only later invaded the area where its ancestor occurred. As we’ll see in chapter 7, the formation of a new species usually begins when populations are geographically isolated from one another.
There are hundreds of other examples of evolutionary change in fossils—both gradual and punctuated—from species as diverse as mollusks, rodents, and primates. And there are also examples of species that barely change over time. (Remember that evolutionary theory does not state that all species must evolve!) But listing these cases wouldn’t change my point: the fossil record gives no evidence for the creationist prediction that all species appear suddenly and then remain unchanged. Instead, forms of life appear in the record in evolutionary sequence, and then evolve and split.
“Missing Links”
Changes in marine species may give evidence for evolution, but that’s not the only lesson that the fossil record has to teach. What really excites people-biologists and paleontologists among them—are transitzonal forms: those fossils that span the gap between two very different kinds of living organisms. Did birds really come from reptiles, land animals from fish, and whales from land animals? If so, where is the fossil evidence? Even some creationists will admit that minor changes in size and shape might occur over time—a process called microevolution—but they reject the idea that one very different kind of animal or plant can come from another (macroevolution). Advocates of intelligent design argue that this kind of difference requires the direct intervention of a creator. 7 Although in The Origin Darwin could point to no transitional forms, he would have been delighted by how his theory has been confirmed by the fruits of modern paleontology. These include numerous species whose existence was predicted many years ago, but that have been unearthed in only the last few decades.
FIGURE 7 . Evolution and speciation in two species of the planktonic radiolarian Eucyrtidium, taken from a sediment core spanning more than 3.5 million years. The points represent the width of the fourth segment, shown as the average of each species at each section of the core. In areas to the north of where this core was taken, an ancestral population of E. calvertense became larger, gradually acquiring the name E. matuyamai as it became larger. E. matuyamai then reinvaded the range of its relative, as shown on the graph, and both species, now living in the same place, began to diverge in body size. This divergence may have been the result of natural selection acting to reduce competition for food between the two species.
But what counts as fossil evidence for a major evolutionary transition? According to evolutionary theory, for every two species, however different, there was once a single species that was the ancestor of both. We could call this one species the “missing link.” As we’ve seen, the chance of finding that single ancestral species in the fossil record is almost zero. The fossil record is simply too spotty to expect
Unfortunately, we have no idea what selective pressures drove the evolutionary changes in these plankton and trilobites. It is always easier to document evolution in the fossil record than to understand what caused it, for although fossils are preserved, their environments are not. What we can say is that there was evolution, it was gradual, and it varied in both pace and direction.
Marine plankton give evidence for the splitting of lineages as well as evolution within a lineage. Figure 7 shows an ancestral plankton species dividing into two descendants, distinguishable by both size and shape. Interestingly, the new species, Eucyrtidium matuyamai, first evolved in an area to the north of the area from where these cores were taken, and only later invaded the area where its ancestor occurred. As we’ll see in chapter 7, the formation of a new species usually begins when populations are geographically isolated from one another.
There are hundreds of other examples of evolutionary change in fossils—both gradual and punctuated—from species as diverse as mollusks, rodents, and primates. And there are also examples of species that barely change over time. (Remember that evolutionary theory does not state that all species must evolve!) But listing these cases wouldn’t change my point: the fossil record gives no evidence for the creationist prediction that all species appear suddenly and then remain unchanged. Instead, forms of life appear in the record in evolutionary sequence, and then evolve and split.
“Missing Links”
Changes in marine species may give evidence for evolution, but that’s not the only lesson that the fossil record has to teach. What really excites people-biologists and paleontologists among them—are transitzonal forms: those fossils that span the gap between two very different kinds of living organisms. Did birds really come from reptiles, land animals from fish, and whales from land animals? If so, where is the fossil evidence? Even some creationists will admit that minor changes in size and shape might occur over time—a process called microevolution—but they reject the idea that one very different kind of animal or plant can come from another (macroevolution). Advocates of intelligent design argue that this kind of difference requires the direct intervention of a creator. 7 Although in The Origin Darwin could point to no transitional forms, he would have been delighted by how his theory has been confirmed by the fruits of modern paleontology. These include numerous species whose existence was predicted many years ago, but that have been unearthed in only the last few decades.
FIGURE 7 . Evolution and speciation in two species of the planktonic radiolarian Eucyrtidium, taken from a sediment core spanning more than 3.5 million years. The points represent the width of the fourth segment, shown as the average of each species at each section of the core. In areas to the north of where this core was taken, an ancestral population of E. calvertense became larger, gradually acquiring the name E. matuyamai as it became larger. E. matuyamai then reinvaded the range of its relative, as shown on the graph, and both species, now living in the same place, began to diverge in body size. This divergence may have been the result of natural selection acting to reduce competition for food between the two species.
But what counts as fossil evidence for a major evolutionary transition? According to evolutionary theory, for every two species, however different, there was once a single species that was the ancestor of both. We could call this one species the “missing link.” As we’ve seen, the chance of finding that single ancestral species in the fossil record is almost zero. The fossil record is simply too spotty to expect
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