In This Chapter
^ Adaptations that you see over and over again ^ Big adaptations resulting from a series of small steps ^ Weird adaptations daptations are evolutionary changes resulting from natural selection.
The environment in which a species lives exerts selective pressures on species such that they change over time.
This chapter lists ten adaptations I particularly like because they represent the types of adaptations that are possible. Some are no-brainers. Want to live in the ocean, and you're a mammal? You'd better be streamlined and insulated. Others are breakthroughs. Photosynthesis, for example, changed the chemistry of the entire world because of all the oxygen it produced. And some are — almost — out of this world, such as the creatures that have evolved to live in the deep-sea thermal vents that you see on the National Geographic Channel.
You have different kinds of teeth, but have you ever thought about how helpful those differences are? According to the fossil record, the ancestral condition in reptiles and the reptilian lineage that led to mammals is to have teeth that are all the same. Think about a crocodile; it has lots of teeth, but they're all the same. The same goes for the dinosaurs.
But then things changed. The Dimetredon, a dinosaur-like creature that had a sail on its back, had two kinds of teeth. Having two kinds of teeth allows for the possibility of division of labor. The teeth in the rear can be used for processing food — for grinding in grazing mammals or for slicing up chunks of flesh in your house cat — and the teeth in the front can specialize in food acquisition or other functions: snipping plants in herbivores, subduing prey in carnivores, or other functions such as defense.
328 Part v:The Part of Tens
What's interesting about the evolution of teeth shape is that the lower jaw has to match the upper jaw. As a consequence, changes to both jaws have to occur at the same time. For this reason, teeth change very slowly. Things get a little more complicated for humans. We've taken to capturing and processing most of our food outside our mouths. We cook it (ever try to eat raw rice?) and use knives and forks to cut it (ever ripped raw meat apart with your teeth?). As a result, our dentition is much reduced.
Evolution has also gone in the other direction: an ancestor with different kinds of teeth evolving into a species with uniform teeth. Example are animals that don't need to process their food because they swallow it whole — like dolphins and some other marine mammals. These creatures are descended from ancestors with non-uniform teeth, but they evolved dentition with one kind of tooth designed for grabbing small, slippery prey, which they then swallow whole.
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