In any giving species, there are a huge number of different possible genotypes, and we can't possibly know the fitness of each different genotype — we just know that each genotype has a fitness. In order to think about all that information, scientists use the adaptive landscape. The adaptive landscape is a three-dimensional imaginary structure that gives us a way of talking and thinking about how differences in genotype affect fitness. Figure 6-1 is an example of a fitness landscape. The letter A represents a mutation that's beneficial (moving the species up the hill to greater fitness); the letter B represents a mutation that decreases a species' fitness.
This landscape is like a 3-D relief map that you can set down on your coffee table. A relief map indicates, through the height of the bumps, the altitude of any region — such a map of the U.S. would show the Rockies and the Sierra Nevadas, the Midwestern plains, the small mountains of Appalachia, and so on. The adaptive landscape has the same bumps, but here, the bumps don't show altitude; instead, they correspond to increased fitness.
Here are some things to know about the adaptive landscape:
1 Each location on the adaptive landscape can be though of as a genotype.
Neighboring points are very similar genetically, distant point less so.
1 The adaptive landscape deals with one species at a time and in one set of conditions. Imagine the adaptive landscape for chickens on an island in the Pacific, for example. For this particular population, some genotypes confer greater fitness and other genotypes confer lower fitness.
1 The top of each bump on the adaptive landscape represents a genotype more fit than those just a little different from it. The different heights of these bumps indicate relative fitness. Higher bumps equal higher fitness levels; lower bumps equal lower fitness levels.
Any particular adaptive landscape represents the fitness of a single species in a particular set of conditions. Move to a different island and the fitness landscape might change. Genotypes that were great on the first island might be hopeless on the next, and therefore, the peaks and valleys would be in different places.
It's impossible to measure the fitness of every genotype of the species being studied. But you can think about how mutations change fitness. A mutation that switches an amino acid from one kind to another, for example, nudges you a little bit on the adaptive landscape. Maybe the protein works better (you move up the hill); maybe it doesn't work as well (you move down the hill); or maybe it works exactly the same way (you move sideways on the hill, meaning that you're in a different place — you have a different genotype — but you have the same fitness. Your genome could change in millions of ways as a result of mutation; it's a big genome. Each of those changes moves you around somewhere in the fitness landscape.
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