Goals: discuss reasons why organisms might or might not be optimally adapted to their environments. Learn about how to test whether traits are adaptations by using the phylogenetic comparative method.
Lab Manual: based on the material in this lecture, you should be able to answer the questions in chapter XI on testing hypotheses of adaptation.
The Lecture
We have looked at a variety of forms of evolution and a variety of ways in which natural selection can occur. We have also looked at studies of phylogeny. Now we will start to put some of this information together. It turns out that we can use what we know about phylogeny to study evolutionary processes.
In this lecture we will consider the following question: "Are organisms optimally adapted to their environments?" To be "optimally adapted" means to have the best possible traits for the environment, given any trade-offs that may be present in these traits (remember from the first chapter of your textbook that a trade-off is a situation in which one trait value might be most beneficial for one reason but a different trait value most beneficial for another; the optimal adaptation would be the trait value with the best balance of these benefits for a given environment.)
Some evolutionary biologists have argued that traits should be optimally adapted because optimal adaptation is what we expect, given enough time for mutations to occur, from natural selection. Since there has been a lot of time for evolution through natural selection to occur, they would argue that most traits should be optimally adapted.
Other evolutionary biologists have argued that many traits may not be optimal adaptations. There are a variety of reasons why natural selection might NOT have had time to lead to optimal adaptation. Some of these follow:
Because species that are not optimally adapted will tend to retain the traits of their ancestors, we can use phylogenies to test for adaptation. We do this using something called the phylogenetic comparative method in which we compare species that occur in various environments or have various characterstics to see whether the traits of organisms are related to their environmental conditions.
If traits are adaptations, we would predict that they would evolve in association with the environment to which they are adapted. Suppose, for example, that we think large bills in birds are an adaptation to eating seeds. We would predict that the large bills have evolved in association with eating seeds. We can test this looking at a phylogeny to see when (in what ancestral species) it is most likely that various traits evolved. We do this the same way we did when we were studying phylogenies -- if several species come from some ancestor and all have the same trait then probably that trait was present in the ancestor. To see how this works, consider the following two phylogenies:
The phylogeny on the left suggests that birds already had large bills before they started eating seeds. If this is true, then they didn't evolve large bills in response to eating seeds so large bills are NOT an adaptation to eating seeds. The phylogeny on the right suggests that within this group of birds large bills evolved when the birds switched to eating seeds. So large bills might be an adaptation to eating seeds.
In addition to predicting that an adaptation would evolve in association with a change in environment, we would also predict that different species that use the same environment in the same way should independently evolve similar traits. In other words, we would predict convergent evolution of similar traits in similar environments if adaptation is occurring.
In contrast, sometimes many species may have the same traits, and occur in the same environment, simply because they are related to each other, not because the traits are adapted to that environment. If this is the case, then these traits will show homology -- related species will have the same traits, suggesting that the traits evolved from a distant ancestor. A hypothesis of phylogenetic contingency predicts homology -- species that have the same traits will be related to each other.
Now let's look at some more phylogenies to see how to distinguish convergent evolution (which suggests adaptation) from homology (which would suggest phylogenetic effects more than adaptation.) Suppose we found the following phylogeny:
In this case, the insect eating birds (BugEater1 - BugEater4) do all have small bills (indicated by blue) and the seed eating birds (SeedEater1 - SeedEater4) do all have large bills. However, all the large billed birds are closest relatives and all the small billed birds are closest relatives. Bill size shows homology; as indicated by the color of the branches on the phylogeny, it is most likely that there has been just one evolutionary change in bill size (either from large to small or from small to large) and the reason birds have the bill size they do is that they inherited that bill size from their ancestor. This pattern suggests phylogenetic contingency more than it does adaptation.
Now, in contrast, suppose we found the following phylogeny:
In this case, large and small bills have evolved independently many times. Bill size shows convergent evolution, not homology. This suggests that birds that eat seeds have independently evolved to have large bills a number of times, and birds that eat insects have independently evolved to have small bills a number of times. This pattern of convergent evolution suggests that bill size is an adaptation to the environment.
Now we will consider another way in which studying phylogeny can help us to study whether or not traits are adaptations. Sometimes the fact that traits are adaptations can be hard to see because differences among related species that reflect adaptation are smaller than differences among more distantly related groups. Suppose for example that one group of related species evolved from a small billed ancestor, another group from a larger billed ancestor, and still another from a very large billed ancestor. Then suppose that within each group, birds that ate seeds of different sizes evolved to have larger or smaller bills than their ancestor, but that there was still a big difference in bill size between the three groups. If we just looked at whether or not bill size in a species was related to seed size, we might not see a strong pattern -- we might see something like this:
Based on this graph it doesn't look like there's much relationshipship between bill size and seed size eaten and we might conclude that bill size has not adapted to seed size. However, suppose these species belong to three different related groups, like the following:
Note that within each group, closest relatives differ in the seed size they eat. Suppose that within each clade, birds that eat larger seeds have larger beaks, as shown by the following graph:
This is the same graph as the one shown above, but the colors of the dots indicate the phylogenetic groups the species come from (they are coded as the same colors on the phylogeny above -- the red dots are the species from the group shown in red on the phylogeny, and so forth.)
This pattern indicates that within each group, birds with larger beaks eat larger seeds. So independently, in three phylogenetic groups, the same trend for birds that eat larger seeds to evolve larger beaks, and birds that eat smaller seeds to evolve smaller beaks, has evolved. This suggests that beak size really IS an adaptation to seed size. Note that we probably would not have seen this pattern if we had not looked at the phylogeny -- it was not obvious in the original graph!
So let's summarize the things we want to look for in comparing species (applying the phylogenetic comparative method) to test for adaptation. In general, we want to know if a trait has evolved in association with some aspect of the environment (that's what we expect if it's an adaptation to that aspect of the environment). We have evidence for adaptation if the trait evolves in association with the environment independently in several different groups, so that we see convergent evolution. If we don't see an obvious association we should check the phylogeny to make sure that there aren't associations that have evolved independently within different phylogenetic groups.
Finally, we should note that if we see what look like strong phylogenetic effects that they really don't rule out the hypothesis of adaptation. This is because it is possible that a trait really did evolve in association with the environment in some ancestral species -- so it was an adaptation -- and then the species that descended from that ancestor all retain the trait AND all stayed in the same habitat so there was no selection to change. So when we see a phylogeny that suggests phylogenetic effects all we can say is that we do not have very strong evidence of adaptation and that the trait may be the way it is because of phylogenetic effects, but we have not disproved the hypothesis of adaptation. That hypothesis turns out to be difficult to disprove. As a result, evolutionary biologists continue to argue as to whether phylogenetic constraints are really important or whether traits really are optimally adapted. These hypotheses are hard to test -- ideally, some of YOU may go on and figure out great ways of testing them!