Species Selection

NOTE: These are lecture notes for Biology 391, Organic Evolution, at The University of Tennesee at Martin. Anyone outside of UT Martin wishing to use these notes or to contact me for additional information should first read the information obtained by clicking here.

Goals: study factors that can affect the rates of speciation and extinction within groups to understand why some groups may have many species and others few species.

Related Textbook Material: Freeman and Herron (2001) Chapter 15 Sections 15.3-15.5

Lab Manual Questions over this material are in Lab Manual Chapter XVII

The Lecture:

Throughout life, within the different phylogenetic groups of life, there is a commonly observed pattern: some groups have many species and others very few species. For example, within plants, there are many, many species of flowering plant compared to other plant groups. Within animals, there are more species of insect than all other groups combined, and within the insects, there are many more species of beetle than of any other group. Within mammals, the rodents and the bats have the most species; other groups have relatively few species compared to these groups.

This uneven pattern of species number is related to another pattern -- some traits are found in a great many species, others in only a few species. Those traits of the groups that have lots of species are found in those species and are, as a result, very common. So we would note that most plants have flowers, that most animals have a hard exoskeleton, and that most mammals have either wings or enlarged incisors (big buck teeth) because there are so many species of flowering plant, insect, bat, and rodent, and these traits occur in these groups. We could ask whether these traits are something that have caused these groups to have many species, or whether something else caused there to be many species and we see these traits simply because species in these groups have them. We will address questions like this in this lecture; generally, we will look at some possible reasons for this pattern -- that is, evolutionary processes that could explain why so many species have evolved, and exist, in some groups compared to others.

Species selection refers to the fact that traits may be unusually common or rare among species because different groups have different rates of two processes: speciation and extinction. We expect many species within a group if it has had a high rate of speciation (producing many species) or if it has had a low rate of extinction (so most of the species that have been produced still exist.) We expect few species within a group if it has had a low rate of speciation (so few species have been produced) or if it has had a high rate of extinction (so few of the species that have been produced still exist.) As a result of these different rates, we expect the traits of those groups with high speciation or low extinction to be common among species and the traits of those groups with low speciation or high extinction to be rare.

Species selection is thus a way to explain the patterns noted above. To understand species selection, we will consider different possible reasons for different rates of speciation and extinction.

We have discussed speciation in some detail already. We need to make sure we understand extinction before we continue to study species selection.

Extinction occurs when all members of species die out leaving no descendents. So there are currently no descendents of species that have really gone extinct. We can contrast this with something called pseudoextinction, which occurs after speciation, when we consider that the ancestral species no longer exists, but that the descendents of that species still exist. Thus we would say that the ancestor to all bird species no longer exists, but it is considered pseudoextinct, not truly extinct, because many of the descendents of that species (the modern birds) do still exist. A dinosaur such as Tyrannosaurus rex we would say, in contrast, is extinct -- it has died out and there are no live descendents of T. rex.

When we study species selection, we want to understand what causes rates of true extinction -- why do species die out - - rather than pseudoextinction. Pseudoextinction is simply a side effect the other process we will be studying as we study species selection -- that is, of speciation.

To understand species selection, we will consider possible reasons for high rates of speciation or low rates of extinction in some groups. These are as follows:

Adaptive radiation. Adaptive radiation occurs when there is a high rate of speciation because there is the potential for each new evolving species to adapt to use a different aspect of the environment. Adaptive radiation occurs in situations in which the environment is relatively empty of potentially competing species. It is possible in such situations because there are no competitors to potentially outcompete newly evolving species (this would tend to prevent them from evolving. The following are situations that make adaptive radiation likely:

On islands. Islands typically have fewer species than the mainland (because it's hard for species to disperse to the island.) As a result, there may be relatively few potential competitors on the island and adaptive radiation can occur. An example is Darwin's Finches -- the many finch species on the Galapagos Islands apparently evolved from a single ancestral finch species that dispersed to the island, and have evolved to use the environment in different ways by evolving different sizes and shapes of beak, which they use to feed in different ways.
After a mass extinction. A mass extinction is an event during which many different species from many different, distantly related groups go extinct (for example, the period when the dinosaurs died out and also many marine invertebrate species died out.) There have been a number of mass extinctions during the history of life. After each, there have been relatively few species left, and some of these species have undergone rapid speciation as newly formed species adapted to different aspects of the environment.
After the evolution of a key innovation (also called a key characteristic.) A key innovation is a characteristic that lets a species use the environment in a way that is sufficiently different from its relatives and other existing species so that many new ways of using the environment become possible. For example, when wings evolved, the species with wings could fly and therefore use the environment in different ways. The evolution of the flower allowed many new ways of reproduction in the environment. The result of this is that adaptive radiation can occur -- because of this new, key, characteristic the species doesn't have many competitors for new ways of using the environment. It is important to note here that the definition of a key innovation is a little fuzzy. You might ask how much differently a characteristic should let a species use the environment for it to be a key innovation. The answer is that we decide it was a key innovation if there's evidence that it really did result in an adaptive radiation -- that is, if it appears that a high rate of speciation occurred after the key innovation evolved.

Body size can affect either the probability of speciation or extinction in a group. Speciation may be higher in species with small body size because there are more likely to be ecological barriers in an environment, making sympatric speciation through ecological isolation more likely (click here if you need to review the lecture material on sympatric speciation.) For example, it's more likely that groups of ants could be isolated from each other by using different parts of the environment (such as tree branches versus the ground, for example) than it is that groups of elephants in the same geographic area could be isolated from each other. Species with small body size are sometimes less likely to be able to cross geographic barriers, too, so that smaller geological events might provide barriers that would allow allopatric speciation (click here if you need to review the lecture material on allopatric speciation.) So both sympatric and allopatric speciation may be more likely for species with smaller body size, resulting in a higher rate of speciation in small species.

Extinction rates, in contrast, may be higher in species with large body size. Large organisms require more resources from the environment and are likely to exist in populations of fewer individuals as a result. They may thus be more subject to genetic drift, which can result in non-adaptive evolution, so they may not be as well adapted to their environments. It is also more likely that environmental changes would cause them to die out -- there are fewer individuals and they require more resources.

If species with small body size have higher rates of speciation or lower rates of extinction than do species with large body size, we would predict there to be more small species than large species. To some extent this is apparently true -- insects are small compared to many other animals, for example, and rodents are some of the smaller mammals. Not all groups with small body size have many species, though, so body size can't entirely explain the observed patterns of species selection.

The above material should have introduced you to some of the ways species selection may occur. It is not a complete list -- there are other factors that could affect rates of speciation and extinction. Later in the course we will consider one more factor: sexual reproduction.

Now that you have seen several species selection hypotheses, you should be asking yourself how they could be tested. How would we test to see if a trait was a key innovation, for example? We would predict that the rate of speciation increased in a group after the evolution of a key innovation. This could be tested using the fossil record for those groups with very good fossil records for which the number of new species evolving can be reliably determined. We can also test for key innovations looking at modern species, by studying their phylogenies.

Since a key innovation leads to adaptive radiation, which means there is a high rate of speciation, we predict that within the same amount of time, more species should evolve in a group with a key innovation than a group without a key innovation. Phylogenetically, this means we predict a pattern like the following:

In this figure, species shown in blue are those with the hypothesized key innovation. The phylogeny shows the history of speciation in the group that evolved from the ancestor that evolved the key innovation, and in the most closely related group without the hypothesized key innovation. The pattern here suggests that the hypothesized key innovation really is a key innovation because when we compare the group that has this (species E- Z) with the most closely related group that does not (species B-D), there are many more species in the group with the trait. This suggests that, within the same amount of time (since the ancestor to E-Z speciated from the ancestor to B-D), there has been much more speciation in the group with the hypothesized key innovation than the group without. Since key innovations are traits that result in adaptive radiation, and adaptive radiation involves a high rate of speciation, this is the pattern predicted if this trait is a key innovation. Note that we also need to identify when the hypothesized key innovation evolved to be able to show that there has been a high rate of speciation since it evolved; we do this by outgroup comparison. Species A is the outgroup to B-Z; it does not have the key innovation, so the key innovation is apparently a derived trait that evolved in the ancestor to E-Z.

Note that we have already looked at how to use studies of phylogeny to test for adaptation (in the previous lecture); this shows another way of using phylogenies to test an evolutionary process. In this case we are looking at rates of speciation.

Study tips:

it would be a good idea to draw for yourself, and keep together, the different kinds of phylogenetic pattern predicted by the different kinds of evolutionary test for which we use phylogenies.
make yourself a table of the different causes we discuss, in this lecture and in future lectures, of factors that would be expected to either increase speciation rates or decrease extinction rates and thus result in species selection
make yourself a table of the different causes of adaptive radiation, and the reasons they cause adaptive radiation
be sure to distinguish between adaptive radiation, which is one cause of increased speciation that can result in species selection, and the general causes of species selection.

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