Evolution concepts and mechanisms

¶ … Wes Sechrest and Thomas M. Brooks and published in the National Academy of Sciences reveals the results of a study they conducted investigating the varying levels of biodiversity distributed throughout the world. The authors employ a fairly novel approach in their measurements of biodiversity, specifically, relying upon two methods approximating the levels of evolutionary history endemic to twenty-five terrestrial "hotspots." The significance of evolutionary history as a measuring stick is that it is associated with the past importance of particular geographic locations, and implies that future evolution is threatened if these locations are threatened. Additionally, Sechrest and Brooks find that their twenty-five defined hotspots house not only disproportionately large amounts of evolutionary history, but are also disproportionately threatened by the activities of man. The article stops short of attempting to identify any possible solutions to this impending problem, however, it does help to illuminate some of the shortcomings of our current conservation strategies; namely, a conservation strategy focused upon preservation of single species ignores much larger trends of extinction that mankind imposes upon the natural world. The authors do not explicitly discuss these problems or any particular reasons why biodiversity should be valued; they stick almost entirely to the manner of their investigation. The ideologies that their study relies upon are implicit and the article itself merely warns that the consequences of ruining seemingly insignificant expanses of land possess the potential to be quite severe.

Biodiversity is, essentially, "The total variation among all living organisms, including their genetic makeup and habitats." (Gallant 2003). It is generally believed that present day levels of biodiversity are at an all time high for the history of the planet earth. However, earth is also experiencing the highest absolute loss of biologic species due, largely, to the activities of man. The practical value of biodiversity is commonly associated with future medicines not yet discovered, or other products that could possibly be derived from species of plants or animals that humans have not yet recognized. Less tangible values include a better understanding of evolutionary history, the process of speciation, resistance to cataclysmic events, and of course, the natural beauty of wild plant and animal life. The study conducted by Sechrest and Brooks shows that not only are these aspects of life in danger on a broad, global scale, but that the most valuable locations are the ones posed with the most imminent threats.

The first premise put forward by Sechrest and Brooks is that, "Species diversity is unevenly distributed across the globe, with terrestrial diversity concentrated in a few restricted biodiversity hotspots." (Sechrest and Brooks 2002). In other words, of the millions of plant and animal species presently inhabiting the planet earth, a significant portion of them reside in geographically small expanses of land. It is important to note that Sechrest and Brooks, with this, their first premise, introduce the first restriction upon their study: it is only concerned with terrestrial life. Oceanic life, which constitutes another large percentage of global biodiversity, is left out of the argument. Yet, similar results could be found in subsequent studies of the ocean. Reefs are home to substantially high levels of biodiversity relative to the other aquatic biomes, and are minuscule in their physical volume.

The consequence of this approach to the issue of global diversity is, Sechrest and Brooks state, "If this history is disproportionately extensive, we may face losses of phylogenic diversity and/or evolutionary ancient lineages even more devastating than reflected by species losses alone." (Sechrest and Brooks 2002). So, if historically significant lineages are restricted to geographically limited spaces, the result of ruining the habitats in these places could be heterogeneous losses of species. We could, for example, wipe out all species of ape quite easily; while the absolute number of animal species lost would be small, the importance of those species from an evolutionary perspective would be enormous. This, to Sechrest and Brooks, is the foundation for the notion that evolutionary history is a valuable lens through which to view our current destruction of the natural world.

In order to measure such a broad concept as evolutionary history, the authors utilized two different methods.

"The first measure incorporates clade evolutionary history, which includes all branches within an included clade in a phylogeny, and hence takes higher-level diversity into account. For species corresponding to an area or set of areas, clade evolutionary history is equal to the amount of branch length uniquely represented by this set of species." (Sechrest and Brooks 2002).

Basically, this is a relative and qualitative measurement rather than a quantitative one. It makes use of the cladistic evolutionary system, which is essentially a family tree of species through time. By this form of measurement the total length of the selected tree with all of its branches is what translates into the overall diversity. Clearly, within any given clade -- group of related species and all their descendents -- this value of evolutionary history is only useful by comparison to other clades.

The second form of measurement that Sechrest and Brooks employ is merely concerned with the length of time any given species has existed as a single population -- the time that has passed since they split from their closest relative's evolutionary lineage. This "represents the species evolutionary history, measured as the branch length from present to the time of last divergence." (Sechrest and Brooks 2002). Again, this process pays homage to the cladistic system -- or family tree system -- of classification, where the length of each branch is proportional to the length of time a particular species has been in existence.

The authors feel that both forms of measurement are necessary because of the multiple ways that the value of evolutionary history can be interpreted. So, by the first method, species that have spawned many, many other species are very valuable; and by the second method, single species that have endured many ages are highly valued. Accordingly, humans would be very low on both of these scales because we have not branched into any other enduring species, nor have we been in existence for very long on a geologic timescale. The crocodilians, by contrast, would rank highly on the second scale but not on the first: they are ancient, but are not responsible for particularly high numbers of species. Bats, on the other hand, are fairly young relative to crocodiles, but have undergone speciation so many times that they account for approximately one quarter of all mammalian species; they would rank highly on the first scale, but less so on the second (Dodson 231). So, Sechrest and Brooks' unique methodology values both biodiversity and longevity in the evolution of life, and they call it a measurement of evolutionary history.

Doubtlessly, such quantities would be useless if there were not some constructive way to employ them. Also, because of the relative nature of both measurements used, Sechrest and Brooks are forced to limit their study to a finite number of species. They write, 'To obtain these measures, we used complete phylogenies (with dated branch lengths) for all extant primate and carnivore species. For both orders, we measured the two PD [evolutionary history] parameters by using lists of species endemic to the hotspots. We then compared these values with those for 1,000 simulations for the some number of species removed at random from the entire species lists for each order. We repeated this analysis by using lists of all primates and carnivores occurring within the hotspots." (Sechrest and Brooks 2002).

The authors compare simulated random distributions of carnivores and primates to the actual evolutionary history values they find for those both endemic and occurring in the hotspots they identify. By necessity, they limited their discussion to just two groups: primates and carnivores. It is likely that this was done because primates and terrestrial carnivores are two fairly common clades with relatively well understood histories. However, it is never explicitly stated why these are the groups chosen, and it is quite likely that these choices introduced the largest error into their subsequent conclusions. Had they selected beetles, for instance, they may not have been able to ultimately say that their hotspots were more valuable by their measurement scales. It is likely that they probably would have achieved similar results, but a rigorous discussion of why carnivores and primates should be used as models for all species within the hotspots would have been beneficial.

Additionally, Sechrest and Brooks never provide a complete listing of the twenty-five hotspots of diversity they have identified. Although they do include the areas that received the highest rankings for primates and carnivores, they fail to list all of the locations they analyzed. Perhaps this is because within the evolutionary biology community these sites are common knowledge, but readers less familiar with the idea of hotspots would probably find this information useful.

Nevertheless, their analysis of the two selected groups is quite thorough. They unambiguously explain the processes by which they went about calculating the evolutionary history values for primates and carnivores. Special care is taken to accurately approximate values that the fossil record or point molecular estimates do not clearly identify. They write, "Date estimates were available for a majority of nodes in both trees. Nodes without times of divergence were dated by interpolation using a pure birth model, whereby a clade's age is proportional to the logarithm of the number of species it contains." (Sechrest and Brooks 2002). In other words, they take advantage of the exponential growth characteristics of speciation to derive data that is probably close to correct. More uniform outcomes are simulated by computers in order to weigh the actual global distribution of species to the null hypothesis.

The remaining bulk of Sechrest and Brooks' article is devoted to the results of their study and the implications it has for the global biodiversity problem. Generally they found that, "Approximately 55% of the world's primates and 22% of carnivores are endemic to hotspots." (Sechrest and Brooks 2002). This is significant because the hotspots they are referring to "represent only 1.4% of the planet's land." (Sechrest and Brooks 2002). Also, "With around 59% of species evolutionary history and 71% of clade evolutionary history, both measures of PD for hotspot endemic primates are significantly greater than expected under the random model." (Sechrest and Brooks 2002). These results seem to back-up the authors' hypothesis that species endemic and occurring in the hotspots have disproportionately high levels of evolutionary history.

Yet, it is mentioned that although the species evolutionary history for primates is particularly high in the hotspots, this is not as true for the carnivores they investigated. This fact is acknowledged and explained within their discussion:

'Carnivore species have unusually large distributions worldwide and few species have restricted ranges. Thus, the amount of evolutionary history endemic to hotspots is less than in primates. Nevertheless, with either measure of PD, hotspots contain a greater number of PD than under a random model." (Sechrest and Brooks 2002).

Apparently, from Sechrest and Brooks' perspective, since they chose two clades from both ends of the spectrum the generalization of all species endemic to hotspots should lie somewhere in between. Carnivores have atypically large ranges and are distributed almost homogeneously throughout the planet, while primates appear to be extremely location specific. Since their results were the same in direction but different in magnitude only implies that they found the upper and lower bounds of a more common trend.

The authors identify what they believe to be the most noteworthy contributions to error within their calculations. They state, "Taxonomic revision can certainly affect the results of interspecific studies. Species level systematics are far from fully resolved even for groups as well-known as primates and carnivores." (Sechrest and Brooks 2002). One of the most significant sources of error inherent in any calculations that make use of the cladistic system of evolutionary classification is the system itself. Since humans are separated from nearly all historic speciation events by thousands and millions of years, the notion that it is even possible to fully recreate all lineages is highly questionable. Inferences must be made based upon fossils, comparative biology, physiology, and genetics. The fact that many species go extinct due to background extinction and cataclysmic events that are also inhomogeneous in their nature suggests that there are many lineages that we may never know of.

The second source of error they briefly discuss, again, is associated with their selection of only carnivores and primates. They say,

"Generalizing these results, it is important to recognize that our analysis is restricted to primates and carnivores, two orders that comprise only 11% of all mammalian biodiversity. Although these taxa do represent comparatively narrow (primates) and broad (carnivores) distributions globally, further mammalian and terrestrial vertebrate taxa should be examined across hotspots to evaluate differences in amounts of phylogenic diversity living within them." (Sechrest and Brooks 2002).

So, although it is implied why they picked these two clades in particular, they never explicitly state their motivations. Sechrest and Brooks acknowledge the limitations that analyzing only 11% of mammalian biodiversity introduces and suggest further research to rectify this problem. The research they publish in this article is quite small in scope but is likely to possess conclusions that can be applied to a much broader sample population.

The corollary argument to the one Sechrest and Brooks thoroughly analyze is that the world's hotspots are disproportionately threatened by humans. Although this point is mentioned many times within the article, it is given relatively space. It is clear, however, that this issue was investigated within their study, and in fact, was probably the motivation behind it. They write that they used the International Union for the Conservation of Nature and Natural Resources Red List "to examine the patterns in threatened species. The majority of the species most likely to become extinct are endemic to the hotspots: 77% of threatened primates and 60% of threatened carnivores." (Sechrest and Brooks 2002). Sechrest and Brooks repeated their analysis using these Red List species and found that the evolutionary history levels for these species and their clades were comparatively threatened. Therefore their ultimate conclusion is reached: "Hotspots are not only vital areas of species-level endemism, but are also highly significant reservoirs of unique and threatened evolutionary history." (Sechrest and Brooks 2002).

The most interesting aspect of this article is that it attempts to address very large issues within evolutionary biology in a rigorous manner. Specifically, not only do the authors attempt to define "biodiversity," and "evolutionary history," but they generate methods of measuring these concepts. When the majority of biologists use the term "biodiversity" they commonly use it as follows: "The disappearance of species is often called a loss of biodiversity, but biodiversity, short for 'biologic diversity,' is almost impossible to define; many scientists avoid using the term." (Allaby 1999). Yet, it appears that Sechrest and Brooks wish to use these concepts to illustrate the fact that the threats to wildlife worldwide need to be understood as more substantial than simply a loss of finite numbers of species: instead, significant lineages are seriously and disproportionately being destroyed. In truth, this study is a valiant attempt to make this problem something tangible to other scientists and the public at large.