When is cladistics more useful than linnaean taxonomy

Lecture 3 Lecture Summary Your text does not cover macroevolution until Chapter 5. If you want you can skip ahead and read the first few pages to help familiarize yourself with what we’ll discuss here. I want to go ahead and start talking about macroevolution now within the context of our discussion on evolution. I’m going to be tossing out a bit of vocabulary here and all of it is important (or I wouldn’t bother to include it). It’s important to understand how evolutionary biologists and paleoanthropologists bring order to the varieties of groups of living organisms. More specifically, we need to understand the framework for looking at the evolutionary relationships between them and placing them into a family tree. Macroevolution We need a way to deal with the million of species that live today and those that are no longer living. We cope with this diversity by grouping organisms together through a classification system. Classification is the ordering of organisms into categories such as phyla, orders, and families to show evolutionary relationships - taxonomy is the field that specializes in the rules of classification. Very simply, animals are organisms that move about and ingest food (but do not photosynthesize as plants do). Multicelled animals are placed in a group called metazoa. Within the metazoa there are more than twenty phyla. One of these phyla are called chordata, animals with a nerve cord, gill slits (at some stage of development), and a stiff supporting cord along the back called a notochord. Most chordates today are Vertebrates, where the notochord has become a vertebral column. They also have a developed brain and paired sensory structures for sight, smell, and balance. Vertebrates are then divided into six classes-the one we’re most concerned with is mammals. Animals are classified first and most traditionally by physical similarities. This is often the starting point but for similarities to be useful they must reflect evolutionary descent. Structures that are shared by species on the basis of descent from a common ancestor are called homologies. We need to be careful in making these assessments, though, e.g. just because birds and butterflies both have wings doesn’t mean they have a common winged ancestor- birds and insects are very different in more fundamental ways. They developed wings independently, their similarities are a product of separate evolutionary responses to similar functional demands. These kinds of similarities are called analogies; they are based strictly on common function and no assumed evolutionary descent. The process leading to analogies is called homoplasy (homo meaning same and plasy meaning growth). The following PBS web site has a fun exercise to test your knowledge on homologies and analogies: http://www.ucmp.berkeley.edu/help/timeform.html There are two approaches or “schools of thought” by which evolutionary biologists interpret evolutionary relationships and produce classifications. Both of these approaches trace evolutionary relationships and construct classifications that reflect these relationships and both recognize that organisms must be compared for specific features and some of these features are more informative than others. They also both focus exclusively on homologies. But they differ in other ways: 1.) Evolutionary systematics (also called gradistic taxonomy) – this is the more traditional approach and uses a phylogeny to illustrate the evolutionary relationships. A

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phylogeny or phylogenetic tree is a tree showing evolutionary relationships as determined by evolutionary systematics-it contains a time component and implies ancestor-descendent relationships. 2.) Cladistics – this approach is relatively new, emerging in the last few decades and is the predominate one among anthropologists. Cladistics more explicitly and more rigorously defines the kinds of homologies that yield the most useful information. For example terrestrial vertebrates share homologies in the number and basic arrangement of bones in the forelimb. This feature is useful for showing that large evolutionary groups (amphibians, reptiles, birds, and mammals) are all related through a common ancestor, it does not provide information useful for distinguishing one from the other (e.g. a reptile from a mammal). Such traits that are shared through common ancestry are called primitive or ancestral. (Careful of thinking of primitive as negatively-it does not mean to reflect evolutionary value but simply that the trait in two organisms comes from a common ancestor). The traits that cladistics focus on, those that are more informative, are ones that distinguish evolutionary lineages-called derived or modified. Strict cladistics shows relationships in a cladogram rather than phylogeny. Cladograms do not indicate time and make no attempt to discern ancestor-descendent relationships in terms of time. Also, both living and fossil forms are shown along the same dimension. In practice, most anthropologists and evolutionary biologists expand cladistic analysis to further hypothesize likely ancestor-descendent relationships shown relative to a time scale. In this way, both aspects of traditional evolutionary systematics and cladistics are combined to produce a more complete picture of evolutionary history. In addition to organizing life forms, time itself has been organized in the geologic time scale (make sure you eventually read the section on this in Chapter 5). Organisms have been profoundly influenced by geographic events during this time scale such as continental drift or the movement of the continents on sliding plates of the earth’s surface. For a quick glance at the geologic time scale click here: http://www.ucmp.berkeley.edu/help/timeform.html Note that we’re only concerned with the Cenozoic Era for the majority of this course. This brings us back to the mammals. The end of the Mesozoic approximately 65 million years ago marks the end of the dinosaurs and the opening of a wide array of ecological niches for the rapid expansion and diversification of mammals. The relatively rapid expansion and diversification of life forms into new ecological niches is more generally called adaptive radiation. This next division in time, the Cenozoic, is thus called the Age of Mammals. Mesozoic mammals were small, resembling mice, but the Cenozoic brought various types of mammals that, along with birds, eventually replaced the reptiles as the dominant terrestrial vertebrates. Why were the mammals so successful? They had larger and more complex brains than reptiles. In order for this kind of brain to develop, a longer period of growth is required. Dentition was also different-having more variable tooth types than reptiles. Finally, mammals have a constant internal body temperature and generate energy internally through metabolic activity. The three major subgroups of living mammals include egglaying, pouched, and placental. The primates are of course placental. But before we discuss early primate evolution (we’ll save this for next week) we need to say a little more about macroevolution.

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The single most important factor underlying macroevolutionary change is speciation – the process by which new species are produced from earlier ones. Species are a group of reproductively isolated organisms- a characterization that follows the biological species concept developed by Ernst Mayr. According to this view, new species are first produced through some form of isolation, e.g. if a group is separated by an ocean or mountain range (geographical isolation). Over time these two populations, because they are unable to mate, will accumulate genetic differences. If the population size is small we can predict genetic drift to cause allele frequencies to change (differently) in both populations. The two populations will diverge through genetic drift and natural selection and eventually become separate species. Isolation can also result from behavioral differences (behavioral isolation), e.g. when behavioral differences prohibit mating. Like interpreting evolutionary relationships, there are two different “schools of thought” concerning the modes of evolutionary change- phyletic gradualism and punctuated equilibrium. The traditional view of evolution has emphasized that change accumulates gradually in evolving lineages – phyletic gradualism. In this view, the entire fossil record of an evolving group (if it could all be recovered) would display a series of forms with finely graded transitional differences between each ancestor and its descendent. The fact that such transitional forms (missing links) are rarely found is attributed to the incompleteness of the fossil record. Anagenesis refers to the gradual, linear change that occurs within a single line. For more than a century this idea dominated evolutionary biology but in the last few decades most biologists have called this notion into serious question. The evolutionary mechanisms operating on species over the long run are often not continuously gradual. In some cases, species persist for thousands of generations basically unchanged. Then suddenly (at least in evolutionary terms) a spurt of speciation occurs. This uneven, nongradual process of long stasis interrupted by quick spurts is called punctuated equilibrium. The idea was introduced by Stephen J. Gould and Niles Eldridge (not “Eldred” like it says in your book!) in the 1970s. For the original paper click here: http://www.nileseldredge.com/pdf_files/Punctuated_Equilibri a_Gould_Eldredge_1977.pdf Advocates of punctuated equilibrium are disputing the tempo (rate) and mode (manner) of evolutionary change. Rather than gradual accumulation of small changes in a single lineage, another mechanism of evolution is necessary to push the process along - speciation. Punctuated equilibrium emphasizes cladogenesis – the formation of branches or clades. The paleontological record seems to support this idea showing long periods of stasis punctuated by rapid change (at an approximated 10,000 to 50,000 years). The primate fossil record shows a bit of both gradualism and punctuated equilibrium. One more thing about species…you may be wondering…how do anthropologists make species determinations from fossils? How do we know if extinct species were interbreeding or not? The answer is we don’t. In modern organisms we have no trouble defining interbreeding groups of organisms-we just observe them. But in the fossil record we need to look at morphological similarities and refer to living animals (their closest living relatives). We need to determine if the variation we see is biologically significant. Either the variation is due to individual, age, or sex differences or the variation represents differences between reproductively isolated groups. Applying strict Linnaean taxonomy to such a situation presents an unavoidable dilemma. Making decisions about paleospecies can be somewhat arbitrary and this is true at the genus level as well. It’s no

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wonder there are “lumpers” (those that group like forms together) and “splitters” (those that focus on the differences to split individuals into groups) in the field. Weekly Reading Summary Wilson 2007 In this week’s readings Wilson reminds us that we are 100% products of evolution just like any other animal and that evolution can, in fact, tell us a lot about ourselves. Again, one doesn’t need to be an expert to apply these concepts to everyday life. He gives the example of pregnancy sickness and homicide, which can both be explained using evolutionary principles. Evolutionary theory, he says, can be a tool for change. Finally, diversity doesn’t end at the species level. Individuals are also diverse-this is where personality comes into play.