Organizing Those Beetles

Linnaeus classified a huge number of plants and animals during his lifetime. His rationale was overall similarity: the more similar organisms were, the more closely together they were placed in the ranked (hierarchical) system familiar to anyone who has taken middle school or high school biology. The highest Linnaean ranking is kingdom, followed by phylum, class, order, family, genus, and species. (There are a variety of mnemonics to remember their order, such as Kings Play Chess On Fine Golden Sets, or Kids Playing Chicken On Freeways Get Smashed.) Any plant or animal can be assigned a series of labels reflecting its membership in a group from each of these categories. Species was the smallest category, consisting of organisms that have the greatest similarity. But all members of the genus—a group of species—have certain characteristics in common as well, and the same can be said for family, and for every other category all the way up to kingdom. Here are the Linnaean classifications for house cats, chimpanzees, and human beings:

House cats

Chimpanzees

Human beings

Kingdom:

Animalia

Animalia

Animalia

Phylum:

Chordata

Chordata

Chordata

Class:

Mammalia

Mammalia

Mammalia

Order:

Carnivora

Primates

Primates

Family:

Felidae

Pongidae

Hominidae

Genus:

Felis

Pan

Homo

Species:

cattus

troglodytes

sapiens

House cats, chimpanzees, and humans all belong to the same kingdom, phylum, and class; they have very many characteristics in common, and their Linnaean classification reflects this. Among other characteristics, they lack chlorophyll, so they are animals; they have a notochord, so they are chordates; and they have a single bone in the lower jaw, so they are all mammals. But chimpanzees and humans have more characteristics in common than either one has with cats, and Linnaeus grouped chimps and humans into the same order and cats into a different order. Humans and chimps were separated at the level of family, indicating that they were quite similar to one another.

Linnaeus's classification is useful, but classifying organisms on the basis of their similarities alone does not truly get at the underlying reality of nature. Why is it that all mammals have a single bone in the lower jaw? Why is it that humans and chimps are able to swing their arms over their heads but horses cannot? Organisms often have the same traits because they share genes. You and your brother or sister are more similar to each other than you are to your cousins because you and your siblings share more of your gene sequences with one another than you share with your cousins. Genes have a lot to do with important traits that an organism exhibits: they are why insects have six legs and spiders have eight, and why you walk on two legs and a monkey on four.

You and your siblings and cousins are similar in some traits (perhaps hair color, or stature, or blood type) because you share genes, and you share genes because you have a genealogical relationship to one another. You have descended with modification from common ancestors: parents in the case of your siblings and grandparents in the case of your first cousins. Similarly, all species are kin to one another in varying degrees because of common descent. The history of life is a branching and splitting genealogy of species changing through time. The Linnaean system, based on similarity and differences, provides an overall shape of this huge family tree of life, but it is not based on the underlying genealogical relationship of species—and thus does not always reflect the true relationship of organisms.

Ideally, a classification scheme would reflect genealogical relationships of organisms rather than just similarity, because similarity can be relatively superficial. Consider dolphins and tuna: both have an overall streamlined shape because that shape is very useful for getting around at high speeds in an open, watery environment. Yet there are many interior differences between dolphins and tuna: the skeletal systems, the circulatory systems, nervous systems, digestive systems, and so on. So, just because creatures are similar in overall shape does not mean that they are very closely related.

A late-twentieth-century classification method that has largely replaced the Linnaean system among biologists today is cladistics. Clade is a Greek word for "branch," and cladistics focuses on the branching of lineages through time. Both cladistics and the classical Linnaean system look at similarities among organisms to establish their relationships, but cladistics seeks in addition to reflect the actual results of evolution. In cladistics, the only groups of organisms that are considered natural are monophyletic, that is, groups comprising a single common ancestral species and all of its descendants. In terms of the tree of life, monophyletic groups correspond to whole branches that can be separated from the tree with a single cut. In contrast, the class of reptiles in the Linnaean system is not monophyletic because it excludes birds, which are descended from reptiles (as I discuss later). Similarly, a group consisting of warm-blooded animals (e.g., birds and mammals) also would not be monophyletic because all warm-blooded animals do not share a recent common ancestor.

Table 2.1

Ancestral traits, derived traits

Table 2.1

Ancestral traits, derived traits

Trait a

Trait b

Trait c

Trait d

Trait e

Trait f

Trait g

Warm blood

Hair

Diversified dentition

Fingernails

Grasping hands

Flat chest

Shoulder mobility

Chimps

x

x

x

x

x

x

x

Humans

x

x

x

x

x

x

x

Monkeys

x

x

x

x

Cats

x

x

x

Letters indicate characteristics. Traits a-c are found in all mammals, traits d-e additionally are found in all primates, and traits f-g are found in chimps and humans. From the standpoint of humans and chimps, traits a-e are ancestral traits, inherited from earlier mammal and primate ancestors. From the standpoint of chimps and humans, traits f and g are shared derived traits, inherited from a more recent ancestor. Looking at traits as ancestral or derived can help us reconstruct the evolutionary relationships of groups.

Letters indicate characteristics. Traits a-c are found in all mammals, traits d-e additionally are found in all primates, and traits f-g are found in chimps and humans. From the standpoint of humans and chimps, traits a-e are ancestral traits, inherited from earlier mammal and primate ancestors. From the standpoint of chimps and humans, traits f and g are shared derived traits, inherited from a more recent ancestor. Looking at traits as ancestral or derived can help us reconstruct the evolutionary relationships of groups.

Unlike the Linnaean system, cladistic taxonomy encourages naming and using only monophyletic groups. For that reason, cladistics focuses on a particular kind of trait (i.e., derived traits) as indicators of evolutionary (phylogenetic) relationships. A cladistic analysis divides traits into two kinds, ancestral and derived, and then constructs evolutionary trees based on the distribution of derived traits. Let me give an example of how that works.

Consider that humans, monkeys, cats, and chimps have many characteristics in common: they all have warm blood; hair; and incisor, canine, premolar, and molar teeth that come in different shapes (compared to, for example, a crocodile, whose teeth all have pretty much the same conical shape). These three traits (Table 2.1, traits a-c) cannot differentiate among these species because they are common to all, and being common to all, they must have been present in the common ancestor of all of these mammals; we call such traits ancestral traits. But note that monkeys, humans, and chimps have traits that cats lack: fingernails rather than claws and hands that can grasp rather than paws (Table 2.1, traits d-e). These traits are associated with the common descent of primates after they separated from other mammalian groups such as the cats and are therefore not shared with cats or other nonprimate mammals. Similarly, a broad, flat chest and the ability to move the arm in a circle at the shoulder (Table 2.1, traits f-g) are traits that chimps and humans share but monkeys lack, which provides evidence that chimps and humans form a separate branch from monkeys. These are derived traits.

Traits are ancestral or derived not in an absolute sense but relative to one group or another. Having fingernails is a derived trait of primates relative to mammals, but having fingernails—common to all primates—can be considered an ancestral trait and thus not useful when one is trying to determine the relationships among different primates, such as between monkeys and apes.

Being able to differentiate ancestral and derived traits makes it possible to reconstruct the evolutionary relationships among organisms. To do so, one must look at the

Figure 2.6

A Cladogram of Primates: A cladogram shows the evolutionary relationship of organisms on the basis of their possession of shared and ancestral traits. Warm blood, hair, and diversified dentition are found in all of the organisms in the diagram; they are ancestral traits. Flat chests and shoulder mobility are found only in the two groups above the mark: chimps and humans. These would be shared derived traits of chimps and humans. Courtesy of Alan Gishlick.

Figure 2.6

A Cladogram of Primates: A cladogram shows the evolutionary relationship of organisms on the basis of their possession of shared and ancestral traits. Warm blood, hair, and diversified dentition are found in all of the organisms in the diagram; they are ancestral traits. Flat chests and shoulder mobility are found only in the two groups above the mark: chimps and humans. These would be shared derived traits of chimps and humans. Courtesy of Alan Gishlick.

presence or absence of traits across a group of organisms, much as we did above with some traits of cats, monkeys, chimps, and humans. When enough traits are examined, certain traits emerge that indicate when a new lineage (a branch of the tree of life) appears—and these obviously are the most informative for reconstructing the tree of life. To determine the traits indicating a separate lineage, it is necessary to find an outgroup: a species or other group that is related to the group you are studying and that shows ancestral traits. To figure out the evolutionary relationships of monkeys, humans, and chimps, we can use cats as an outgroup: cats and primates both are mammals, and cats exhibit the mammalian ancestral traits. This allows us to set aside a very large number of traits that primates and cats share (like warm blood and diversified dentition) and focus on those derived traits that distinguish monkeys, humans, and apes from one another.

We can illustrate the relationships among these animals using a diagram called a cladogram, which indicates the characteristics that distinguish clades (Figure 2.6). Traits apply to all species to the right of where their labels appear, so mammalian characteristics such as warm blood and hair will occur in the animals named at the bottom of the diagonal line, because they are found in all of the organisms on the diagram: cats, monkeys, chimps, and humans. Cats lack fingernails and grasping hands, though, and because those characteristics set off primates from other mammals, they are shared derived traits of primates. Only humans and chimps have the flat chest and mobile shoulders that allow the arm-over-arm locomotion called brachiation, so these traits are shared derived traits for humans and chimps.

Mammals form a clade because they share a common ancestor, all of whose descendants are mammals; primates are a clade within the mammal clade because they share a common ancestor, all of whose descendants are primates, and humans and chimps form a clade within primates because they share a common ancestor, all of whose descendants are hominids (the technical name for the animals descended from the last common ancestors of chimpanzees and humans). We often will lack the fossil evidence for an actual ancestor of a lineage, but by using cladistic reconstruction, we can reconstruct many of the traits this ancestor would have had. For example, the first member of the lineage leading to humans, separate from chimpanzees, would be a biped, because that is a derived trait of our lineage, as is the presence of relatively small canine teeth. Such reconstructive expectations also help us interpret fossil remains.

This is a very brief and necessarily incomplete introduction to cladistic taxonomy. I used anatomical characteristics, but one can also use biochemical similarities, genetic or chromosomal similarities, and even developmental (embryological) characteristics to form cladograms of evolutionary relationships. Most evolutionary biologists use the cladistic approach to classify organisms because it avoids grouping organisms together on the basis of characteristics that do not reflect evolutionary relationships. When I was in high school and college, the Linnaean system was used. Birds were considered a separate branch of vertebrate life at the same level (i.e., class) as mammals or reptiles, partly because they had warm blood. Yet cladistic analysis, which separates ancestral from derived traits, shows that birds have a large number of traits that they share with a group of dinosaurs, and evolutionarily are closer to them than to mammals. Indeed, because cladistic taxonomy produces nested monophyletic groups, birds are dinosaurs—think about that during your next Thanksgiving dinner! Warm blood turns out to be a trait that has evolved more than once in the lineage of tetrapods (the descendants of the fish that adopted a terrestrial lifestyle about 365 million years ago). So, warm blood is a derived trait of both the mammal lineage and the reptile lineage that gave rise to dinosaurs and birds. Warm blood is a trait birds and mammals share— but not a trait that indicates close relationship. The division of traits into ancestral and derived clears up the confusion. To classify birds as a separate class, parallel to mammals and amphibians, would not reflect what really happened in evolutionary history. If we want all of our clades to reflect monophyly, we need to include birds as a subgroup of reptiles.

So cladistics is preferred to traditional Linnaean taxonomy because, by forcing us to classify according to monophyletic relationships, it better reflects the true genealogical relationship of living things. It also focuses on clades, or branches of the tree of life, and especially on the traits that distinguish clades, rather than on difficult-to-obtain ancestors. Cladistics is also considered superior to the Linnaean system because it does not depend on hunches about relationships among species, but rather allows—and requires—rigorous testing of hypotheses of evolutionary relationships. If you are interested in cladistic analysis, a good place to begin is the Web site of the University of California Museum of Paleontology (http://www.ucmp.berkeley.edu/IB181/ VPL/Phylo/PhyloTitle.html).

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