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Figure 5.7 Reconstructed phylogeny of African antelopes. Two lineages diverged 6-7 Ma, the slowly evolving impalas and the rapidly speciating gnus and hartebeests. The second group could be said to be evolutionarily more successful than the first, and this might be interpreted as a result of species selection of species-level characters - the rate of speciation. However, the gnus and hartebeests have more specialized ecological preferences than do the species of impalas: perhaps selection has occurred at the individual level (natural selection), and this has had an effect at the species level. Species numbers 14 and 26 are omitted in this study. (Based on Vrba 1984.)

where their ecological tolerances determine their evolutionary rates, and produce a superficial appearance of species selection.

Is evolution hierarchical? And, if so, was Darwin wrong? The case has been overstated by critics: evolution occurs by natural selection, as Darwin said in 1859. Many proposed examples of species selection can be explained by natural selection, coupled with rapid asymmetric geographic speciation and the effect hypothesis. Nonetheless, species selection is a possibility, and convincing examples may be found in the future.

THE TREE OF LIFE Tree thinking

As noted at the start of this chapter, Darwin was the first to picture evolution as a great branching tree, and to point out that all species had evolved from a single ancestor at the base of the tree. The idea of the tree of life has come to the fore recently, with a massive effort by biologists and paleontologists to discover the whole tree. From small-

scale questions like "Which species of ape is closest to humans - the gorilla or chimp?", large teams of researchers are hastening to put together complete trees of all species of mammals, angiosperms, amphibians, spiders and many other groups. As each complete tree, that is, a tree containing all species, is published, systematists are getting closer to Darwin's ideal of understanding the shape of the whole tree of life.

It is important to distinguish trees from ladders. Many people think that all plants and animals are arranged in a series from simple to complex, or "lower" to higher". The pattern of evolution is then like a ladder, a single long line of progression from one species to the next, an idea that was popular 200 years ago and termed the Scala naturae (see p. 13). But all the evidence shows that evolution is a process of splitting and so the tree is the correct analogy, not the ladder.

Fossils offer fundamental information on the history of life and on large-scale patterns of evolution. There has been a revolution in the ways in which paleontologists interpret evolutionary aspects of the fossil record, and this is true of biologists as well. Nearly all studies of ecology, behavior and evolution are tied to a phylogenetic tree of the organisms involved. Since 1990 phylogenetic trees have been springing up everywhere, both because of new techniques for discovering trees and a realization that nothing in biology means anything without a tree.

Cladistics: reconstructing life's hierarchy_

For centuries, biologists have struggled with the search for the true pattern of relationships among organisms: how is the tree of life to be discovered? Debates about whether birds originated from dinosaurs, whether annelids and arthropods are close relatives or not, or whether the gorilla or chimp is the closest relative of humans, all hinge on the need to identify patterns of relationships correctly.

If you had the task of sorting out the relationships among 100 species of parrots, where would you begin? You might note the color of their feathers, and classify them into a blue group, a red group and a green group. But then you might notice that body size or beak shape gives a different classification. If you then looked at the internal anatomy of the 100 parrots, you might find an entirely different classification based on the shape of the skull, the bones of the wing, or the arrangement of muscles or arteries. Up to 1960, sys-tematists had a hard task in seeking to decide which characters were "good" and which were "bad". Good characters are phyloge-netically informative, that is, indicative of the true phylogeny, but what about the bad, or uninformative, characters?

Phylogenetically uninformative characters fall into two main categories: convergences and plesiomorphies. Convergence in evolution is when features, or organisms, evolve to look the same perhaps because they live the same way. The marsupial mole of Australia looks just like the northern hemisphere mole, with great paddle-like limbs, poor eyesight and an excellent sense of smell, because they both burrow and eat worms, and yet they are not closely related. Two species of parrots might have convergently evolved a red patch of feathers on their wings as a signal. Plesio-morphies are characters that are shared by the organisms of interest, say parrots, but also by other groups. So, all parrots have beaks, but this is not a helpful character in sorting out the phylogeny of parrots because all other birds have beaks too. True parrots have blue and green feathers that have a special iridescent quality not seen in the feathers of cockatoos. But such special light-reflecting feathers are seen also in many other bird groups, and so are plesiomorphic for parrots.

Phylogenetically informative characters identify clades, or monophyletic groups. These are groups that had a single origin and include all the descendants of that common ancestor. A good example of a clade is the Psittaci-formes, the parrots, a group that has long been identified as real and distinct from all others by naturalists. Clades are distinguished from two kinds of non-natural groups: (i) paraphyletic groups, which had a single common ancestor, but do not include all descendants, such as the Reptilia, which excludes birds and mammals; and (ii) poly-phyletic groups, which are random assemblages of organisms that arose from more than one ancestor, and so have no place in the search for the tree of life.

Willi Hennig (1913-1976), an eminent German entomologist, realized the difference between phylogenetically informative and uninformative characters, and between mono-phyletic and paraphyletic/polyphyletic groups. He stressed the need to develop a new, more objective method in systematics, which has come to be called cladistics. The fundamental aim of cladistics is to identify clades, and so to discover, or reconstruct, the tree of life. Patterns of relationships are shown as branching diagrams, or cladograms (e.g. Fig. 5.8), in

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