Evolution Organic evolution is a fact

By saying organic evolution is a fact, we mean that, if one accepts that the human mind, with its strengths and limitations, is capable of understanding aspects of the natural world, and that scientific

3 Cladistic methods were first developed and articulated by an entomologist, Willi Hennig, in Grundzuge einerTheorie der phylogenetischen Systematik ( 1950). Hennig's work had a minor impact on European biologists, but it was not until the 1966 publication of an English translation (entitled simply Phylogenetic Systematica) of a revised version of the 1950 work that cladistic methods became relatively well known. During the late 1960s and throughout much of the the 70s and early 80s cladistic methods became a kind of cause célèbre as a host of determined advocates foisted it upon a host of equally determined scientists unimpressed by the method (see Box 4.3).The real strengths of the method eventually triumphed, and today, virtually all phylogenetic reconstruction is done by means of cladograms.

methods are an appropriate tool for this type of inquiry, the biota has undergone evolution.4 Evolution refers to descent with modification: organisms have changed and modified their "morphology" (morph -shape; ology - the study of) through time, and each new generation is the most recent bearer of the unbroken genetic thread that connects life. In this sense, each new generation is forward looking in that its members potentially contain changes relevant for the future, but is connected to the past by features that they have inherited.

That evolution has occurred is not a particularly new idea; it was articulated by a variety of enlightenment and post-enlightenment philosophers and natural historians. The unique contribution of Charles Darwin and Alfred Russel Wallace (who j ointly presented similar ideas at an 1858 meeting of the Linnean Society of London) was that the driving force behind evolution is natural selection. Here, however, we are most concerned with the record of evolution - an observable pattern of descent with modification - regardless of the process (natural selection) that may be responsible for it. It is important to intellectually decouple evolution (fundamentally a pattern) itself from natural selection (the process driving the pattern), and our efforts will be directed largely to evolution and not to natural selection.

Evolution amounts to modifications (in morphology, in genetic make-up, in behavior, etc.), so that while some changes are developed in descendants, many of the ancestral features are retained. Clearly implicit in this are the relationships between anatomical structures. For example, we postulate a special relationship between the five "fingers" in the human "hand" and the five "toes" in, say, the front "foot" of, say, a lizard. Here, the English language is confusing; we are really talking about the digits of the forelimbs, a particular feature that happens to have been conserved (or maintained) through time in these two lineages (humans and lizards). In theory, the digits on the forelimbs of lizards and humans can be traced back in time to digits in the forelimb of the common ancestor of humans and lizards. We call these anatomical structures "homologues," and two anatomical structures are said to be "homologous" when they can, at least in theory, be traced back to a single original structure in a common ancestor (Figure 3.5). Thus we infer that the digits in the forelimbs of all mammals are homologous with those of, for example, dinosaurs. That is because these digits can be traced back to the digits in the forelimbs of the common vertebrate ancestor of mammals and dinosaurs. The wings of a fly, however, are not homologous with those of a bird, since they cannot be traced to a single structure on a common ancestor. Because the wings of a fly and the wings of a bird perform in similar fashion (they allow flight to take place), they are considered to be "analogues," and are said to be "analogous" (Figure 3.6). Obviously, the concept of evolution is intimately tied to the concept of homology.

4 Scientific debate about the "theory of evolution" is not about whether evolution actually occurred, but rather about the underlying causal mechanisms behind evolution.

Figure 3.5. Homologues. Homologues are anatomical structures that can, at least theoretically be traced back to a single structure in a common ancestorThe front limbs of humans, bats, birds, and pterosaurs are all homologous, and retain the same basic structure and bone relationships even though the appearance of these forelimbs may be outwardly different Homology forms the basis for hypotheses of evolutionary relationships.

Figure 3.5. Homologues. Homologues are anatomical structures that can, at least theoretically be traced back to a single structure in a common ancestorThe front limbs of humans, bats, birds, and pterosaurs are all homologous, and retain the same basic structure and bone relationships even though the appearance of these forelimbs may be outwardly different Homology forms the basis for hypotheses of evolutionary relationships.

Human

Human

Figure 3.6. Analogues. Analogues may perform similar functions, and may even look outwardly similar but internally they can be very different. Here, a human leg is contrasted with that of a grasshopper Although both have legs, the two structures are different. Human muscles are on the outside of the skeleton, whereas grasshoppers' muscles are on the inside of their skeleton.

Figure 3.6. Analogues. Analogues may perform similar functions, and may even look outwardly similar but internally they can be very different. Here, a human leg is contrasted with that of a grasshopper Although both have legs, the two structures are different. Human muscles are on the outside of the skeleton, whereas grasshoppers' muscles are on the inside of their skeleton.

An obvious, yet often-ignored clue to the fact that evolution has taken place is the hierarchical distribution of characters in nature. If descent with modification has taken place, what patterns of character distributions might one expect to find? Modification of ancestral body plans through time would produce exactly the distribution of characters that we observe: a hierarchical arrangement in which some homologous characters are present in all organisms, in which other characters are found in somewhat smaller groups, and in which still other characters have a very restricted distribution and are found in only a few organisms.

Chopping down Accepting the fact of evolution, there must be a single phylogeny - a the "tree of life" single genealogy - that documents the interrelatedness or connectedness of all life. This is not an unfamiliar concept, because most of us have seen "trees" that purport to document who came when and from whom. Such "trees of life" are common in textbooks and museum displays, and deeply influence most people's ideas about the pattern of evolution (Figure 3.7). These trees commonly show an ancestral protoplasmoid rising out of primordial sludge and giving rise to everything else. But how does one make a tree of life? How do we figure out who gave rise to whom? After all, no human was present to observe the appearance of the first dinosaur on the face of the earth. And is it reasonable to suppose that, with the fossil record as incomplete as it is, one fossil that we happen to find seren-dipitously turns out to be the very ancestor of other fossils that we happened to find? Because of their rarity, the chances of that occurring, especially among vertebrates, are vanishingly small. Thus the oldest hominid fossil known is very unlikely to be the direct ancestor of all subsequent humanity. On the other hand, it is likely to have many features that the real ancestor possessed. In this book, therefore, we avoid trees of life, and instead use cladistic analysis to reconstruct evolutionary events.

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