Some examples of vestigial structures The fact of extinction The law of succession in the fossil record Structural homology Developmental homology The occurrence of closely related species in groups of islands Age of Earth known to be much greater than 6,000 years (but absolute dates not known)
Not available to Darwin:
Many more examples of all the above Transitional fossil forms Direct observation of populations in the wild changing through time (however, Darwin did know about, and paid close attention to, the changes that animal breeders can cause in domestic animal populations) Ring species Genetic and molecular information of any kind, including vestigial molecular traits, molecular homologies, and basic genetics Radiometric dating and absolute dates for the geologic time scale
The answer to this question is entirely subjective and is left to the reader. For perspective, however, we can offer that the discovery of Archeopteryx was quite influential and convinced many people, both scientists and laypeople, that the theory of common ancestry was probably correct. However, most people, including biologists, doubted that natural selection was a significant factor in evolution until the "modern synthesis" of the 1930s united genetics with natural selection.
Many different approaches are possible. Anatomical traits of dogs or cats (e.g., snout length in dogs, ear shape, leg length, and so on), could be traced through time examining archeological remains of the domesticated dogs and cats throughout history, and assessing whether the anatomical changes appear to lead back to the putative wild ancestor. The question can also be tested with molecular data-for example, by comparing DNA sequences in modern dogs and cats and in wild canids and felids. In either case we would use the data to construct a phylogenetic tree, as discussed in further detail in later chapters. If all modern breeds are descended from a common ancestor, the phylogeny should show that all modern breeds are more closely related to the purported wild ancestor than to any other wild canid or felid. Furthermore, the modern breeds should show a branching phylogeny indicating the pattern and relative timing of their divergence from each other.
Many answers are possible. Several examples that paleontologists are eager to find are: a common chimp-human ancestor from approximately 7-8 mya (just before the hypothesized split of chimps and humans); a transitional bat fossil showing incipient development of bat wings; a transitional turtle fossil demonstrating an intermediate stage in development of the turtle shell; and very early representatives of the major animal phyla. (Some possible examples of these cases have been discovered, but are still controversial.)
The fossil record is a case of "Absence of evidence is not evidence of absence." Many species do not leave any discoverable fossils at all, due to such factors as our lack of access to deeply buried strata and the complete destruction of large sections of the earth's crust in subduction zones. Thus, presence of a fossil obviously proves that the predicted species once existed, but absence of known fossils does not prove that it did not exist.
Birds are endothermic ("warm-blooded"), and their body feathers are critically important as insulation. Feather coloration is also important in species and sex recognition, and feathers are frequently used in behavioral displays. Any of these functions may have played a role in the feathered dinosaurs, with insulation generally suspected to have been particularly important. Mathematical models of the thickness and potential insulative value of the dinosaur feathers and close inspection of other anatomical and locomotory features associated with endothermy could clarify whether the dinosaurs' feathers had a thermoregulatory benefit.
Under the modern definition of homology, a kiwi's wing is homologous to an eagle's wing, and the rubber boa's spurs are homologous to a kangaroo's hind legs. Owen's classical definition is only applicable if the organs are subjectively judged to be "the same organ." Owen would likely have agreed that a kiwi's wing and kangaroo's wing are the same organ, but he might not have perceived the rubber boa's spurs as being essentially the "same organ" as a quadruped's hind legs.
a. Analogous b. Homologous c. Analogous d. Homologous e. Homologous f. Analogous
Both types of trees show relationships, and show patterns of descent from ancestors. However, phylogenetic trees are fundamentally about populations, while genealogical trees are about individuals. Phylogenetic trees cover much huger spans of time than genealogical trees. In phylogenetic trees, all living representatives, even "elderly" ones, are shown at the tips of the tree; in genealogical trees, "elderly" living individuals will be in the middle of the tree, not just at the tips. Phylogenetic trees branch only when populations acquire distinct new traits; genealogical trees develop a new branch for every individual. Branches of genealogical trees merge when parents mate; branches on phylogenetic trees never merge. In general, phylogenetic trees tend to be a bush that begins from a single oldest point (the common ancestor), and develops branches going forward in time, while genealogical trees typically begin with a bush (all the individual ancestors) and converge on a single point, or just a few points, in the present (the individual under study, and perhaps his or her siblings).
Jaguarundis are more closely related to tigers than to bobcats, because the common ancestor of jaguarundis and tigers lived more recently than did the common ancestor of jaguarundis and bobcats. (The easiest way to see this on the phylogenetic tree is to imagine all other branches removed, so that the phylogeny just shows bobcats, jaguarundis, and tigers.)
The assumptions behind relative dating were clearly correct. The clean consistency of these two dating systems is widely viewed as an impressive testament to the meticulous fieldwork and logical reasoning of the 18th- and 19th-century geologists who developed relative dating. Interestingly, many of these early geologists were creationists who were expecting to find evidence of worldwide floods (see next).
If most fossils were formed by worldwide floods, as the Theory of Special Creation proposes, fossils should be found in thick bands of flood-type sediment (i.e., no annual layers of slow sedimentation, as are seen in deep ocean deposits, rivers, and lakes). In any given worldwide flood, fossils should also be either not sorted at all (i.e., all species mixed together), or possibly sorted according to body size or buoyancy (e.g., dinosaurs and elephants together in the lowest layers, small beetles and trilobites higher up). Fossils should not be sorted by tiny internal anatomical details. Extinct and modern species should be mixed together. The Theory of Evolution, in contrast, predicts that fossils should be highly sorted, according to their order of descent from a common ancestor. The existing fossil record, with its clear "law of succession," the very precise order in which various animal taxa are found (no elephants in the lowest layers; no trilobites in the upper layers; ichthyosaurs always in Mesozoic sediments, but dolphins always in Cenozoic sediments; etc.), and the common occurrence of slowly deposited riverine and oceanic sediments, refutes these predictions of Special Creation.
