Chapter 23 Summary
Core Concepts Summary
23.1 A phylogenetic tree is a reasoned hypothesis of the evolutionary relationships among organisms.
The nested pattern of similarities seen among organisms is a result of descent with modification and can be represented as a phylogenetic tree. page 465
The order of branches on a phylogenetic tree indicates the sequence of events in time. page 465
Sister groups are more closely related to one another than they are to any other group. page 465
A node is a branching point on a tree, and it can be rotated without changing evolutionary relationships. page 466
A monophyletic group includes all the descendants of a common ancestor, and it is considered a natural grouping of organisms based on shared ancestry. page 466
A paraphyletic group includes some, but not all, of the descendants of a common ancestor. page 466
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A polyphyletic group includes organisms from distinct groups based on shared characters, but it does not include a common ancestor. page 466
Organisms are classified into domain, kingdom, phylum, class, order, family, genus, and species. page 467
23.2 A phylogenetic tree is built on the basis of shared derived characters.
Characters, or traits, existing in different states are used to build phylogenetic trees. page 468
Homologies are similarities based on shared ancestry, while analogies are similarities based on independent adaptations. page 468
Homologies can be ancestral, unique to a particular group, or present in some, but not all, of the descendants of a common ancestor (shared derived characters). page 469
Only shared derived characters, or synapomorphies, are useful in constructing a phylogenetic tree. page 469
Molecular data provide a wealth of characters that complement other types of information in building phylogenetic trees. page 471
Phylogenetic trees can be used to understand evolutionary relationships of organisms and solve practical problems, such as how viruses evolve over time. page 473
23.3 The fossil record provides direct evidence of evolutionary history.
Fossils are the remains of organisms preserved in sedimentary rocks. page 474
The fossil record is imperfect because fossilization requires burial in sediment, sediments accumulate episodically and discontinuously, and fossils typically preserve only the hard parts of organisms. page 474
Radioactive decay of certain isotopes of elements provides a means of dating rocks. page 477
Archaeopteryx and Tiktaalik are two fossil organisms that document, respectively, the bird–
The history of life is characterized by five mass extinctions that changed the course of evolution. page 481
The extinction at the end of the Cretaceous Period 66 million years ago led to the extinction of the dinosaurs (other than birds). page 481
The extinction at the end of the Permian Period 252 million years ago is the largest documented mass extinction in the history of Earth. page 481
23.4 Phylogeny and fossils provide independent and corroborating evidence of evolution.
Phylogeny makes use of living organisms, and the fossil record supplies a record of species that no longer exist, absolute dates, and environmental context. page 482
Data from phylogeny and fossils are often in agreement, providing strong evidence for evolution. page 482
Self-Assessment
Draw a phylogenetic tree of three groups of organisms and explain how a nested pattern of similarity can be seen in the tree and how it might arise.
Self-Assessment 1 Answer
Species, shown at the tips of the branches, diverge from one another, as genetic differences between separated populations accumulate over time. The most recent common ancestor of two species or two branches is represented by a node in the tree. The successive branching of a group shows which species most recently shared a common ancestor, revealing a nested pattern of similarity.
Distinguish among monophyletic, paraphyletic, and polyphyletic groups, and give an example of each.
Self-Assessment 2 Answer
Monophyletic groups are groups in which all members share a single common ancestor not shared with any other species or group of species, like amphibians. Paraphyletic groups include some, but not all, of the descendants of a common ancestor, like birds being excluded from the reptile group even though there is evidence that they share a common ancestor. A polyphyletic group does not include the last common ancestor of all members, like putting bats and birds in a single “flying vertebrates” group.
List the levels of classification, from the least inclusive (species) to the most inclusive (domain).
Self-Assessment 3 Answer
The levels of classification are as follows: species, genus, family, order, class, phylum, kingdom, and domain.
Define “homology” and “analogy” and describe two traits that are homologous and two that are analogous.
Self-Assessment 4 Answer
A homologous trait is one that results from shared ancestry, such as an amniotic egg and lungs. An analogous trait is a similarity that results from convergent evolution, such as bird and bat wings or echolocation.
Name a type of homology that is useful in building phylogenetic trees and explain why this kind of homology, and not others, is useful.
Self-Assessment 5 Answer
Synapomorphies, or shared derived characters between some members of a group, are useful in building phylogenetic trees because the homologies are shared by some but not all of the members of the group. If an entire group shared the same homologous trait, we would not be able to construct a meaningful phylogenetic tree.
Describe three ways that an organism can leave a record in sedimentary rocks and explain why this means that there are gaps in the fossil record.
Self-Assessment 6 Answer
Organisms can be recorded by the preservation of their morphology, especially parts such as mineralized skeletons that are resistant to destruction; they can leave a record as trace fossils, such as tracks and trails; and they can be recorded by molecular fossils, especially decay-
resistant lipids. In most environments, sedimentation is discontinuous, also creating gaps in the fossil record. In addition, for an organism to be fossilized, it must be buried and have features that resist decay after death. Not all organisms meet these two criteria, so they do not become fossils, leading to gaps in the fossil record. Explain how the fossil record can be used to determine both the relative and the absolute timescales of past events.
Self-Assessment 7 Answer
A relative timescale is established by the relationships among the sedimentary rock beds that contain fossils. Because younger sediments are deposited on top of older sediments, fossils in beds higher in a sedimentary sequence will be younger than those in lower beds. Radiometric age dating provides an estimate of absolute (calendar) age because unstable isotopes decay at a known rate.
Describe the significance of Archeopteryx and Tiktaalik.
Self-Assessment 8 Answer
Archeopteryx is a fossil organism that shows the transition from dinosaur to bird, and Tiktaalik is a fossil organism that shows the transition from fish to tetrapod in the fossil record. Transition fossils are significant because they give us evolutionary clues of morphological and physiological phylogenetic shifts through time.
Describe how mass extinctions have shaped the ecological landscape.
Self-Assessment 9 Answer
Mass extinctions have shaped the ecological landscape by removing dominant organisms, thereby changing the competitive landscape of the remaining ones. Some survivors diversify in the wake of mass extinction, establishing new ecological relationships in ecosystems.