CHAPTER
15
Tracing Evolutionary History
Lecture Outline
I. Introduction
1. Bird feathers are marvelously adapted for life on the wing.
2. In flight, the shapes and arrangements of various feathers
a. produce lift,
b.smooth airflow,
c. help with steering and balance, and
d.provide a waterproof, lightweight covering.
3. Dinosaurs were the first feathered animals on Earth.
4. Thousands of fossils of feathered dinosaurs have been found and classified into more than 30 different species.
5. The evolution of birds is an example of macroevolution, the major change recorded in the history of life over vast tracts of time.
II. Early Earth and the Origin of Life
A.15.1 Conditions on early Earth made the origin of life possible
1. The Earth formed about 4.6 billion years ago.
2. As the Earth cooled and the bombardment slowed about 3.9 billion years ago, the conditions on the planet were extremely different from those today.
a. The first atmosphere was probably thick with water vapor and various compounds released by volcanic eruptions, including nitrogen and its oxides, carbon dioxide, methane, ammonia, hydrogen, and hydrogen sulfide.
b.Lightning, volcanic activity, and ultraviolet radiation were much more intense than today.
3. The earliest evidence for life on Earth comes from 3.5-billion-year-old fossils of stro-matolites, built by ancient photosynthetic prokaryotes still alive today.
4. Because these 3.5-billion-year-old prokaryotes used photosynthesis, it suggests that life first evolved earlier, perhaps as much as 3.9 billion years ago.
5. The first life may have evolved through four stages.
a. The abiotic (nonliving) synthesis of small organic molecules, such as amino acids and nitrogenous bases.
b.The joining of these small molecules into polymers, such as proteins and nucleic acids.
c. The packaging of these molecules into “protocells,” membrane-enclosed droplets that maintained an internal chemistry different from that of their surroundings.
d.The origin of self-replicating molecules that eventually made inheritance possible.
1. In the 1920s, two scientists, Russian chemist A. I. Oparin and British scientist J. B. S. Haldane, independently proposed that conditions on early Earth could have generated organic molecules.
2. Our modern atmosphere is rich in O2, which disrupts chemical bonds.
3. The early Earth likely had a reducing atmosphere.
4. In 1953, graduate student Stanley Miller, working under Harold Urey, tested the Oparin-Haldane hypothesis.
5. Miller identified a variety of organic molecules that are common in organisms, including
a. hydrocarbons (long chains of carbon and hydrogen) and
b.some of the amino acids that make up proteins.
6. Hypothesis about the origins of life include
a. deep-sea environments near submerged volcanoes or hydrothermal vents or
b.meteorites as sources of organic molecules.
C.15.3 Stages in the origin of the first cells probably included the formation of polymers, protocells, and self-replicating RNA
1. The abiotic synthesis of small organic molecules would have been a first step in the origin of life.
2. But what is the evidence that the next three stages could have occurred on early Earth?
a. Synthesis of polymers
b.Formation of protocells
c. Self-replicating RNA
3. The abiotic synthesis of small organic molecules would have been the next step in the origin of life.
a. Before enzymes, hot sand, clay, or rock may have helped monomers combine to form polymers.
b.Waves may have splashed organic molecules onto fresh lava or other hot rocks and then rinsed polypeptides and other polymers back into the sea.
4. A key step in the origin of life would have been the isolation of a collection of organic molecules within a membrane-enclosed compartment.
a. Laboratory experiments demonstrate that small membrane-bounded sacs or vesicles form when lipids are mixed with water.
b.These abiotically created vesicles are able to grow and divide (reproduce).
5. The origin of self-replicating molecules
a. Today’s cells transfer genetic information from DNA to RNA to protein assembly. However, RNA molecules can assemble spontaneously from RNA monomers.
b.Furthermore, when RNA is added to a solution containing a supply of RNA monomers, new RNA molecules complementary to parts of the starting RNA sometimes assemble.
c. Some RNA molecules, called ribozymes, can carry out enzyme-like functions, sup-porting this hypothesis.
III. Major Events in the History of Life
A.15.4 The origins of single-celled and multicellular organisms and the colonization of land were key events in life’s history
1. Macroevolution is the broad pattern of changes in life on Earth.
a. The Archaean and Proterozoic eons lasted about 4 billion years.
b.The Phanerozoic eon includes the last half billion years.
3. Prokaryotes lived alone on Earth for 1.5 billion years, from 3.5 to 2 billion years ago.
a. During this time, prokaryotes transformed the atmosphere.
b.Prokaryotic photosynthesis produced oxygen that enriched the water and atmosphere of Earth.
c. Aerobic cellular respiration allowed prokaryotes to flourish.
4. The oldest fossils of eukaryotes are about 1.8 billion years old.
5. The common ancestor of all multicellular eukaryotes lived about 1.5 billion years ago.
6. The oldest fossils of multicellular eukaryotes are about 1.2 billion years old.
7. The first multicellular plants and fungi began to colonize land about 500 million years ago.
8. Humans diverged from other primates about 6 to 7 million years ago.
9. Our species, Homo sapiens, originated about 195,000 years ago.
10. If the clock of Earth’s history were rescaled to represent an hour, humans appeared less than 0.2 seconds ago!
B. 15.5 The actual ages of rocks and fossils mark geologic time
1. Radiometric dating is based on the decay of radioactive isotopes.
2. The rate of decay is expressed as a half-life, the time required for 50% of an isotope in a sample to decay.
3. Carbon-14 is useful for dating relatively young fossils—up to about 75,000 years old. Radioactive isotopes with longer half-lives are used to date older fossils.
4. The fossil’s age can also be inferred from the ages of rock layers above and below the stratum in which it is found.
C.15.6 The fossil record documents the history of life
1. The geologic record is based on the sequence and age of fossils in the rock strata.
2. The most recent Phanerozoic eon
a. includes the past 542 million years and
b.is divided into three eras:
i. Paleozoic,
ii. Mesozoic, and
iii. Cenozoic.
3. The boundaries between eras are marked by mass extinctions.
IV. Mechanisms of Macroevolution
A.15.7 Continental drift has played a major role in macroevolution
1. According to the theory of plate tectonics, the Earth’s crust is divided into giant, irregularly shaped plates that essentially float on the underlying mantle.
2. In a process called continental drift, movements in the mantle cause the plates to move.
3. Since the origin of multicellular life roughly 1.5 billion years ago, there have been three occasions in which the landmasses of Earth came together to form a
supercontinent.
4. About 250 million years ago,
a. plate movements brought all the landmasses together and the supercontinent of
Pangaea was formed,
b.the physical environment and climate changed dramatically, and
5. During the Mesozoic era, Pangaea started to break apart, causing a geographic isolation of colossal proportions.
6. By the end of the Mesozoic era, some 65 million years ago, the modern continents were beginning to take shape.
7. The history of continental merges and separations explains many patterns of
biogeography, the study of the past and present distribution of organisms.
8. Continental drift solves the mystery of marsupials, mammals whose young complete their embryonic development in a pouch outside the mother’s body, such as
kangaroos, koalas, and wombats.
9. Australia and its neighboring islands are home to more than 200 species of marsupials, most of which are found nowhere else in the world.
B. 15.8 CONNECTION: Plate tectonics may imperil human life
1. Volcanoes and earthquakes result from the movements of crustal plates.
a. The boundaries of plates are hotspots of volcanic and earthquake activity.
b.California’s frequent earthquakes are a result of movement along the infamous San Andreas Fault, part of the border where the Pacific and North American plates grind together and gradually slide past each other.
C.15.9 During mass extinctions, large numbers of species are lost
1. Extinction is inevitable in a changing world.
2. The fossil record shows that the vast majority of species that have ever lived are now extinct.
3. Over the last 500 million years, five mass extinctions have occurred and in each event, more than 50% of the Earth’s species went extinct.
4. The Permian mass extinction
a. occurred about 251 million years ago,
b.defines the boundary between the Paleozoic and Mesozoic eras,
c. claimed 96% of marine animal species, and
d.took a tremendous toll on terrestrial life.
5. The Cretaceous mass extinction, about 65 million years ago,
a. caused the extinction of more than half of all marine species and many lineages of terrestrial plants and animals.
b.resulted in the extinction of almost all of the dinosaurs except birds, and
c. was likely caused by a large asteroid that struck the Earth, which would have blocked light and severely disturbed the global climate for months.
6. Mass extinctions affect biological diversity profoundly.
7. It took 100 million years for the number of marine families to recover after Permian mass extinction.
8. Is a sixth extinction under way?
a. The current extinction rate is 100–1,000 times the normal background rate.
b.It may take millions of years for life on Earth to recover.
D.15.10 Adaptive radiations have increased the diversity of life
2. Adaptive radiations have also followed each mass extinction, when survivors became adapted to the many vacant ecological roles, or niches, in their communities.
a. For example, mammals underwent a dramatic adaptive radiation after the extinction of terrestrial dinosaurs 65 million years ago.
E. 15.11 Genes that control development play a major role in evolution
1. The fossil record can tell us
a. what the great events in the history of life have been and
b.when they occurred.
2. Continental drift, mass extinctions, and adaptive radiation provide a big-picture view of how those changes came about.
3. We are now increasingly able to understand the basic biological mechanisms that underlie the changes seen in the fossil record.
4. The field of evo-devo
a. addresses the interface of evolutionary biology and developmental biology and
b.examines how slight genetic changes can produce major morphological differences.
5. Genes that program development control the
a. rate,
b.timing, and
c. spatial pattern of change in an organism’s form as it develops.
6. Many striking evolutionary transformations are the result of a change in the rate or timing of developmental events.
7. Paedomorphosis
a. is the retention in the adult of body structures that were juvenile features in an ancestral species and
b.occurs in the axolotl salamander in which sexually mature adults retain gills and other larval features.
8. Slight changes in the relative growth of different body parts can change an adult form substantially.
9. Skulls of humans and chimpanzees are more similar as fetuses but quite different as adults, due to different rates of growth.
10. Homeotic genes
a. are called master control genes and
b.determine basic features, such as where pairs of wings or legs develop on a fruit fly.
11. Profound alterations in body form can result from
a. changes in homeotic genes or
b.how or where homeotic genes are expressed.
12. New developmental genes that arose as a result of gene duplications may have facilitated the origin of new body forms.
a. For example, a fruit fly has a single cluster of homeotic genes that direct the development of major body parts.
b.Two duplications of these gene clusters appear to have occurred in the evolution of vertebrates from invertebrates.
c. Mutations in these duplicated genes may then have led to the origin of novel vertebrate characteristics, such as a backbone, jaws, and limbs.
13. Additional evidence for this type of change in gene regulation is seen in studies of the three-spine stickleback fish.
15. Stickleback fish that live in the ocean have bony plates that make up a kind of body armor and a large set of pelvic spines that help deter predatory fish.
16. Stickleback fish that livein lakes have reduced or absent bony plates and pelvic spines, resulting from a change in the expression of a developmental gene in the pelvic region.
F. 15.12 Novel traits may arise in several ways
1. Most complex structures have evolved in increments from simpler versions having the same basic function—a process of refinement.
2. But sometimes we can trace the origin of evolutionary novelties to the gradual adaptation of existing structures to new functions.
3. As an example of the process of gradual refinement, consider the amazing camera-like eyes of vertebrates and squids.
4. Although the eyes of vertebrates evolved independently of those of squids, both evolved from a simple ancestral patch of photoreceptor cells, through a series of incremental modifications that benefited their owners at each stage.
5. There appears to have been a single evolutionary origin of light-sensitive cells in all animals with eyes, vertebrates and invertebrates.
6. They all share the same master genes that regulate eye development.
7. Figure 15.12 illustrates the range of complexity in the structure of eyes among molluscs living today.
8. Evolutionary novelty can also arise when structures that originally played one role gradually acquire a different one.
9. Such structures that evolve in one context but become co-opted for another function are sometimes called exaptations. Examples of exaptations include
a. feathers that may have first functioned for insulation and later were co-opted for flight and
b.flippers of penguins that first functioned for flight and were co-opted for underwater swimming.
G.15.13 Evolutionary trends do not mean that evolution is goal directed
1. The fossil record seems to show trends in the evolution of many species, for example, toward larger or smaller body size.
2. The evolution of horses reveals a potential misunderstanding.
a. If we select only certain species in this family tree, it appears that there was a general trend toward the reduction in the number of toes, larger size, and teeth modified for grazing.
b.However, if we consider all of the known members of this family tree, this apparent trend vanishes.
3. Branching evolution can lead to real evolutionary trends.
4. The species selection model of long-term trends compares species to individuals.
a. Speciation is their birth, extinction their death, and new species that diverge from them are their offspring.
b.Unequal survival of species and unequal generation of new species play a role in macroevolution similar to the role of unequal reproduction in microevolution.
6. Whatever its cause, it is important to recognize that an evolutionary trend does not imply that evolution progresses toward a particular goal.
7. Evolution is the result of interactions between organisms and the current environment.
V. Phylogeny and the Tree of Life
A.15.14 Phylogenies based on homologies reflect evolutionary history
1. Phylogeny is the evolutionary history of a species or group of species.
2. Phylogeny can be inferred from
a. the fossil record,
b.morphological homologies, and
c. molecular homologies.
3. Homologies are similarities due to shared ancestry, evolving from the same structure in a common ancestor.
4. Generally, organisms that share similar morphologies are closely related.
a. However, some similarities are due to similar adaptations favored by a common environment, a process called convergent evolution.
b.A similarity due to convergent evolution is called analogy.
B. 15.15 Systematics connects classification with evolutionary history
1. Systematics is a discipline of biology that focuses on
a. classifying organisms and
b.determining their evolutionary relationships.
2. Carolus Linnaeus introduced taxonomy, a system of naming and classifying species.
3. Biologists assign each species a two-part scientific name, or binomial, consisting of
a. a genus and
b.a unique part, the specific epithet, for each species within the genus.
4. Genera are grouped into progressively larger categories.
5. Each taxonomic unit is a taxon.
6. Biologists traditionally use phylogenetic trees to depict hypothesis about the evolutionary history of species.
a. The branching diagrams reflect the hierarchical classification of groups nested within more inclusive groups.
b.Phylogenetic trees indicate the probable evolutionary relationships among groups and patterns of descent from the last common ancestors.
C.15.16 Shared characters are used to construct phylogenetic trees
1. Cladistics
a. is the most widely used method in systematics and
b.groups organisms into clades.
2. Each clade is a monophyletic group of species thatincludes an ancestral species and all of its descendants.
3. Cladistics is based on the Darwinian concept that organisms share characters with their ancestors and differ from them. Thus, there are two main types of characters.
a. Shared ancestral characters, characters that originated in an ancestor.
b. Shared derived characters, unique evolutionary novelties.
5. As an example, consider a frog representing the outgroup and four other tetrapods representing the ingroup.
6. The presence or absence of characters is indicated as
a. 1 if the character is present or
b.0 if the character is absent.
c. In our example, the phylogenetic tree is constructed from a series of branch points (called nodes) represented by the emergence of a lineage with a new set of derived characters.
d.When constructing a phylogenetic tree, scientists use parsimony, looking for the simplest explanation for observed phenomena.
7. Systematists use many kinds of evidence. However, even the best tree represents only the most likely hypothesis based on available evidence.
8. As new data accumulate, hypotheses are revised and new trees drawn.
9. The phylogenetic tree of reptiles shows that crocodilians are the closest living relatives of birds.
a. They share numerous features:
i. they have four-chambered hearts,
ii. they “sing” to defend territories, and
iii. they build nests.
b.These traits were likely present in the common ancestor of birds, crocodiles, and dinosaurs.
D.15.17 An organism’s evolutionary history is documented in its genome
1. Molecular systematics uses DNA and other molecules to infer relatedness.
a. Scientists have sequenced more than 110 billion bases of DNA from thousands of species.
b.This enormous database has fueled a boom in the study of phylogeny and clarified many evolutionary relationships.
2. The more recently two species have branched from a common ancestor, the more sim-ilar their DNA sequences should be.
3. The longer two species have been on separate evolutionary paths, the more their DNA is expected to have diverged.
4. Molecular evidence has also begun to sort out the relationships among the species of bears.
5. Different genes evolve at different rates.
a. DNA coding for ribosomal RNA (rRNA)
i. changes relatively slowly and
ii. is useful for investigating relationships between taxa that diverged hundreds of millions of years ago.
b.In contrast, DNA in mitochondria (mtDNA)
i. evolves rapidly and
ii. can be used to investigate more recent evolutionary events.
6. The remarkable commonality of molecular biology
a. demonstrates that all living organisms share many biochemical and developmental pathways and
E. 15.18 Molecular clocks help track evolutionary time
1. Molecular clocks
a. rely on genes that have a reliable average rate of change,
b.can be calibrated in real time by graphing the number of nucleotide differences against the dates of evolutionary branch points known from the fossil record, and
c. are used to estimate dates of other evolutionary episodes not documented in the fossil record.
2. Molecular clocks have been used to date a wide variety of events.
3. In one fascinating example, published in 2011, researchers studied the divergence of human body lice from head lice and estimated that people began to wear clothing between 83,000 and 170,000 years ago.
4. Some biologists are skeptical about the accuracy of molecular clocks because the rate of molecular change may vary
a. at different times,
b.in different genes, and
c. in different groups of organisms.
F. 15.19 Constructing the tree of life is a work in progress
1. Molecular systematics and cladistics are remodeling some trees.
2. Biologists currently recognize a three-domain system consisting of
a. two domains of prokaryotes: Bacteria and Archaea, and
b.one domain of eukaryotes, called Eukarya, including the kingdoms i. Fungi,
ii. Plantae, and
iii. Animalia.
3. Molecular and cellular evidence indicates that
a. the two lineages of prokaryotes (bacteria and archaea) diverged very early in the evolutionary history of life and
b.archaea are more closely related to eukaryotes than to bacteria.
4. Comparisons of complete genomes from all three domains show that, especially during the early history of life, there have been substantial interchanges of genes between organisms in different domains and these took place through horizontal gene transfer, a process in which genes are transferred from one genome to another through mechanisms such as plasmid exchange and viral infection.
5. Some biologists suggest that the early history of life may be best represented by a ring, from which the three domains emerge.
6. Some scientists have argued that horizontal gene transfers were so common that the early history of life should be represented as a tangled network of connected branches.
