Lecture Outline
I.
Biologists have two major tools for studying the past.
A. Using Phylogenies
1. What is a phylogeny?
a. A phylogeny is an evolutionary history of a group of organisms.
b. A phylogenetic tree shows evolutionary relationships of groups of
organisms.
2. Reading phylogenies (Fig. 26.1)
a. Branches represent populations through time.
(1) Adjacent branches are sister taxa.
(2) At branch tips are groups living today, or those that have gone extinct.
b. Nodes (forks) occur when an ancestral group split into multiple
descendant groups.
c. The root or base represents the most ancient ancestor that all of the
branches have in common.
(1) The root is determined by an outgroup that is thought to have
diverged before the other taxa on the tree.
(2) A phylogenetic tree illustrates monophyletic groups that consist of an
ancestor and all of its descendants. (Fig. 26.2)
3. How Do Researchers Estimate Phylogenies?
a. The phenetic approach
(1) Based on statistical analysis of similarities among populations
(2) Usually uses genetic analysis to calculate genetic distance or average
percentage of bases in a DNA sequence that differs between two
populations
b. The cladistic approach (Fig. 26.4)
(1) Based on morphological similarities among populations
(2) Involves the identification of shared derived characteristics or
synaptomorphies
(3) Based on the logic that when an ancestral population splits, the
descendant populations acquire new traits that are derived from traits
in the ancestor, and the populations are therefore similar
(4) Convergent evolution can present a problem for cladistic analysis
because analogous traits can be mistaken for synaptomorphies.
4. Whale evolution: a case history
a. Whales, cows, and hippos are all Artiodactyla.
b. All Artiodactyla have an ankle bone called an astralagus; it is the shared
derived characteristic that identifies them as a monophyletic group.
c. Cladistic data indicated that whales were the outgroup in the Artiodactyla
group because they lacked the astralagus. (Fig. 26.5a)
d. Phenetic data conflicted with the cladistic data.
(1) DNA analysis indicated that whales are most closely related to
hippos.
(2) Identification and analysis of SINE sequences illustrated that whales
are not the outgroup for Artiodactyla, but camels are. (Fig. 26.5b)
B.
II.
Using the Fossil Record
1. How do fossils form?
a. Fossils form when part or all of an organism is buried in ash, sand, mud, or
some other sediment before it can decompose. (Fig. 26.6)
b. Once burial occurs, several things can then happen:
(1) If decomposition does not occur, the organic remains can be preserved
intact. (Fig. 26.7a)
(2) If sediments accumulate on top of the material, they can compress the
organic material. (Fig. 26.7b)
(3) If the remains decompose after they are buried, the hole that remains
can fill with minerals and create a cast of the remains. (Fig. 26.7c)
(4) Dissolved minerals can gradually infiltrate the interior of the cells and
harden into stone, forming a permineralized fossil. (Fig 26.7d)
c. After many centuries have passed, fossils can be exposed at the surface
through erosion, a road cut, quarrying, etc.
d. If a fossil is found, researchers can prepare it for study. (Fig. 26.8)
e. Fossilization is a relatively rare event, since rapid decomposition and slow
burial occur more often than slow decomposition and rapid burial do.
2. Limitations of the fossil record
a. Habitat bias: Because burial of sediments is so crucial to fossilization, there
is strong habitat bias in the record.
b. Taxonomic basis: Organisms with hard parts are much more likely to leave
fossil evidence than those with soft bodies are.
c. Temporal bias: Recent fossils are much more common than ancient ones.
d. Abundance bias: Because fossilization is improbable, the record is
weighted toward common species.
3. Life’s timeline (Fig. 26.9)
4. Molecular data from living species can supplement fossil data (Box 26.1;
Figs. 26.10, 26.11)
The Cambrian Explosion
A. The first animals appear in the fossil record about 563 million years ago.
1. Their early diversification occurred at the start of the Cambrian period.
2. This diversification occurred so rapidly that it is called the Cambrian explosion.
3. This incredible diversification occurred in only 40 million years. (Fig. 26.12a)
4. The Cambrian explosion is documented by three major fossil assemblages:
a. The Doushantuo microfossils (570 million years ago)
b. The Ediacaran faunas (565–544 million years ago)
c. The Burgess Shale faunas (525–515 million years ago)
B.
The Doushantuo Microfossils
1. These deposits come from the Doushantuo formation in China.
2. Chia-Wei Li et al. were able to identify several dozen individual sponges.
3. Other researchers found clusters of cells that were animal embryos.
(Fig. 26.12b)
4.
5.
Cyanobacteria, multicellular algae, and other cell types were also found.
Fossils of what are thought to be the first animals on Earth were also found.
C.
The Ediacaran Faunas
1. These deposits come from the Ediacara Hills in southern Australia.
2. The specimens included the compressed bodies of large sponges, jellyfish, and
comb jellies.
3. Many burrows, tracks, and other traces from unidentified species were also
found. (Fig. 26.12c)
4. No animals with shells were present.
5. The data here indicate that shallow-water marine habitats contained a diversity
of animal species.
6. None of these specimens have limbs, heads, mouths, or feeding appendages.
7. Thus, the Ediacaran animals burrowed in sediments, sat immobile on the sea
floor, or floated in the water; they did not actively hunt.
D.
The Burgess Shale Faunas
1. These deposits come from the Burgess Shale in British Columbia, Canada.
2. Few species from the Ediacaran faunas were also found in this site.
4. New species of sponges, jellyfish, and comb jellies were present.
5. Arthropods and mollusks were also present for the first time.
6. Echinoderms, worms, and even a few chordates were also present. (Fig. 26.12d)
III. The Genetic Mechanisms of Change
A. Gene Duplications and the Cambrian Explosion
1. Proteins coded by homeotic loci are responsible for laying out the threedimensional pattern of multicellular organisms as they develop.
2. These genes may have triggered the origin and elaboration of animal body
plans and appendages that occurred during the Cambrian explosion.
a. Scientists are working to determine the number and identity of homeotic
loci found in different animal groups.
b. There may be a strong correlation between the phylogenetic history, the
number of homeotic genes present in each group, morphological
complexity, and body size.
(1) This is known as the “new genes, new bodies” hypothesis.
(2) Gene duplication events could have occurred before and during the
Cambrian explosion, making new copies of existing homeotic genes.
(3) The number of homeotic loci present should correlate directly with
morphological complexity.
c. Data from the homeotic genes called the Hox loci (Fig. 26.13)
(1) The number and identity of Hox loci varies widely among animals.
(2) Groups that branched off early and are simpler have fewer Hox genes
than more complex groups that branched off later.
(3) New Hox loci may have been created by gene duplication events.
(4) Vertebrates have several copies of the entire Hox cluster of genes.
(5) These data supports the “new genes, new bodies” hypothesis.
B.
Changes in Gene Expression
1.
2.
3.
4.
The origin of the vertebrate foot is a case study of how these changes can
clearly affect evolution.
A major innovation during vertebrate evolution was the origin of a limb with
feet, leading to the evolution of tetrapods.
The tetrapod limb evolved from the fins of fish.
a. But fish and tetrapods have the same number of Hox loci.
b. Changes in gene expression are likely responsible for the transition from
fish to tetrapod.
Sordino and colleagues explored this question by comparing the expression of
two genes, hoxd-11 and Shh, in zebrafish and mice.
a. When limb buds were specially treated early in development, there was no
difference in the pattern of gene expression between mice and zebrafish.
(Fig. 26.14a)
b. When the experiment was performed late in development, there was a
dramatic difference in the timing and location of gene expression in mice.
(Fig. 26.14b)
c. The late and “reoriented” expression of hoxd-11 and Shh may have added
an entirely new element to the limbs of mammals. (Fig. 26.14c)
IV. Adaptive Radiations
A. When the tree of life is examined in a broad sense, dense, bushy outgrowths are
seen scattered among the branches. (Fig. 26.15a)
1. This phenomenon results when many large, distinctive groups of organisms
branch from a lineage in a short time.
2. Due to its shape, the pattern is sometimes called a star phylogeny. (Fig. 26.15b)
3. A leading cause of this phenomenon is known as adaptive radiation.
a. It occurs when a single lineage produces descendants with a wide variety
of adaptive forms.
b. When this diversification occurs quickly, it results in a star phylogeny.
c. The hallmark of adaptive radiation is ecological diversification within a
single lineage.
B.
Colonization events may trigger adaptive radiations.
1. One of the most consistent themes in adaptive radiations is opportunity.
a. Adaptive radiations frequently occur when habitats are unoccupied by
competitors.
b. Example: The radiation of mammals following extinction of the dinosaurs
2. Losos et al. documented this by studying radiations triggered by colonization
events on islands with distinct habitats that were free of competitors.
a. Their study focused on Anolis lizards. (Fig. 26.16a)
b. Their results indicated that the original colonist on each island belonged to
a different ecological type. (Fig. 26.16b)
c. From different starting points, an adaptive radiation had occurred on each
island.
d. On all islands, the same four ecological types of lizards eventually evolved.
C.
The Role of Morphological Innovation
1. Extinction and colonization are not the only triggers for adaptive radiation.
2.
V.
New traits, or “morphological innovations,” are a driving force behind many
adaptive radiations.
a. Evolution of wings, six legs, and an exoskeleton triggered massive
diversification in insect populations. (Fig. 26.17a)
b. Evolution of flowers triggered the diversification of angiosperms.
(Fig. 26.17b)
c. An extra set of jaws in the throat triggered the emergence of cichlids.
(Fig. 26.17c)
d. Feathers and wings stimulated the diversification of birds. (Fig. 26.17d)
Mass Extinctions
A.
Mass extinction are the rapid extinction of a large number of lineages scattered
throughout the tree of life, whereas background extinction is the lower, average rate
of extinction observed over the entire history of life.
B.
Biologists typically recognize five major mass extinction events. (Fig. 26.18)
C.
How Do Background and Mass Extinctions Differ?
1. Background and mass extinctions have contrasting causes and effects.
a. Background extinctions occur when normal environmental change or
competition reduces certain populations to the point where they die out.
b. Mass extinctions result from extraordinary, sudden, and temporary
changes in the environment.
2. Mass extinctions are much rarer than background extinctions.
D.
Who Killed the Dinosaurs?
1. K-T refers to the extinction that wiped out the dinosaurs at the end of the
Cretaceous and beginning of the Tertiary periods.
2. Impact hypothesis: The cause of this mass extinction was the impact of an
asteroid about 10 km across.
3. Once controversial, this hypothesis is now supported by strong evidence:
a. Sedimentary rocks that formed at the K-T boundary contain very high
amounts of iridium (rare on Earth, common in meteorites). (Fig. 26.19a)
b. In Haiti, shocked quartz and microtektites are abundant in rock layers
from this time period. These minerals are found only at documented
meteorite impact sites. (Fig. 26.19b)
c. A huge crater exists just off the northwest coast of Mexico’s Yucatan
Peninsula. (Fig. 26.19c)
4. The distribution of shocked quartz and microtektites indicates that the meteor
hit Earth at an angle and splashed over much of southeastern North America.
5. Computer models and geologic data indicate that the impact unleashed a
devastating chain of events.
a. A fireball of hot gas spread from the impact site, triggering extensive
worldwide wildfires.
b. The SO4–2 released from the impact site reacted with the atmosphere to
form sulfuric acid, triggering extensive acid rain.
c. Massive amounts of dust, ash, and soot blocked the Sun for long periods of
time, leading to rapid global cooling and a crash in plant productivity.
6.
7.
d. From 60% to 80% of all species went extinct during this time.
Selectivity
a. The asteroid impact did not kill indiscriminately.
(1) Dinosaurs and all large-bodied marine reptiles perished.
(2) Mammals, crocodilians, amphibians, and turtles survived.
b. Traditionally, the leading hypothesis to explain this was that the extinction
event was size selective; but no data support this.
Recovery
a. After the K-T extinction, fern fronds and spores dominate the plant fossil
record.
b. Within 5–10 million years, all the major mammalian orders had appeared,
replacing the ecological niches left open by the extinct dinosaurs.