Download Chapter 27

Phylogenies and the
History of Life
27
BIOLOGICAL SCIENCE
FOURTH EDITION
SCOTT FREEMAN
Lectures by Stephanie Scher Pandolfi
© 2011 Pearson Education, Inc.
Key Concepts
Phylogenetic trees document the evolutionary relationships
among organisms and are estimated from data.
The fossil record provides physical evidence of organisms that
lived in the past.
Adaptive radiations are a major pattern in the history of life. They
are instances of rapid diversification associated with new
ecological opportunities and new morphological innovations.
Mass extinctions have occurred repeatedly throughout the history
of life. They are environmental catastrophes that rapidly eliminate
most of the species alive.
© 2011 Pearson Education, Inc.
Introduction
• In biology we must consider profound changes in the nature of life
on Earth over immense periods of time.
• There are two major analytical tools that biologists use to
reconstruct the history of life: phylogenetic trees and the fossil
record.
© 2011 Pearson Education, Inc.
Tools for Studying History: Phylogenetic Trees
• The evolutionary history of a group of organisms is called a
phylogeny.
• Phylogenies are usually summarized and depicted in the form of a
phylogenetic tree, which shows ancestor-descendant relationships
among populations or species.
© 2011 Pearson Education, Inc.
Reading a Phylogenetic Tree
• A branch represents a population through time.
• The point where two branches diverge, called a node (or fork),
represents the point in time when an ancestral species split into two
or more descendant species
• A tip (or terminal node), the endpoint of a branch, represents a
group (a species or larger taxon) that is living today or ended in
extinction.
© 2011 Pearson Education, Inc.
How Do Researchers Estimate Phylogenies?
Phylogenetic trees are an extremely effective way of summarizing
data on the evolutionary history of a group of organisms.
• Researchers analyze morphological and/or genetic characteristics
to infer phylogenetic relationships among species.
• There are two general strategies for using data to estimate trees:
the phenetic and the cladistic approaches.
© 2011 Pearson Education, Inc.
The Phenetic Approach to Estimating Phylogenies
• The phenetic approach is based on computing a statistic that
summarizes the overall similarity among populations.
• A computer program then compares the statistics for different
populations and builds a tree that clusters the most similar
populations and places more divergent populations on more distant
branches.
© 2011 Pearson Education, Inc.
The Cladistic Approach to Estimating Phylogenies
• The cladistic approach to inferring trees focuses on
synapomorphies, the shared derived characters of the species
under study.
• Synapomorphies allow biologists to recognize monophyletic
groups—also called clades or lineages. Synapomorphies are
characteristics that are shared because their common ancestor had
them.
• When many such traits have been measured, traits unique to each
clade are identified and the groups are placed on a tree in the
appropriate relationship to one another.
© 2011 Pearson Education, Inc.
Ancestral and Derived Characters
• An ancestral trait is a characteristic that existed in an ancestor.
• A derived trait is one that is a modified form of the ancestral trait,
found in a descendant.
• Ancestral and derived traits are relative.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Distinguishing Homology from Homoplasy
• Problems can arise with both phenetic and cladistic analyses
because similar traits can evolve independently in two distant
species rather than from a trait present in a common ancestor.
• Homology occurs when traits are similar due to shared ancestry.
• Homoplasy occurs when traits are similar for reasons other than
common ancestry.
– For example, ichthyosaurs (extinct aquatic reptiles) and
dolphins (extant mammals) are very similar, but these
similarities are not due to common ancestry.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Distinguishing Homology from Homoplasy
• Convergent evolution occurs when natural selection favors similar
solutions to the problems posed by a similar way of life.
• Convergent evolution is a common cause of homoplasy.
• If similar traits found in distantly related lineages are indeed similar
due to common ancestry, then similar traits should be found in
many intervening lineages on the tree of life.
© 2011 Pearson Education, Inc.
Evidence for Homology
• Even though insects and vertebrates diverged some 600–700
million years ago, biologists argue that their Hox genes are derived
from the same ancestral sequences.
• There are several lines of evidence to support this hypothesis:
– Groups of Hox genes are organized on chromosomes in a
similar way.
– All of the Hox genes share a 180-base-pair sequence called the
homeobox.
– The products of the Hox genes have similar functions.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Evidence for Homology
• If similar traits found in distantly related lineages are indeed similar
due to common ancestry, then similar traits should be found in
many intervening lineages on the tree of life—because all of the
species in question inherited the trait from the same common
ancestor.
© 2011 Pearson Education, Inc.
Using Parsimony
• Parsimony is a principle of logic stating that the most likely
explanation or pattern is the one that implies the least amount of
change.
• Convergent evolution and other causes of homoplasy should be rare
compared with similarity due to shared descent, so the tree that
implies the fewest overall evolutionary changes should be the one
that most accurately reflects what happened during evolution.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Whale Evolution: A Case History
• Artiodactyls, including hippos, cows, deer, and pigs, are mammals
that have hooves, an even number of toes, and an unusual pulleyshaped ankle bone (astragalus).
• Traditionally, phylogenetic trees based on morphological data place
whales as the outgroup—that is, a species or group that is closely
related to the monophyletic group but not part of it.
© 2011 Pearson Education, Inc.
Whale Evolution: A Case History
• DNA sequence data, however, suggest a close relationship between
whales and hippos. This tree would require two changes to the
astragalus trait.
• Recent data on gene sequences called short interspersed nuclear
elements (SINEs) show that whales and hippos share several SINE
genes that are absent in other artiodactyl groups.
• These SINEs are shared derived traits (synapomorphies) and
support the hypothesis that whales and hippos are indeed closely
related.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Tools for Studying History: The Fossil Record
• A fossil is the physical trace left by an organism that lived in the
past.
• The fossil record is the total collection of fossils that have been
found throughout the world.
The fossil record provides the only direct evidence about what
organisms that lived in the past looked like, where they lived, and
when they existed.
© 2011 Pearson Education, Inc.
How Do Fossils Form?
• Fossilization preserves traces of organisms that lived in the past.
• Most of the processes that form fossils begin when part or all of an
organism is buried in ash, sand, mud, or some other type of
sediment.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Preservation after Burial
• There are four main types of fossils:
1. Intact fossils form when decomposition does not occur.
2. Compression fossils form when sediments accumulate on top
of the material and compress it into a thin film.
3. Cast fossils form when the remains decompose after burial
and dissolved minerals create a cast in the remaining hole.
4. Permineralized fossils form when the remains rot extremely
slowly and dissolved minerals infiltrate the interior of the
cells and harden into stone.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Fossilization Is a Rare Event
• Fossilization only occurs under ideal conditions.
• There are 10 specimens of the first bird to appear in the fossil
record, Archaeopteryx.
• As far as researchers currently know, only 1 out of every
200,000,000 individuals fossilized.
© 2011 Pearson Education, Inc.
Limitations of the Fossil Record
• Paleontologists—scientists who study fossils—recognize that they
are limited to studying tiny and scattered segments on the tree of
life, yet they also know that this is the only way to get a glimpse of
what extinct life was like.
• There are several limitations of the fossil record:
1. Habitat bias occurs because organisms that live in areas
where sediments are actively being deposited are more likely
to fossilize than are organisms that live in other habitats.
© 2011 Pearson Education, Inc.
Limitations of the Fossil Record
2. Taxonomic bias is due to the fact that some organisms (e.g.,
those with bones) are more likely to decay slowly and leave
fossil evidence.
3. Temporal bias occurs because more recent fossils are more
common than ancient fossils.
4. Abundance bias occurs because organisms that are abundant,
widespread, and present on Earth for a long time leave
evidence much more often than do species that are rare, local,
or ephemeral.
© 2011 Pearson Education, Inc.
Life's Timeline
• The best data available indicate that the Earth started to form about
4.6 billion years ago, and that life began around 3.4 billion years
ago.
• To organize the tremendous sweep of time between then and now,
researchers divide Earth history into segments called eons, eras, and
periods.
• Radiometric dating allows researchers to assign absolute dates—
expressed as years before the present—to events in the fossil
record.
© 2011 Pearson Education, Inc.
The Precambrian
• The Precambrian encompasses the Hadean, Archaean, and
Proterozoic eons. This period spans from the formation of the Earth
to the appearance of most animal groups about 542 million years
ago (mya).
• In the Precambrian era, almost all life was unicellular and hardly
any oxygen was present.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
The Phanerozoic Eon
• The Phanerozoic eon spans the interval between 542 mya and the
present. It is divided into three eras—the Paleozoic, the Mesozoic,
and the Cenozoic—that are further divided into periods.
• The Paleozoic era covers the interval from 542 to 251 mya.
– Many animal groups—including fungi, land plants, and land
animals—appeared in the Paleozoic era. This era ends with the
obliteration of almost all multicellular life-forms at the end of
the Permian period.
© 2011 Pearson Education, Inc.
The Phanerozoic Eon
• The Mesozoic era (Age of Reptiles) covers the interval from 251 to
65.5 mya.
– This era saw the rise and dominance of the dinosaurs and ended
with their extinction.
• The Cenozoic era (Age of Mammals) includes the interval from
65.5 mya to the present.
– During this time the mammals diversified after the
disappearance of the dinosaurs.
– Events that occur today are considered to be part of the
Cenozoic era.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Changes in the Oceans and Continents
• Earth’s crust is broken into enormous plates that are in constant
motion, driven by heat rising from the planet’s core.
• Movement of these plates has dramatically shifted the extent and
position of the continents over time.
• There have been major changes in climate as well.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Adaptive Radiations
• An adaptive radiation is when rapid speciation in a single lineage
is followed by divergence into many different adaptive forms.
• The Hawaiian silverswords fulfill the three hallmarks of an
adaptive radiation:
– They are a monophyletic group.
– They speciated rapidly.
– They diversified ecologically.
• Biologists use the term niche to describe the range of resources that
a species can use and the range of conditions that it can tolerate.
Silverswords occupy a wide array of niches.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Why Do Adaptive Radiations Occur?
Two general mechanisms can trigger adaptive radiations: new
resources, and new ways to exploit resources.
© 2011 Pearson Education, Inc.
Ecological Opportunity as a Trigger
• One of the most consistent triggers of adaptive radiations is
ecological opportunity—the availability of new types of resources.
• For example, biologists have documented adaptive radiations of the
Anolis lizards of the Caribbean islands.
• On the two islands studied, the same four ecological types
eventually evolved, because the islands had similar varieties of
habitats. Therefore, similar adaptive radiations took place
independently on the two islands, triggered by the available
environment and lack of competition.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Morphological Innovation as a Trigger
• Morphological innovation can also be a trigger for adaptive
radiation, as was seen in the Cambrian explosion.
• Many of the other important diversification events in the history of
life started off with the evolution of a key morphological trait that
allowed descendants to live in new areas, exploit new food sources,
or move in new ways.
© 2011 Pearson Education, Inc.
Examples of Morphological Innovations
• The evolution of wings, three pairs of legs, complex mouthparts,
and a strong external skeleton helped insects move and feed
efficiently. Today insects are the most diverse lineage on Earth,
with perhaps well over 3 million species in existence.
• Flowers are a unique reproductive structure that are particularly
efficient at attracting pollinators, making angiosperms more
reproductively efficient. Today angiosperms are far and away the
most species-rich lineage of land plants. Over 250,000 species are
known.
© 2011 Pearson Education, Inc.
Examples of Morphological Innovations
• Cichlids are a lineage of fish that evolved a unique set of jaws in
their throat, making food processing extremely efficient. Different
species have throat jaws specialized for feeding on different foods.
Over 300 species of cichlid live in Africa’s Lake Victoria alone.
• Feathers and wings gave some dinosaurs the ability to fly. Today
the lineage called birds contains about 10,000 species, with
representatives that live in virtually every habitat on the planet.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Adaptive Radiation
© 2011 Pearson Education, Inc.
The Cambrian Explosion
• The first animals—sponges, jellyfish, and simple worms—appear
in the fossil record around 565 mya, at the end of the Proterozoic
eon.
• Soon after that in geologic time, by about 50 million years later,
animals had diversified into almost all the major groups living
today.
• This diversification is known as the Cambrian explosion.
• This period saw what was arguably the most evolutionary change in
the history of life.
© 2011 Pearson Education, Inc.
Cambrian Fossils: An Overview
• The Cambrian explosion is documented by three major fossil
assemblages, called the Doushantuo, Ediacaran, and Burgess Shale
fossils.
• The presence of these exceptionally rich deposits before, during,
and after the Cambrian explosion makes the fossil record for this
event extraordinarily complete.
© 2011 Pearson Education, Inc.
The Doushantuo Microfossils
• From the Doushantuo formation in China, researchers identified
microfossils (tiny fossils) of sponges, cyanobacteria, and
multicellular algae in samples dated 570–580 mya. They also found
what they concluded were animal embryos in early stages.
• These were examples of the first types of animals on Earth.
© 2011 Pearson Education, Inc.
The Ediacaran Faunas
• In the Ediacara Hills in Australia, paleontologists identified fossils
of sponges, jellyfish, comb jellies, and traces of other animals dated
544–565 mya.
• These were small, soft-bodied animals that burrowed in sediments,
sat immobile on the seafloor, or floated in the water.
© 2011 Pearson Education, Inc.
The Burgess Shale Faunas
• Virtually every major living animal group is represented in the
Burgess Shale fossils from British Columbia, Canada, which date to
515–525 mya.
• These fossils indicate a tremendous increase in the size and
morphological complexity of animals, accompanied by
diversification in how they made a living.
• This diversification filled many of the ecological niches still found
in marine habitats today.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
What Triggered the Cambrian Explosion?
• There are four non-mutually-exclusive hypotheses:
1. Increased oxygen levels made aerobic respiration more efficient.
2. The evolution of predation exerted selection pressure for prey
defense strategies, driving morphological divergence.
3. New niches beget more new niches. The ability to exploit new
niches created new niches for predators, driving speciation and
ecological diversification.
4. New genes, new bodies. Mutations increased the number of Hox
genes in animals and made it possible for larger, more complex
bodies to evolve.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Mass Extinctions
• A mass extinction is the rapid extinction of a large number of
lineages scattered throughout the tree of life. A mass extinction
occurs when at least 60 percent of the species present are wiped out
within 1 million years.
• Mass extinctions are caused by catastrophic events.
• Paleontologists traditionally recognize five mass extinctions ("The
Big Five").
• Background extinction is the lower, average rate of extinction,
representing the relatively constant, normal loss of some species.
© 2011 Pearson Education, Inc.
How Do Background and Mass Extinctions Differ?
• Background extinctions typically occur when normal environmental
change, emerging diseases, or competition reduces certain
populations to zero.
• Mass extinctions result from extraordinary, sudden, and temporary
changes in the environment; they cause extinction randomly with
respect to individuals’ fitness under normal conditions.
In a general sense, background extinctions are thought to result
primarily from natural selection. Mass extinctions, in contrast,
function like genetic drift.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
The End-Permian Extinction
• The largest mass extinction was the end-Permian extinction, which
resulted in the disappearance of 90 percent of all species.
• There are four hypotheses about the cause of this event:
1. Flood basalts called the Siberian traps added enormous
quantities of heat, CO2, and sulfur dioxide to the atmosphere.
This led to intense global warming and the formation of toxic
sulfuric acid.
2. Oceans became completely or largely anoxic—meaning
that they lacked oxygen. These conditions are fatal to
organisms that rely on aerobic respiration.
© 2011 Pearson Education, Inc.
The End-Permian Extinction
3. Sea level dropped dramatically during the extinction event,
reducing the amount of habitat available for marine
organisms.
4. Low oxygen concentrations and high CO2 levels in the
atmosphere may have restricted terrestrial animals to small
patches of low-elevation habitats.
• The cause of the end-Permian extinction may be the most important
unsolved question in research on the history of life. In contrast, the
cause of the dinosaur’s demise is settled.
© 2011 Pearson Education, Inc.
What Killed the Dinosaurs?
• The impact hypothesis for the extinction of dinosaurs proposes
that an asteroid struck Earth 65 mya, resulting in the extinction of
an estimated 60–80% of the multicellular species alive.
• Conclusive evidence—including iridium, shocked quartz, and
microtektites found in rock layers dated to 65 mya, as well as a
huge crater off Mexico’s Yucatán peninsula—has led researchers to
accept the impact hypothesis.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Selectivity
• Some evolutionary lineages were better able than others to
withstand the environmental change brought on by the asteroid
impact.
• For example, among vertebrates, the dinosaurs, pterosaurs (flying
reptiles), and large marine reptiles perished, while the mammals,
crocodilians, amphibians, and turtles survived.
• Researchers are currently testing the hypothesis that organisms that
could remain inactive for long periods of time, such as by
hibernating, were able to survive.
© 2011 Pearson Education, Inc.
Recovery
• After the asteroid impact, recovery was slow.
• Terrestrial ecosystems around the world were radically simplified,
and the diversity of marine environments remained low for 4–8
million years afterward.
• Mammals diversified to fill the niches left empty by the extinction
of the dinosaurs. Within 10–15 million years, all of the major
mammalian orders living today had appeared.
© 2011 Pearson Education, Inc.