Transcript The Tree of Life

The Tree of Life
Introduction to Biological
Diversity
Tools for Studying Historyof Life:
Phylogenies and the Fossil Record
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The evolutionary history of a group of organisms is
called a phylogeny
A phylogenetic tree shows ancestor-descendant
relationships among evolutionary groups (usually
species or populations).
Fossils are physical evidence left by organisms from
the past. The fossil record includes all fossils that
have been found and recorded.
The Parts of a Phylogenetic Tree
A Field Guide to Reading
Phylogenetic Trees
Populations are represented by branches
 Nodes show where ancestral groups split into
descendant groups.
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a
polytomy is a node where more than two descendant
groups branch off.
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Adjacent branches are sister taxa
a
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taxon is any named group of organisms
Tips are branch endpoints
represent
living groups or a group’s end in extinction.
Field Guide Continued
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Rooted
The most ancient node of the tree is
shown at the bottom
 Location of this node is determined using
an out group
a taxonomic group that diverged before
the rest of the taxa being studied.
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An
ancestor and all its descendants
form a monophyletic group
a
clade or lineage
Phylogeneticic Tree Illustrating
Some of the Great Apes
Alternative Phylogenetic Trees
representing the same evolutionary relationships
Ways to Estimate Phylogenies
Morphological and genetic characteristics are
used to estimate phylogenetic relationships among
species.
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Two
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approaches
Phenetic approach
Cladistic approach
The Phenetic Approach
Computes a statistic that summarizes the
overall similarity among populations based on the
data.
 A computer program then compares the
similarities among populations and builds a tree
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clusters
the most similar populations
places more divergent populations on more distant
branches.
The Cladistic Approach
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Focus on synapomorphies
the
shared derived characters of the species under
study
A computer program is used to identify which
traits are unique to each monophyletic group
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many such traits measured
then place the groups on a tree in the correct
relationship to one another
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Difficulties with the Cladistic
Approach
Cases of convergent evolution.
Biologists then use parsimony to try to identify
the phylogenetic tree that minimizes the overall
number of convergent evolution events.
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Principle
of logic stating that the most likely
explanation or pattern is the one that implies the least
amount of change or the least complexity.
Assumes that convergent evolution should be much
rarer than similarity due to shared descent.
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Whale Evolution: A Case
History
Traditionally, phylogenetic trees based
on morphological data place whales outside
of the artiodactyls
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mammals
such as cows, deer, and hippos
have hooves
 an even number of toes
 unusual pulley-shaped ankle bone (astralagus)
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Whale Evolution
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DNA sequence data
suggest a close relationship between whales
and hippos
A phylogenetic tree showing closely related
whales and hippos is less parsimonious than the
tree based on morphological data because it
requires the evolution and then loss of the
astralagus in whales.
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Recent Data
Recent data on short interspersed
nuclear elements (SINEs) show that
whales and hippos share several that are
absent in other artiodactyl groups.
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These SINEs are shared derived traits
(synapomorphies) and support the
hypothesis that whales and hippos are
indeed closely related.
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SINE Genes
Environment Changes Life/Life
Changes Environment
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Geological events that alter environments
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Change the course of biological evolution
Conversely, life changes the planet that it inhabits
EARLY EARTH
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Earth formed about 4.5 billion years ago
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Along with the rest of the solar system
Earth’s early atmosphere
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Contained water vapor and many chemicals
released by volcanic eruptions
A. Oparin and J. Haldane
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In the 1920s, independently postulated that
conditions on the early Earth favored the synthesis
of organic compounds from inorganic ones.
 The environment in the early atmosphere would
have promoted the joining of simple molecules to
form more complex ones.
 The energy required to make organic molecules
could be provided by lightning and UV radiation in
the primitive atmosphere.
 The lack of an ozone layer in the early atmosphere
would have allowed this radiation to reach the
Earth.
They reasoned that this cannot happen today because
high levels of oxygen in the atmosphere attack
chemical bonds.
Stanley Miller and Harold Urey,
1953
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tested the Oparin-Haldane hypothesis
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They discharged sparks in an “atmosphere” of
gases and water vapor.
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creating, in the laboratory, the conditions that had
been postulated for early Earth
H2O, H2, CH4, and NH3
a more strongly reducing environment than is currently
believed to have existed on early Earth
Produced a variety of amino acids and other
organic molecules
Miller and Urey experiment
Water vapor
CH4
H2
Electrode
NH3
Condenser
Cold
Water
H2O
Cooled water
containing
organic
molecules
Sample for chemical
analysis
Deep-Sea Vents
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Instead of forming in the atmosphere
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The first organic compounds on Earth may have
been synthesized near submerged volcanoes and
deep-sea vents
Extraterrestrial Sources of
Organic Compounds
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Some of the organic compounds from
which the first life on Earth arose
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May have come from space
Carbon compounds
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Have been found in some of the meteorites
that have landed on Earth
Protobionts
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Laboratory experiments demonstrate
that protobionts
aggregates of abiotically produced molecules
surrounded by a membrane or membrane-like
structure
 Could have formed spontaneously from
abiotically produced organic compounds
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Example: small membrane-bounded
droplets called liposomes
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Can form when lipids or other organic
molecules are added to water
Liposomes
Glucose-phosphate
20 m
Glucose-phosphate
Phosphorylase
Starch
Amylase
Phosphate
Maltose
Maltose
(a) Simple reproduction. This lipo-
some is “giving birth” to smaller
liposomes (LM).
(b) Simple metabolism. If enzymes—in this case,
phosphorylase and amylase—are included in the
solution from which the droplets self-assemble,
some liposomes can carry out simple metabolic
reactions and export the products.
The “RNA World” and the Dawn
of Natural Selection
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The first genetic material
Was probably RNA, not DNA RNA
molecules called ribozymes have been found
to catalyze many different reactions,
including
 Self-splicing
 Making complementary copies of short
stretches of their own sequence or other
short pieces of RNA
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Ribozyme
(RNA molecule)
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Template
Nucleotides
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Complementary
RNA copy
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How life actually began is speculative
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There are clues in the molecules and anatomical
developments of each species. These clues together
with the fossil record have produced several theories
about how life evolved on Earth.
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Earth formed about 4.5 billion years ago,
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The oldest fossils
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embedded in rocks from western Australia
about 3.5 billion years old.
resemble bacteria, so scientists think that life originated
much earlier.
may have been as early as 3.9 billion years ago
 when Earth began to cool to a temperature at which
liquid water could exist.
Using the Fossil
Record
The fossil record is the only source of
direct evidence about what prehistoric
organisms looked like, where they lived, and
when they existed.
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Careful
study of fossils opens a window
into the lives of organisms that existed long
ago and provides information about the
evolution of life over billions of years.
Most Fossils Form
When an Organism
is Buried in Sediment
Before
Decomposition
Begins
Four Types of
Fossils
Index fossils
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Are similar fossils found in the same
strata in different locations
Allow strata at one location to be
correlated with strata at another
location
Absolute Ages of Fossils
Can
be
determined by
radiometric
dating
Limitations of the
Fossil Record
There are several features and
limitations of the fossil record that must
be recognized
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habitat bias
taxonomic bias
temporal bias
and abundance bias
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Limitations are Recognized
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Paleontologists
recognize that they are limited to asking
questions about tiny and scattered
segments on the tree of life
 Yet analyzing fossils is the only way
scientists have of examining the physical
appearance of extinct forms and inferring
how they lived.
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The Geologic Record
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By studying rocks and fossils at many
different sites
Geologists have established a geologic
record of Earth’s history
 Three Eonss
 the Archaean
 the Proterozoic
 the Phanerozoic
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Life’s Timeline
Major events in the history of life are
marked on the timeline which has been
broken into four segments
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the
Precambrian
the Paleozoic
the Mesozoic
the Cenozoic).
Precambrian
Almost all life was unicellular.
Little or no oxygen in atomosphere.
Paleozoic
Paleozoic era
Mesozoic
Ended with
extinction of
the dinosaurs
Cenozoic
The Cambrian Explosion
Animals first originated around 565
million years ago
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Animals diversified into almost all the
major groups extant today
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Known as the Cambrian explosion.
Cambrian Fossils
•Three major fossil beds record this explosion of animal life
The Doushantuo
Microfossils
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Researchers identified
Sponges
 Cyanobacteria
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Multicellular algae
Samples dated 570–580 Ma
Also Animal Embryos
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In early stages of development
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Samples contained
one-celled,
two-celled, four-celled, and eight-celled
fossils
individuals containing larger cell numbers whose
overall size was the same
exactly the pattern that occurs during cleavage in
today’s animals.
The Ediacaran Faunas
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found in these Australian deposits
Sponges
 Jellyfish
 Comb jellies
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Traces of other animals dated 544–565 Ma
Indicate
that shallow-water marine habitats
contained a diversity of animal species
•Virtually every
major animal group is
represented in the
Burgess Shale fossils
Compelling Picture of Life in the
Oceans 525–515 Ma
Few,
if any, species in the Ediacaran faunas are
also found in the Burgess Shale–type
assemblages 20–40 million years later
New species of sponges, jellyfish, and comb
jellies are abundant
 Entirely new groups as well
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arthropods
mollusks.
Oldest Fossils
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As prokaryotes evolved, they exploited
and changed young Earth
The oldest known fossils are
stromatolites
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Rocklike structures composed of many
layers of bacteria and sediment
Which date back 3.5 billion years ago
Stromatolites
Lynn Margulis (top right), of the University of Massachussetts, and
Kenneth Nealson, of the University of Southern California, are
(a) collecting bacterial mats in a Baja California lagoon. The
shown
mats are produced by colonies of bacteria that live in environments
inhospitable to most other life. A section through a mat (inset)
shows layers of sediment that adhere to the sticky bacteria as
the bacteria migrate upward.
Some bacterial mats form rocklike structures called stromatolites,
such as these in Shark Bay, Western Australia. The Shark Bay
stromatolites began forming about 3,000 years ago. The inset
shows
a section through a fossilized stromatolite that is about
(b)
3.5 billion years old.
e 26.11a, b
Oxygenic photosynthesis
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Probably evolved about 3.5 billion years ago in cyanobacteria
Figure 26.12
The First Eukaryotes
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The oldest fossils of eukaryotic cells
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Date back 2.1 billion years
Endosymbiotic Origin of
Mitochondria and Plastids
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The theory of endosymbiosis
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Proposes that mitochondria and plastids
were formerly small prokaryotes living
within larger host cells
Probably gained entry to the host cell as
undigested prey or internal parasites
The host and endosymbionts would have
become a single organism
Eukaryotic Cells as Genetic
Chimeras
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Additional endosymbiotic events and
horizontal gene transfers
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May have contributed to the large genomes
and complex cellular structures of
eukaryotic cells
The Earliest Multicellular
Eukaryotes
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Molecular clocks
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Date the common ancestor of multicellular
eukaryotes to 1.5 billion years
The oldest known fossils of eukaryotes
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Are of relatively small algae that lived about
1.2 billion years ago
The Colonial Connection
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The first multicellular organisms were
colonies
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Collections of autonomously replicating cells
Figure 26.16
10 m
Specialized Cells
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Some cells in the colonies
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Became specialized for different
functions
The first cellular specializations
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Had already appeared in the prokaryotic
world
Mass Extinctions
The rapid extinction of many
groups
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loss
of at least 60% of all species
within 1 million years
caused by catastrophic episodes.
traditionally recognize five mass
extinctions
Background and
Mass Extinctions
 Background extinctions typically occur when normal
environmental change or competition reduces a
population to the point where it dies out.
 Mass extinctions occur when unusual large-scale
environmental change causes the extinction of many
normally well-adapted species.
 Natural selection causes most background
extinctions, whereas random chance plays a large role
in mass extinctions.
Permian
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The Permian extinction
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Claimed about 96% of marine animal species
and 8 out of 27 orders of insects
Is thought to have been caused by enormous
volcanic eruptions
Cretaceous
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The Cretaceous extinction (K-T)
Doomed many marine and terrestrial
organisms, most notably the dinosaurs
 Is thought to have been caused by the
impact of a large meteor
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NORTH
AMERICA
Yucatán
Peninsula
Figure 26.9
Chicxulub
crater
Evidence
Conclusive evidence—including iridium,
shocked quartz, and microtektites found in
rock layers dated to 65 million years ago,
as well as a huge crater off the Yucatán
Peninsula—has led researchers to accept
the impact hypothesis.
The large-scale environmental change
triggered by the asteroid impact caused
the extinction of 60% to 80% of all
species.
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Selectivity
Some evolutionary lineages
were better able than others to
withstand the environmental
change brought on by the
asteroid impact. Why certain
groups survived while others
perished is still a mystery.
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Recovery
Ferns appear to have replaced
diverse woody and flowering
plants in many habitats following
the K-T extinction. Mammals
diversified to fill the niches left
empty following the dinosaur
extinctions.
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Reconstructing the Tree of Life:
A Work in Progress
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A three domain system
Has replaced the five kingdom system
 Includes the domains Archaea, Bacteria, and
Eukarya
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Each domain
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Has been split by taxonomists into many
kingdoms
Adaptive Radiations
One broad pattern that can be
observed in the tree of life
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Dense groups of branches scattered
throughout the tree
 Star phylogenies
 represent major diversification
over a relatively short period of time
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a process known as adaptive
radiation.
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Colonization Events as a
Trigger
Adaptive radiations occurred
following the colonization of
unoccupied island habitats
 Example: Anolis lizards of the
Caribbean islands
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The Role of
Morphological Innovation
Morphological innovation
 Opportunities for adaptive
radiation
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Examples: Important new traits such
as limbs, wings, flowers, and jaws
 Allowed descendants to live in new
areas, move in new ways, and exploit new
sources of food
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