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Phylogenies

and theHistory

of Life

CHAPTER 27

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Key Concepts & Study Guide

I. Phylogenies and the fossil record are the major tools

that biologists use to study the history of life.

What are phylogenetic trees? What are the different aspects (terminology) of

phylogenetic trees and how do these terms relate to organisms on the

phylogenetic tree?How does the whale example in the text exemplify problems associated with

constructing phylogenetic trees?

How are fossils formed?

What are the limitations of the fossil record?

What are the biases and how can they impact our understanding of evolution?

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Key Concepts & Study Guide

II. The Cambrian explosion was the rapid

morphological and ecological diversification of

animals that occurred during the Cambrian period.

What are the four major eras with respect to the timeline of life?

Can you summarize the major (basic, keep it simple) events in each era?

What was the Cambrian explosion?

What are the three major faunas representing the Cambrian explosion

Genetic Mechanisms of Evolutionary Change

Homeotic Gene Duplication

Changes in gene expression patterns

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Topic Outline & Study Guide

III. 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.What is an adaptive radiation?

Can you give an example of an adaptive radiation?

How and why do they occur?

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Topic Outline & Study Guide

IV. Mass extinctions have occurred repeatedlythroughout the history of life. They rapidly eliminate

most of the species alive in a relatively random

manner.

What is a mass extinction?How do we define the term?

What is the evidence for mass extinctions over the years?

Online Discussion: on BB

Is a Mass Extinction Event Underway Now?

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IntroductionWhat are the two major analytical tools that biologists use to reconstruct the

history of life?

Phylogenetic trees-A phylogenetic tree or evolutionary tree is a branchingdiagram or "tree" showing the inferred evolutionary relationships amongvarious biological species or other entities based upon similarities and

differences in their physical and/or genetic characteristics. The taxa joinedtogether in the tree are implied to have descended from a common ancestor.

The fossil record- Ever since recorded history began, and probably before,people have noticed and gathered fossils, including pieces of rock andminerals that have replaced the remains of biologic organisms, or preservedtheir external form. Fossils themselves, and the totality of their occurrence

within the sequence of Earth's rock strata, is referred to as the fossil record.The fossil record was one of the early sources of data relevant to the study ofevolution and continues to be relevant to the history of life on Earth.Paleontologists examine the fossil record in order to understand the processof evolution and the way particular species have evolved.

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Phylogenetic Trees

Phylogeny

evolutionary history of a group of organisms is called a

Phylogenetic tree

shows ancestor-descendant relationships among populations or species

Monophyletic group

and ancestor and all its descendants form a

AKA called a clade orlineage

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Reading a Phylogenetic Tree

Branches

represent populations through time

sister taxa

are adjacent branches

a taxon

Is any group of organisms

Tips

Are the trees endpoints and representliving groups or a groups end in

extinction The names at the tips can represent

species or larger groups

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Reading a Phylogenetic Tree

Nodes

A polytomy

In rooted phylogenies the most ancient node of the tree is ?

The location of this node is determined using an ?

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Estimating Phylogenies

There are two general strategies for using data to estimatetrees:

The phenetic approach is

ased on computing a statistic that summarizes the overallsimilarity among populations

The cladistic approach Inferring trees focuses on synapomorphies, the shared derived characters

of the species under study.

Said another way, a synamorphy is a trait that certain groups of organisms

When many such traits have been measured, traits unique toeach monophyletic group are identified and the groups areplaced on a tree in the appropriate relationship to oneanother.

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Synapomorphies Identify Monophyletic Groups

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This is a monophyletic

groups that shares aderived trait (the c in

the thir position)

This is a

monophyletic group

that shares a derivedtrait (G, in the fifth

position)

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Distinguishing Homology from Homoplasy

Problems can arise with both phenetic and cladistic analyses

because similar traits can evolve independently in two distantspecies rather than from a trait present in a common ancestor.

Homoplasy

occurs when traits are similar for reasons other than common ancestry.

Figure 27.2a shows an example comparing the similar traits of dolphins andextinct marine reptiles called Ichthyosaurs.

Homology

Occurs when traits are similar due to shared ancestry.

Figure 27.2b shows an example using the hox genes

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HomoplasyandHomology

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Homoplasy and Homology

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Homoplasy and Homology

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Distinguishing Homology from Homoplasy

Convergent evolution

Occurs when natural selection factors similar solutions to the problemsposed by a similar way of life, as shown by the dolphin and ichthyosaur

Is a common cause of homoplasy

If similar traits found in distantly related lineages are indeedsimilar due to common ancestry, then similar traits should befound in many intervening lineages on the tree of life.

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Distinguishing Homology from Homoplasy

Parsimony

Is a principle of logic stating that the most likely explanation or patter s theone that implies the least amount of change

Convergent evolution and other cause of homoplasy should be rarecompared with similarity due to shared descent

So the tree that implies the fewest overall evolutionary changes should bethe one that most accurately reflects what is happening during evolution

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Choosingamong theManyPossibleTrees

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Whale Evolution: A Case History

Traditionally, phylogenetic trees based on morphological dataplace whales outside of the artiodactyls-mammals that havehooves

DNA sequence data, however, suggest a close relationshipbetween whales and ? WHY?

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Evidence ThatWhales and Hippos Form a Monophyletic Group

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Evidence That Whales and Hippos Form a Monophyletic Group

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Evidence That Whales and Hippos Form a Monophyletic Group

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Evidence ThatWhales and Hippos Form a Monophyletic Group

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Whale Evolution: A Case History

Recent data on gene sequences called short interspersed nuclear

elements (SINEs) show that whales and hippos share several SINEgenes that are absent in other artiodactyl groups.

These SINEs are shared derived traits

and support the hypothesis that whales and hippos are indeedclosely related

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Tools for Studying History: The Fossil Record

A fossil Is the physical trace left by an organism that live in the pasts

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 pastlooked like, where they lived, and when they existed

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How Do Fossils Form?

Most fossils form when An organisms is buried in sediment before decomposition occurs.

Four types of fossils are

Intact fossils, compression fossils, cast fossils, and premineralizedfossils.

Fossilization is an extremely rare event.

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Fossilization Preserves Traces of Past Organisms

HOW FOSSILIZATIONOCCURS

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Fossilization Preserves Traces of Past Organisms

HOW FOSSILIZATION

OCCURS

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Limitations of the Fossil Record

There are several limitations of the fossil record that need to berecognized:

habitat bias occurs because organisms that live in areas where sediments

are actively being deposited are more likely to form fossils than are

organisms that live in other habitats

taxonomic bias is due to the fact that some organisms (e.g.,

temporal bias occurs because more recent fossills are more common than

ancient fossils

abundance bias evidence much more than other species that are rare

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Limitations of the Fossil Record

Paleontologists

recognize that they are limited to studying tiny and scattered

segments on the tree of life, yet they also know that this is theonly way to get a glimpse of what extinct life was like.

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Life's Timeline

Major events in the history of life are marked on the timelineshown in Figure 27.8, which has been broken into four segments.

Precambrian encompasses the hadean, archaean, protorozoic,

Paleozoic era

Mesozoic era

Cenozoic era

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Lifes Timeline: The Precambrian

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Lifes Timeline: The Paleozoic Era

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Lifes Timeline: The Mesozoic Era

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Lifes Timeline: The Cenozoic Era

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The Cambrian Explosion

The first animalssponges, jellyfish, and simple wormsappearin the fossil record around ???? mya, at the end of the Proterozoiceon.

Soon after that in geologic time, by about ?????? years later,animals had diversified into almost all the major groups livingtoday.

This diversification is known as the ?

This period saw what was arguably the most evolutionary changein the history of life.

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Cambrian Fossils: An Overview

The Cambrian explosion is documented by three major fossilassemblages, called the

The presence of these exceptionally rich deposits before, during,and after the Cambrian explosion makes the fossil record for thisevent extraordinarily complete.

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Fossils Document the Cambrian Explosion

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Doushantuofossils aremicroscopic

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From the Doushantuo

formation in China,

researchers identified

microfossils (tiny fossils) of

sponges, cyanobacteria, andmulticellular algae in

samples dated 570580 mya.

They also found what they

concluded were animalembryos in early stages.

These were examples of the

first types of animals on

Earth.

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Small, soft-

bodied

animals fromEdiacara

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In the Ediacara Hills in

Australia, paleontologists

identified fossils of sponges,jellyfish, comb jellies, and

traces of other animals

dated 544565 mya.

These were small, soft-

bodies animals that

burrowed in sediments, sat

immobile on the sea floor, or

floated in the water.

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Diverse, largeanimals withhard parts fromBurgess Shale

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Virtually every major living animal

group is represented in the Burgess

Shale fossils from British Columbia,

Canada, which date to 515525 mya. These fossils indicate a tremendous

increase in the size and

morphological complexity of

animals, accompanied by

diversification in how they made aliving.

This diversification filled many of

the ecological niches still found in

marine habitats today.

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Did Gene Duplication Trigger the Cambrian Explosion?

What is gene duplication?

What are hox genes?

Does the phylogenetic tree of Hox genes in animals in generalsupports this hypothesis?

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http://evolution.berkeley.edu/evosite/evo101/IIIC6cComplexity2.shtml

http://www.nature.com/scitable/topicpage/hox-genes-in-development-the-hox-

code-41402

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Hox Genes in Animals

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Did Gene Duplication Trigger the Cambrian Explosion?

What conclusions can be drawn from this phylogeny?

Both duplication of Hox genes and changes in expression andfunction of existing genes have been important in making theelaboration of animal body plans possible.

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Adaptive Radiation

can be triggered by ecological opportunity

morphological innovation.

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Adaptive Radiations

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Dense groups of bushy branches

called star phylogenies or polytomies

can be observed in the tree of life

(Figure 27.11a).

These star phylogenies represent speciation eventsthat were so rapid that the order of branchingcannot be resolved.

If rapid speciation in a single lineage is followed bydivergence into many different adaptive forms,

then a process known as adaptive radiation hastaken place.

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Ecological Opportunity as a Trigger

One of the most consistent triggers of adaptive radiations isecological opportunity, meaning

For example, biologists have documented adaptive radiations ofthe Anolis lizards of the Caribbean islands.

On the two islands studied, the same four ecological typeseventually evolved, because the islands had similar varieties of

habitats.

Therefore, similar adaptive radiations took place independentlyon the two islands, triggered by

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Th d i di i f A li h d

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The same adaptive radiation ofAnolis has occurred

on different islands, starting from different types ofcolonists

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Morphological Innovation as a Trigger

Morphological innovation can also be a trigger for adaptiveradiation, as was seen in the Cambrian explosion.

Many of the other important diversification events in the historyof life started off with the evolution of a key morphological traitthat allowed descendents to live in new areas, exploit new foodsources, or move in new ways.

Animation: Adaptive Radiation

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Mass Extinctions

A mass extinction is the

Mass extinctions are caused by

Paleontologists traditionally recognize five mass extinctions("The Big Five").

Background extinction is the lower, average rate of extinction,representing the normal loss of some species that always occurs.

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The Big Five Mass Extinction Events

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How Do Background and Mass Extinctions Differ?

Background extinctions typically occur when normalenvironmental change, emerging diseases, or competitionreduces certain populations to zero.

Mass extinctions result

Background extinctions are thought to result from

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What Killed the Dinosaurs?

The impact hypothesis for the extinction of dinosaurs proposes

that an asteroid struck Earth 65 mya and caused the extinction ofan estimated 6080% of the multicellular species alive.

Conclusive evidenceincluding iridium, shocked quartz, and

microtektites found in rock layers dated to 65 mya, as well as ahuge crater off Mexicos Yucatn peninsulahas led researchersto accept the impact hypothesis.

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Evidence of an Asteroid Impact 65 MYA

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Selectivity

Some evolutionary lineages were better able than others towithstand the environmental change brought on by the asteroidimpact.

For example, among vertebrates, the dinosaurs, pterosaurs(flying reptiles), and large marine reptiles perished, while themammals, crodilians, amphibians, and turtles survived.

Researchers are currently testing the hypothesis that organisms

that could remain inactive for long periods of time, such as byhibernating, were able to survive.

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Recovery

After the asteroid impact, recovery was slow.

Terrestrial ecosystems around the world were radically simpified,and the diversity of marine environments remained low for 48million years afterward.

Mammals diversified to fill the niches left empty by the extinctionof the dinosaurs. Within 1015 million years, all of the majormammalian orders living today had appeared.

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