Chapter 15Chapter 15

Tracing Evolutionary History

Are Birds Really Dinosaurs with Feathers?

• For decades, evolutionary biologists debated whether birds evolved from dinosaurs

– Fossil Archaeopteryx supported this view

– Conflicting view posited birds evolving from a very different reptile group

• Bird-dinosaur link was supported by cladistics and corroborated in the 1990s by fossil evidence

• Debate continues on how birds learned to fly

MACROEVOLUTION AND EARTH'S HISTORY

15.1 The fossil record chronicles macroevolution

• Macroevolution is the main event in the evolutionary history of life on Earth

– Documented in the fossil record

• The geologic record is based on the sequence of fossils

– Earth's history divided into three eons

– Within the most recent eon, eras and periods marked by mass or lesser extinctions

• Some major events in the history of life

– Precambrian period: oldest known fossils- prokaryotes from 3.5 billion years ago

– Paleozoic era: lineages that gave rise to modern life forms

– Mesozoic era: age of reptiles, including dinosaurs

– Cenozoic era: Explosive evolution of mammals, birds, and flowering plants

Animation: The Geologic RecordAnimation: The Geologic Record

15.2 The actual ages of rocks and fossils mark geologic time

• Radiometric dating can gauge the actual ages of fossils and the rocks in which they are found

– Based on the decay time of radioactive isotopes relative to other isotopes

• Carbon-14 for relatively young fossils

• Isotopes with longer half-lives for older fossils

15.3 Continental drift has played a major role in macroevolution

• Continental drift is the slow, incessant movement of Earth's crustal plates on the hot mantle

• World geography changes constantly

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Eurasian PlateNorth

American Plate

Pacific Plate

Nazca Plate

African Plate

SouthAmerican

Plate

ArabianPlate Indian

Plate

Antarctic Plate

Splitdeveloping

Australian Plate

Edge of one plate being pushed over edge ofneighboring plate (zones of violent geologic events)

• Continental movements have greatly influenced the distribution of organisms around the world

– Formation of Pangaea 250 million years ago altered habitats and triggered extinctions

– Breakup of Pangea beginning 180 million years ago created a number of separate evolutionary arenas

• Explains the geographical distribution of diverse life forms

– Examples: marsupials, lungfishes

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North Americ

aEurasia

IndiaAfrica

SouthAmerica

Australia

Antarctica

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Gondwana

Pangaea

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NorthAmerica

SouthAmerica

Africa

Living lungfishes

Fossilized lungfishes

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Europe

Asia

Australia

CONNECTION

15.4 Tectonic trauma imperils local life

• Plate tectonics are the forces involved in movements of Earth's crustal plates

– The geologic processes that result include volcanoes and earthquakes

• Can create devastation or opportunities for organisms

– The boundaries of plates are hot spots of such geologic activity

Video: Galápagos Islands OverviewVideo: Galápagos Islands Overview

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San Francisco

NorthAmericanPlate

Santa CruzPacificPlate

Los Angeles

California

San Andreas Fault

15.5 Mass extinctions were followed by diversification of life-forms

• Extinctions occur all the time, but extinction rates have not been steady

• Over the last 600 million years, at least six periods of mass extinctions have occurred, including

– Permian extinction (250 million years ago); claimed 96% of aquatic life

– Cretaceous extinction (65 million years ago); eliminated dinosaurs

• Cause of mass extinctions is unclear

– Permian extinction occurred at a time of enormous volcanic explosions

– Cretaceous extinction may have been caused by an asteroid

• Mass extinctions have been followed by an explosive increase in diversity

– Provide surviving organisms with new environmental opportunities

– Example: rise of mammals after extinction of dinosaurs

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NorthAmerica

ChicxulubcraterYucatan

Peninsula

YucatanPeninsula

Video: Lava FlowVideo: Lava Flow

Video: Volcanic EruptionVideo: Volcanic Eruption

PHYLOGENY AND SYSTEMATICS

15.6 Phylogenies are based on homologies in fossils and living organisms

• Phylogeny is the evolutionary history of a group of organisms

– Traced partly from the fossil record

– Also inferred from morphological and molecular homologies among living organisms

• May reveal common ancestry

• Not all likenesses are inherited from a common ancestor

– Analogy: similarity due to convergent evolution

• Species from different evolutionary branches may come to resemble each other if they live in similar environments

• Systematics is the analytical study of diversity and phylogeny

15.7 Systematics connects classification with evolutionary history

• Systematics includes binomial designation of species and hierarchical classification

• A binomial gives each species a two-part name

– Genus (a group of related species)

– Species within the genus

• Genera are grouped into progressively more inclusive categories (taxa)

– Family, order, class, phylum, kingdom, domain

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Species

Genus

Family

Order

ClassMammalia

Carnivora

Felidae

Felis

Feliscatus

Phylum

Kingdom

Domain

Chordata

Animalia

Eukarya

• A phylogenetic tree is a hypothetical hierarchy of evolutionary relationships

(domesticcat)

Feliscatus

Mephitismephitis

(striped skunk)

Lutralutra

Canisfamiliaris

(domestic dog)(Europeanotter)

Canislupus(wolf)Species

FelisGenus

Family

Order Carnivora

MustelidaeFelidae

Mephitis Lutra Canis

Canidae

15.8 Cladograms are diagrams based on shared characters among species

• Cladistics is concerned with the order of branching in phylogenetic lineages

– Each branch (clade) on a cladogram represents an ancestral species and all its descendants

– Each clade consists of taxa that are monophyletic (from a "single tribe")

• All the taxa on a clade share one or more homologous features

– Shared derived characters: New traits unique to each lineage

– Shared primitive characters: Traits present in the ancestral groups

• Comparison of ingroup and outgroup is important in cladistics

– Ingroup: Group of taxa being analyzed

– Outgroup: Closely related to the ingroup but not a member of it

• Parsimony seeks the simplest explanation of observed data

– The simplest (most parsimonious) hypothesis of relationships creates the most likely phylogenetic tree

Lizards Snakes

Common reptilian ancestor

BirdsCrocodiles

15.9 Molecular biology is a powerful tool in systematics

• Molecular systematics uses DNA and RNA to compare relatedness

– The closer the nucleic acid sequences between two organisms, the more likely they are to share a common ancestor

– Molecular trees cover long and short times based on the different rates at which different genes evolve

• Humans are more closely related to fungi than to plants

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Brown bear Polarbear

Asiaticblack bear

Americanblack bear

Sunbear

Slothbear

Spectacledbear

Giantpanda

Lesserpanda

Raccoon

Pleistocene

Pliocene

Ursidae

Procyonidae

Common ancestralcarnivorans

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Student Mushroom Tulip

Common ancestor

• Computer DNA analysis can show exactly how many bases are alike in homologous regions

• Some regions of DNA change at a rate consistent enough to serve as molecular clocks to date evolutionary events

• Comparison of entire genomes reveals interesting homologies

– Humans and chimpanzees are 99% identical

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Human Chimpanzee Gorilla Orangutan

Common ancestor

15.10 Arranging life into kingdoms is a work in progress

• Five-kingdom system

– All prokaryotes are in kingdom Monera

– Eukaryotes are grouped into four kingdoms: Protista, Plantae, Fungi, Animalia

– Molecular studies have found flaws in this system

• The domain system

– Prokaryotes are in two domains: Bacteria and Archaea

– All eukaryotes are in domain Eukarya

• All classification systems are human constructions, not facts of nature

– Will always be refined by new data

Animation: Classification SchemesAnimation: Classification Schemes

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Monera Protista Plantae Fungi Animalia

Earliestorganisms

Prokaryotes

Eukaryotes

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Birds MammalsLizards