Chapt15 Lecture

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Chapter 15The History

and Classification

of Life on Earth

The Fossil Record Reveals the History of Life on Earth

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15.1 The geologic timescale is based on the fossil record

Because all life-forms evolved from the first cell or cells, life has a history that is revealed by the fossil record The geologic timescale depicts the history of life based on the

fossil record Divisions of the Timescale - the timescale divides the

history of Earth into eras, then periods, and then epochs The epochs have the shortest time frames The three eras span the greatest time

Paleozoic, Mesozoic, and Cenozoic Dating Within the Timescale - the timescale provides

both relative and absolute dates References to events during a timescale are in relative time Dates given in millions of years (MYA) are in absolute time

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Limitations of the Timescale Evolution is not a series of events leading only

from the first cells to humans Timescales list mass extinctions, but don’t tell

when specific groups became extinct Extinction is the total disappearance of a species or

a higher group Mass extinction occurs when a large number of

species disappear in a few million years or less

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APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES 15.2 The geologic clock can help put Earth’s history in perspective

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15.3 Continental drift has affected the history of life

Continental Drift We now know that continents are not fixed Their positions and the positions of the oceans have changed

over time Plate Tectonics

Earth’s crust is fragmented into slablike plates that float on a hot, liquefied metallic core that lies directly beneath the Earth’s crust

At deep oceanic ridges, seafloor spreading occurs as molten mantle rock rises and material is added to plates

Seafloor spreading causes the continents to move and causes the Atlantic Ocean to get wider

Where the plates meet, the forward edge of one sinks into the mantle and is destroyed, creating a subduction zone

When continents collide the result is often a mountain range

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Figure 15.3A The continents have drifted through time

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15.4 Mass extinctions have affected the history of life

At least five mass extinctions occurred throughout the history of life End of the Ordovician, Devonian, Permian, Triassic, and

Cretaceous periods Causes of Mass Extinctions

Meteorite - A piece of rock from outer space that strikes the Earth and creates a crater

The date of a large crater on the Yucatán Peninsula in Mexico corresponds to the timing of the K-T extinction

Climate Changes - Severe climate change can cause an extinction

Marsupials died from the cold as the continent of Antarctica drifted to the South Pole

Human Activities - Some scientists believe we are currently in the midst of a mass extinction due to human activities

This modern-day extinction is due to our manipulation of the environment, such as modern agricultural methods and industrialization and its demand for energy and the resultant global warming 15-9

Figure 15.4 Mass extinctions

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Figure 15.4 Mass extinctions

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Systematics Traces Evolutionary Relationships

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15.5 Organisms can be classified into categories

Classification is the grouping of extinct and living species into the categories: domain, kingdom, phylum, class, order, family, genus, and species Taxon (pl., taxa) - group of organisms that fills a

particular category of classification Character - any trait that distinguishes one group from

another Taxonomy - the science of naming species

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Scientific Names They are called binomial because they have two

parts The first word is the genus, and the second word is the

specific epithet Example: Parthenocissus quinquefolia

It preferable to use an organism’s scientific name instead of the common name The scientific name is based on Latin, which doesn’t

change Common names often differ between countries and even

within the same country

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Figure 15.5 Hierarchy of taxa for Parthenocissus quinquefolia

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15.6 Linnaean classification reflects phylogeny

Systematics - study of the diversity of organisms at all levels of biological organization

Phylogeny - evolutionary history of a group of organisms Often represented by a phylogenetic (evolutionary)

tree, a diagram indicating common ancestors and lines of descent

Each branch point in a phylogenic tree is a divergence from a common ancestor, a species that gives rise to two new groups

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Derived and Ancestral Characters Derived Characters - individual characteristics

unique to a group of organisms

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Ancestral Characters - those shared by a common ancestor

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Figure 15.6 Linnaean classification and phylogeny

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15.7 Certain types of data are used to trace phylogeny

Fossil Record - It is possible to use the fossil record to trace the history of life in broad terms One of the advantages of fossils is that they can be dated, but it is

not always possible to tell which group a fossil is related Homology - character similarity that stems from having a common

ancestor Homologous structures are related to each other through

common descent Molecular Data - systematists assume that when two species are

closely related, a comparative study of their DNA will show few differences in base-pair sequences

Molecular Clocks - When nucleic acid changes are neutral and not tied to adaptations they accumulate at a fairly constant rate These changes can be used as a molecular clock to determine

when two species diverged from a common ancestor

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Deciphering homology can be difficult because of convergent evolution

Convergent evolution - the acquisition of the same or similar characters in distantly related lines of descent Similarity due to convergence is analogy Analogous Structures have the same function in different

groups, but do not have a common ancestry Example: the wings of an insect and the wings of a bat are

analogous Parallel evolution - the acquisition of the same or a

similar character in two or more related lineages without it being present in a common ancestor Example: the flying squirrel (a placental mammal) and the flying

phalanger (a marsupial)

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Figure 15.7B Convergent evolution

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Figure 15.7C Molecular data

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15.8 Phylogenetic cladistics and evolutionary systematics use the

same data differently Phylogenetic cladistics - a method of

determining evolutionary relationships based on shared characters derived from a common ancestor Uses shared derived characters to classify organisms

and arrange taxa in a diagram called a cladogram, which traces the evolutionary history of the group being studied

In a cladogram, a clade is an evolutionary branch that includes a common ancestor, together with all its descendant species

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Figure 15.8A Data for constructing a cladogram

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Figure 15.8B In a cladogram, a clade (colors) contains a common ancestor and all its descendents with shared derived characters

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Evolutionary Systematics The traditional method of using characters and

judgment to classify and determine evolutionary history

Evolutionary systematists mainly use structural data and the Linnaean system to classify organisms and construct phylogenetic trees

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Figure 15.8C Evolutionary systematics versus cladistic view of reptilian phylogeny

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The Three-Domain System Is Widely Accepted

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15.9 This text uses the three-domain system of classifying organisms Domain Bacteria

Bacteria are a prokaryotic group that is so diversified and plentiful they are found in large numbers nearly everywhere on Earth

The cyanobacteria are photosynthetic, but most bacteria are heterotrophic

Domain Archaea Like bacteria, archaea are prokaryotic unicellular organisms that

reproduce asexually Archaea do not look that different from bacteria under the

microscope, and the extreme conditions under which many species live has made it difficult to culture them

Domain Eukarya Eukaryotes are unicellular to multicellular organisms whose cells

have a membrane-bounded nucleus Sexual reproduction is common and various types of life cycles are

seen Protists, plants, fungi, and animals are all eukaryotic 15-30

Figure 15.9 The three-domain system of classification

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Connecting the Concepts:Chapter 15

The geologic timescale describes the history of life on Earth The species that are alive today are the end product of all the

changes that occurred on Earth as life evolved Every known species that has evolved is given a two-part

name consisting of a genus and a specific epithet All sorts of data are used to classify organisms and develop

tree diagrams that show evolutionary relationships among species Cladistics is a widely accepted way to determine evolutionary

relationships Most biologists today have adopted the three-domain

system of classifying species The archaea are structurally similar to bacteria, but their rRNA differs

from that of bacteria and is instead similar to that of eukaryotes The domain Eukarya contains four kingdoms: protists, fungi, plants,

and animals 15-32