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CHAPTER 26LECTURE

SLIDES

Prepared by

Brenda LeadyUniversity of Toledo

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Taxonomy and systematics Taxonomy

Science of describing, naming, and classifying living and extinct organisms and viruses

SystematicsStudy of biological diversity and the

evolutionary relationships among organisms, both extinct and modern

Taxonomic groups are now based on hypotheses regarding evolutionary relationships derived from systematics

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Taxonomy

Hierarchical system involving successive levels

Each group at any level is called a taxon Domain

Highest levelAll of life belongs to one of 3 domainsBacteria, Archaea, and Eukarya

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EukaryaBacteriaDomains: Archaea

Plantae Fungi Animalia

Excavata Alveolata Stramenopila Rhizaria Amoebozoa OpisthokontaLand plants and algal relativesEukaryoticsupergroups:

Large eukaryotickingdoms:

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Eukarya

lupus

Canis

Canidae

Carnivora

Mammalia

Chordata

Animalia

Domain

Species

Genus

Family

Order

Class

Phylum

Kingdom

~270

1

7

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~5,000

~50,000

>1 million

OpisthokontaSupergroup >1 million

Taxonomicgroup

Gray wolffound in

Number ofspecies

~ 4– 10 million

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Binomial nomenclature

Genus name and species epithet Genus name always capitalized Species epithet never capitalized Both names either italicized or underlined Rules for naming established and

regulated by international associations

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Phylogenetic trees

Phylogeny – evolutionary history of a species or group of species

To propose a phylogeny, biologists use the tools of systematics

Trees are usually based on morphological or genetic data

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Phylogenetic tree

Diagram that describes phylogeny A hypothesis of evolutionary relationships

among various species Based on available information New species can be formed by

Anagenesis – single species evolves into a different species

Cladogenesis – a species diverges into 2 or more species

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Present

5

10

A

B

B

CD

E

F G HI J K

Mill

ion

s o

f ye

ars

ago

(m

ya)

Tim

e

10

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Monophyletic group or cladeGroup of species, taxon, consisting of the most

recent common ancestor and all of its ancestors Smaller and more recent clades are nested

within larger clades that have older common ancestors

Paraphyletic groupContains a common ancestor and some, but not all,

of its descendents

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D E F G

B C

A

D E F G

B C

A

D E F G

B C

A

(a) Monophyletic (b) Paraphyletic (c) Polyphyletic

H I J K L M N O H I J K L M N O H I J K L M N O

Over time, taxonomic groups will be reorganized so only monophyletic groups are recognized

Reptiles were a paraphyletic groups because birds were excluded

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Reptiles Reptiles

Tu

rtle

s

Liz

ard

san

d s

nak

es

Cro

cod

iles

Bir

ds

Tu

rtle

s

Liz

ard

san

d s

nak

es

Cro

cod

iles

Bir

ds

Orders

Classes

(a) Reptiles as a paraphyletic taxon

(b) Reptiles as a monophyletic taxon

KEY

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Homology

Similarities among various species that occur because they are derived from a common ancestor

Bat wing, human arm and cat front leg Genes can also be homologous if they are

derived from the same ancestral gene

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Morphological analysis

First systematic studies focused on morphological features of extinct and modern species

Convergent evolution (traits arise independently due to adaptations to similar environments) can cause problems

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Molecular systematics

Analysis of genetic data, such as DNA and amino acid sequences, to identify and study genetic homologies and propose phylogenetic trees

DNA and amino acid sequences from closely related species are more similar to each other than to sequences from more distantly related species

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Cladistics

Study and classification of species based on evolutionary relationships

Cladistic approach discriminates among possible phylogenetic trees by considering the various possible pathways of evolutionary changes and then choosing the tree that requires the least complex explanation for all of the available data

Phylogenetic trees or cladograms

Cladistic approach compares homologous traits, also called characters, which may exist in two or more character states

Shared primitive character or symplesiomorphy Shared by two or more different taxa and inherited

from ancestors older than their last common ancestor Shared derived character or synapomorphy

Shared by two or more species or taxa and has originated in their most recent common ancestor

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A

B

GD E F

C

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Branch point – 2 species differ in shared derived characters

Ingroup – group we are interested in Outgroup – species or group of species

that is assumed to have diverged before the species in the ingroup

An outgroup will lack one or more shared derived characters that are found in the ingroup

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(b) Cladogram based on morphological traits

Lamprey Salmon Lizard Rabbit

(a) Characteristics among species

Notochord Yes Yes Yes Yes YesNo Yes Yes Yes YesNo No Yes Yes YesNo No No Yes YesNo No No No

VertebraeHinged jawTetrapod

Lancelet Lamprey Salmon Lizard Rabbit

YesMammaryglands

Vertebrae

Hinged jaw

Tetrapod

Lancelet

Notochord

Mammaryglands

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Cladogram can also be constructed with gene sequences

7 species called A-G A mutation that

changes the DNA sequence is analogous to a modification of a characteristic

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Constructing a cladogram

1. Choose species

2. Choose characters

3. Determine polarity of character states Primitive or derived?

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4. Analyze cladogram based on All species (or higher taxa) are placed on tips in the

phylogenetic tree, not at branch points Each cladogram branch point should have a list of one

or more shared derived characters that are common to all species above the branch point unless the character is later modified

All shared derived characters appear together only once in a cladogram unless they arose independently during evolution more than once

5. Choose the most likely cladogram among possible options

6. Choose a noncontroversial outgroup as root

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Principle of parsimony

Preferred hypothesis is the one that is the simplest for all the characters and their states

Challenge in a cladistic approach is to determine the correct polarity of events It may not always be obvious which traits are primitive

and came earlier and which are derived and came later in evolution

Fossils may be analyzed

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Example

4 taxa (A-D) A is the outgroup

Has all the primitive states

3 potential treesTree 3 requires fewest

number of mutations so is the most parsimonous

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According to the principle of parsimony, tree

number 3 is themore likely choice

because it requires only five mutations.

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Molecular clocks Favorable mutations rare and detrimental mutations

eliminated Most mutations are neutral If neutral mutations occur at a constant rate they can be

used to measure evolutionary time Longer periods of time since divergence allows for a

greater accumulation of mutations Not perfectly linear over long periods of time

Not all organisms evolve at the same rate Differences in generations times

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Nucleotidedifferences ina homologousgene betweendifferent pairsof species

Evolutionary time since divergence of pairs of species(millions of years)

Primate evolution example

Evolutionary relationships derived by comparing DNA sequences for cytochrome oxidase subunit II Tends to change fairly rapidly on an evolutionary

timescale 3 branch points to examine (A, D, E) Ancestor A

This ancestor diverged into two species that ultimately gave rise to siamangs and the other five species

23 million years for siamang genome to accumulate changes different from other 5 species

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Ancestor D This ancestor diverged into two species that eventually

gave rise to humans and chimpanzees Differences in gene sequences between humans and

chimpanzees are relatively moderate

Ancestor E This ancestor diverged into two species of chimpanzees Two modern species of chimpanzees have fewer

differences in their gene sequences

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Cooper and Colleagues Extracted DNA from Extinct Flightless Birds and Modern Species to Propose a New Phylogenetic Tree

Ancient DNA analysis or molecular paleontology Under certain conditions DNA samples may be

stable as long as 50,000 – 100,000 years Discovery based sciences- gather data to

propose a hypothesis Sequences are very similar New Zealand colonized twice by the ancestors of

flightless birdsFirst by moa ancestor, then by kiwi ancestor

Horizontal gene transfer

Any process in which an organism incorporates genetic material from another organism without being the offspring of that organism

Vertical evolutionChanges in groups due to descent from a

common ancestor

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Due to Horizontal Gene Transfer, the Tree of Life Is Really a “Web of Life”

Vertical evolution involves changes in species due to descent from a common ancestor

Horizontal gene transfer is the transfer of genes between different species

Significant role in phylogeny of all living species Still prevalent among prokaryotes but less common

in eukaryotes Horizontal gene transfer may have been so

prevalent that the universal ancestor may have been a community of cell lineages

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Bacteria Archaea Eukarya

Common ancestral community of primitive cells

AnimalsFungi Plants

Horizontal gene transfer

KEYVertical evolution

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