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Part 5: Evolution and biodiversity

Chapter 31: Evolving life

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Phylogeny• Evolutionary relationships depicted in a branching

diagram– phylogenetic tree or cladogram

• Pattern of relationships uncovered using cladistic analysis– relationships between taxa identified by shared derived

characters

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Fig. 31.1: Phylogenetic tree

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Characters• Ancestral or primitive character

– plesiomorphic character or plesiomorphy

• Shared plesiomorphies– symplesiomorphies

• Example– platypus, koala, dingo

hair, internal fertilisation, suckle young

– these characters are symplesiomorphies do not show the pattern of relationships between the taxa

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Characters (cont.)• Derived or advanced character

– apomorphy

• Shared apomorphies– synapomorphies

• Example– koala and dingo

anal and urogenital openings separate, mammary glands with teats, egg shell absent

– these characters are synapomorphies indicate that koala and dingo are more closely related to

each other than either is to platypus

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Synapomorphies• Each branch of a cladogram is supported by one

or more synapomorphies• Each species has one or more unique characters

that distinguish it from other species– autapomorphy– contains no information about relationships

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Types of characters• Range of character types used in exploring

evolutionary relationships• Morphology

– anatomy– embryology

• Molecules– proteins (amino acid sequences)– DNA and RNA (nucleotide sequences)

Question 1:

What kinds of evidence would be required to support the theory that life evolved from a common ancestor?

a) DNA evidence

b) Morphological evidence

c) Fossil evidence

d) Embryological evidence

e) All of the above

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Comparing morphology• Anatomy

– comparison of body form example: pentadactyl (five-digit) limb of vertebrates

• Embryology– comparison of developmental patterns

example: protostome vs. deuterostome

– comparison of embryonic form example: notochord in chordates

Fig 31.2: The embryos of vertebrates

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Comparing morphology (cont.)• Vestigial organs

– rudimentary (poorly-developed) organs without functions

• Evolutionary remnants of structures that were functional in ancestral species– indicate phylogeny

• Example– pythons possess cloacal spurs

vestigial hind limbs

– descended from limbed ancestors

Fig 31.3: Vestigial structures

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Comparing morphology (cont.)• Fossils

– preserved remains of organisms

• Comparative morphology + geological time– indicate minimum age of taxa

• Reduced range of characters because of fossilisation process– usually bones, teeth, shells, wood

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Characters and information• All characters are not equally useful in

reconstructing phylogeny• Need to distinguish between

– characters that carry information– characters that carry no information

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Divergent evolution• Pattern of evolution is divergence from a common

ancestral form– example: crocodile, bird, bat and whale differ but are all

descended from a common ancestral form

• Change in form – example: crocodile, bird, bat and whale forelimbs differ in

external appearance but have same basic structure pentadactyl limb

• Homologous characters– same plan, different function– indicate common ancestry

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Fig. 31.4: Forelimbs of vertebrates

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Convergent evolution• Convergence in form in unrelated organisms

– example: cacti, euphorbs, aloes and other succulent plants are similar in form but are descended from different ancestral forms

• Similarity due to similar environments– example: succulent plants inhabit arid areas

• Analogous characters– different plan, same function– do not indicate common ancestry

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Fig. 31.5: Convergence in succulent plants

(a) cacti

(b) euphorbs

(c) Aloe

Copyright © Dennis Stevenson, New York Botanic Garden

Copyright © Professor Pauline Ladiges, University of Melbourne Copyright © Professor Pauline Ladiges, University of Melbourne

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Reconstructing phylogenies• Homologous characters

– provide information for examining phylogeny

• Analogous characters– do not provide information for examining phylogeny

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Parallel evolution• Closely related organisms may evolve similar

features because they face similar environments– features are analogous not homologous– parallel evolution

• Emphasises need to examine more than one set of characters

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Comparing molecules• Proteins

– comparison of amino acid sequences example: chimpanzees and humans have identical

sequences in several important proteins

• DNA– comparison of nucleotide sequences

example: phylogeny of apes reconstructed using mitochondrial DNA

Table 31.1: Comparing DNA sequences

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DNA and RNA sequences• Rates of change differ between sequences• Nuclear-encoded RNA

– ribosomal RNA (rRNA) genes– highly conserved (slow to change)– used to reconstruct phylogenies back to the origin of life

• Mitochondrial DNA (mtDNA), chloroplast DNA (cpDNA)– variable rates of change– can be used for more closely related taxa

Question 2:

Why do some parts of DNA accumulate mutations and evolve rapidly, while other parts are conserved and evolve more slowly?

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Fig. 31.8: Phylogeny of great apes and humans

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Classification• Grouping organisms with common characters• Classification reflects phylogeny

– grouped according to pattern of common ancestry

• Classification is dynamic– changes as more information about relationships

becomes available

Fig. 31.10: A phylogenetic tree

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Taxonomy• Methods and principles of classification• Rules for naming organisms

– written codes for consistency– international codes of nomenclature

• Taxon = group of organisms, level (rank) of classification– kingdom (highest level, most inclusive)– species (lowest level, least inclusive)

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Taxonomic hierarchy

kingdom

phylum

class

order

family

genus

species

intermediate ranks indicated by prefix sub- (e.g. subphylum, subfamily, subspecies)

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Monophyly• Hierarchical classification reflects phylogeny

– expresses the branching pattern of cladograms

• Each named group should be monophyletic– containing all descendants of a common ancestor– non-monophyletic groups: paraphyly, polyphyly

• Traditional classifications often include non-monophyletic groups– changing as we uncover more data about morphological

and molecular characters

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Paraphyly• Paraphyletic group

– does not include all descendants of a common ancestor– example: paraphyletic family Pongidae (great apes) does

not include humans (family Hominidae) although apes and humans form a monophyletic group

• Polyphyletic group– includes descendants from unrelated groups– based on convergent evolution– example: kingdom Protista is composed of many

unrelated lineages

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Fig 31.13: Monophyletic, paraphyletic and polyphyletic taxons

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The binomial system• Codes of nomenclature set rules for naming ranks

of taxa– example: animal families end in -idae, plant families end

in -aceae

• All taxa except species have a single-word name– example: Chordata (phylum), Mammalia (class), Felidae

(family)

• Only species are identified by two words– genus + specific epithet– this is also called the binomial system

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Species names• Species names are composed of two parts

– genus name (capitalised)– specific epithet (never capitalised)

• Combination of genus + specific epithet are unique for each species– Tachyglossus aculeatus (echidna)– Aquila audax (wedge-tailed eagle)– Litoria nasuta (rocket frog)

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Species names (cont.)• May be more than one species in a genus

– Aquila audax (wedge-tailed eagle)– Aquila chrysaetos (golden eagle)– Aquila heliaca (imperial eagle)

• Specific epithet may be used in several genera– Litoria nasuta (rocket frog)– Perameles nasuta (long-nosed bandicoot)– Acropora nasuta (staghorn coral)

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What is a species?• Basic unit of taxonomy• Different concepts of how the species taxon is

defined– biological species concept

based on reproductive isolation between species

– morphological species concept based on phenotypic difference between species

– many other concepts based on different aspects

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Kingdoms of life• Highest (most inclusive) rank of classification• Linnaeus classified all organisms into two

kingdoms– plants and animals

• Today six major lineages are recognised– Bacteria, Archaea, Protista, Plantae, Fungi and Animalia

• Changing classification

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Fig. 31.16: Schemes of classification

Bacteria: copyright © Kwangshin Kim/Photo Researchers, Inc.; Archaea: copyright © James King-Holmes/Science Photo Library/Photo Researchers, Inc.; Protista: copyright © Professor Geoff McFadden, University of Melbourne; Plantae: copyright © K Thiele, University of Melbourne; Fungi: copyright © H Swart; Animalia: copyright © Martin Harvey/ANT Photo Library

Summary• Phylogeny is the evolutionary history and

relationships of organisms• Phylogeny is discovered using homologous

features and cladistic analysis• DNA and amino acid sequencing contribute to

discovering phylogeny• Taxonomy is based on phylogenetic relationships• Major groups of life are classified in kingdoms

31-39Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and SaintSlides prepared by Karen Burke da Silva, Flinders University