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Chapter 18
Organizing
Information
About Species
Albia Dugger • Miami Dade College
18.1 Bye Bye Birdie
• Over millions of years, unique forms and behaviors evolved in
many different lineages of Hawaiian finches – the Hawaiian
honeycreepers
• Variations in traits allowed the birds to exploit special
opportunities presented by their island habitats
• Polynesians arrived on the Hawaiian Islands around 1000
A.D. – followed by Europeans in 1778
The Hawaiian Honeycreepers
• By 1778 at least 43 honeycreeper species that had thrived on
the Hawaiian islands before humans arrived were extinct
• Conservation efforts began in the 1960s, but 26 more species
have since disappeared – today, 35 of the remaining 68
species are endangered
• They are pressured by invasive, non-native species of plants
and animals, and by rising global temperatures that allow
disease-bearing mosquitoes to invade higher-altitude habitats
Endangered: The Palila
Endangered: The Akekee
Extinct: The Poouli
18.2 Phylogeny
• Evolutionary history can be reconstructed by studying shared,
heritable traits
• Phylogeny is the evolutionary history of a species or a group
of species – a kind of genealogy that follows a lineage’s
evolutionary relationships through time
Characters
• Each species bears evidence of its own unique evolutionary
history in its characters
• A character is any heritable physical, behavioral, or
biochemical feature that can be measured or quantified
• Examples: Number of segments in a backbone, the
nucleotide sequence of ribosomal RNA
Table 18-1 p296
Traditional Classification
• Traditional classification groups organisms based on shared
characters, such as feathers in birds
• Traditional classification does not always reflect phylogeny –
species that appear very similar are not necessarily closely
related
Evolutionary Classification
• Evolutionary biologists try to pinpoint the source of shared
characters: a common ancestor
• Common ancestry is determined by derived traits –
characters present in a group, but not in that group’s
ancestors
• A group whose members share one or more defining derived
traits is called a clade – a monophyletic group consisting of
an ancestor with a derived trait, and all of its descendants
Cladistics
• Making hypotheses about evolutionary relationships among
clades is called cladistics
• Parsimony analysis is used to find the simplest and most
likely evolutionary pathway – the one in which defining
derived traits evolved the fewest number of times
Parsimony Analysis
Parsimony Analysis
Parsimony Analysis
Cladograms
• Cladistic analysis produces a cladogram – an evolutionary
tree that diagrams evolutionary trends and patterns
• Data from an outgroup (a species not closely related to any
member of the group) may be included to “root” the tree
• Each line represents a lineage, which may branch into two
lineages at a node – a common ancestor of two lineages
• Every branch on a cladogram is a clade; the two lineages that
emerge from a node are sister groups
earthworm
tuna
lizard
mouse
human
earthworm
multicellular
tuna
multicellular with a backbone
lizard
multicellular with a backbone and legs
mouse
multicellular with a backbone, legs, and hair
human
Figure 18-3 p297
ANIMATED FIGURE: Interpreting a
cladogram
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Take-Home Message: How do evolutionary
biologists study life’s diversity?
• Evolutionary biologists study phylogeny to understand how all
species are connected by shared ancestry
• A clade is a monophyletic group whose members share one
or more derived traits; cladistics is a method of making
hypotheses about evolutionary relationships among clades
• Cladograms and other evolutionary tree diagrams are
hypotheses based on our best understanding of the
evolutionary history of a group of organisms
18.3 Comparing Form and Function
• Physical similarities are often evidence of shared ancestry,
but sometimes a trait evolves independently in two groups
• In many cases, comparative morphology can be used to
unravel evolutionary relationships
Morphological Divergence
• Homologous structures are similar body parts in separate
lineages that evolved in a common ancestor
• Homologous structures may be used for different purposes,
but the same genes direct their development
• Change from the body form of a common ancestor is an
evolutionary pattern called morphological divergence
• Example: Vertebrate forelimbs vary in size, shape, and
function, but are alike in structure
32
1
pterosaur
1
2
chicken
3
2
3
21
4
5
3
penguin
1
stem reptile
23
4
5
porpoise
1
2
bat
3 4
1
2
3
4
5
1
2
3
4
5
5
human
elephant
Figure 18-4 p298
Morphological Convergence
• Analogous structures are body parts that look alike but did
not evolve in a shared ancestor – they evolved independently
in lineages subject to the same environmental pressures
• The independent evolution of similar body parts in different
lineages is called morphological convergence
• Example: Bird, bat, and insect wings all perform the same
function, but are derived from different structures
Morphological Convergence
Take-Home Message: What does comparative
morphology reveal about phylogeny?
• In morphological divergence, a body part inherited from a
common ancestor becomes modified differently in different
lines of descent (homologous structures)
• In morphological convergence, body parts that appear alike
evolved independently in different lineages, not in a common
ancestor (analogous structures)
18.4 Comparing Biochemistry
• The kind and number of biochemical similarities among
species are clues about evolutionary relationships
Molecular Clocks
• A molecular clock is used to estimate how long ago two
lineages diverged by comparing DNA or protein sequences
• Over time, neutral mutations that have no effect on survival or
reproduction accumulate at a constant rate
• The accumulation of neutral mutations in the DNA of a
lineage act as a molecular clock
• The number of differences between genomes can be used to
estimate the relative times of divergence
DNA and Protein Sequence Comparisons
• Some essential genes are highly conserved (their DNA
sequences have changed very little over evolutionary time) –
other genes are not conserved at all
• Comparing the nucleotide sequence of a gene or the amino
acid sequence of a protein can provide evidence of an
evolutionary relationship
• Generally, two species with many identical proteins are likely
to be close relatives – the number of amino acid differences
give us an idea of evolutionary relationships
Comparison of an Amino Acid Sequence
DNA Comparisons
• DNA from nuclei, mitochondria, and chloroplasts can be used
in nucleotide comparisons
• Mitochondria are inherited intact from a single parent, usually
the mother – any differences in mitochondrial DNA sequences
between maternally related individuals are due to mutations,
not genetic recombination during fertilization
Comparison of a DNA Sequence
Cladogram Based On DNA Sequence
Take-Home Message:
How does biochemistry
reflect evolutionary history?
• Mutations change the nucleotide sequence of a lineage’s
DNA over time
• Lineages that diverged long ago have more differences
between their DNA and amino acid sequences than do
lineages that diverged more recently
18.5 Comparing Patterns of Development
• Similar patterns of embryonic development are an outcome of
highly conserved master genes
• A mutation in a master gene typically halts development
Similar Forms in Plants
• Homeotic genes encode transcription factors that determine
details of body form during embryonic development
• Example: A floral identity gene, Apetala1, affects petal
formation across many different lineages – it is likely that this
gene evolved in a shared ancestor
Developmental Comparisons in Animals
• The embryos of many vertebrate species develop in similar
ways – directed by the very same genes
• Differences are brought about by variations in expression
patterns of master genes that govern development
• Example: All vertebrates go through a stage in which they
have four limb buds, a tail, and a series of somites – divisions
of the body that give rise to a backbone
Comparison of Vertebrate Embryos
Hox Genes
• Hox genes are homeotic genes of animals
• The pattern of expression of Hox genes determines the
identity of particular zones along the body axis
• Hox genes occur in clusters on a chromosome, in the order in
which they are expressed in a developing embryo
• Example: Legs develop wherever the antennapedia gene is
expressed in an embryo
Expression of the Antennapedia Gene
Vertebrate Hox Genes
• In vertebrates, expression of the Hoxc6 gene causes ribs to
develop on vertebrae of the back – not the neck or tail
• The Dlx gene encodes a transcription factor that signals
embryonic cells to form buds that give rise to appendages
• Hox genes suppress Dlx expression in all parts of an embryo
that will not have appendages
Expression of the Hoxc6 Gene
Persistent Juvenile Features
• A chimpanzee skull and a human skull appear quite similar an
early stage
• As development continues, both skulls change shape as
different parts grow at different rates
• A human adult skull is proportioned more like the skull of an
infant chimpanzee than the skull of an adult chimpanzee
• Human evolution involved changes that caused traits typical
of juvenile stages to persist into adulthood
Proportional Changes During
Skull Development: Chimpanzee
adult
proportions in infant
Proportional Changes During
Skull Development: Human
proportions in infant
adult
Persistent Juvenile Traits in Salamanders
• In axolotls, external
gills and other larval
traits persist into
adulthood
Take-Home Message: Why
are similarities in
development indicative of shared ancestry?
• Similarities in patterns of development are the result of master
genes that have been conserved over evolutionary time
• Some differences between closely related species are a result
of master gene mutations that change the rate or onset of
development
ANIMATED FIGURE: Mutation and
proportional changes
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18.6 Applications of Phylogeny Research
• Studies of phylogeny reveal how species relate to one
another and to species that are now extinct
• We use information about phylogeny to understand how to
preserve the species that exist today
Conservation Biology
• The reservoir of genetic diversity among Hawaiian
honeycreepers is diminishing along with its numbers
• Lowered diversity means the group as a whole is less resilient
to change, and more likely to suffer species losses
• Deciphering honeycreeper phylogeny can tell us which ones
are most valuable in terms of preserving genetic diversity
Diversity of Hawaiian Honeycreepers
Evolutionary Relationships
Among Honeycreepers
Conservation Biology (cont.)
• Cladistics analyses are also used to correlate past
evolutionary divergences with behavior and dispersal patterns
of existing populations
• Example: A cladistic analysis of mitochondrial DNA
sequences suggests that blue wildebeest populations are
genetically less similar than they should be
• Using a combination of data, conservation biologists can
recommend measures to improve gene flow
Medical Applications
• Researchers study the evolution of infectious agents by
grouping their biochemical characters into clades
• Example: A phylogenetic analysis of H5N1 influenza virus
isolated from pigs showed that the virus “jumped” from birds
to pigs at least three times since 2005, and that one group
had acquired the potential to be transmitted among humans
Take-Home Message:
How is studying phylogeny useful?
• Phylogeny research is yielding an ever more specific and
accurate picture of how all life is related by shared ancestry
• Among other applications, phylogeny research can help us
preserve species in danger of becoming extinct, and to
understand the spread of infectious diseases