Download Systematics

This week in lab - trip to Green Oaks!
- dress appropriately (sturdy shoes, long pants, etc.)
Arrive promptly at lab - need to leave quickly
Studying diversity of forest trees in 2 habitats
Last day… started talking about systematics: study of
relationships between organisms & their
classification & naming
Defined 3 types of possible groups that can be created
from the taxa that are arranged in a phylogeny
Most systematists today try to use only monophyletic
groups in classification, but traditional
classifications often include paraphyletic groups
Paraphyletic groups are most controversial
- usually considered ‘bad’, but still frequently used
- e.g. ‘reptiles’ or ‘Reptilia’
Share many characters, but mostly ancestral…
‘Reptiles’ excludes some descendants (birds, mammals?)
What does a ‘tree’
represent?
Relationships
between
taxa
What does the
horizontal axis
represent?
Relative time since
taxa split
(sometimes
absolute time or
genetic distance)
Phylogeny often has
branches at top
What does ‘x’ axis
represent?
nothing….
Branches can be rotated without changing meaning,
only branching pattern is important
Phylogenies constructed using similarities between species,
because related species are expected to share most traits
If two taxa share many traits, we expect them to be
closely related
- alternative: same characters occur in unrelated spp.;
character may have evolved multiple times
Not impossible! Convergent evolution…
Actually,
Alluaudia
Try to use only homologous
characters: characters whose
similarity is due to inheritance
from a common ancestor
- recognized by detailed
similarity in structure,
organization, development,
etc. (but not always easy!)
However, we assume it is more likely that shared
characters indicate related species
Principle of Parsimony suggests simplest explanation
preferred
Most parsimonious phylogeny has characters evolving
fewest times possible
(Mutations and evolutionary change assumed to be
relatively rare)
William of Ockham, of Occam’s Razor fame
e.g. mosses &
tracheophytes
share many
characters
- if not closely
related, those
characters
would each have
to evolve twice,
many more
evolutionary
changes
Instead, we assume they are closely related
Not all characters are equally useful for indicating
relationships
- e.g number of legs in mammals
Try grouping cats, whales, deer, bats
& rodents based on # of legs…
Can’t group mammals that have
4 legs together because
ancestor had 4 legs… all
mammal groups inherited
this trait
Shared ancestral traits are
not useful
Instead, try to use shared
derived traits:
traits that have been
modified from ancestral
trait since the group
started evolving
(AKA synapomorphies)
Modification of front legs
into flippers, hind legs
reduced to vestigial bones
can be used as shared
derived trait for whales
To determine ancestral trait, examine outgroups: taxa that
are not part of the focal group but are closely related
Look at an example of
constructing
a phylogeny
Phylogenies built
by finding which
taxa share the
largest number of
derived traits
An example…
‘bird-hipped’
dinosaurs
What sort of characters can be used in phylogenetic
analysis?
- almost anything that can be measured and has a
genetic basis
- some characters more useful than others because
more or less stable, or easier to compare
- may depend on breadth of study (for phylogeny of
a phylum use slow-evolving characters,
for phylogeny of a genus use fast-evolving
characters)
Morphological characters
- easy to measure, many traits available
- usually only characters available for fossils, if used
can combine modern & fossil taxa in analysis
Developmental characters
Essentially morphological, but traits seen in embryo
may disappear in adult
Behavioral characters
Variable, may be hard to compare but can add much
potential information
e.g. duck courtship displays
King Eiders
Different wasp families differ in their nest structures
Crabronidae
Vespidae
Molecular characters
Probably most current studies use molecular characters
- potential characters include:
- enzyme frequencies
use electrophoresis to determine variation
Amino acid
sequence of
proteins
Each position
in sequence is
a character
- having same
amino acid
is a shared
trait
Must align
sequences
first…
DNA hybridization
- compares similarity of whole strands of DNA from
2 spp. by how tightly they bond together
- heat DNA until 2 strands separate, when cooled they
join together again in double helix
DNA from 2 different
species will form
hybrid DNA, but will
separate again at
lower temperatures
Which of these pairs do you think is most closely related?
A) American Black Bear & Asiatic Black Bear
B) Sloth Bear & Spectacled Bear
C) Grizzly Bear & Polar Bear
American Black Bear
Sloth Bear
Asiatic Black Bear
Grizzly Bear
Spectacled Bear
Polar Bear
Phylogeny must be based on overall DNA similarity
(measured by dissociation temperature), not by
number of shared characters