Fish Taxonomy and Systematics
Lecture 4:
Cladograms & Speciation
Cladograms
Phylogenetic relationships expressed in
cladograms - branching representation of the
evolutionary relationships among taxa based
on shared common traits and shared unique
traits
Constructing a Cladogram
 Listing of traits
 Coding of each taxon by presence/absence of
each trait
 Assemble groupings based on trait conditions
 Use the simplest branching structure possible:
principle of parsimony
Which states that “cladogram (tree) having the
fewest number of “steps” (evolutionary changes)
is the one accepted”
Sequentially group taxa by
shared derived character states (apomorphies)
TAXA
Tuna
Leopard
Lancelet
(outgroup)
Vertebral column
(backbone)
0
1
1
1
1
1
Hinged jaws
0
0
1
1
1
1
Lamprey
Tuna
Vertebral
column
Salamander
Hinged jaws
Four walking legs
0
0
0
1
1
1
Turtle
Four walking legs
Amniotic (shelled) egg
0
0
0
0
1
1
Hair
0
0
0
0
0
1
Amniotic egg
(a) Character table
Leopard
Hair
(b) Phylogenetic tree
More on traits...
• Meritic-count it!
• Morphometric-measurable shape
fin lengtheye shape
head length
ratios between such measures...
anatomical characteristics
molecular characteristics
Which traits do I use?
Relationship
Recency of common ancestry
i.e., taxa sharing a common ancestor
more recent in time are more closely related
than those sharing common ancestors more
distant in time.
Ingroup – group studied
Outgroup – group not part of
ingroup, used to “root” tree
 The out-group serves as a base line for comparisons with
the other organisms being evaluated, the in-group
Branch point
(node)
Taxon A
Taxon B
Taxon C
ANCESTRAL
LINEAGE
Taxon D
Taxon E
Taxon F
Common ancestor of
taxa A–F
Polytomy
Sister
taxa
Apomorphy (derived trait)

= a new, derived feature e.g., for this
evolutionary transformation
scales
--------> feathers
(ancestral feature)
(derived feature)

Presence of feathers is an apomorphy
for birds.
Taxa are grouped by apomorphies
Apomorphies are the result of evolution.
Taxa sharing apomorphies underwent same
evolutionary history should be grouped
together.
Example: Are fish more closely related to sharks or to humans?
Shark
TIME
Fish
Humans
Shark
Fish
Humans
TIME
common ancestor of
Fish and Humans
common ancestor of
Sharks, Fish, and Humans
Vertebrata
Osteichthyes
Shark
TIME
Fish
Humans
monophyletic
group
common ancestor of
Fish and Humans
common ancestor of
Sharks, Fish, and Humans
Constructing a Cladogram
This example shows the evolutionary relationships
among plants.
1. In the table below, the traits in the row for the
out-group are marked with a zero.
 When a plant has a trait not found in the outgroup, the trait is considered a derived trait and is
marked with a one.
 Next the numbers of shared derived traits are
totaled.
Constructing a Cladogram
Derived Traits
Organisms
Vascular tissue
Seeds
Flowers
Mosses (Out-group)
0
0
0
Pine trees
1
1
0
Flowering plants
1
1
1
Ferns
1
0
0
Total
3
2
1
Constructing a Cladogram
2. Starting with a diagonal line, as shown on the
next slide, the out-group (mosses) is placed on the
first branch of the cladogram.
 Just past this first branch, the most common
derived trait is listed—vascular tissue.
 Vascular tissue is a series of tubes and vessels
within a plant.
Constructing a Cladogram
3. Next the second most common derived trait is
determined, which in this case is seeds.
 The ferns lack seeds and so are placed in the
second branch of the cladogram.
4. The third most common derived trait is flowers.

Conifers do not have flowers and so are
placed in the third branch above the second
branch on the cladogram.

The flowering plants are placed at the end of
the cladogram.
Constructing a Cladogram
Pine trees
Flowering plants
Ferns
Mosses
Flowers
Seeds
Vascular
tissue
Speciation
Q. How do populations become distinct species?
 Speciation - process by which a new species originates.
 Involves the creation of a population of organisms that are
novel enough to be classified in their own group.
The process whereby gene flow is reduced sufficiently
between sister populations to allow each to become
different evolutionary lineages
Allopatric (with geographic isolation): speciation resulting
from divergent evolution of populations that are geographically
isolated from each other.
Non-allopatric (without geographic isolation)
Speciation
Allopatric
speciation:
(with
geographic
isolation)
Vicariant - large populations geographically isolated
(little inbreeding) (United States)
Founder - small population becomes geographically
isolated and then reproductively isolated via
inbreeding, selection, drift (Gilligan’s Island)
Reinforcement - early isolation followed by sympatry,
but selection against hybrids
Speciation
Non-allopatric (without geographic isolation)
Sympatric - sister species evolve within the
dispersal range of each other, but adapt to
different habitats - habitat-dependent assortive
mating (tribes)
Parapatric - sister species evolve in segregated
habitats across a narrow contact zone - little
mixing in spite of proximity
Speciation Based on Gene Flow
(a) Allopatric speciation
(b) Sympatric speciation
Allopatric
In allopatric speciation, gene flow is
interrupted or reduced when a population
is divided into geographically isolated
subpopulations
The definition of barrier depends on the
ability of a population to disperse
Separate
populations
may
evolve
independently through mutation, natural
selection, and genetic drift
Allopatric speciation in the Grand Canyon
A. harrisi
A. leucurus
Examples of Allopatric speciation:
Allopatric speciation of squirrels in the Grand Canyon.
Animals like birds do not show speciation like those animals
that are barred from breeding by the canyon.
Key question about allopatric species is whether they are indeed
different enough that viable, fertile offspring would not be produced
by mating. This can be tested sometimes as in…
Allopatric speciation
Reproductive
isolation
between
populations generally increases as the
distance between them increases
Barriers to reproduction are intrinsic;
separation itself is not a biological
barrier
Mantellinae
(Madagascar only):
100 species
Rhacophorinae
(India/Southeast
Asia): 310 species
Other Indian/
Southeast Asian
frogs
100
60
80
1
2
20
40
0
3
Millions of years ago (mya)
1
3
2
India
Madagascar
88 mya
65 mya
56 mya
Sympatric Speciation
Sympatric means “same country”
Speciation takes place in geographically
overlapping populations
Occurs because of
Polyploidism
Habitat differentiation
Sexual selection
Polyploidism
Polyploidy is the presence of extra sets of
chromosomes due to accidents during cell
division
1. Autopolyploidy
2. Allopolyploidy
Autopolyploidy in plants
2n=6
4n=12
Failure of cell division after chromosome duplication
gives rise to tetrapliod cells which
may be viable and self-fertile
Allopolyploid
An allopolyploid is a species with multiple
sets of chromosomes derived from different
species
Many important crops (oats, cotton,
potatoes, tobacco, and wheat) are
polyploids
Ancestral species:
AA
Triticum
monococcum
(2n = 14)
BB
Wild
Triticum
(2n = 14)
Product:
AA BB DD
T. aestivum
(bread wheat)
(2n = 42)
DD
Wild
T. tauschii
(2n = 14)
Habitat differentiation
Sympatric speciation can also result from
the appearance of new ecological niches
 North American maggot fly can live on native
hawthorn trees as well as more recently
introduced apple trees, so food preference
may isolate the two populations
Sexual selection
 Sexual selection can drive sympatric speciation
 Sexual selection for mates of different colors has
likely contributed to the speciation in cichlid fish in
Lake Victoria
Example: Lake Victoria has 200 closely related species of
Cichlids (fish) which probably all arose from one ancestor
with the driving force for speciation being:
Competition for a limited resource (food) within the lake, and
adaptation to new food sources. This gave rise to different species
that are kept from breeding with each other by distinctive
coloration pattern.