Phylogenetics of the Invertebrates
D. L. A. Underwood
Biology 316 - General Entomology
A. Overview of phylogenetic reconstruction and classification
1. The goal
a. Ideally we name things and place them in taxonomic groups that reflect common
ancestry.
2. Team Project - C takes notes, D
speaks
a. Why do we have this goal? Give
two reasons.
3. Traditionally, classification schemes
did not always follow evolutionary
relatedness.
a. The figure to the right is from
Purves et al., a popular biology text
book. It shows two ways to view
the evolutionary relationships of
some of the vertebrates. It also
shows two ways to classify them.
b. In (a), four major splits occurred
through time and are represented by
five extant (living) taxa.
c. In (b), a traditional classification
scheme that recognized three major
classes of vertebrates, Aves,
Reptilia, and Mammalia.
d. Finally, in (c), a classification
scheme that reflects common
ancestry.
Phylogenetics of the invertebrates - Page 1 of 10
4. Team project - B takes notes, A speaks
a. Describe two ways that a classification scheme influences your thinking about the
evolutionary history of the organisms in question. You may contrast the “traditional”
and the “phylogenetic” classifications depicted in the previous figure.
B. Terminology and how to read a cladogram or phylogenetic tree
1. An example using the familiar vertebrates (taken from Hickman et al. 1995)
a. In the cladogram below, a nested hierarchy of taxa is depicted.
b. Amphioxus is the outgroup, and the study group comprises four vertebrates (bass,
lizard, horse, and monkey).
c. Four characters that vary among the vertebrates are used to generate a simple
cladogram: presence versus absences of four legs, amniotic eggs, hair, and mammary
glands.
d. For the four characters, absence is the ancestral state in vertebrates because this is the
condition found in the outgroup, Amphioxus.
e. For each character, presence is the derived state in vertebrates.
f. Because they share presence of four legs and amniotic eggs as synapomorphies, the
lizard, horse, and monkey form a clade relative to the bass. These characters are also
called shared, derived traits.
g. This clade is subdivided further by two synapomorphies (presence of hair and
mammary glands) that unite the horse and monkey relative to the lizard.
h. We know from comparisons involving even more distantly related animals that
presence of vertebrae and jaws constitute synapomorphies of the vertebrates and that
Amphioxus, which lacks these features, falls outside the vertebrate clade.
i. The horse and monkey are sister taxa. The horse-monkey clade and lizards are sister
taxa.
Phylogenetics of the invertebrates - Page 2 of 10
C. Tools used in phylogenetic reconstruction
1. Types of traits
a. Morphological, physiological, behavioral
i Traits expressed during development
ii In juvenile forms
iii In adult forms
b. Molecular data
i Amino acid substitutions in proteins
ii Genomic genes and gene products
iii Mitochondrial genes
2. Evolution as descent with modification
a. Ancestral traits
i Organisms that share a common ancestor retain some of the traits of that ancestor.
ii These traits are only useful in determining membership into a potential
phylogenetic tree. For example, the presence of four limbs unites the tetrapods,
but doesn't tell us anything as the sequence of their evolutionary divergence.
b. Derived traits
i If a trait changes from the ancestral trait, it is derived. For example, snakes have
lost their four limbs. Loss of limbs is the derived state.
ii If organisms exhibit the same change in an ancestral trait, it is termed a shared
derived trait (synapomorphy).
c. Homologous traits
i Traits that are derived from the same ancestral trait.
ii For morphological traits, the structure is derived from the same embryological
tissue.
d. Homoplastic traits (analogous traits)
i Traits may resemble one another not due to common ancestry, but due to other
evolutionary processes.
ii Convergent evolution results in traits arising independently. The same selective
forces that drove the evolution of the trait in one group of organisms can drive the
evolution of a similar looking trait in an unrelated group of organisms. For
example, loss of limbs has evolved numerous times in both reptiles and
amphibians.
iii Evolutionary reversals occur when a character changes from a derived state back
to the ancestral state. Seastar adults have radial symmetry, yet they are placed
within the clade of animals whose shared derived trait is bilateral symmetry. The
larval forms of seastars are bilaterally symmetrical.
3. Team project - D takes notes, C speaks
a. Give two attributes of traits that tend to homoplastic. In other words, what about
certain traits that make them more susceptible to convergent evolution or reversals?
Phylogenetics of the invertebrates - Page 3 of 10
4. Data processing
a. Parsimony is usually employed. This idea merely reflects our feeling that changes
through time do not occur that frequently and a phylogenetic tree should have the
fewest changes. For example, in the phylogeny above, jaws evolved only once, rather
than each major vertebrate class independently evolving jaws. We will explore this in
lab.
b. Molecular data require the use of computer programs that calculate the probability of
a given set of mutations occurring. Nucleotide substitutions are common and not all
substitutions are equally likely. The most common method is called maximum
likelihood.
c. Ideally, a researcher will use both morphological and molecular data in constructing a
phylogeny.
d. Computer programs generate numerous possible phylogenies (sometimes hundreds,
depending upon the data set). All of these trees are compared and a consensus tree is
constructed that reflects only those lineage splits that are found in the majority of the
most parsimonious trees.
D. When classification does not reflect phylogeny
1. There are many ways to lump organisms into categories.
a. "Reptiles" may include birds as well as snakes, lizards, crocodiles, and turtles, or it
may not.
b. A monophyletic taxon includes all of the descendents of a single common ancestor.
This is the ideal classification.
c. A paraphyletic taxon includes a single common ancestor but not all of the
descendents. Excluding birds
from the Class Reptilia renders
it a paraphyletic grouping.
Many of our names and
groupings of creatures were
originally coined long before
the advent of reconstructing
phylogenies. Even though we
all know that birds are reptiles,
albeit highly derived, most
people do not include them
when discussing reptiles.
Intellectual inertia is an
extremely difficult force from
which to break!
d. A polyphyletic grouping contains members who share more than one most recent
common ancestor. These groupings are generally the result of insufficient data.
Phylogenetics of the invertebrates - Page 4 of 10
E. Where does the Phylum Arthropoda fit within the Kingdom Animalia?
1. Classification before the use of molecular data was based strictly on morphology,
primarily types of body cavities, larval forms, segmentation, etc.
a. The bilaterally symmetrical animals were divided into two major clades, the
protostomes and the deuterostomes. (See cladogram on penultimate page.)
b. Current thoughts retain the protostome and deuterostome clades, but a major
reorganization of the protostomes has occurred.
c. Protostomes are now divided into two clades, the Lophotrochozoa and the Ecdysozoa,
based on ecdysis (growth by molting) and cleavage pattern.
Phylogenetics of the invertebrates - Page 5 of 10
F. The Ecdysozoa
a. Note that the Arthropoda is depicted as have four subphyla, not the five that we
covered in lecture previously. Folks who study these groups no longer recognize the
Uniramia. Even though the five subphyla have been agreed upon in the primary
literature, it will take some time before this is reflected in textbooks (again,
intellectual inertia!).
b. The placement of the Onychophora and the Tardigrada are also depicted above as
being sister taxa. This is no longer a well-supported hypothesis.
G. Two "minor" phyla of interest
1. The two phyla below are considered to be the sister taxa to the Phylum Arthropoda.
a. A sister taxon is the most closely related taxon to the group in question.
b. All three phyla share the ability to grow by molting (ecdysis).
2. Phylum Onychophora (velvet worms)
a. The velvet worms comprise 70 species of caterpillar-like animals that live in moist
warm environments.
Phylogenetics of the invertebrates - Page 6 of 10
b. They have an over 500-million-year history - some of the coolest fossils found in the
Burgess Shale, located in Yoho National Park, in the Rocky Mountains, near Field,
British Columbia, Canada, are onychophorans.
c. Characteristics shared with the arthropods include a chitinous cuticle, an open
circulatory system with hemocoel, and a tracheal system.
d. Recent ribosomal RNA sequence data suggest that they are
indeed arthropods.
3. Phylum Tardigrada (water bears)
a. The water bears are minute, usually less than a millimeter in
length.
b. There are 300-400 species living in water film surrounding
mosses and lichens; some are freshwater and a few are
marine.
c. The tardigrades show segmentation, "malpighian tubules",
arthropod-like musculature and setae.
4. The placement of these two phyla differs depending upon
the researcher.
a. Biology textbooks place them as sister taxa to one another with the tardigradeonychorphora clade a sister to the arthropods (as was seen in the Ecdysozoa tree
above).
b. Others place either one of them as the sister taxon to the arthropods.
c. Here is one possible phylogeny.
H. A phylogenetic view of the Phylum Arthropoda
1. On the last page of the outlines you will find a traditional phylogeny for the
arthropod subphyla and the major classes based on morphology.
a. I include it
here as an
example of
how our
interpretation
has been
dramatically
altered with
Articulating, jointed
the relatively
appendages
recent
addition of
Setae
molecular
Malpighian tubules
data.
b. This view is
Growth by ecdysis
represented
Appendages
in tree (A)
Hemocoel and open circulatory system
below.
Loss of ectodermal cilia
Phylogenetics of the invertebrates - Page 7 of 10
2. Below are four different hypotheses combining both morphological and molecular
data.
a. Tree (C) currently garners the strongest support.
Phylogenetics of the invertebrates - Page 8 of 10
One Possible Metazoan Phylogeny
A traditional phylogeny of the Kingdom Animalia
Cnidaria Platyhelminthes
Arthropoda
13
Annelida
16
15
12
Mollusca
19
18
17
14
Echinodermata
23
22
Chordata
25
26
24
11
5
10
4
9
21
8
3
20
7
6
1
2
1- multicellularity
2- collagen
3- radial symmetry
4- cnidae
5- planula larva
6- bilateral symmetry
7- triploblastic
8- spiral cleavage
9- mesoderm arises from the 4d cell
10- complete gut with mouth arising
from the blastopore
11- coelom by schizocoely
12- compound eyes
13- articulated appendages with chitinous exoskeleton
14- trochophore larvae
15- head of prostomium and peristomium
16- serially arranged body setae
17- coelom reduced; body cavity a hemocoel
18- radula
19- mantle which secretes a shell
20- coelom arises by enterocoely
21- blastopore becomes the anus
22- water vascular system
23- secondarily pentaradial symmetry
24- dorsal hollow nerve cord
25- notochord
26- pharyngeal gill slits
Phylogenetics of the invertebrates - Page 9 of 10
A traditional phylogeny of the Arthropoda
Subphyla
Crustacea
10
7
9
a
14
26
25
22
16
Ins
ect
a
24
23
15
po
d
op
Ch
ilo
Me
17
Di
pl
hn
Ar
ac
18
8
od
a
Uniramia
ros
tom
Ma
ata
lac
ost
rac
Ma
a
xil
lip
od
a
Br
anc
hio
po
Ce
da
ph
alo
car
Re
id
mi
ped a
ia
Chelicerata
ida
Trilobitomorpha
21
20
6
13
12
5
4
19
3
11
Phylum
Arthropoda
2
1
1- articulated appendages with chitinous exoskeleton
2- compound eyes
3- body divided into prosoma and opisthosoma, without distinct
head
4- first appendages chelicerae
5- second appendages pedipalps
6- typically with four pairs of walking legs
7- opisthosomal appendages reduced, lost, or modified as
spinnerets or pectines
8- with trachae, book lungs, or both
9- opisthosomal appendages modified as book gills
10- telson long and spiked
11- mandibles present
12- biramous anntenules and antennae
13- nauplius larvae
14- fully developed carapace
15- maxillae reduced or absent
16- thorax shortened to fewer than 11 segments
17- thorax shortened to fewer than 6 segments
18- male gonopores fixed on thoracomere
19- tracheal gas exchange system
20- malpighian tubules
21-one pair of antennae
22- loss of compound eyes
23- two pair of walking legs per somite
24- one pair of walking legs per somite
25- loss of abdominal appendages
26- formation of distinct three-segmented body head, thorax and abdomen
Phylogenetics of the invertebrates - Page 10 of 10