Evolutionary Relationships of the Arthropoda I

Evolutionary Relationships of the Arthropoda I
Insects are a class (Insecta) in the phylum Arthropoda
The Arthropoda is the largest phylum of organisms and
accounts for over 75% of all species on Earth. Major groups
(subphyla) within the Arthropoda are:
1.  Trilobitomorpha (extinct) — trilobites
2.  Chelicerata — horseshoe crabs, spiders, scorpions,
ticks, mites, solifugids
3.  Crustacea—brine shrimp, barnacles, fish lice,
lobsters, crabs and shrimp
4.  Myriapoda — centipedes, millipedes
5.  Hexapoda — insects, entognathans
Representative Arthropods
Trilobitomorpha
Hexapoda
Chelicerata
Crustacea
Myriapoda
Major Characteristics of the Arthropoda
•  External and internal body segmentation with regional specialization
(tagmosis).
•  Paired articulated (jointed)
appendages surrounded by chitinous
cuticle. Arthropoda means jointed
(arthro) leg (poda).
•  Cuticle forms a well developed
exoskeleton, generally with thick
sclerotized plates.
•  Paired compound eyes usually
present (sometimes lost secondarily).
•  Growth by the process of ecdysis
(molting).
•  Coelom reduced to portions of the
reproductive and excretory systems.
•  Muscles are striated and arranged in
isolated, segmental bands.
Three major questions regarding the
evolutionary relationships of the Arthropoda
1. How are arthropods related to other major phyla of
invertebrates, particularly the Annelida (segmented worms)
and the Mollusca (snails, bivalves, octopus and squid)?
2. Are arthropods a single evolutionary lineage, or have the
characteristics that unite them evolved multiple times?
3. What is the evolutionary relationship of insects to the other
major groups (subphyla) in the Arthropoda?
To answer these questions we need to review some terms and
concepts used in analyzing evolutionary relationships. In particular,
we need to define what we mean by an evolutionary relationship,
what kind of data we use to determine evolutionary relationships,
and how we represent these evolutionary relationships.
Three Kinds of Phylogenetic Relationships
•  Monophyletic. A group of species that
includes an ancestral species and all of its
descendants. These species comprise a single
evolutionary lineage and share a unique history
of descent. Monophyletic groups are called
“natural” because they represent the “true”
evolutionary history of the groups.
•  Paraphyletic. A group in which member
species are all descendent from a common
ancestor, but which does not contain all the
species descended from that ancestor. Class
Reptilia (turtles, snakes, lizards and
crocodilians) in the vertebrates is a good
example of a paraphyletic group because it
excludes birds, which is the sister group of the
crocodilians.
•  Polyphyletic. A group in which member
species share more than one immediate
ancestor. Polyphyletic groups are “artificial”
because they do not shared a common
immediate ancestor. They occur when
convergent or non-homologous characters are
used to define or diagnose a group.
Endothermic vertebrates is an example of a
polyphyletic group because birds and mammals
do not share an immediate common ancestor.
Characters Used in Phylogenetic Analysis
•  Homologous characters are features that have
the same evolutionary origin as determined by
positional, developmental and genetic studies. Only
homologous characters are useful in recovering the
evolutionary history of a group of taxa.
•  Convergent (analogous) characters are features
that perform similar functions, but have different
evolutionary origins. Convergent characters cannot
be used to reconstruct evolutionary history, but they
are very useful in comparative studies of
performance (e.g., insect versus vertebrate flight).
Homologizing structures
in the heads of stalkeyed flies in two different
families
Fossorial forelegs in
five different genera
Lepidoptera
Trichoptera
Other Neoptera
Other Pterygota
•  Plesiomorphies are characters that arose in a distant common
ancestor, or “primitive” features that are shared by distantly related
species. Plesiomorphic characters that are shared between two or
more taxa are called symplesiomorphies.
Thysanura
•  Apomorphies are characters that arose in a most recent common
ancestor, or a recently evolved (“advanced”) feature that appears
only in a group of closely related species. Apomorphies that are
unique to a particular taxon are called autapomorphies.
Autapomorphies are useful in identifying a taxon and distinguishing it
from other groups (a diagnostic trait). Apomorphies that are shared
among taxa are called synapomorphies.
Other Apterygota
Two Kinds of Similarities in Homologous Characters
•  Determining which characters are apomorphic and which are
pleisomorphic is accomplished by a character polarity analysis.
Examination of character distribution in groups known to be basal
relative to the one under study is one popular way to polarize
characters (outgroup comparison). Traits shared between the
outgroup and the ingroup are plesiomorphies, whereas those share
within the ingroup are apomorphies.
•  Plesiomorphy and apomorphy are relative terms. Each homologous
character is a synapomorphy at only one level of a phylogeny and is
a sympleisomorphy at a deeper level of the phylogeny. For example,
wings are a plesiomorphy of butterflies because they are shared with
butterflies (ingroup) and with their closest relatives (outgroup). Wings
are an apomorphy of pterogyotes (winged insects) because they are
not shared with pterogyotes and their closest relatives (Thysanurans).
•  Taxa based on apomorphies are monophyletic, whereas taxa based
on plesiomorphies are paraphyletic.
Wingless
Winged
•  Each split or dichotomy in the cladogram produces a
pair of newly derived taxa that are called sister-taxa or
sister-groups.
•  The more synapomorphies that are nested in a
consistent manner, the higher the level of congruence
for the cladogram. However, not all cladograms show a
completely consistent nested set of synamorphies. Low
levels of congruence may be due to mistakes in
determining which characters are homologous and
which are homoplastic, or the result of evolutionary
convergence. Low levels of congruence may also result
from mistakes in determining which characters are
plesiomorphic and which are apomorphic.
Lepidoptera
Trichoptera
Other Neoptera
Other Pterygota
•  A cladogram is a graphic representation of the origins
of synapomorphies. In its ideal form a cladogram
depicts a completely nested set of synapomorphies. A
cladogram is a very general evolutionary tree that
indicates only relative relationships and not the timing
evolutionary events. A phylogeny is a cladogram
calibrated with the fossil record and the geological time
scale.
Thysanura
•  Evolutionary trees are constructed by analyzing the
topological arrangement of the homologous traits
(apomorphies and plesiomorphies) identified in the taxa
under study (ingroup) in comparison with the outgroup.
Other Apterygota
Phylogenetic Analysis
Wings
covered
in scales
Wings covered
Wings folded
Wings present
Dicondylic mandibles
Major Branches in Animal Phylogeny
Multicellular Ancestor
Radial
Ctenophores
Radial
Porifera
Cnidaria
Deuterostomes
Bilateral
Protostomes
Echinoderms
Sea Squirts
Lancets
Vertebrates
Molluscs
Annelids
Rotifers
Flatworms
Nematodes
Tardigrades
Onychophorans
Arthropods
Characteristics of Phylum Annelida
•  Annelids (from Latin annellus
for “little ring”) are the
segmented worms the include
earthworms, marine worms
(polychaetes) and leeches.
There are about 15,000
species worldwide.
•  Characteristics shared with the
Arthropoda include serial
arranged body segmentation
(metamerism), double ventral
nerve cord, dorsal and ventral
longitudinal muscles, and a
dorsal blood vessel with
forward-going peristalsis.
Characteristics of the Mollusca
•  Molluscs (from Latin molluscus
for “soft”) include the
gastropods (snails and slugs),
bivalves (clams and mussels)
and the cephalopods (squids
and octopus). There are about
93,000 species worldwide.
•  Characteristics shared with the
Annelida include pelagic larvae
(trochophore) with one or more
bands of locomotory cilia
located equatorially (near the
mouth) and formed before
gastrulation, pelagic larva with
para- or circumanal ciliary tuft,
and paired excretory organs
and ducts that open externally
(nephridiopores).
Apical tuft (cilia)
Prototroch (cilia)
Stomach
Mouth
Metatroch (cilia)
Mesoderm
Anus
Trochophore larva
Relationship of Arthropoda to Other Phyla
•  Hypothesis 1. Arthropoda is the sister group of
the Annelida, which together comprise the
Articulata. Mollusca is the sister group of the
Articulata.
•  Hypothesis 2. Annelida and Mollusca are sister
groups, which together comprise the
Eutrochozoa. Arthropoda is the sister group of the
Eutrochozoa.
•  Hypothesis 3. Arthropoda and Mollusca are
sister groups and Annelida is the sister group of
the Arthropoda + Mollusca clade.
•  These 3 phylogenetic hypotheses can be
“tested” by mapping apomorphic characters on to
cladograms and counting up the number of steps
required. By the principle of parsimony, the
hypothesis with the least number of steps is more
likely to be true.
•  Parsimony analysis provides equal support for
hypothesis 1 (Articulata) and hypothesis 2
(Eutrochozoa). In each instance, a minimum of
three evolutionary changes are required.
Hypothesis 3 requires at least four evolutionary
changes and is therefore less parsimonous.
Most Recent Phylogeny for Protostomes
•  Recent phylogenetic analysis based on
molecular characters (Dunn et al 2008)
suggest two major lineages within the
Protostoma: 1) the Lophotrochozoa, which
include the molluscs, the annelids several other
phyla, and 2) the Ecdysozoa, which include the
Arthropoda, Onychophora, Tardigrada and
Nematoda.
•  The lophotrochozans are split into two groups
those that have lophophores, a fan of ciliated
tentacles surrounding the mouth (bryozoans,
brachiopods, ectoprocts, and phoronids) and
those that have trochophore larvae (molluscs
and annelids, and several other worm-like
groups). There is debate on whether these
subgroups are monophyletic, or even whether
the lophophorates are protostomes!
•  The ecdysozoans all share a three-layered
cuticle composed of organic material, which is
periodically molted as the animal grows, hence
the name. There is still controversy over
whether this group is monophyletic and some
researchers still place the annelids as the sister
group to the arthropods or the panarthropods
(arthropods + onychophorans).
Alternative Phylogeny for the Ecdysozoa
•  One problem with the previous
phylogeny of the protostomes is that
it places the tardigrades as the sister
group to the nematodes, which
complicates our understanding of the
evolution of segmentation and other
characters associated with
arthropods (paired limbs with claws).
•  Telford et al. (2008) proposed an
alternative phylogeny of the
Ecdysozoa which places the
tardigrades + onychophorans as the
sister group to the Arthropods
(Eurarthropods).
•  Note there are other differences
between the Dunn et al and the
Telford et al phylogenies, including
the placement of the Myriapoda. We’ll
cover that topic in the next lecture.
Conclusions
•  We answered the first of our three questions regarding the
phylogenetic relationships of the Arthropoda, at least tenetatively.
Molluscs and annelids are more closely related to each other than
either is to arthropods. This means that segmentation as observed in
arthropods and annelids is homoplastic and not homologous. In
contrast, the trochophore larvae present in molluscs and annelids is
probably a true homology.
•  We also learned that having more characters in a phylogenetic
analysis does not necessarily make things easier. More characters
may mean more chances for conflicts in terms of the hierarchical
nesting of them. Molecular characters may improve phylogenetic
analysis, but errors may occur when the rates of evolution are vastly
different in different lineages (e.g., the long branch attraction
problem, a problem seen in nematodes).
•  We will address our other two questions in the next couple of lectures
when we examine the phylogenetic relationships within the
Arthropoda and the relationships of insects to the other groups.