Exercise 2
Bivalve Phylogeny: Cladistics
The focus of this exercise is to introduce students to cladistics and how it is a tool for analyzing
taxonomic and phylogenetic relationships among Bivalvia.
SUGGESTED ELEMENTS FOR AN INTRODUCTORY LECTURE
Cladistics is an approach used to diagram relationships between taxa that share a common
evolutionary history.
Apomorphies describe taxa that share a unique set of features.
Plesiomorphies are primitive characters.
ACTIVITIES
1. To code multiple characters of bivalve species
2. To create a data matrix of character and character states of bivalve species
3. To construct a cladogram using 5 pre-selected characters from 5 representative bivalve species.
4. To construct a cladogram from bivalve species based on characters you select.
VOCABULARY
Ancestral
Apomorphy
Coding
Parsimonious
Cladogram
Clade
Derived
Plesiomorphy
Cladistics
Character state
Outgroup
Synapomorphy
Phylogeny
MATERIALS FOR ALL PROCEDURES
Equipment
None
Supplies
Colored pencils
Lab notebook
Organisms
Mercenaria mercenaria
Crassostrea virginica
Mimachlamys sp.
Mytilus edulis
Pecten sp.
Nucula sp.
Various bivalve species
SUPPLEMENTAL MATERIALS
Mikkelsen, P. and R. Bieler (2008). Seashells of southern Florida: Living marine mollusks of the Florida
keys and adjacent regions, bivalves. Princeton: Princeton University Press.
Abbott, R. (1996). Seashells of North America: A guide to field identification. New York: St. Martin's
Press.
Rehder, H. (1981). The Audubon Society field guide to North American seashells. New York New York:
Knopf Distributed by Random House.
Dance, S. (2002). Shells: The photographic recognition guide to seashells of the world. New York:
Dorling Kindersley.
Abbott, R. (1995). A field guide to shells: Atlantic and Gulf coasts and the West Indies. Boston: Houghton
Mifflin.
VENDORS FOR MATERIALS
Shells can be collected locally or ordered from Deltona shells, http://www.deltonaseashells.com
Field guides can be purchased at amazon.com
Bivalve Phylogeny: Cladistics
LAB OBJECTIVES:
1. To code multiple characters of bivalve species
2. To create a data matrix of character and character states of bivalve species
3. To create a cladogram of bivalve species
Systematics is a discipline within biology whose goal is the determination of the evolutionary history and
relationships among organisms, or phylogeny, and then the use of that phylogeny in classifying
organisms. To achieve that goal, a systematist will utilize evidence from a wide variety of sources
including paleontology, embryology, morphology, behavior, and molecular biology.
This exercise is designed to give you practice in the use of cladistics for analyzing taxonomic and
phylogenetic relationships. Cladistics is an alternative approach to diagramming relationships
between taxa. The basic assumption behind cladistics is that members of a group share a
common evolutionary history, and are thus more closely related to one another than they are
to other groups of organisms. Related groups of organisms are recognized because they share a
set of unique features called apomorphies, which were not present in distant ancestors, but
which are shared by most or all of the organisms within the group. These shared derived
characteristics are called synapomorphies.
Species belonging to the family Nuculidae have a deep temporal record and are the first bivalve mollusks
to appear stratigraphically. Consequently, it is appropriate to view them as the most primitive family of
bivalves. The character states that Nucula possesses, therefore, define the primitive states.
Nucula: nut clam
Your goal is to construct a cladogram for the bivalve species we examined in previous labs.
Your first step will be to construct a data matrix based on morphological characters and their
character states. Below is a table of the characters you will be examining. For each character
determine the “ancestral” and “derived” character state. Create a data matrix table for each
species and place a “0” for ancestral and “1” for derived characters, respectively. This is known
as “coding” features or “characters” for each bivalve.
Steps for Constructing a Cladogram
1. Select a taxonomic group to be analyzed; for example, a group of bivalves.
2. For each member of the group, determine some observable traits, characters, and note their
"states." A "character state" is one of two (or more) possible forms of that character. For
example, for a character "auricles" the possible states may be "present" and "absent."
3. For each character, determine which state is ancestral (primitive or plesiomorphic) and which is
derived (apomorphic). This is usually done by comparison with a more distantly related
organism termed the outgroup. It is hypothesized that traits shared with the more distantly
related organism(s) are likely to be "ancient" or plesiomorphic traits. Similarly, traits that differ
from the outgroup are postulated to have arisen since the group being considered branched from
its shared common ancestor with the outgroup, and thus are likely to be "derived" or apomorphic.
In this exercise, members of Family Nuculidae will be the outgroup for all species as they are the
most primitive family of bivalves. The character states that Nucula possesses, therefore, define
the primitive states.
4. Group taxa by shared derived character states (synapomorphies).
5. When in doubt, choose the most parsimonious tree. While similar structures may evolve
independently in separate lineages facing similar selective pressures (convergent evolution), this
is assumed to be a rare event. Most major structures (eyes, horns, tails, fur, etc.) are assumed to
have evolved or to be lost only rarely. Thus, when in doubt, choose a pathway that minimizes the
number of times a feature must be postulated to have arisen (or lost) separately.
Exercise 1
Use the 5 bivalve species below to test the hypothesis that Pecten and Mimachlamys – two “scallops” –
form a natural group. In other words, test whether “scallops” are really related or just look alike due to
convergent evolution. If Pecten and Mimachlamys form a branch, or “clade,” then they are related and
the hypothesis is true.
If Pecten and Mimachlamys do not form a clade, then they superficially look alike, perhaps because of
their environment, and the hypothesis is false.
Complete the following data matrix for the 5 bivalve species and their characters list below. Once you
have completed the data matrix, construct a cladogram.
Mercenaria mercenaria
Mytilus edulis
Pecten sp.
Character
1. Purple marking
2. Auricles
3. Radial sculpture
4. Muscle scars
5. DNA sequence
Species
Crassostrea virginica
Mimachlamys sp.
Character State
pres/abs
pres/abs
pres/abs
one/two scars
A/T
Char 1
Purple
Data Matrix
Char 2
Char 3
Auricles Radial sculpture
Char 4
Muscle scars
Mercenaria
Mytilus
Crassostrea
Pecten
Mimachlamys
Character #1. Is there dark purple or blue color on the inside of the shell?
Char 5
DNA Sequence
Mercenaria:
Mytilus:
Crassostrea:
Pecten:
Mimachlamys:
Character #2. Does the shell have auricles or “ears”?
Mercenaria:
Mytilus:
Crassostrea:
Pecten:
Mimachlamys:
Character #3. Does the shell have radial sculpture on the outside?
Mercenaria:
Mytilus:
Crassostrea:
Pecten:
Mimachlamys:
Character #4. Does the shell have two muscle scars or one on each valve?
Mercenaria:
Mytilus:
Crassostrea:
Pecten:
Mimachlamys:
Now code a gene sequence – a “molecular character.” Laboratory work indicates that these bivalves
have the following sequences in one of their genes. There are two different base pair sequences
differing at position #10. You can code this character just like auricles and muscle scars.
Character #5. What is the base pair at position #10?
Mercenaria:
||
AATTTGCGCAATTAG
Mytilus:
AATTTGCGCAATTAG
Crassostrea:
AATTTGCGCAATTAG
Pecten:
AATTTGCGCTATTAG
Mimachlamys:
AATTTGCGCTATTAG
||
Based on your coding construct a bivalve tree. Look carefully at the arrangement of the bivalves on your
tree. Remember your hypothesis: Pecten and Mimachlamys – two “scallops” – form a natural group.
Do Pecten and Mimachlamys form a clade on your tree? If so, then what do you conclude?
If the tree is not what you expected, you can go back and think about your coding to make sure that you
have not made an error. If there are no errors, then the tree is correct.
Exercise 2
Now that you have completed a cladogram testing the hypothesis that scallops are an artificial group, next
use the shell species we have examined in previous labs to construct a data matrix and cladogram. For
this cladogram you will be using Nucula as the outgroup possessing ancestral characters. Compare your
new cladogram with the previous one. Below are a list of character and character states you can use to
construct your cladogram. Below (and on Blackboard) you will find diagrams of the internal anatomy of
various bivalve species that you can use to create your cladogram.
Bivalve Characteristics and States
Characteristics
States
1.
2.
3.
4.
5.
6.
Byssus
Siphon
Foot
Margin
Adductor muscle
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
Pallial sinus
Symmetry
Ctenidia (gills)
Labial palps
Abdominal sense organs
Pallial tentacles
Pallial eyes
Shell shape
Costae (radial ridge)
Growth lines
Ligament
Aortic bulb
Adductor scar shape
Foot extension
a/p
a/short/long
small/large
smooth/crenulated
2 equal/2 unequal/ 1
Dentition
taxodont/heterodont/dysodont/isodont/desmodon
t/ edentate
a/small/large
bilateral/asymmetrical
small/large
small/large
a/p
a/p
a/p
oblong/round
a/slight/prominent
a/slight/prominent
external/internal
a/p
round/oblique/both
not able/able