1997 WWLPT Biology Institute:
Life Cycles: Reproduction & Embryological Development
Embryology as Evidence of Evolution:
Discussion Questions to Accompany Sea Urchin Embryology
Mira Loma High School
Target Age or Ability Group Audience
Teacher Instructions/Special Precautions
Materials & Equipment Needs
Background [Prior Knowledge
or vocabulary necessary to complete activity]
The Student Lab
Method of Evaluation/Assessment
After observation of the early stages of sea urchin development, students
will read about the two major developmental pathways found in coelomate
animals. Next, students will observe data regarding differences in
the DNA that codes for 18s rRNA in five organisms from five different phyla.
Based on the two types of data (embryological and molecular), students
will construct a phylogenetic tree that shows the evolutionary relationship
between these five organisms. Since sea urchins are frequently used
in biology classrooms as a model of early animal development, students
will be asked to comment on their relevance to the study of vertebrate
1. observe the two major developmental pathways (protostome
2. analyze data regarding differences in nucleotide
3. construct a phylogenetic tree
4. observe the similar evolutionary history shared
by echinoderms and chordates
Target Audience or Age Group
Classes: Biology (probably most appropriate
for honors/advanced classes)
Notes to the Teacher:
Unless you want to provide additional diagrams/sketches/pictures for students
to use, no preparation time is required.
This activity could be completed in one class period. It may be more
appropriate to send home with students and use as a discussion tool in
the following class period.
This activity is most appropriately used in conjunction with a sea urchin
fertilization lab, although it also could be used during a discussion of
taxonomy or evolution.
Materials & Equipment
Needs to top
No materials necessary, unless you have access to good diagrams/sketches/pictures
of the early developmental stages of the five organisms listed.
In order to complete this activity, students should:
have an introductory knowledge of evolution, including the ways in which
evolutionary relationships are determined
be familiar with the structure and function of DNA
be able to identify the early stages of development in animal embryos
be familiar with the fate of the blastopore in different animal embryos
be familiar with the concept of a phylogenetic tree
The Student Lab to
Embryology as Evidence of Evolution
The embryos of animals from different phyla share many common characteristics
during the early stages of development. Each embryo is formed when
a sperm cell fertilizes an egg to create a zygote. This zygote then
goes through subsequent cleavages to form two cells, then four, then eight,
and so forth. Eventually the embryo is solid ball of cells called
a morula. Cells in the morula begin to migrate and the embryo changes
from a solid ball to a hollow sphere of cells known as a blastula.
Soon thereafter a group of cells begins to migrate inward. The point
on the embryo where this migration takes places is called the blastopore.
Other cells will follow in the migration, eventually forming a tube called
the archenteron. This process of migration is called gastrulation,
and an embryo in this stage is called a gastrula.
The stages described thus far can be observed in any animal embryo.
After gastrulation, however, the development of embryos varies from phylum
to phylum. One example of this variation can be seen when we observe
the fate of the blastopore in different animals.
As mentioned earlier, a tube called the archenteron forms as gastrulation
takes place. In embryos of any phylum, this tube will develop into
the gut of the animal. It is the orientation of this gut that varies
from phylum to phylum. In animals such as mollusks, annelids and
arthropods, the blastopore (the opening of the archenteron) will develop
into the mouth of the gut. In animals such as echinoderms and chordates,
the blastopore will form the anus instead of the mouth.
In attempting to determine the evolutionary relationships between organisms,
scientists look at several different types of data. One such type
is embryological data. It is assumed that organisms with similar
developmental patterns share a common ancestor. Is this a safe assumption?
Why or why not?
If similarities in development are a reliable means by which to determine
evolutionary relationships, what would you conclude about the different
phyla in the animal kingdom? Which are most closely related?
In an attempt to determine the relatedness of several different organisms,
two researchers (Hiroshi Wada and Noriyuki Satoh) analyzed the nucleotide
sequence in a segment of DNA from five different organisms. The DNA
used was a segment that codes for the production of 18S rRNA, a nucleic
acid that forms part of the ribosome. Why would Wada and Satoh choose to
examine DNA sequences to determine relatedness of organisms?
Think about the function of rRNA, and decide why the selection of 18S rRNA
was a good choice for comparing the biochemical makeup of organisms.
The table below shows the results obtained by Wada and Satoh.
The numbers listed indicate the number of substitutions (or differences)
in the DNA nucleotide sequence between any two possible combinations of
Based on the data above, which two organisms are most closely related?
Which are least closely related?
Construct a phylogenetic tree that shows the relationship between the five
organisms examined. This tree should show in graphic form the relationships
between organisms and how they diverged over time. In making your
tree, be sure you consider both embryological and molecular data.
Suppose you constructed a phylogenetic tree for these five organisms using
only morphological (physical) characteristics. Would this tree differ
from the one constructed in question 1? If so, how?
Discuss which of the two trees (embryological/molecular vs. morphological)
shows a more accurate relationship between the five organisms.
If there was a conflict between embryological and molecular data, describe
the conflict and how you decided which piece of data was more significant.
Make a statement as to why it is or is not appropriate to use sea urchin
embryos in the study of animal (especially vertebrate) development.
If there is a more appropriate organism to study, name it and then list
the pros and cons of using this organism in the classroom. Also list
the pros and cons of using sea urchins.
Methods of Evaluation/Assessment
Students' interpretation of data will be assessed in looking at the phylogenetic
Conclusion questions will assess the process used by students in constructing
Several activities produced at the 1995 Woodrow Wilson Summer Biology Institute
in Evolution focus on the creation of phylogenetic trees using molecular
evidence. These activities can be found athttp://outcast.gene.com/ae/AE/AEPC/WWC/1995?
References Including Web Addresses
Wada, H. and Satoh, N. 1994. Details of the evolutionary history
from invertebrates to vertebrates, as deduced from the sequences of 18S
rDNA. Proceedings of the National Academy of Sciences of the USA