Lab two – Individual adaptations to the environment: leaf convergence in plants
Objectives – after completing this laboratory, you should:

Clarify your understanding of what an adaptation is.

Understand the concept of morphological convergence and its underlying
causes.

Become familiar with the specialized leaf structures of carnivorous plants.

Be familiar with evolutionary tree diagrams, or cladograms, and what they
represent

Be able to use a cladogram to test hypotheses of convergence and adaptation.
Background – When two organisms share a feature, there are really two explanations for
how this came to be: One possibility is that the organisms share the feature because they
both inherited it from a common ancestor they shared some time in the past. In
evolutionary biology, a feature that is shared by two different species through common
descent from the same ancestor is known as a homology, and the features in each organism
are described as being homologous. However, a second possibility exists: it is also possible
that two species share the same feature because they each evolved it independently. In this
case, if you were to travel back in time to observe the last common ancestor of these two
species, this ancestor would NOT have the feature they share. We call this phenomenon
(similar yet independently evolved features) convergence, because the two species arrived
at the same feature from different evolutionary starting points.
There are several explanations for the phenomenon of convergence. One possibility
is that the species converged on a particular trait or suite of traits because they both
encountered a similar environment and evolved similar adaptations to that environment.
Alternatively, the two species could share the same feature for purely non-adaptive
reasons. Convergence could occur simply by chance; for instance, if two species have
similar physical limits on the types of features their development can actually produce. It is
also possible that one species evolved the feature as an adaptation to a particular
environment while the other species only possesses the feature because it inherited it from
an ancestral species; while the feature may have been an adaptation to a particular
environment in this ancestral species, if the modern species no longer encounters the same
environment, then the feature is no longer considered an adaptation in the modern species
(it’s more like biological baggage that the species is stuck with).
At this point, you may be thinking that this is all a nice intellectual exercise, but that
there is no feasible way to differentiate between homology and convergence, or between
the different causes of convergence in any concrete, scientific way. After all, in order to test
hypotheses of homology, convergence and adaptation, we would need to be able to look
back thorough time and see when and where features first evolved! Until the later half of
the last century (i.e., the 1960s and ‘70s), this was the case: evolutionary ecologists really
had no method or theoretical framework for testing these sorts of hypotheses. Around this
time, however, the work of a German entymologist named Willi Hennig gained recognition
and became a new and powerful tool for testing evolutionary hypotheses. Hennig
introduced a method for analyzing the features of species (called characters) to produce an
evolutionary diagram called a cladogram. The components of a cladogram and what they
represent biologically are described below:
In this cladogram, the tips of the lines (denoted by the !, %, #, @ and *) represent
the organisms that have been included in the analysis; the cladogram depicts the
evolutionary relationships of these organisms. The lines on this cladogram represent
evolutionary lineages (usually species) through time, so as you travel from the tips of the
lines down to the point where the lines all join (D), you are traveling back through
evolutionary time. The points where the separate lines connect are called nodes. Nodes
represent the most recent common ancestor or (MRCA) of evolutionary lineages. In this
cladogram, the node “D” is the MRCA of all of the species in the analysis. “C” is the MRCA of
species !, % and #, while “A” is the MRCA of % and # and “B” is the most recent common
ancestor of @ and *. As you can see on the cladogram, nodes are really the points in time
where one ancestral species splits into two daughter species.
An important point to remember when learning to read cladograms is that they are
all about relative ancestry. What this means is that the real information about the
relationships between species at the tips of a cladogram is found in the nodes: namely, how
many nodes (or lineage splitting events) separate one tip species from another. Because
cladograms represent relative relatedness of species, they can’t be fixed in space – in other
words, the diagram shown above can be flipped 180 degrees around any of the nodes
without changing the relative relatedness (the MRCAs) of any of the species at the tips. It’s
key to remember that the order and distance between the species along the tips of the
diagram do NOT contain any information about the actual relationships of the species. As
an example, in the cladogram above, # and @ appear physically closer to each other along
the top of the diagram than # and !, but # and @ are separated by four nodes, while # and !
are only separated by two nodes. This means that # and ! are actually much more closely
related (they’re separated by fewer speciation events) than # and @.
One final note on cladograms – it is also important to realize that these diagrams
really represent a hypothesis of evolutionary relationships. The relationships portrayed by
a cladogram can change when additional data or more sophisticated analytical techniques
are utilized. For this reason, it is always a good idea to regard cladograms as hypotheses in
the process of continual improvement and refinement, and not as static, unchanging fact
(this is true of all the products of scientific research).
How can cladograms help us test for convergence?
Sometimes it can be very hard to decide whether two species that share many
features are similar due to common ancestry (homology) or convergence. Because
cladograms are created based on the analysis of many different characters (most
commonly DNA sequence data in modern analyses), the evolutionary “signal” present in
the majority of the characters tends to overwhelm any false indication of evolutionary
relationship that might be present in a few convergent characters. This means that once a
cladogram has been obtained, the evolution of a particular feature or character of interest
can be re-examined in the context of the cladogram.
This sort of analysis of character evolution is usually done by “mapping” the
presence or absence of the character of interest back onto the species at the tips of the
cladogram, and then, based on the relationships of the species on the cladogram,
calculating how many times the character would have to either evolve (be gained) or be
lost in order to explain its distribution in the tip species. The basic principle used in this
process is called parsimony. Simply stated, “parsimony” is the expectation that the simplest
answer to a problem is probably the best answer. In our case, this means that if the
distribution of a trait in two species on the cladogram can either be explained by two
independent gains of the trait or one gain and seven subsequent losses of the trait, the first
explanation (which requires only two events as opposed to eight) is preferred. We will
explore this idea a bit further using plants that share the feature of stem succulence.
The cladogram above represents our most current understanding of the relationships
between a large group of flowering plants known as the core eudicots. A group of species
(or, in this case, orders of plants) such as this that all descend from a single common
ancestor (the node at the base of this cladogram) is called a clade. All of the orders of
plants at the tips of this cladogram are themselves clades composed of many species
(orders are shown at the tips here, because we would need hundreds of pages to display all
the species at the tips!).
Can you think of how we might use this cladogram of the core eudicots to determine
whether stem succulence in the cacti and the euphorbs is due to homology or convergence?
The cacti are part of the Caryophyllales, and the euphorbs are found in the Malpighiales. In
order for stem succulence in the cacti and euphorbs to be a homology, the most recent
common ancestor of the Caryophyllales and the Malpighiales would have to have possessed
this trait. Find the most recent common ancestor of the Caryophyllales and Malpighiales.
Now look at the other orders (which don’t contain stem succulents) that also descended
from this ancestor. Is it more parsimonious to assume convergence (independent gains of
stem succulence in cacti and euphorbs) or homology (one gain and multiple losses of stem
succulence) for this trait?