Mammalogy!
Definition of a mammal:
Basic mammalian characteristics include:
Mammary glands
Endothermy
Hair
Sweat glands
Divided into about 29 orders, 125 families, 1,229 genera, and over 5,420 species
(more on taxonomy below)
see also table 2-2, p. 18 of text
Taxonomy:
You can't study mammalogy (or herpetology, ichthyology, ornithology, etc. without taxonomy).
One needs a way to discuss different animals so that everyone knows what one is talking about.
This course covers “mammals” (as loosely defined above).
Taxonomy is the science of classifying organisms (making sense of their relationships, since modern taxonomy is based on evolution).
Traditionally, taxonomy is based on the system developed by Linnaeus. You should all be familiar with it, but here's a quick summary:
To classify the cheetah, we would do the following:
Kingdom Animalia Phylum Chordata Order Carnivora Family Feliday Genus Acinonyx species jubatus
As you go down in categories, the organisms begin to look more and more like each other.
Incidentally, notice the italics. Genus and species names are always italicized (or underlined) when written (this is actually part of the official rules of the English language!) We will be using this system in this class, as it's the easiest to use from a teaching perspective, and is still used widely by zoologists, botanists, and other organismal biologists.
However, this approach has several problems, particularly if you're strictly investigating evolutionary relationships:
1 - everything above species is entirely subjective. Is an “order” for mammals the same as an “order” for insects? Does it reflect similar evolutionary relationships?
(As an aside, our species concept needs revision, but no one can agree on how to do this!)
2 - it covers up branching points. For example, in the family Felidae, do all cats of the genus Panthera have the same evolutionary relationship to each other?
3 -
To address some of these issues, many modern biologists use a cladistic approach.
Note that cladistics can be used to both classify and establish relationships (the two are complementary).
Using cladistics, organisms are classified solely based on their evolutionary relationships.
Cladistics (strictly speaking) does away with the entire Linnaean system, as it's impossible to force a cladistic classification into the Linnaean categories.
Instead, branching points are used, where each branching point describes characters that are (supposedly) shared by all subsequent branches.
[ show cladogram ]
This makes it very difficult to use in teaching as there are as many branching points as species.
It makes it hard to know what to call a particular group of organisms.
Fortunately, your text doesn't do much with cladistics except talk about it. But here are a few more examples from herpetology this spring as to the problems a pure cladistic classification can cause.
[ show picture of lizard ]
[ show picture of reptile ] Please note that for determining relationships or constructing a classification based solely on relationships (sort of the idea), cladistics is a very valuable tool.
For teaching a course like this cladistics is horrible.
Determining (analyzing) relationships:
There are actually several different ways of doing this:
1) Phenetics:
Measures as many different characters as possible, then tries to establish relationship based on the similarities of these characteristics.
Often uses fairly complicated statistical techniques such as cluster analysis.
The problem is that a “blind” phenetic approach does so without regard to the relationships of the characters.
In other words, what is causing the similarities? Evolution? Convergence? Homologies?
A blind phenetic approach can lead to classifications based on similarities, regardless of where those similarities come from.
(e.g., classifying dolphins and sharks together)
2) Cladistics:
When used to construct relationships, it uses “shared derived” characteristics to determine relationships.
If two organisms have the same characteristic due to homologies, then they are in some way related.
Supposedly, this provides an objective approach.
Unfortunately, what happens is practice is that there is a lot of argument over:
- which characters are homologous
- and which characters should be used
- also, many of the algorithms used to do cladistic classification allow one to rate characters on an importance scale before analysis.
- for example, if the researcher believes that two skull openings is more important than color pattern, then he or she can “assign” relative weights to these characters. - this can obviously remove the objectivity (although the example above seems reasonable).
In fairness, it should be mentioned that more recent classifications rely more and more on molecular relationships which are (supposedly) more objective.
Cladistics is partly responsible for the state of modern taxonomy. Whether that's a good or bad thing is for other people to decide.
3) Traditional:
There is a third approach that is still used. It's the “expert” approach.
Someone will study a particular group of organisms for a long time and become an acknowledged expert in the field.
Based on his or her knowledge, he or she puts together a taxonomy.
This may or may not be combined with one of the two approaches above.
Regardless of what approach one uses, one ought to be able to explain what happened or is happening.
For example, someone decides to subdivide two species of gopher.
Why? Just because? Is there a reason for doing so (not just because “cladistics tells me so”)
Is there some type of geographic barrier that can explain why there should be two species?
Characters used:
Morphology:
Basically, the appearance of individuals. Various measurements are taken. For example:
Skull shape, foot length, total length, various ratios, color, etc.
All of these can be used to differentiate different species. You will be using some of these in lab to differentiate the different species.
Biochemistry/genetics:
Using proteins, DNA, immune system responses, or other techniques to differentiate species and/or groups.
These days a lot of taxonomy is done using DNA sequences. Obviously, the more similar DNA sequences, the more related.
This is without a doubt the most powerful approach to establishing relationships and usually works very well.
However, there are two problems in using this blindly:
1) Sometimes the people doing the analysis have no idea what the organisms even look like.
a) if you are trying to establish the relationship between x and y, you ought to make sure you can identify x and y to begin with!!
A long time ago some folks almost made a mess of gopher classification because they didn't even know what a gopher looked like.
2) It is not always clear what a specific difference in DNA means. Does a difference of “x” imply two different species?
a) For example, if x hybridizes easily with y, they shouldn't be separate species, regardless of what the DNA is telling you!
(This happened with painted turtles, but I can't think of a good example from mammals).
Still, doing DNA analyses intelligently is probably one of the best methods for establishing taxonomic relationships.
Has been used successfully many, many, times.
Still, keep in mind that DNA is one of many techniques. It may be better than most, but it's not the only one!
We will not learn how to do this in this course.
Some taxonomic nomenclature:
Monophyletic - all members of a taxon are in the same group and descended from a single “node” or individual.
Paraphyletic - Not all members of a taxon are in the same group
This is the issue with birds vs. reptiles
Cladists do not like to see paraphyletic groups Polyphyletic - Members of the same taxon have different ancestors (really screwed up).
No one likes to see polyphyletic groups
Recent taxonomic changes:
One final problem is the current state of taxonomy.
In recent years much of the taxonomy has changed.
This is not just true in mammalogy, but also in herpetology, ornithology and other fields of zoology.
Unfortunately, the current nomenclature is a bit of a mess, and not all of it is accepted yet.
Many new families and even orders exist. Most are simply a re-organization of previous families and orders.
Some species have also been put into different genera:
Mustela vison --> Neovison vison
Pipistrellus subflavus --> Perimyotis subflavus
(This doesn't seem to be quite as bad as in herpetology).
What does that mean for us?
Two things are obvious:
Scientific names are not stable. They should be. But what you learn today may change tomorrow.
English (=common) names often seem more stable than anything else.
As a result, you will be required to learn English names as well as current scientific names.