Remember the Linnaean system of classification you probably learned in grade school or high school (heck, maybe even college)? It went something like this…
Kingdom -> Phylum -> Class -> Order -> Family -> Genus-> Species
Linnaeus started using this classification system in the 1700s and he had a good run; we still use his system in certain situations. However, we’ve moved away from this ranking system mostly because of the discovery of transitional fossils that screwed with Linnaeus’s idea of neat little boxes for all of life to be categorized into. This system falls apart, for example when you have two equivalent ranks, let’s say class Osteichtyes (boney fish) and class Amphibia (amphibians), and find a transition between the two.
In the case of our example, let’s say we find a transitional fossil like Tiktaalik with both fish and amphibian characteristics. Now what? Does this thing belong to class Osteichtyes or class Amphibia? Which box do we put it into? For reasons like this, most modern evolutionary biologists prefer to organize species on a diagram called a cladogram. These take a little getting used to but once you understand them, they give a person a much better sense of how evolution is actually operating.
Cladograms are braching diagrams where each branch represents an evolutionary lineage.We put the names of specific taxa at the end of each branch and we put the names of groups to which those taxa belong at the intersection of branches (nodes). Check out the example below that uses mammals.
The diagram doesn’t really have axes, but implied in the x direction is some sort of evolutionary distance from each other and implied in the y direction is relative time (the first branch at the bottom happened long ago the latter branches higher up happened more recently). This diagram tells you that all the animals listed are mammals (see the group name Mammalia at the bottommost node). The koala and kangaroo are more closely related to one another than to anything else on the diagram. Their branches intersect each other before they intersect any other lineage. This indicates that they diverged from each other more recently than they diverged from any of the other animals on the diagram. Furthermore, they both belong to the group Marsupialia. Similarly, the bat and the lion are more closely related to each other than they are to either of the marsupials. They are both placental mammals (eutherians).
Now lets try a slightly more complicated example…
Here we can see some interesting relationships. Notice you (assuming you are a human) are more closely related to a turtle than you are to a frog (you and the turtle are both amniotes, part of the group Amniota). Now, as you work your way down the diagram things start to really get interesting. The tuna is more closely related to humans than it is to sharks. Surprising right? I know what your thinking “Is that really true?” Sure it is. See the distance between the ‘Vertebrata’ node and the ‘Osteichtyes’ node? That distance is shared evolution between humans and tunas that sharks don’t share (they’ve already branched off). Put another way, this means that tunas and humans have a few extra million years of evolution in common that tunas and sharks don’t. The fact that sharks and tunas both live in the ocean and kinda look a lot alike doesn’t count for all that much.
As long as we are examining this diagram notice that the Linnaean ranks we used before as equivalent, Osteichthyes and Amphibia, are certainly not equivalent on this diagram. Amphibia is contained within Osteichthyes. In fact all land dwelling vertebrates are within Osteichthyes (this is why some evolutionary biologists are fond of saying we’re all ‘fish’).
Now finally, I’ll show you where Tiktaalik fits on the cladogram. It’s closer to being an amphibian than to being a tuna, but then again it’s just as close to being human as it is to being an amphibian.
Update: If you want to read a sort of sequel to this post involving dinosaurs, you can check out this post.
As a Ecology, Behavior and Evolution Major in the Bio dept at UCSD I do love a good cladogram.
They are pretty handy, huh?
What I don’t understand about taxonomy is that they try so hard on things like this, argue about which classification is better or which is not… when it all boils down on perspective really.
TBH, this is like the chicken and the egg – how can you be sure that one trait is evolutionarily earlier than the other? Where does the “outlier” go? And how would you define an outlier trait? Does one getting closer to another species dependent on what? Does an absence of fossil indicates a recent evolution trait? Have we even seen all those fossils?
And we have no knowledge how many species really are there on our planet – especially taking into account how many of those species evolved into new species and species that extinct without a trace *think microbes*. Everyday is a challenge, and I don’t think it’s worthwhile to classify all those species.
The egg came first…as reptiles laid eggs long before chickens existed.
Sure classification schemes are not perfect. But they are essential for thinking about whether traits evolved through ecological pressure or whether they are around because of they were inherited from a previous ancestor. For example, how do we know if every whale evolved flippers separately or not? If you know that they are all close evolutionary relatives then it is much simpler to suppose that flippers evolved once and that each whale species did not re-enter the ocean and evolve flippers independently.
I’m still a bit confused with the evolutionary trait – obtained from environmental pressure and from parents.
I can understand this in terms of microbes… but not necessarily on eukaryotes (or let’s say big animals?!).
If you happen to have obtained a trait of making your neck a bit longer than usual because you have to find a way to cheat every single exam, will your children obtain this “special” trait because of your adaptation to your environment? That’s a bit vague for me… how does one species slowly evolve into a new species? From the experience I have, there’s no one example that showed me how one species slowly turn into another one… unlike in microbes, which is so easy, most of them just use the plasmid to transfer from one another and you have some sort of genetic swap and there you have it, a new species.
We need a half hour to go into this. I’ll try to give you a more in depth reply or maybe another post on this topic a little later.
“From the experience I have, there’s no one example that showed me how one species slowly turn into another one”
Shades of the tired old “micro vs. macro evolution” argument. Ugh. This person may not be debating in good faith.
Xerophytes is legit, he’s just not a biologist.
LoL.
At least, I’m legit.
Hey, I’m in one of the branches of biology. So that puts me into a sort of cladogram of biology!
I think we should classify animals by how they taste!
oh yes.
i am willing to defend that argument, by the way. I have all sorts of novel, ingenious arguments that will make you wish that you hadn’t wasted time taking me up on this.
you will wish that you had the time back
you will say “christ. I could have spent this time cleaning my cat’s litter boxes. that’s 2 hours i won’t get back”
Oh for god’s sake, FINE.
I say you are wrong, sir! Clearly the best way to classify meat is by nutritional values.
Lets see what you got.
aha.
tissue makeup, chemistry and muscle and fat distributions will all vary continuously as species evolve into each other. so the closer two animals are related, the more they will taste alike.
chickens taste like alligators more than cows for this reason. turkeys and geese taste alike for this reason.
and i bet seals taste more like bears than whales.
Do your arguments involve Canadian bacon? I assume that is classified by the way it tastes.
no, but it’s true that back bacon is classified by the way it tastes. that’s why each time you buy a pack, there’s a bite out of one of the pieces. we have “premium grade”, “satisfactory grade”, “economy grade”, and “ship to the united states grade”
We further sub-categorize our imported Canadian bacon into Grade A and Grade F But Edible. (I HAVE SEEN THIS DEMARCATION. Good for colleges and prisons, apparently…)
Ben, et al.
PLEASE make a cladogram for foods/animals based on taste and include ALL bacons. That would be so epic…
I dunno… have you voted for SSO in the podcast awards recently?
I have a couple times and JUST did again!
Bacoooooon
i’ll see what i can do then.
there. dont’ say i never do anythign for you
http://www.super-robot.ca/srpics/Cladogram.jpg
Holy crap! Its glorious!
Thanks!
Este articulo fue etupendo. Me encanto como el autor describio como los animailes bofan y como canbian el tamano de sus testiculos. Grasias por su information y tiempo.
~Mari De La Paz
This is really helpful for my biology test!! I’ve learned a lot. But I was confused on one of the parts. This explains in a very childish way!!