Rather than stretch out the thread on Evolution: Backward Reasoning? any farther, I’m taking this question to a new post.
Can anyone explain to me, in three paragraphs or less, exactly what cladistics is? This is as far as I understand the concept:
The famous system of bilogical classification first developed by Linnaeus is based on anatomical observation, and strongly implies that all living things are related to each other in a hierarchal organization. For example, apes and dogs are similar enough to each other that it makes sense to say that they’re both “mammals”. Mammals and birds are similar enough to each other that it makes sense to call them both “Vertebrates”; and so on.
Darwin’s theory of Evolution by Natural Selection proposes a plausible mechanism for how this could have come about: by variation from common ancestors.
Supposedly, there’s a problem with this. Evolution is supposed from anatomical observation. But if you presume evolution, and then use that to infer relationships between organisms (i.e., “anatomical feature in speciman A evolved from anatomical feature of speciman B”, then you have circular reasoning.
Supposedly, cladistics is a system of using certain objective(?) criteria for determining evolutionary relationships. But I couldn’t follow the reasoning in what I’ve read about it. Any “Cladistics for Dummies” authors out there?
Cladistics attempts to provide a structure to determine descent. It looks at common features that derive from a common ancestor and tries to determine where they “branched” to associate common groups. The important aspect of cladistics (if I don’t need to be corrected by Dr Fidelius) is that the branching must occur on a time line or “time axis” for the graph (cladogram).
Among mammals, for example, the “earliest” appear to have laid eggs. A branch is constructed that places monotremes (egg-laying platypuses, etc.) on one side and all other mammmals on the other side. A second branch occurs with the development of placental mammals. No animal with a placenta giving live birth has had a descendent that returned to the marsupial method of having the young move from the uterus to the marsupial pouch for their final development, so another branch occurs between marsupial and placental mammals.
One issue that arises from cladistics can be seen by examining the cladogram for a lungfish, a salmon, and a cow. Cladistics looks at the fact that the branching traits that led to the development of lungfish and cows was a later branch than that which led to salmon and lungfish, so (on a cladogram) the lungfish and the cow have a branch that is closer than the branch that separates the lungfish and the salmon. This is pretty counter-intuitive, and cladistics is still suspect in some circles for this reason.
The purpose of cladistics is to organize the data into logical categories. It is an organizing principle, not a reality (although it expresses “reality” very well in certain areas).
Another point on the time-line dependent aspect of cladistics:
(You can check this out more closely in Gould’s Dinosaur in a Haystack, Chapter 19, “Evolution by Walking.”)
Using the time-line/development guideline, the hoof developed among mammals after the development of the specific stirrup-shaped stapes in the ear. Since the hoof is a later development, it appears later in a cladogram of mammals. Since most people intuitively associate “later” with “higher” or “better,” it is disconcerting to see a cladogram with elephants and manatees at the “top” while humans are down in the “middle.” Of course, since we should have long ago gotten rid of the idea that evolution is a ladder with better and better animals being produced, this should not be a problem–but it is still disconcerting to see it visualized.
(If someone is going to jump on my last statement with “But I thought that is what evolution was supposed to do?” please save us all a lot of bandwidth by realizing that Darwin’s Theory of Evolution through Natural Selection makes no such claim. The point of Natural Selection is that it will convey on succeeding generations a better method to survive and multiply in the ecological niche in which the animal or plant exists. A succession of species that clearly gets “better” as it proceeds to shed mass and fur to become the swiftest predator on the prairie is no longer better if an ice age makes fur and mass a necessary component to prevent freezing to death. Evolution is not interested in building the “best” animal, it is interested in building animals that will better survive in current conditions.)