Hey Chuck,
I noticed one question that seemed to pop up repeatedly in all your last bunch of posts. You want to know how scientists decide on their classifications. If we have a bat, a pigeon and a mouse, then the bast and pigeon have wings, while the bat and mouse have breasts. How do we decide which factor is most important in splitting them into two clades?
This is actually quite a difficult question, and is one of the reasons that classifications have changed over the years with more study, in some cases quite dramatically. Let's take a look at the sugar glider and flying squirrel again. At first glance, it's hard to tell them apart. But, when you look a lot closer, you realise the similarities really are only skin deep. Under the skin, there are all sorts of differences.
Sugar gliders have a pouch, where the young complete their development, just like a kangaroo. They also have an epipubic bone, a bone that sticks forward from the pelvis which supports the pouch, again, like a kangaroo. Their kneecap is cartilage, like a kangaroo's, but not like the flying squirrel, whose is made out of bone.
Moving on from bones, you notice major differences if you start looking at sex organs. Flying squirrel females have a single vagina, which leads up to a single uterus, just like in a human. In the sugar glider, however, a glance at the naughty bits would reveal that the vagina splits into two seperate passages inside, just like in a kangaroo. To match up with this, the head of the sugar glider's penis is branched into two (like a kangaroos), while the flying squirrel has a boring, straight penis, again like humans.
There are many more details of anatomy and physiology in which sugar gliders match up with kangaroos, but not with flying squirrels (forgive the focus on genitalia, but I just yesterday read the chapter on the genital system in my comparative anatomy book, so this stuff is fresh!). The sugar glider and flying squirrel look very similar on the outside, since they live the same sort of lifestyle and need to be able to do similar things. But they clearly have a different fundamental body plan inside - this is why even creationist taxonomists were happy to put the sugar glider together with it's cousins, the wombats and kangaroos.
And, for a long time, this was pretty much how taxonomy was done. Since the details of internal similarity between the Australian marsupials seems both more numerous and more fundamental than the superficial similarities many share with placental mammals, it made sense to put them together.
But things aren't always this clear cut. It's not always simple to agree which feature is most informative taxonomically. Are all animals with a gibbety bone closely related, or are gibbety bones just the type of thing that's easy for lots of unrelated animals to evolve independently? The modern techniques of cladistics, as described by RAZD in a previous post, are an attempt to make this all a bit more scientific. They take advantage of the calculating powers of computers to classify groups of animals based around huge databases of characteristics.
When done well, you don't just pick and choose the features that you consider the most important, Instead, you include lots of features, and then calculate which nested relationships are the most likely. The more features you include, and the more animals (or plants, or microbes, or whatever) you include the more accurate your answer will be, since it's less likely to be tricked by odd convergences.
You asked before whether DNA was more important, and many of the modern analyses do rely heavily on DNA. One of the reasons for this is simply that it gives you a lot more data, and the more data you have the less likely your analysis is to be fooled by chance similarities. There's also the fact that convergence is less of a problem in the world of DNA. Since there are many arrangements of DNA that can produce functionally similar organisms, you don't need to worry about superficial similarites between distantly related organisms living different lifestlyes. It's through DNA that we've been able to confirm that some of the suspicious classifications of the past, like Insectivora (small, furry things that usually eat insects) are in fact an artificial assemblage of distantly related small, furry insect eaters. Elephant shrews and tenrecs are actually closer to elephants than to shrews, while colugos wind up nearer to primates.
Hope this helps!
™ Version 4.2
(1)
