Pleisiomorphy and Synapomorphy: Evidence for evolution
In biology there is a Taxonomy which classifies families and groups of species. It separates others, but tells us how the lines are drawn. One important feature of evolution to understand is Pleisiomorphies and Synapomorphies. These are how things arose, who shares them and who doesn't.
Pleisiomorphy is an appendage which has a similar function, but arose from different lines of evolution.
Synapomorphy is an appendage which is similar in form and arose from common ancestors.
The importance in these is expressed in the photo above. The Bat, which is a mammal, has five digits on its upper appendages, it has evolved to have them at different lengths, and four of the five compose the wing portion of the bat, while the fifth digit is used for grabbing.
These share a common ancestor with mammals, and though it has wings like a bird, it is not a bird.
These two lines don't even come close in evolution, seeing as how the ancestor for mammals and lizards split over 300million years ago. When mammals became placental, and had internal pregnancy, except for marsupials, lizards continued to lay eggs.
It is through looking at the similarity in appendages that we can understand the relation of one species to another. Just like looking at the skeleton of a whale.
We have the entire line of whale evolution, which shows their ancestor, the move to the water, the change in the hind appendages and finaly the evolution away from them. As they evolved their snout moved further up on their head, until it was on top, but they never gave up the air breathing lungs.
We find a lot of these similarities in nature. Only the mammals which have undergone extreme evolution have lost some or most of their synapomorphies. While other lines of evolution have gained their pleisiomorphies by the same method, evolving.
Birds and butterflies both have wings, but butterflies are not mammals, or lizards, they are arthropods. The belong to insects, and they don't get their wings from the same place, but from divergent evolution. Other insects, that can fly, do share a common ancestor.
This was part of what Darwin saw during his exploration of species. He had a complex understanding of how features arose, who shared them and who didn't. Which is how he came to note the forelimbs of the chimp are very similar to that of humans. He noted the brain form, the skull, the lack of a post anal tail.
Darwin didn't have genetics though, and he didn't need them to start the journey for looking at how things arose similarly from common ancestors and how they didn't share these at all. We now have complex systems that are ever being updated, which try to define a species and put it in the groups where it belongs.
The hard fact is, evolution happened, it is happening and it will continue. Much of what happens in evolution is due to environmental stresses. While evolution can slow and even stop for periods of time, it can oscillate. It can speed up to tens of years, and slow down to millions, as in the case of crocodiles. One thing s for certain though, the evidence is astounding, true, and verifiable.