This idea that there is a clear "pathway" of evolution from one type of organ to another has got to be a major delusion. These differences are not built into any genome, say the reptilian genome, so that the mammalian genome could have a different kind of heart by evolution from the reptilian, which means that evolution would have to proceed by trial and error, and just try to wrap your mind around that for a few minutes and it will drive you screaming bonkers. There is no way you could ever get a four chambered heart with its own peculiarities by evolution from a three-chambered heart, merely by trial and error. Unless you can point to something in the genome that makes this possible and I know you can't. I tried to think this through for the necessary changes that would have to happen to evolve the mammalian ear from the reptilian. You have to depend on mutations to come up with the right sequence in the right order and that is simply impossible. I'll try to elaborate this more later if you want to contest it.
But why do you think this change of heart () is impossible? After all, the 3-chambered reptilian heart is made up of two atrial chambers and a single ventricular chamber. All that is required is for a partition to form across ventricle to split it in two and you have a 4-chambered heart. And this is exactly what we see happen in the last stages of our own heart during foetal development.
The problem is that there is nothing in a genome that describes what organs or even a final body plans will look like, which is why analogies like blueprints or computer programs fail as they create this false impression. Instead one way I’ve heard the genome described which is somewhat more fitting is origami, since it is a series of simple folds which on their own don’t mean much, until they all come all come together to create a final form. The genome has protein-coding genes which provide a toolset of different ‘folds’, and regulatory genes which tell cells when these different ‘folds’ are to be performed. And these folds are being performed by many generations of cells, from a single fertilised egg cell to a multicellular adult, a different page of the instructions at a time. From this it doesn’t seem so far-fetched how a colony of cells can change how they distribute themselves to alter organs or body plans.
My point is that if you sincerely try to think through the trial and error required to get from one type of heart to the one you think it evolved to, you will discover that it is simply impossible. Try it for the task of evolving the mammalian ear from the reptilian. Sincerely, I said.
I am just trying to describe what is possible using my own understanding of biology. Is there any reason to think I’m being insincere in this? In the case of the heart we know a thickening of the ventricular wall will split it into two ventricles because that is what happens to our own heart during foetal development and results from how and when our genes are expressed. This change prevents oxygenated blood from the lungs mixing with deoxygenated blood returning from the rest of the body, which makes it more efficient, especially as we need a constant supply of oxygen as endotherms to maintain our internal body temperature. This is not as important for frogs and reptiles as they use the environment to control body temperature, but even a partial change in the shape of the ventricle wall, as in the 3.5 chambers mentioned by dwise1, will help to control the flow and reduce this mixing. As you mentioned, the formation of the ear is another example of this principle. During foetal development we see formation of the Meckel's cartilage. Most of this cartilage becomes absorbed into our lower jaw, but part of it splits off and migrates to form bones of the middle ear. Again this is controlled by expression of our genes, and some of the intermediary steps are reflected in some reptile fossils, so we know it is possible.
The point is that changes is gene expression during development can affect an individuals physiology, which would be described as variation within the population, but over many generations and favourable selection this variation can come to dominate the population. So a 3-chamber heart can become 4-chambered, or to give another example, a reptiles jaw can become broader and the musculature of their intestine can be modified. This is why I tried the analogy of origami, but I’m guessing it wasn’t as helpful as I hoped. Such is the way with analogies.
Re: Ordinary selection of built in variation is not species to species evolution
it can't be mutations because at the rate you impute to them there would never be a stable population at all, it would always be mutating into something else, but we have lots of phenotypically stable populations, especially daughter populations after a series of splits. Like domestic breeds in many cases.
Just to add to what’s already been said, you also have to remember that the selective pressures which initially moulded the phenotype of a population will continue to act on it, so as new mutations occur they they will also be subjected to these same selective pressures and so maintain a phenotypically stable population. Therefore, just because many mutations occur in each generation, this does not translate into the population always mutating into something else.
This is Linnaeus 1735 classification of animals. Aves is the second column which is then divided into seven orders, with the seventh being the Passeriformes that is further subdivided into ten genera, the second being Turdus (the thrushes). Hope it helps you see what others are referring to.