I think you’re getting tripped on definitions. It seems that paleontologists have adopted a new convention in their definitions and illustrations, and we have to “adapt” (hah!) to that convention.
A series of transitional fossils demonstrates the history of transitions that have occurred. A transitional form is not necessarily a direct ancestor of a modern form; it might be, but it might also be the “cousin” of as-yet undiscovered contemporaries that were on the direct line to modernity. Those as-yet undiscovered cousins would be roughly similar to the discovered cousins.
The first time I saw this convention used in a cetacean evolution diagram, it stunned me for a minute. Then I realized that the paleontologists were trying to distinguish between what’s highly certain (the general path of the transitions) and what’s less certain (the exact placement of fossil species on the tree), and I appreciated the effort. Of course, the risk is that the new convention might lead to confusion amongst folks who did not learn this kind of diagram during their education.
The other source of confusion might lie in the way that the Homo group is depicted. I believe that a “group” was chosen rather than branches with leaves because we know from genetic evidence that various members of the different Homo species mated. So it’s not that no transitional form exists; rather, the transitions do not have sharply defined boundaries.
Yeah, I’m having a hard time keeping up. Hopefully this post is mostly in agreement with what anthopologists have figured out. I invite any experts in the field to correct anything I might have missed or misportrayed.
I saw a while ago a picture arranging antelope in a series of smallest to largest. A series of transitional forms but no ancestor-descendant relationship.
A series of transitional forms by itself means absolutely nothing for support of the theory of evolution. Likewise a series of ancestor-descendants means nothing by itself. It only supports the theory if it is a series of ancestor-descendants that also shows a transition in form.
I am about to leave for a three day retreat so I might not be able to post anything for a few days.
I’m sorry but I don’t understand what you meant by what you typed.
When you say “a modern form and an ancestor form” do you mean that the species represented by the ancestor form was actually an ancestor of the species represented by the modern form? I.e. that both are in the one line of descent?
And are you then claiming that the species represented by the transitional fossil is in the same line of descent?
Try this definition, perhaps: a form that shows transition between an older form and a newer one. Relatedness is necessary; a direct line of descent is not.
@aarceng
What Lynn said.
“Same line of descent” is not the same thing as common ancestor. Transitional forms share features and ancestors. They don’t have to be directly descended from one another.
Let me try to combine those. Feel free to correct me if I get it wrong.
Transitional Fossils (T) are ones that have traits in common with a modern form (M) and an ancestor form (A). M is a descendant of A. T is a descendant of A but not necessarily an ancestor of M.
Cladogenesis is splitting of one species into several different ones; such as a common ancestor splitting into chimps, gorillas, and humans. However if you follow one branch, say common ancestor to chimp then it will be anagenesis. Similarly the line from common ancestor to gorilla will show anagenesis, and the line from common ancestor to human. (If (and oh what a big if) the theory is correct and there is a common ancestor.) So within the one branch there will be both cladogenesis and anagenesis. It’s not an either/or choice; that’s a false dichotomy.
So along the line leading from common ancestor to humans there should be a population that slowly changes from one to the other through innumerable slight modifications. The transitions should have existed although they might not necessarily have been preserved as fossils.
This is a big part of the definition. The only thing I would add is that the later of the two forms connected by a transition is not necessarily modern. For example, Tiktaalik is a transitional form between lobe-finned fishes and early amphibians.
You understand the definitions correctly. However, the example of CA to humans is quite muddled by the significant amount of inter-breeding among the Homo clades (Neanderthal, Denisovan, Sapiens).
So perhaps
Transitional Fossils (T) are ones that have traits in common with a descendant form (D) and an ancestor form (A). D is a descendant of A. T is a descendant of A but not necessarily an ancestor of D.
So using your Tiktaalik example what is the ancestor species that both Tiktaalik and amphibians have descended from? We can see from this example that the transitional fossil can come after the descendant since the oldest tetrapod trace fossils (tracks and trackways) predate Tiktaalik by a considerable margin. (Tetrapod - Wikipedia). In other cases could it possibly come before the ancestor?
I think it’s also important to clarify that while the ancestor species would have existed, we have no guarantee that it would correspond to any particular fossil we have uncovered, and we would not be able to identify such a fossil with 100% certainty if we had it.
This is true if you mean A to refer to a hypothetical ancestor that they must have shared. But if you are looking to use A to mean an actual fossil, it would be better to say that M and T share traits with A but are not necessarily descended in a direct line from A.