What important assumption does parsimony make when assessing which phylogenetic tree is most accurate? Why was parsimony misleading in the case of the astragalus during the evolution of artiodactyls?

Video transcript

Welcome back. Here's the next question. Why is the optimal tree being chosen for maximum parsimony. So here we're talking about the creation of philo genetic trees. So parsimony in general as a general term uh in science says refers to the fact that the simplest explanation for a set of data is preferred. So in the case of villa ginny, when you're talking about drawing this tree that explains relationships among different species and how they evolved, we would say that the hypothesis of relationships that requires the smallest number of character changes is most likely to be correct. When we talk about character changes. When we're looking at um philology, we're talking about the gain or loss of certain attributes or characteristics. So you know the the a gain of wings or the loss of certain bony structures or the gain. It would also be um morphological or physiological or even behavioral traits being gained or lost. So when we posit those relationships, the tree that overall so on all its branches has the fewest number of character changes. Is the one most likely to be correct. So you have to look at the whole picture and not just one branch. So why would we say that? We would choose um the one choose for maximum parsimony to get the optimal tree. Well let's look at our answer choices choice A says to lessen home a place. Well let's recall that home a place E. Is similar structures with different ancestry. So our classic example is bird wings and bat wings. Similar structures, They do the same thing but they don't arrive, they arise. There was independently in the two lines. They're not due to common ancestry. Well let's think about this. If we have a tree where for instance, you know, so we draw different different species arising here coming out. And we're going to say that species A. Here has evolved to have a bony skeleton but that none of these other species out here have bony skeletons including its ancestral species here. And then we say that species B over here also has a bony skeleton, but its common ancestor does not. Well in this case we've drawn a tree where a bony skeleton has evolved independently two different times. Whereas if we drew a tree where the common ancestor of A. And B. Had a bony skeleton. So fewest number of character changes. So here we had to go from no skeleton too bony skeleton, A non bony skeleton too bony one. And here we also did that. Whereas here we have that character change occurring once and then the descendants both having that characteristic. This is going to be a more likely answer than the other. So we've minimized home a place, see the independent evolution of the same characteristic versus descent from a common ancestor. And that is precisely what we're looking at when we are looking for maximum parsimony. So that is going to be our answer. And just to make sure we can look at our other answers. Sometimes a question like this can be a little confusing because it's not it can be a little nebulous. So sometimes what we want to do is look through the other answers and make sure nothing seems more likely. Choice B says to avoid more errors. Well no that's not the answer. We we can have cases where you have convergent evolution in two different lines. Um but this is to say which is the most likely explanation. Choice C. Says to lessen hm Ology. Hm Ology is similar structures due to a common ancestor and we don't want to minimize that. We don't want to lessen that we want to actually have that occur more often because it requires fewer character changes. So in this case the bony skeleton in A. And B. Is a similar structure due to sharing a common ancestor. So choice C. Is not our answer and finally, Choice D. To maximize the data. Um That's not our correct answer. We're looking to get the simplest explanation for the data set. We have no choice D. Not correct. So again, why is the optimal tree being chosen for maximum parsimony choice A two lesson home? A place you in the next video