Clint Explains Phylogenetics - There are a million wrong ways to read a phylogenetic tree

[Music] well hi there got some big news lions are not tigers but they are both cats they're both big cats and if you breed them together they can produce offspring ligers and tigons so it probably won't come as any big shock to learn that lions and tigers share common ancestors if i were to show this as a diagram i might draw something like this in this case i can see that today i have two distinct organisms lions and tigers but if i follow the lines back in time long enough i will run into a point where the two lines intersect this point of intersection is called a node and it represents the last common ancestor of both lions and tigers to be clear this likely wasn't a lion or a tiger it also wasn't a single animal or a pair of animals but a population whose descendants include both lions and tigers this population at this point in time was the last time that lions and tigers shared common ancestors before their two lineages went their separate ways this could have happened because part of the population became physically isolated from the rest or because some of them made choices like which mates they picked to isolate themselves genetically from the rest of the population either way this population that was previously a single population became after this point represented by the node two separate populations in this diagram called a phylogeny visually represents my hypothesis that these two species alive today are related and share common ancestors like any hypothesis this one makes predictions for example i would predict that if lions and tigers share common ancestors they should have considerable genetic and anatomical similarities i would also make many predictions about fossils that could be found to show steps in the transition between their common ancestors and modern lions and tigers the phylogeny itself is a hypothesis a model that would explain the evolutionary relationship between these two organisms however a phylogeny showing the relationships between only two species is not that interesting this is like a family tree depicting the relationship between two people we can be pretty confident that they're related so somewhere they share common ancestors but that's about all that it says it doesn't get interesting until we add in a third person timmy and wanda are both humans and of the two people here timmy's closest relative is wanda well other than timmy himself they also share ancestors cool but the story doesn't get interesting until i add in spencer timmy wanda and spencer are all humans as such they all share ancestors but the two that share ancestors more recently will be more closely related to one another than they are to anybody else for example if timmy and wanda are siblings then they share parents and if spencer also shares ancestors with them but 10 generations back then timmy and wanda will be more closely related to one another than they are to spencer also worth noticing is that spencer is no more closely related to timmy than he is to wanda he's equally related to both of them because he shares the same ancestors with both of them also if i flip the way that i drew the diagram so that timmy is on the top and wanda on the bottom that doesn't change the story at all if i flip it so that spencer is on the very top instead of the very bottom that also changes nothing their position on the diagram doesn't mean anything all that matters is how recently they shared ancestors with the other individuals on the tree if i add in a new person janet and find out that she goes here on the diagram what i know is that janet shares ancestors more recently with timmy and wanda than she does with spencer but spencer is equally related to janet timmy and wanda because the last ancestors that he shares with any of them he shares with all of them this is exactly the same way that our phylogenetic trees work yes lions and tigers share common ancestors but what happens if i add in a leopard a cheetah and a grizzly bear which one is the closest relative to the tiger this is where we need to remember what we discussed with the family tree there are a million wrong ways to interpret a phylogenetic tree and only one is the right way so when i'm trying to answer the question which is the closest living relative to the tiger i need to remember the one right way even phylogenetics graduate students often make mistakes with this because the more you know about these diagrams the more wrong ways you can come up with to interpret them the only right way is to identify the last time that tigers share common ancestors with any other living group on this phylogeny and everything else that shares those ancestors with the tiger are equally related to the tiger and the closest living relatives to the tiger in this case the last time that tigers share ancestors with any other living organisms on this phylogeny was at this node it shared those ancestors with both lions and leopards therefore lions and leopards are both the closest living relatives to the tiger remember when the ancestors of tigers split off from the ancestors of leopards and lions there was no difference between the ancestors of leopards and the ancestors of lions they were the same thing so one of them is not more closely related to tigers than the other leopards and lions are more closely related to one another than they are to tigers because they share more recent common ancestors with one another than they do with tigers but they are both equally related to tigers and now you know but to make sure that you understand how to interpret phylogenetic trees i have a couple of questions for you to answer down in the comments please feel free to ask questions and help one another with these questions as well so first looking at our phylogeny of lions tigers leopards cheetahs and grizzly bears what are the closest living relatives on this phylogeny to cheetahs second if i would be to flip all of these nodes resulting in this phylogeny does it depict the same hypothesis about the relationships between these organisms or a different hypothesis again please answer in the comments and let's make sure we've all got this if you've learned something today please like this video if you'd like to learn more about this topic and other topics in the future please subscribe and click the little bell and we hope to see you real soon you

Channel: Clint Explains

Views: 120,336

Keywords: Phylogenetics, Cladogram, Clade, Phylogenetic tree, family tree, evolutionary relationships, diagram, hypothesis, Science, Biology, Evolution, Monophyletic, monophyly, hybrid, interpreting phylogenetic trees, Clint's Reptiles, paraphyletic, polyphyletic, education, teaching, lesson plan, made easy, science help, Blacktip reef shark the best pet fish, Clint Explains, bill nye the science guy, biology education, understanding phylogenies, evolutionary tree, science rules, Kahn Academy, big cats

Id: SqmblucbOzk

Length: 7min 45sec (465 seconds)

Published: Sat Feb 27 2021