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Discovering the tree of life

AP Bio: EVO‑3 (EU), EVO‑3.B (LO), EVO‑3.B.1 (EK), EVO‑3.C (LO), EVO‑3.C.1 (EK), EVO‑3.C.2 (EK), EVO‑3.C.3 (EK)

Video transcript

(light music) - [Instructor] We're looking at the main types of biodiversity, genetic, ecological, and evolutionary. Here we want to really focus on the latter, talk about evolutionary lineages. Something that I live and breathe in my own scientific research. The study of evolutionary lineages is nothing less than analyzing biodiversity over time. We always want to know where did biodiversity come from, where is it going and what is the role of humans in the future of these patterns. Lineages allow us to look at the evolutionary history of different species, so the study of evolutionary lineages is also the study of the tree of life. Scientists use that tree metaphor to depict actual relationships among organismal groups in a branching diagram, but how do you make that diagram, what do you need to know that will allow us to assemble that tree of life? What we're talking about is a science known as phylogenetic systematics. Phylogenetic patterns are made up of characters, features you can observe in organisms. Sounds pretty simple but what it really means is that a unique feature of an organism represents a unique event in the evolutionary history of that organism an event that marks the first appearance of that feature. And if you look at several organisms and list the unique features of those organisms, you are actually looking at the unique events in their histories that tell you something about their relationships. This is because these events can be shared with other organisms, if they are shared, their histories are shared. In other words the species are related. Think about sea urchins for example, who wouldn't want to think about sea urchins, they're pretty neat animals, they have long spines, nice rounded bodies supporting those spines, but did you know that sand dollars are basically flat sea urchins that have adapted to life on the beach, which I think is pretty nice work if you can get it. So if sand dollars are sea urchins, can we identify some evolutionary novelty that joins all the sand dollars together to the exclusion of all other types of sea urchins? Can we put them together in a single lineage? Here's our sea urchin and here is our sand dollar, but here's another type of sand dollar, the obvious feature that links these two guys is that they are as we said, really flat but no other sea urchins are flat like this, that's a character that uniquely connects all of the sand dollars together to the exclusion of all other sea urchins. A feature that's arisen only once in the evolutionary history of the sea urchins that led to the sand dollars. The suggestion is that these two guys share common ancestry, a common history because right at this point they evolved as characteristic of being flat. With the sand dollar group, all of which we now know share a common ancestry, you can have further elaborations on this flat form. For example, some will have weird holes through their bodies that also represent unique evolutionary events within the sand dollar grouping. And this is another critical thing to recognize about the tree of life. Every group is nested or included within another group, nature is a hierarchy that can be represented by these branching diagrams, diagrams known as cladograms or phylogenetic trees. But things can be really complicated, we know that there is some 250 living species of sand dollars and over 750 extinct fossil species. We also know there's a single phylogenetic tree for sand dollars showing how they're all related one to another. But we don't know the precise shape the branching order also known as the topology of that tree. Each sub-grouping can be supported by a unique evolutionary novelty or even several if you have lots of data. The aim is to best arrange all the unique characters to resolve or support all of the relationships. Ultimately the idea is to make a branch point for every single one of the relationships so we can document the unique evolutionary events or characters among all the species being studied. One thing that emerges from all of this is the simple fact that some species have appeared on earth more recently than others, you can read this from the topology of the tree when you realize that there's a time axis along here, oldest to most recent. The important thing is the relative branching order, the topology of the tree. This branch point occurred before that one and that branch point occurred before both of those. But let's face it, with ten million species on earth, the tree of life can get pretty complicated which is why it takes a lot of data and a lot of studies to place as accurately as possible each group of organisms on that big tree. So for phylogenetic systematics, you need to use a computer, taking lots and lots of character data, coding character traits for every species in your analysis and feeding that into a computer program. There are mathematical processes to produce trees that can be tested and tested again with new characters with the aim of arriving at the most supportable hypothesis for the topology of the tree. Sometimes these new characters can lead to changes in the branching order of the tree. And there's something relatively new on the scene to help phylogeneticists test these hypotheses of relationships and that of course is DNA. Evolutionary pathways and novel features are recorded just as much in DNA as they might be in the physical traits of an organism that you can see with your eyes. Phylogeneticists rely more and more on the analysis of large amounts of molecular data to develop trees. With the grand view of these trees in hand, you get an even more powerful way of looking at how evolution happens you can actually read life's history book which I think is one of the most exciting things you can possibly do in this field of study. For example, in the case of the sand dollars, I can explore why they got flat or why they have these bizarre holes in them, using the tree to tell a story of evolution, it's what I live for in my science. One of the first ever cladograms appeared in the work of none other than Charles Darwin, who recognized the importance of evolutionary trees. I always think of old Chuck when I think of this grand picture of life, this grand picture of biodiversity and how trees show that. Darwin knew that these trees reveal lineages and big picture biodiversity against a background of enormous time. What he didn't know was just how powerful our modern techniques could become in telling a spectacular story and how useful these trees are in helping us protect earth's amazing but threatened biodiversity. (light music)