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This episode’s topic is a subject that underlies all of our conversations about the evolution and relationships of living things: The Tree of Life!
In the News:
Ground Sloth Diet
Chemical investigation shows that giant ground sloths were strict herbivores, countering previous suggestions of meat- or insect-eating. [Scientific Paper]
Why Are Whales So Big?
An analysis of whale evolution found that whales began attaining their modern-day colossal sizes around the time of the global changes leading into the Ice Age. [News Report]
The Mighty Jaws of T. rex
Evidence from T. rex bite marks and jaw mechanics reveal that these predators were probably excellent bone-crunchers. [News Report]
The Evolution of Soldier Bees
Only a few bee colonies have dedicated guards. This study finds that the evolution of soldier bees lines up with the evolution of parasitic robber bees. [Press Release]
Classifying Living Things
In biology, taxonomy is the practice of classifying and naming living things. Groups like “mammals,” “reptiles,” or “invertebrates” reflect the way we lump organisms into categories to make it easier for us to study and discuss them.
The most famous classification system is the one developed by Carl Linnaeus. This is a nesting system, where each group fits inside a larger group.
Phylogeny is the shared history of life. In much the same way that your family tree reflects your ancestry and genealogy, an evolutionary tree (or phylogenetic tree) reflects the ancestry of groups of living organisms.
Charles Darwin was the one who made famous the notion of “common descent” (sound familiar?), the idea that all life is part of one big related family. In fact, Darwin was the first person to draw a phylogenetic tree!
Classic taxonomy and modern phylogeny don’t always match up. Ideally, we want to name our categories based on real-life relationships, but nature doesn’t fit nicely into categories. Like the colors of a rainbow, the boundaries between different “categories” of life are blurry.
The evolutionary tree of life is something we are constantly working to figure out. Cladistics is a method we use to do this. Essentially this is a technique used to combine our understanding of different creature’s traits and their evolutionary history to put together the best approximation of their family history that we can.
From techniques like these, we’ve developed some handy vocabulary words (we use these a lot in the podcast):
Clade – any group of related organisms, as long as it includes the earliest ancestor and all of its descendants (eg. Mammalia, Crocodylia, Primates, Homo sapiens).
Basal Traits – Traits that have been around since the early ancestry of a clade. Example: the earliest reptiles laid eggs, so egg-laying in crocodiles and turtles is a basal (or ancestral) trait among reptiles.
Derived Traits – Traits that are different from the ancestral condition. Example: some lizards and snakes give live birth, a trait that is different (derived) from the ancestral egg-laying feature of reptiles.
The clearer our understanding of phylogeny, the better a picture we develop of evolutionary processes. When did this trait arise? What were the steps in the development of this group of animals? What patterns do we see across the tree of life?
This also allows us to make predictions.
Understanding where on the tree a certain trait evolved allows us to predict what creatures may have inherited it. This is how we predict, for example, that saber-toothed cats had fur, or that T. rex had feathers, despite no direct fossil evidence.
We can also make predictions genetically! A molecular clock is a technique where we look at the genetic differences between species, check their phylogenetic relationships, and estimate the rate of genetic change – this allows us to predict how old these species’ common ancestor is, and predict when certain traits evolved.
Our understanding of phylogeny is not complete.
-Lots of information is missing, especially in the fossil record.
-It can be difficult to study many species or many traits at the same time.
-Convergent evolution can be very confusing! Is a trait shared because of common ancestry, or because it evolved multiple times?
As a result, there’s always a lot of discussion about the precise relationships of different groups, and new research is constantly trying to refine exactly where we place the branches on the tree of life.
The “Tree” also isn’t a perfect analogy. Even as branches split, interbreeding can persist for a long time. And the activities of some microorganisms and viruses can exchange genes “horizontally” between separate branches of the tree.
Taxonomy can get confusing, too! Since our categories are somewhat arbitrary, there’s a lot of debate about what groups should get what names to best reflect our understanding of evolutionary relationships. Species and genus names change quite often, and it can be a bit overwhelming trying to keep up with all of it!