Changing the way we look at the world
The problem with Cladistics
It has been a long time since I wrote a blog. But I got the urge to write another when a few weeks ago I watched a fascinating video of a talk by Professor Richard Buggs (Professor of Evolutionary Genomics at Queen Mary University of London) about trees. The lecture was about trees of life and you can find it here.
He covered a lot of ground, including ordinary trees you will see every day, phylogenetic trees (“Darwin’s tree of life”) and the Biblical tree of life.
During his lecture Richard highlighted the problem with phylogenetic trees (or something called a cladogram which is similar). They are easy to construct from a set of known characters of a group of just about anything. Plug the data into a computer programme for generating phylogenetic trees and a tree can be constructed. If the theory of evolution i.e. common descent is correct then phylogenetic trees and cladograms constructed for a group of organisms using different types of data should all be the same. For example, a tree constructed using the gene for cytochrome C will be the same as one constructed from morphological data. The problem is that this is very rarely the case.
Another problem that is very common when fossils are used to construct a cladogram is that they seem to change every time a new fossil species is discovered.
I became acutely aware of this problem when reading the scientific literature on dinosaurs.
The overall picture is fairly constant (most of the time) but even that has changed as recently as 2017. Before that date there were only two types or clades of dinosaur, Ornithischia and Saurischia, but now it seems that there are three clades Ornithischia, Theropoda and Saurischia (Baron, M. G., Norman, D. B., & Barrett, P. M. (2017). A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature, 543(7646), 501-506.)
The problem is even worse for families of dinosaurs. When reading papers published over a period of 10 or more years it seems that every new fossil results in a revised cladogram.
Part of the problem is down to the fact that some dinosaurs are know from very fragmentary fossils (sometimes a tooth and a bit of jaw bone!).
One publication admitted the problem very clearly by stating that : “ [..] the origins of the derived bauplan of the Sauropoda remains poorly understood. This is because of an early sauropod record in which only fragmentary forms are known from rocks earlier than the Toarcian [upper lower Jurassic], as well the instability of several derived sauropodomorph taxa within a number of recent cladistic analyses” (McPhee, Yates, Choiniere, & Abdala, 2014). The situation is largely unchanged today.
It is not only a problem for that part of the fossil record. Go on Wikipedia and look up a random dinosaur group and you will find, more often than not, several alternative cladograms to choose from. This is in part due to that fact that many fossil species of dinosaur are based on rather fragmentary specimens with some exceptions like Tyrannosaurs and Allosaurs.
I came across a clear example of this when looking up Dreadnoughtus schrani. The authors of the initial report tell you that this is the most complete large sauropod ever found with 45.5% of the skeleton. There is another sauropod which is 44.1 % complete and after that they are 15.2% or less.
You will also find odd things like basal dinosaurs which have derived features. One extreme example (but by no means not the only one) is Heterodontosuarus which has a jaw full of mammal-like teeth!
All this notwithstanding should not result in ever changing cladograms. If evolution really did occur then the cladograms should be stable when adding new information and / or new species.
All this suggests that we should be very careful when hearing and reading about dinosaur evolution in the popular scientific media.
It has been a long time since I wrote a blog. But I got the urge to write another when a few weeks ago I watched a fascinating video of a talk by Professor Richard Buggs (Professor of Evolutionary Genomics at Queen Mary University of London) about trees. The lecture was about trees of life and you can find it here.
He covered a lot of ground, including ordinary trees you will see every day, phylogenetic trees (“Darwin’s tree of life”) and the Biblical tree of life.
During his lecture Richard highlighted the problem with phylogenetic trees (or something called a cladogram which is similar). They are easy to construct from a set of known characters of a group of just about anything. Plug the data into a computer programme for generating phylogenetic trees and a tree can be constructed. If the theory of evolution i.e. common descent is correct then phylogenetic trees and cladograms constructed for a group of organisms using different types of data should all be the same. For example, a tree constructed using the gene for cytochrome C will be the same as one constructed from morphological data. The problem is that this is very rarely the case.
Another problem that is very common when fossils are used to construct a cladogram is that they seem to change every time a new fossil species is discovered.
I became acutely aware of this problem when reading the scientific literature on dinosaurs.
The overall picture is fairly constant (most of the time) but even that has changed as recently as 2017. Before that date there were only two types or clades of dinosaur, Ornithischia and Saurischia, but now it seems that there are three clades Ornithischia, Theropoda and Saurischia (Baron, M. G., Norman, D. B., & Barrett, P. M. (2017). A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature, 543(7646), 501-506.)
The problem is even worse for families of dinosaurs. When reading papers published over a period of 10 or more years it seems that every new fossil results in a revised cladogram.
Part of the problem is down to the fact that some dinosaurs are know from very fragmentary fossils (sometimes a tooth and a bit of jaw bone!).
One publication admitted the problem very clearly by stating that : “ [..] the origins of the derived bauplan of the Sauropoda remains poorly understood. This is because of an early sauropod record in which only fragmentary forms are known from rocks earlier than the Toarcian [upper lower Jurassic], as well the instability of several derived sauropodomorph taxa within a number of recent cladistic analyses” (McPhee, Yates, Choiniere, & Abdala, 2014). The situation is largely unchanged today.
It is not only a problem for that part of the fossil record. Go on Wikipedia and look up a random dinosaur group and you will find, more often than not, several alternative cladograms to choose from. This is in part due to that fact that many fossil species of dinosaur are based on rather fragmentary specimens with some exceptions like Tyrannosaurs and Allosaurs.
I came across a clear example of this when looking up Dreadnoughtus schrani. The authors of the initial report tell you that this is the most complete large sauropod ever found with 45.5% of the skeleton. There is another sauropod which is 44.1 % complete and after that they are 15.2% or less.
You will also find odd things like basal dinosaurs which have derived features. One extreme example (but by no means not the only one) is Heterodontosuarus which has a jaw full of mammal-like teeth!
All this notwithstanding should not result in ever changing cladograms. If evolution really did occur then the cladograms should be stable when adding new information and / or new species.
All this suggests that we should be very careful when hearing and reading about dinosaur evolution in the popular scientific media.
