If I’m not mistaken, a lot of the relationships between different organisms (tree of life) were first established in the 19th century and were done so by looking at similarities in form and function and is the basis for scientific names. Since the development of genetic analysis, has there been an effort to go back and confirm that these educated guesses were correct? If so, were there any huge surprises? Like learning that animals that were thought to be closely related based on how they look are actually much more distant
In: Biology
Simply, yes. Researchers have programs to analyze the genetic codes of every documented species and confirm how similar the DNA of 2 related species are. They then trace back their evolutionary history based on DNA sequences (pieces of genetic code) that are gained/lost from one species to another.
There are multiple questions here, and the answer to all of them is “yes”. The way this is done is by sequencing genomes and comparing them. The more differences between them (particularly in highly conserved bits of DNA), the further back they diverged. It’s a tricky thing to do though as there are a lot of places for human and even machine error to come into play. It should be noted that “a species” is really a gene pool, not one genome, which means results can vary a lot depending on which members of the species you sample, since some members of a species are going to be more genetically similar to a given related species than other members.
I can’t think of any good examples of surprises, unfortunately, but I do know they exist from encountering them previously (and if I’d have known this question would be asked one day, I’d have bookmarked them). If I recall correctly. most of the ones I found were plants.
Your question here is mainly about the differences between cladistics (grouping things by attributes) and phylogeny (grouping things by evolution/ DNA/ common ancestors). So, the differences between cladograms and phylograms.
The historical tree of life isn’t actually as historical as most people seem to think. While Darwin conceptualised it, it was never given a formal introduction like evolution was. Its popularity mainly spiked around the 1960s, which wasn’t too far before genetic sequencing anyway. By the 1960s we also had good non-evolutionary knowledge of most common species (e.g. we had things like skeletons of most animals, which do make certain things more obvious). We also had a lot more knowledge of common ancestors through fossil records, which had a big impact on the understanding of things like dolphins and whales.
Also worth noting: a lot of modern cladograms do cheat. They use knowledge of genetic links to make biased decisions in clade differentiation. And since cladograms can take different attributes into account, there are multiple ways of comparing groups of animals (however in functional terms, historically the most obvious differences were taken – although there are issues with bias here too).
Some big deviations between the two:
1. Cladograms usually have humans as very separate. We are bipedal, relatively bald and don’t really look like anything else. We have a lot of attributes distinct from anything else. However, we know that the evolutionary difference between a human and an ape are actually tiny. Genetic comparison between human and ape DNA did cause quite a big stir – we previously assumed we were more separated.
2. Another big difference was snakes vs lizards. Again, the observed difference here is much larger than the actual DNA-measured split – probably because of how extreme “leg vs not leg” is.
3. Splits under the hoofed mammals are pretty exaggerated. Particularly marine mammals. The evolutionary distance/ difference between a whale and a hippo and a horse is relatively small considering the massive visual differences.
4. Flight was traditionally a struggle in cladograms. Wasps and bees are very closely related to (non-flying) ants, but this evolutionary difference often wasnt reflected. The connection between bats, birds, flightless birds and dinosaurs is another very clear example of taking the wrong indicators for clades.
5. (perhaps the most important) – cladograms basically entirely ignore how horizontal gene transfer works in separating simpler organisms.
To give some credit to cladograms; they get a lot of things right, and in general tracking small changes in attributes is a good approximation of evolutionary differences. The introduction of a fossil record (although strictly speaking shouldn’t have made a big difference) did massively help steer the construction of cladograms – so a cladogram from 1910 vs 1930 vs 1950 vs 1970 would have had increasing “accuracy” or agreement with a phylogram.
Yes. Modern taxonomy is mostly based on molecular biology and genetic similarity, not form and function, and has indeed revealed that many of the original groups weren’t descendants of common ancestors after all. Of course, form isn’t a *complete* coincidence, so a lot of this took the form of small adjustments moreso than totally remaking the taxonomy.
Probably the biggest exception to this rule is birds not being relatives of dinosaurs as originally thought, but just being, well, dinosaurs.
>Since the development of genetic analysis, has there been an effort to go back and confirm that these educated guesses were correct?
Yes, but it’s not like an original “Tree of life” was invented and then only lately did people go back to revise it once gene sequencing was invented. Relationships between organisms have been under constant debate and revision ever since they were first proposed. Genetics contributed to this but it’s just another tool for an ongoing process. And physical traits are still important, because among other reasons you can’t genotype long extinct groups. Genes also don’t always give clear answers, especially for distantly related groups.
>If so, were there any huge surprises? Like learning that animals that were thought to be closely related based on how they look are actually much more distant
Yep. I wish there was a big list of these to point to, but I’m going to just try to remember off the top of my head. The biggest surprise is probably the discovery of the archaea. Look like bacteria, definitely aren’t.
There’s also the discovery the relationships between different phyla…we already had some figured out (like the deuterosomes, uniting vertebrates, echinoderms, and some other things), but others were surprising. For example, it was once widely thought that arthropods and annelids were closely related, but it turns out that annelids are closer to mollusks and arthropods are closer to nematodes. And speaking of arthropods, we are pretty sure insects come from within crustacea now, which was a bit unexpected. Some people also think that comb jellies are actually less related to other animals even than sponges, which would be _really_ shocking, but the genetic evidence is still highly debated on that.
In mammals, the relationship between different groups of mammals has been shuffled around, although for the most part individual groups remain untouched. So, for example, we long knew that all primates were primates, and all bats were bats (a few people disagreed but the idea was never mainstream). But we used to think bats and primates were closely related…now we realize they aren’t thanks to genetic work. But in a few cases our actual groups were messed up, for example it was thought that sloths and armadillos and pangolins and aardvarks were related, but now we know the new world animals like armadillos and sloths are in a group but the others are unrelated. We were fooled because they convergently look similar, but the genes tell the real story. There are some similar things that happened in birds.
So yeah, I would say that genetic evidence has mostly backed up our guesses based on physical form in the broad sense, but there have definitely been plenty of surprises. And the work isn’t over, not by a long shot.
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