The current view is that Hexapods/Insects are a subgroup within the “crustaceans”, which are now called Pancrustacea. Pancrustacea is the sister-group to Myriapoda, together forming Mandibulata, and Mandibulata is the sister-group to Chelicerata.
Hexapods, Arachnids, and Myriapods all moved to land separately from each other (and Arachnids possibly multiple separate times, but this is still controversial and hinges on the uncertain phylogenetic position of horseshoe crabs), so the common ancestor was marine, but the evolution of jointed, segmented appendages is believed to have evolved only once. The old view that Myriapods and Hexapods were sister-groups was based only on certain morphological characters that were all simply convergent terrestrial adaptations.
Relatedness doesn’t have anything to do with physical appearance. You are more closely related to your 1st cousin than to your 2nd cousin because the ancestor that you share with your 1st cousin is a grandparent while the ancestor that you share with your 2nd cousin is a greatgrandparent. Degrees of relationship are defined by the how far back in the family tree you share a common ancestor—nothing more.
Nope. All Placental mammals evolved from a common placental ancestor around 90 million years ago and have had the same amount of time to evolve since then, so all placental mammals alive today have had the same amount of time evolving up to this point from their further distant pre-synapsid (the group that contains all mammals as well as groups including Dimetrodon, Edaphosaurus, Lystrosaurus, Gorgonops, etc) origins.
I assume you might mean something like being more “earlier diverging” from the group of mammals, like how Marsupials diverged before Placentals, or how monotremes diverged even earlier than that. Those terms tend to be incredibly loaded and perspectively relative, but no Pangolins aren’t really that earlier diverging than other modern placentals. By the time Pangolins arised, many different groups of placental mammals had already branched out. We already had a split for Xenarthrans (Armadillos, Sloths, Anteaters), Afrotherians (Manatees, Elephants, Tenrecs), Euarchontoglirians (Rabbits, Rats, Monkeys, Humans), and other clades when Laurasiathere split into Pan-Eeungulata (Horses, Rhinos, Cows, Camels, Whales, Pigs) and Ferae, which includes Pangolins and the species that make up Carnivora.
So the short story is that Pangolins are neither closer related to reptiles or an earlier diverging group relative to other modern day Placental Mammals. They are right in the middle of a moment where a lot of placental mammal groups had already formed into the bases of the branches that would give rise to the living placentals we know today, during a time where a lot of radiation and evolution was taking place as a response to the vacuum left from the extinction event that ended non-avian dinosaurs.
If you want a rudimentary breakdown of placental mammals, its basically this. We have the base of Placenta. Here there are 3 idea of what happened:
A group that includes Xenarthrans and Afrotherians separated from Boreoeutheria, the rest of the placental mammals, and formed Atlantogenata.
Xenarthrans split, then Afrotheria split, then we have Boreoeutheria.
Xenarthra, Afrotheria, and Boreoeutheria all trifurcated from each other simultaneously.
Either way, those are the 3 initial groups. Then we’ll move to what happened to Boreoeutheria, and that splits into Euarchontogliria and Laurasiatheria. Euarchontogliria splits into Glires (Rodents, Rabbits, Squirrels) and Euarchonta (primates, colugos, tree shrews) Laurasiatheria then splits into Eulipotyphla (Hedgehogs, true-Shrews, Shrew-like Moles) and Scrotifera.
Somewhere in Scrotifera we get Chiroptera (Bats) and then we get another major split between Ungulata and Ferae. Ungulata you get Perissodactyla (Horses, Rhinos, Tapirs) and Artiodactyla (Pigs, Camels, Deer, Whales). Lastly, you get Ferae splitting into Carnivora (Dogs, Cats, Bears, Seals) and Pholidota (Pangolins).
I hope, I did understand this correctly, but I think there might be an example: Take a duplicated sequence/gene. Let’s call the original sequence A. After duplication the sequences are allowed to evolve more or less freely to A’ and a. A’ might resemble more A than a. During evolution it can also happen that genes are lost again. In a rare case in can happen that one species/lineage keeps A’ and another closely related keeps a. A phylogeny based on A sequences then would look like A and A’ are more closely related than A and a.
That will usually be true, but like everything in biology, there are exceptions and some messiness. For example, mutation rates - the rate that the DNA sequence changes - can be quite variable, and can change due to environmental changes (for example, if the species is exposed to more radiation) or can evolve (for example, if the species evolves better or worse DNA proofreading machinery). Let’s say there are three species: A, B, and C, and the species A and B are sisters (they are are closest to each other on the tree and share a more recent common ancestor). If A evolves a much faster mutation rate, it may change enough that now B and C are genetically more similar in terms of percent divergence in DNA sequence, even though A and B are phylogenetically sisters and share a more recent common ancestor.
Hybridization throws a whole other layer of complication, but that is more a problem of not being able to properly represent the species in any single phylogenetic tree.
[Haven’t read the whole thread, so I apologize if I’m overlooking a point made elsewhere, but here goes.]
If T1, etc are all measurements of time, and the organisms we’re looking to compare are all alive at the same time (e.g. today), then any two organisms on one side of the divergence at node A are more closely related to each other than to any organism in the other lineage. In the diagram you give, nodes D, E, F and G should all be at the same point on the x axis (assuming that we’re considering living organisms and that x measures time). If the tree is drawn that way, it becomes clear that no route that requires going back from today to node A can be shorter than one that reaches the most recent common ancestor before (to the right of) that point.
All the common formulations that I have seen of “Animal X is more closely related to Animal Y than it is to Animal Z” appear to use that time-based approach.
If we’re trying to compare the relatedness of organisms that aren’t all alive at the same time, then there’s a valid case to make that looks more like the one you have illustrated. The proto-Coelacanth lobe-finned fishes a few steps down the branch from node C that ultimately led to modern Coelacanths at node F were likely more closely related to the very early ray-finned fishes (a separation of maybe 100 m years) than those proto-Coelacanths were to modern humans (a separation of maybe 360 m years).
When you get into differing rates of genetic change and reticulate evolution the whole thing becomes a lot more messy. In reality, each gene has its own evolutionary tree and the most recent common ancestor of one gene traceable in a certain human and a particular coelacanth could be significantly older than the pair of lobe-finned fish that were their most recent common ancestors.
i know i picked T as the abbreviation here, but it’s not really meant to indicate clock time. i know that all of these organisms in the example have evolved until today – so evolution as measured by days, hours, etc. will be the same for siblings. instead, T is really meant as more of a measure of genetic divergence or genetic change. i was thinking something like an evolutionary clock where some organisms run a little faster than others in terms of genetic change.
I think we are in agreement.
“Genetic divergence” is usually quantified as percent difference, so it is one minus percent similarity. My example was meant to show that it is possible to have a situation where A has a lower genetic similarity to B than it does to C, under circumstances where B has a higher mutation rate. This can be true of the average for the whole genome, not just for individual genes. (Whether you consider that to mean less closely related depends on your definition of closely related - if you define it strictly on time to the common ancestor rather than genetic similarity, then A will by definition always be closer to B than C).
Because of this, biologists have to use methods that look for changes that are shared between species (“derived traits”) rather than just rely on overall percent similarity/divergence when building phylogenetic trees. These days it is usually done with some fancy models of DNA evolution that can account for things like variation in mutation rate between species and between parts of the genome.
It might sound like mostly edge cases, but mutation rates really do vary quite widely in some lineages, and it is a well-studied phenomenon in phylogenetic biology that it can cause errors when trying to figure out the relationships between taxa.
Ah ok, I was thinking of a different setup where A and B are “sisters” and C is a third species - I guess that would correspond to A and B being F and G in the diagram, and C being D.
Using that diagram, it is possible for the DNA to be more similar (higher percent similarity) between F and D than between F and G, if G has a much higher mutation rate than D. An example of this would be if G has evolved to be a parasite (either parasitic plant or parasitic animal), as parasites tend to have much faster rate of change in DNA than their non-parasitic relatives.
A specific example could be the ghostplants (Voyria). There is an image of their phylogeny here: https://bsapubs.onlinelibrary.wiley.com/cms/asset/a2c54b74-ac4a-41a4-b7bb-b33f3ad3aba9/ajb20712-fig-0002-m.jpg
The Voyria branches stick out much farther than the other branches because they seem to have much higher mutation rates than their relatives. This made it very difficult for botanists to figure out where they belong in the tree. Even though they share a more recent common ancestor with Gentianeae than Exaceae does, it looks like the DNA sequences are actually more similar between Gentianeae and Exaceae than between Gentianeae and the ghostplants (Voyria). (disclaimer: this is an older study so they didn’t look at the entire genome, but I suspect it will be true if someone does decide to sequence their whole genomes)
There is a very small-scale way in which something like this may be true. If you have a speciation event involving a widespread species, then the parent population may be more closely related to the new species than it is to other populations of it’s own species. Mallards and their various offshoots may well be a good example of this.
Pangolins and Armadillos look so similar that Pangolins used to be considered Xenarthans (the group that contains Armadillos, Sloths, and Anteaters), alongside Aardvarks, despite having no evidence of South American origins. Pangolins share a lot of convergent traits with Xenarthrans too. And when it was kicked out of Xenarthra, it was nested sister to the clade. Then molecular evidence clarified its position as closest to Carnivora.
Darren Naish covered the history of Pangolin phylogeny and taxonomy here. I recommend the read, he is very knowledgable and great at dispensing that knowledge. He operates on his website tetzoo.com now, but still posts new articles regularly, as well as a podcast about the same kinds of topics (paleontology, zoology, phylogeny, etc). I recommend him to every naturalist I meet.
