Evolution: Why do traits cluster taxonomically?

[naita]

@md-2000

I would argue - not exactly.

The point here is not that they are what defines the group, but why are there no closely related groups that do not have these characteristics? Why aren’t there 2 types of mice, those with 3-boned ears and those without?

Yes, there’s an evolutionary advantage, that was my point number two. But it’s obviously not so great that there aren’t any animals without the feature. It seems plausible to me that separate mammalians (not different types of mice, they’re way too young a group within mammals) lineages could have existed where two of the bones had fused into one for instance. And in that case we wouldn’t have been talking about the three bones in the middle ear being a defining characteristic of mammals along with milk and hair.

I agree that stability of that feature indicates it’s a very good one though. There’s a lot more variety in the number of toes and design of limbs and feet for instance. Like the eye, mentioned earlier in the thread, they’re a feature that arose early in the mammalian evolution, and was pretty great from the start.

[DesertDog]

@puzzlegal

For that matter, “honey” could be called “bee milk”

I’ve called it “bee barf” at times – rolls off of the tongue easier.

A big factor in linkage is how close the genes are in a particular chromosome. The advantage of sexual reproduction is the meiosis stage is where the genes from both sides of the chromosome are swapped back and forth at random spots. Two genes that happen to be close together – even though they have nothing to do with each other – on the same chromosome are less likely to be split apart than two that are further apart.

To use Mendel’s pea plant experiments as an example, if the plant-tallness gene and the flower-color gene are side by side on the same chromosome, the odds of them being separated are less than if they are further apart and much less if they are on different chromosomes entirely.

[Chronos]

Wow, I checked this thread before going to bed, so of course all the replies came while I was asleep or working.

@Riemann

To your question - presumably the answer would be that the traits you’re considering conferred a sufficiently significant advantage that all lineages without the traits died out. (I’m assuming there’s some cause - random chance extinction of certain lineages is always also a possibility, of course.)

If all of those traits are that advantageous, then how is it that plenty of animals survived with none of those traits? I could see it if the traits were inherently tied together in some way, and some of those might be: For instance, maybe live birth is good, if and only if you have some way of feeding the live-birthed young, and so it was only advantageous enough to evolve for the milk-producers. And fur for insulation might be related to warm-bloodedness. But it’s a lot harder to come up with some way to connect milk and fur, and I can’t see any connection at all between either and earbones.

@Darren_Garrison

(Who knows when milk-vs.-hair came about, but monotremes also predate nipples and the X-Y sex chromosome system.)

Ah, thanks, I knew there was another trait I was forgetting: The X-Y sex chromosome system (birds have a system where the female has the mismatched chromosomes, and most other vertebrates have non-genetic sex).

@puzzlegal

For what it’s worth, lots of caterpillars have hair. For that matter, “honey” could be called “bee milk”, although if course it’s created by a different organ than what mammals use, and is nutritionally suited for bees, not mammals. I think pigeons create a milky substance to feed their young, too.

Similar in function, but clearly not homologous. No taxonomist would ever be confused into calling caterpillars or bees “mammals”, and everything that’s remotely close to mammals (i.e., vertebrates) that has hair or milk is a mammal. Now, taxonomists might be confused by platypoi, but they’re a very rare case (in both number of species and number of individuals).

@RitterSport

Wouldn’t it be the case that the ancestor of all mammals had hair, three ear bones, and produced milk? My assumption is that the ancestor also laid eggs, then at some point, some ancestor of placentals and marsupials went down the marsupial path, and some ancestor of placentals went down that path, all without modifying the hair, ear bones, and milk production.

The ancestor of all currently-extant mammals, yes. But there should at some point have been ancestors of that creature that had some but not all of those traits. What happened to them?

@dtilque

It could be something as simple as the lineages without all the traits did not survive Chicxulub.

Maybe that’s it… Maybe, 65 million and one years ago, there were all of these varieties of proto-mammals, some with all of those traits, some with only some or one of them, and then the only one that survived happened to be one with all of them? And then that one and its descendants diversified into all of the mammals we see today. Is there evidence that that’s when the last common ancestor of the mammals was?

[puzzlegal]

I thought the mammals got their start right after one of the giant extinction events, so yes, I’d guess our “close relatives” were wiped out, possible just through random chance.

[RitterSport]

Tons of info here:

Evolution of mammals - Wikipedia.

Looks like it was the cynodonts, more than 250 million years ago.

[Mangetout]

You named some traits that are the exclusive property of mammals (ear bones, milk, hair), and some traits that are only present in some mammals (eggs, live birth). Is it really surprising that amongst a huge array of traits not even mentioned, this should be the case?

If the traits had been assigned by dice rolls, this would probably still be the case. It’s almost like the significance of the exclusivity of these traits is an artifact of them being picked out as significant.

[zimaane]

Are warm-bloodedness and having hair independent? Doesn’t the presence of hair as an insulator make it easier to be warm-blooded?

[penultima_thule]

@zimaane

Are warm-bloodedness and having hair independent?

Yes.
Birds are warm-blooded.

[Tim_T-Bonham.net]

Having no expertise in this field, and being 60 years from schooling on this, still, it seems to me that some of these traits are somewhat connected.

Like having hair – that is an evolutionary advantage in cold climates where there is a winter. And carrying young internally and giving live birth vs. laying eggs in a nest is also an advantage in cold climates. Producing young from eggs has some problems. Eggs would freeze solid in a Minnesota winter, killing the young. So the parents have to lay on the nest to keep them warm (thus removing one parent from food gathering, etc…) or they stop reproducing during the winter months, or they take a long, risky journey to migrate south.

So it occurs to me that these traits have similar advantages in cold climates, so would tend to evolve together.

[sunstone]

The basis for taxonomy is that types of living critters can be placed in groups based on characteristics that they share. For example, the phylum Chordate members all have four characteristics at some point in their lives. So humans are Chordates because we all have a notochord, a dorsal hollow nerve chord, a post anal tail, and pharyngeal (gill) slits at some time in our lives. These may develop into other features as the embryo matures. So lampreys, fish, birds, mammals, and so forth all have these Chordate features in common.

Taxonomists have 7 primary levels. In school we learned Kingdom, Phylum, Class, Order, Family, Genus and Species. Each of these levels has a set of characteristics in the same way as we see in the phylum Chordata.

The phylum is broken up into a number of classes, and in turn the resulting Classes are broken up into Orders, etc until we finally arrive at Family, then Genus and ultimately Species.

So in the Order Primate, all members have binocular vision with eyes directed forward, a poor sense of smell, five fingered grasping hand, nails rather than claws, and a long pregnancy period followed by a long post pregnancy developmental period.

So taxonomy by its basic nature places critters into groups with similar characteristics, and consider them related by previous shared characteristics. Evolutionary Theory and scientific evidence confirms this basic principle.

It is no surprise that related critters share many features, and fewer features are shared by critters that are not closely related. You are not very much like an oak tree, and are in different Kingdoms.

[DesertDog]

@Chronos

Now, taxonomists might be confused by platypoi, but they’re a very rare case (in both number of species and number of individuals).

Part of the problem is, people keep trying to stuff nature into neat little boxes and she keeps climbing out of them. For example we’ve gone from Aristotle’s two biological kingdoms (animal and plant) to Hogg adding protists to Copeland adding Monera ti finally fungi being recognized as different enough to have their own branch.

Then the cladistic classification system was come up with, dropping everything back to three – bacteria, archaea, and eukariota. Life is a continuum and where the lines are drawn is arbitrary.

[md-2000]

@Chronos

@dtilque

It could be something as simple as the lineages without all the traits did not survive Chicxulub.

Maybe that’s it… Maybe, 65 million and one years ago, there were all of these varieties of proto-mammals, some with all of those traits, some with only some or one of them, and then the only one that survived happened to be one with all of them? And then that one and its descendants diversified into all of the mammals we see today. Is there evidence that that’s when the last common ancestor of the mammals was?

I think it was Stephen Jay Gould’s book A Wonderful Life where he discusses the immense variety of the life forms to be found in the Burgess Shale, one of the first multicellular explosions of evolution. He suggests that it’s not just a matter of winning the evolution race, but that pure random events like survival by chance from events like meteors or happening to be in the right place at the right time where resources are available that help determine the overall direction of evolution.

[zimaane]

@sunstone

The basis for taxonomy is that types of living critters can be placed in groups based on characteristics that they share.

A related approach, which is now quite common, is base groups on descent from a common ancestor. So, primates, for example, are in the primate groups because all primates are the descendants of a single animal that lived sometime in the distant past.

This approach has disrupted the bird groups somewhat. Falcons used to be classed with other birds of prey, but cladistic analysis has suggested that they are more closely related to parrots.

[Chronos]

@DesertDog

Part of the problem is, people keep trying to stuff nature into neat little boxes and she keeps climbing out of them.

But what’s remarkable here is that the mammals do fit so neatly into their little box.

[outlierrn]

Nitpick, the ossicles are in the middle ear.

[penultima_thule]

@Chronos

the mammals do fit so neatly into their little box

Yes, but give nature time …

[Pleonast]

@zimaane

This approach has disrupted the bird groups somewhat. Falcons used to be classed with other birds of prey, but cladistic analysis has suggested that they are more closely related to parrots.

For example, the peregrine falcon and the blue-and-yellow macaw have a common ancestor at 75.6 million years ago. The bald eagle and the red-headed woodpecker have a common ancestor at 77.7 Mya. Those two clades have a common ancestor at 78.4 Mya, while the common ancestor of all extant birds is at 113.3 Mya. There was some crazy radiation once therapods learned to fly.

More fun at

OneZoom Tree of Life Explorer

[sunstone]

I would like to add that taxonomy is a human construct, like math and physics. These constructs enable us to organize the physical word, but do not occur in nature themselves. We have invented these tools to help us understand the natural world. In a sense, they do not exist outside of our minds. Remember that science is a creative effort to make sense and take advantage of our physical world.

[Andy_L]

I think we built the box around the mammals - so it’s not surprising that they fit that box. Evolution produces a radiating pattern as groups of creatures change to match their temporary environments. There are some cases of convergence: bats and birds have the similarities required for them both to fly - but because bats and birds bear the marks of their distinct paths into the flying lifestyle, we can figure out what box they fit into, just like a close examination of a tree will reveal which twigs go with which branch - that’s just the way trees grow

For mammals, it may be that hair and live-birth and mammary glands happened to be a set of characteristics that fit together particularly well, so that creatures with those characteristics easily outcompeted anything “nearly” like them but not them - or it may be that chance played a role in killing off the near-mammal that could have out-competed us. But it’s not particularly surprising that we built boxes that group together characteristics that happen to go together in the outside world.

[Darren_Garrison]

@Chronos

But what’s remarkable here is that the mammals do fit so neatly into their little box.

I don’t see how it is remarkable at all—or unique to mammals. You pick a group of animals, you describe them using traits that are shared by everything in that group. You don’t, for instance, define mammals as living on land because some of them no longer live on land.

There are two different types of traits: primitive and derived. Primitive traits are ones inherited by a more distant ancestors and are useless for determining more recent groups—for instance, “having four limbs” is useless for defining mammals because so do reptiles, amphibians, birds, and some fish. Same with “made up of lots of nucleated cells”, which stretches back even further and is even more useless for defining mammals. Derived traits are the ones that appeared more recently and are more useful.

Again, Linnean thinking is limiting with its KPCOFGS grouping. Cladistics offers for a (very complicated and unwieldy) system of new groupings every time a new trait appears and splits into two lineages. For instance, if you look at the wiki for theraspids, you will see that theraspids contants the cynodonts, which includes mammals. But if you click on cynodonts and you look at the cladogram, you will see more than a dozen branching points between being a cynodont and being a mammal.

Therapsid

Cynodont

And even that is just a thumbnail sketch. And as easily as you can say “why do mammals fit into one little box”, you can say “why do prozostrodontia fit into one little box”

Prozostrodontia