Evolution: Why do traits cluster taxonomically?

Different groups of organisms, obviously, are similar to each other in some ways, and distinct in other ways. But it seems to me that a lot of those difference cluster together, for reasons that aren’t immediately apparent.

Take mammals, for instance. All mammals produce milk, and no non-mammals do. Well, OK, that’s the definition of “mammal”. But there are a bunch of other traits, unrelated to milk, that are strongly or even perfectly correlated with that: All mammals have three bones in the inner ear, and no non-mammals do. All mammals have hair of some form, and no non-mammals do. Almost all mammals, with only two exceptions, bear live young, and very few non-mammals do. All mammals are warm-blooded, but (aside from the birds, who I think evolved their warm-bloodedness independently from us) very few other animals do.

Logically, these traits must have evolved one at a time. Let’s say, for the sake of argument, that the order went milk - earbones - hair - live birth. Why aren’t there any extant animals with milk but without the three earbones, or animals with the milk and earbones but without the hair? Why only two extant animals with milk, earbones, and hair, but who lay eggs? Why are the only, or nearly only, milk-bearers who survived the ones with all of those other traits?

The simplest explanation is that a common ancestor had all 3 trails and that’s why the resulting related mammals all have them. If you’re saying that you know that’s not the case then I can’t think of any other reason other than coincidence.

Never been much good at answers containing “evolution” and “why”.

Is the question why a species say with milk, earbones, feathers and livebirth (MEFLs) are extinct rather than extant or have MEFLs never existed?

I would guess that individuals with genetic capability of MEFLs would have existed, even if not expressed morphologically. Indeed doesn’t that exist within our own DNA with the F component(s) switched off?

Given that MEFLs would have arisen from a genetic mutation there wouldn’t have been many of them, and they would have to exist for enough generations in their parental species (which assumes only minor if any survival disadvantage) to build up sufficient numbers. And then for a environmental change occurred that conferred a survival advantage on the MEFLs to displace within some niche the generic population. Maybe there haven’t been sufficient environmental choke points for whatever advantage MEFLs have to become material?

And even if those ducks lined up it could end dead end if by a roll of the dice all the MEFLs turned out to be males. Or happened to be all in once location and taken out by a natural disaster. Survival being a numbers game, or failing that needing an inordinate amount of luck.

Obviously, as you know, when a trait is shared by animals with a common ancestor, the parsimonious assumption is that the trait evolved once in the common ancestor. (The other possibility is convergent evolution.)

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.)

In other words, in your hypothetical - let’s say “milk” evolved first. Let’s call this the defining characteristic of mammals for our hypothetical phylogenetic scenario. There are surviving modern lineages without milk, but we don’t call them mammals. Then, among the mammals, which we will now assume had radiated into multiple milk-bearing species, one mammal lineage evolved “earbones”. At the time, there were obviously still lineages of mammals with milk but without the new “earbones” feature, since “earbones” arose only once in one mammal lineage. But it turned out that “earbones” was a big advantage, and all those other lineages went extinct. Hence, all non-extinct mammals have both “milk” and “earbones”.

I think this is probably the crux of the question. I can’t see any reason one trait has to wait for another to evolve.
There seems to a lot of evidence for periods of rapid development and evolution and then periods of stability. You only need the most successful to out compete the others, and during a time where many mutations and selection are occurring there may be lots of combinations and culling. In times of ecological plenty evolution will favour the novel improvements and provide slack for changes that have no effect or are even slightly disadvantageous.
Not every mutation has been actively selected for. We are not evolved into perfect beings.

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

This article has a very interesting commentary on the evolution of the ear structures. In particular the differences between the sub-groups of the mammals and also the stem mammals.

To add - the vast majority of species that have existed are extinct. So, whether there’s a causal relationship between the trait under consideration or whether it’s a chance event, it’s not an improbable scenario that there are no surviving modern mammals except those on the sole lineage that evolved all of the various traits you’re considering at different times.

All mammals produce milk because there were no non-milk producing but close-to-mammilian animals alive at the time the group “mammal” was being defined. If platypuses and echidnas had been extinct (or not discovered) at the time “mammal” was being defined, then all mammals would have live birth. And if fossil platypuses and echidnas were then later found and somehow determined to be egg layers, they wouldn’t have counted as mammals. (Who knows when milk-vs.-hair came about, but monotremes also predate nipples and the X-Y sex chromosome system.)

If something had survived with milk but no hair, maybe it would have been placed in a different group, or maybe the definition of mammal would be wider than it is now, not requiring hair like it doesn’t require live birth. (As for hair but no milk, that also could have been included in a group that includes all current mammals plus those things, just been given a different name than “mammal.” Perhaps named based on hair. Or three inner ear bones.)


You seem to be thinking in the old terms of Linnaen taxonomy, where the vertebrate groups of fish, amphibians, reptiles, birds, and mammals were all placed at the same level (what we now call “class”) but modern taxonomy leans more towards cladistics, which is a branching pattern based on shared derived traits. A clade is a group that contains a species that evolved a novel trait and all species that later evolved from that species. Everything with three inner ear bones evolved from a single species that had three inner ear bones. Same goes with hair. Same goes with milk. Each evolved at seperate times (and we don’t know for sure the order.) As for why representatives of some groups are still around and others aren’t, who knows? There are lots of mammalish things that are extinct (but might have extended the definition of “mammal” if they weren’t.)

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.

No, I don’t think this is the issue. I think @Chronos question was a perfectly valid one. It is certainly true that there are some traits shared by all modern mammals, features that evolved at different times in evolutionary history. Defining the clade more rigorously using modern phylogenetics does not change this. And the point is that this would be statistically improbable absent the knowledge that most lineages go extinct. There were once mammal species with some of the earlier traits but not the later ones, but they have no modern descendants.

They have features that people call hair (in English, at least) but it isn’t developmentally or evolutionarily related to mammal hair.


Perhaps because biological systems only build on existing structures (ie…gills to wings). So, animals that share a common ancestor are all independently modifying the same precursors and coming up with similar results.

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.

Perhaps we can take a lesson from eyes. Some things convey such a strong evolutionary advantage that intermediate examples die out. There are no animals from fish on down I’m aware of that have half-assed eyeballs - it seems to have taken the evolutionary tree by storm… The eyeball seems to be one such trait, although some have lost that characteristic in unlit environments. (Although not perfectly - we see proto-light-sensitive organs in other branches of the evolutionary family tree, in very specialized niches.)

The same would go for hair - I would argue the opposite; that feathers are such a specialized development with distinct advantages compared to hair that feathered birds out-competed hairy birds. OTOH, hair is a lot cheaper to grow unless the stiffness of feathers is needed for flight. So perhaps we can hypothesize the “mother of all birds” was a flying one.

Similarly, live both has advantages over eggs - but if an animal needs to fly, weight is far more important, so not having to carry around a developing fetus has a distinct advantage; whereas for ground animals being able to carry around the developing fetus is easier than being tied to an egg that needs warming and simplifies the feeding required - to create an egg, the mother needs to assemble all that extra “food” ahead of time instead of as the fetus develops.

In some cases one trait wins, in some cases the other. In many cases, that is who won the evolution battle.

Right- if they weren’t maladaptive enough in some way, they’ll most likely be retained.

And apparently the triple ear bones have evolved more than once in mammals, so it’s likely that all the variations you mentioned, except possibly hairlessness were at one point extant. I’m not familiar enough with the fossil record to know for sure.

I think there are two major elements at play here:

  1. We’re picking the features that are common in the taxonomic group and ignoring all the ones that either aren’t common or aren’t isolated to the taxonomic group. There’s an element of circular reasoning there. Why are these features important → Because they uniquely define this group → Why are they unique to this group? → Because that’s how we define the group.
  2. An advantageous mutation doesn’t just give one subset of a species an advantage over other members of that species, or that species an advantage over other species. It gives all descendant species an advantage over neighboring branches of the evolutionary tree without that adaptation. Some of those branches may have different mutations that make them competitive without it, but some branches will just die out. And then we’re back to issue one, we’re picking the common traits and then defining them as more important than the ones that aren’t “group defining”.

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?

The implication is that the characteristic developed so early in the cladistic tree(?), and conveyed such an advantage, that all subsequent surviving members kept that characteristic. Parallel but unendowed competitors… couldn’t compete.

IANABiologist, but I recall reading something that suggested there are two major branches of the bird family - the ones with duckbills and those peckers. Is there any analogous characteristics that could replicate this argument there and help illustrate the OP’s point? Obviously both branches come from a proto-avian with feathers and wings, since that’s a common characteristic?