Sex and the Trouble with Clades
The Case Against Ancestor-Based Groupings
When it comes to clades, a particular category of sayings comes to mind which includes such questions as “chicken or egg?” and “nature or nurture?” Not to suggest that those classic questions are grouped together because they arose from a single common origin, only that they are of a type: questions posing binary options about causality. Which takes us to another set of questions within that same “clade,” ironically raising the central issue in deciding how to populate clades: “Similar because related, or related because similar?”
If it’s “family resemblance” we’re talking about, the resemblance is obviously due to family relatedness. (As many of us might deem fortunate, even family relatedness doesn’t always guarantee similarity!) On the flip side, we wouldn’t think of saying that, merely because two people resemble each other, they must be related. How many times have we seen a total stranger who looks strikingly like a friend or acquaintance and said, “I could have sworn that was her!” Similarity alone doesn’t indicate relatedness.
Art forgeries may be deceptively similar, perhaps even virtually identical. (Isn’t that the whole point!) Yet, clearly it would be foolish to say that, simply because the original and the forgery are indistinguishable to the untrained eye, they must have come from the same artist. Similar isn’t same. Or necessarily same kind. Or same kin. Or same ancestry.
If one were to enter the room of a ten-year-old and discover a table full of castles, cars, rockets, and superheroes—all made from Lego blocks—there would be obvious similarity in the basic (pimpled and colored) building blocks. But nobody’s going to insist that any of those objects came from any other of those objects, even if one had reason to believe that there was some chronological order in which they had been constructed. Common motif alone doesn’t imply descent—even if some “precursor” object had been dismantled or altered to become one of the current objects.
In biology, similar might refer to any number of otherwise-unrelated correspondences, including such functional (non-taxonomic) categories as evergreen, annual, predator, glider, vine, and parasite. Uncontroversially, such categories are legitimate groupings in aid of various beneficial analysis. Regardless of clime or culture, fundamental groupings of organisms haven’t changed dramatically from antiquity. The obvious similarities are there for all the world to see.
Interestingly, creatures of all sorts may have a variety of corresponding similarities that admit them to multiple groupings. Just as individuals with diverse interests might well be members of numerous clubs (say, political, social, or civic), members of clade-like “clubs” are both homogenous and diverse. Most important of all—no matter how much we insist on shoving them into pigeon-holes of our own making—they never lose their own, one-of-a-kind individuality. However vaingloriously we presume to label them, one by one they are what they are!
Frustratingly for dedicated cladists, groupings of similar creatures are thrown into confusion by the seemingly limitless ways in which the millions of distinct species on the planet are found in multiple, overlapping patterns of similarity with other creatures while remaining resolutely dissimilar individuals. Just when you think two organisms are similar and possibly related, you discover a dissimilarity that puts one of those two organisms in bed with yet a third organism having similarities with the second organism, but not with the first. On and on marches the parade of both the similar and the dissimilar—sometimes in perfect lockstep, but more often not. How, then, determine a truly reliable clade?
Clades by Ancestry
In his thoughtful note “On Being a Fish,” Bret Weinstein brings caution to the use of nomenclature in correctly categorizing creatures which might share similar traits but shouldn’t be classed together in the same clade. “To be a clade,” says Weinstein, “a group of creatures must include an ancestor and all of its descendants.” Typically, that means all of its lineal, or “blood-line” descendants, as in (humanly speaking) children, grandchildren, great-grandchildren, etc., in contrast to collateral descendants. So, a clade is a monophyletic group (complete and closely related), not merely a paraphyletic group (only partially linked and more distantly related).
The plea for ancestor-based clades is a sensible attempt to avoid purely arbitrary rules based on similar or dissimilar characteristics—rules, for example, that logically might exclude penguins from the bird category since they are flightless. Yet, ancestor-based clades are no less arbitrary, insisting (to extend the example) that penguins must be birds, not because of analogous features (bar flight), but by virtue of mutual descent—which introduces a whole new set of problems, not the least being the highly speculative and circular nature of the enterprise.
Among those attempting the daunting task of classifying species, the current working assumption is that similar creatures have inherited their similar traits from the “last common ancestor” (which, oddly, can be a population or a species, not only a single organism), whether known or unknown, whether extant or extinct.
It is said, for instance, that we humans are mammals because we descend from the most recent common ancestor of all mammals—an observation which (coming dangerously close to circularity) begs an important question about the working assumption itself. Could all mammals (or creatures in other clades, for that matter) be, in actual fact, the descendants of a common progenitor?
Before we can begin to think about comparative traits and similarities suggesting common descent, there is the no small matter of descent itself. To be a progenitor (a word derived from the Latin for beget or begotten), a creature must first have descendants, which is to say, biological offspring. If that seems blindingly obvious, it is by no means straightforward. Before there could be a clade including snakes, bats, birds, and whales that have “lost” their quadrupedal nature, there first must have been creatures having a quadrupedal nature that were genuine, genetic descendants from some earlier progenitor, which itself was the descendant of some still-earlier progenitor.
Similarly, before one can reasonably conclude that we humans are permanent members of every fully-nested set from which we have arisen (as in Homo sapiens, ape, primate, mammal, vertebrate, and animal), there must first be some demonstrable assurance (apart from an omnipresent circular argument) that we have in fact “arisen,” and specifically that we have arisen alongside apes and primates, as assumed. Scientific convention aside, on what factual basis are we to accept that members of any clade or nested clade have “arisen”?
By some grand irony, virtually every offer of a “factual basis” for common descent within a clade hearkens back to the self-same arguments from shared characteristics that ancestor-based clades are meant to replace! We hear Weinstein saying, for instance, “we can often infer a great deal about a common ancestor simply by looking at the characteristics shared by the clade’s living representatives. The most recent common ancestor of all currently living birds, for example, was almost certain to have had feathers, a four-chambered heart, hollow bones, and no teeth. And it is pretty clear that it flew.” And again, “the most recent common ancestor of all living mammals lived on land, did not fly, had fur, made milk, and had good hearing, warm blood, a four-chambered heart, and a single bone for its lower jaw.” So, how do we determine the relatedness upon which clades are based? Extrapolate from the common characteristics we were just told to ignore!
Sex: A Niggling Problem
Given the need for offspring to perpetuate descent from one generation to the next, and then from one species to another within a clade or nested clade, there’s a pesky, problematic fly in the ointment—namely, sexual reproduction. To be sure, sexual reproduction is certainly about relatedness and similarity. But, compared to all other sexually-reproducing organisms, the sexual reproduction of any single organism is even more about dissimilarity and unrelatedness. Ironically, what all sexually-reproducing creatures have in common is the similarity of being sexually dissimilar. Whatever other similarities there might be, it is sex—both a creature’s distinct sexual organs and its unique reproductive processes—that most definitively distinguishes each of millions of species.
As everybody knows, when it comes to mating equipment, it’s not “one size fits all.” Size matters! As does compatibility between the male and female. Without precisely the right mating equipment (such as the male marsupial which has a forked penis used to fertilize eggs in the female’s twin uteruses), reproduction is a non-starter. In the words of the old rock song, “I hear you knocking, but you can’t come in!”
Reporting for the BBC in an article intriguingly titled, “The Twisted World of Sexual Organs,” Colin Barras provides an incredible description of some of the world’s most bizarre sexual organs. Barras begins the article with a bit of TMI, describing how a Korean woman eating squid had triggered a release of the dead male squid’s spermatophores, which in the normal mating process act as tiny guided missiles to implant the payload of sperm into the soft tissue of the female squid near her genital opening, releasing sperm as the female lays her eggs. In the case of the Korean woman, the target ended up being her tongue, cheeks and gums! (Squid tonight, anyone?)
Barras then takes us on a wild ride, noting generally about insect genitalia that:
There is a baffling level of variety in their shape and size. So much variety, in fact, that some closely related species that otherwise look identical can be distinguished purely because of differences in the shape of their sexual organs. This doesn’t just apply to insects. A quick look at the sexual organs can help distinguish between closely related species of reptiles and mammals too. Many male mammals—although not humans—carry a bone in their penis. The variation in shape and size of this penis bone is astonishing, if a little eye watering from the female perspective.
There’s more. Since flatworms are hermaphrodites carrying both male and female genitalia, a “fencing match” ensues in which “each flatworm attempts to stab the other with its two-pronged, fork-like penis. Sexual intercourse ends when one flatworm registers a successful hit on its opponent and transfers a packet of sperm.”
And how about the male damselfly, with a penis whose tip carries two horn-like structures, each coated in tiny spines for brushing away any rival sperm from the female’s reproductive tract. Or the male weevil’s penis that has spines which sink into the lining of the female’s reproductive tract, causing her injury.
“You dirty rat, you!” might well be applicable to the Norway rat that hooks together hundreds of little fellas with tails to form a mega-sperm that can power its way towards the eggs faster than a single sperm. Or perhaps even more so those other rats that have “kamikaze” sperm which are thought to form a tangled mess to ensnare the sperm of rival males.
When it comes to cataloguing the fascinating world of sex from unique species to crazy-unique species, get your hands on a copy of Dr. Tatiana’s Sex Advice to All Creation, by Olivia Judson (Imperial College London), who just might know more about sex in nature than anyone. By any measure, Judson’s book is an encyclopedic description of the fascinating, often bizarre mating and reproductive habits of countless plant and animal species. No book could better emphasize the point that, from creature to creature, sex is unique, idiosyncratic, and—apart from some occasional frisky (non-reproductive) hanky-panky—reserved exclusively to one’s own species. As Judson reminds us, the working definition of a species is a group of organisms capable of interbreeding. Others need not apply.
And what about those often-bizarre mating instincts? Sex doesn’t get any weirder than with the praying mantis. After the male mounts the female, clasps her sides with his forelegs, and rocks his abdomen up and down, it takes about half an hour before his sperm is transferred to the lovely lady…who (perhaps spurred on by his antennae violently beating her) suddenly twists around and bites off her partner’s head! Relieved of his brain, the male loses all his inhibitions and begins rocking his decapitated body like there’s no tomorrow (which, for him, there isn’t!).
Certainly solves the problem of potential unfaithfulness by one’s sexual partner, but it doesn’t solve the problem of having that unique physical apparatus and those corresponding mating rituals in Generation One so as to progress to Generation Two. From what closest living being would the praying mantis have emerged? How did we get both the (unsuspecting) male and the (cannibalistic) female developing simultaneously—each having complete bodily form and full sexual functioning, including the bizarre cannibalism bit?
And then there’s the Argentine lake drake that, in the blink of an eye, explosively releases his 40-centimetre-long [16-inch] penis, using it to lasso the female and force himself on her. Even beyond the lake drake’s admirable male organ, the instinct to use that 16-incher is unknown among all other creatures on the planet. As far as we can tell, there are no precursors with 16-inchers, and no descendants except offspring having both those remarkable16-inchers and the instinct to lasso their lovers.
On a deeper level, of course, is the far more important necessity for reproductive compatibility, not the least being the need for the same number and type of chromosomes before meiosis normally has any hope of success. And just in case one creature happens to fancy another from a different species, there’s that built-in security feature: “pin-number-protected” sex making sure no deviant interloper contaminates the line of descent. (Even notorious hybrids such as infertile mules bred from female horses and male donkeys turn out to be exceptions proving the rule.)
Dubious Lines of Descent
The point to all of this? In millions of weird and wonderful different species, the male and female have compatible sexual organs, microbiotic reproductive cells, precise DNA, morphological capacity, and—not to be overlooked—the unique instincts for the mating, conception, and development of the offspring peculiar to that species. But, crucially, not for any other offspring. Given the exclusivity of sex (species by sexually-unique species), determining clades based on assumed common lines of descent is operating on a premise that could not in actual fact be true.
For old-timers who may remember the hilarious “parts is parts” commercial, there’s no such thing as “sex is sex.” More to the point, there’s no such thing as “offspring is offspring” or “descendants is descendants.” Taken as a whole, mammals have an abundance of common identifying features. Nevertheless, each individual mammal species maintains its own unique line of descent as against all other mammal species. What, then, of the operating assumption that clades are related descendants of a common progenitor? How possibly could that be when no single species along the way could have propagated or procreated anything other than its own distinct species?
Take, for instance, the claim that we mammals, being tetrapods, descended from the same common progenitor as fish (gnathostomes), a category which also sweeps in creatures as diverse as snakes, bats, birds, and whales. Knowing what we know about sexually-disparate species, what unique form of sex do fish of any kind have? (Obviously, each distinct fish species has its own distinctive offspring.) And, how, uniquely, do snakes reproduce? (Which specific species are we talking about? Those that lay eggs, or those that give birth to live young?) What are the unique sexual features of bats? (How about that delayed ovulation and sperm-storage routine!) What kind of unique offspring come from, not just birds generally, but specifically finches, robins, and hummingbirds? (Any chance that a pair of mating hummingbirds are going to produce finches?) Whether with their species-specific mating and reproductive routines or their distinctive, species-exclusive offspring, “mix and match” is simply not an option. Nor, even more obviously, halfway-in-between mating processes or offspring.
Homology’s Guessing Game
Among the most frequent assumptions about the linkage of species is the matter of morphology or homology. How do we know two species share a common origin? “Just look at those two strikingly-similar jaws.” Or, “Is there not a clear correspondence between the bone arrangement in the human hand, bat wings, and the whale’s flipper?”
One would not be surprised to learn that seals, sea lions, and walruses are a monophyletic group, Pinnipedia. What might be surprising is the suggestion that pinnipeds have been linked by similarity with terrestrial bears, the strongest evidence being morphological similarity between the skull of a bear and that of a seal. Yet, we’ve already been cautioned by art forgeries and Lego blocks to realize that similarity alone is not always a safe indicator of relatedness. Whether we’re talking about comparative morphology or homology, “looks can be deceiving.”
Speaking of seals and bears, it’s said that the only easily-identifiable difference between a seal and a bear is the seal’s noticeably distinctive teeth. Actually, there is another, far more crucial difference, which (as you can guess by now) is their noticeably-different external and internal reproductive equipment. The same would also be true of each and every supposed intermediate creature from which an uninterrupted lineage would be indispensable in order to satisfy any definition of a clade dependent upon “an ancestor and all its descendants.”
In more recent times, of course, a far more sophisticated form of homology has emerged, based upon the groundbreaking work of Watson and Crick. Correspondences between the DNA of various species has been offered in support of genetic relatedness, either on the basis of outright genome commonality, or perhaps either similar “lost features” or “retained features” in the genome. Here again, there’s the danger of circularity. In the quest to demonstrate a mutuality of descent, any reference to “lost” or “retained” features already assumes mutuality of descent as the starting point.
Take, for example, the commonly-accepted lineages of gorillas, chimpanzees, and humans. It is said that (following an assumed “population separation event”) the variant “A” was lost in the chimpanzee lineage, leaving only the variant “a;” whereas in both the human and the gorilla lineages, the variant “a” was lost, leaving only the variant “A.” So, despite chimpanzees being our closest relatives on the “species tree,” we are said to be closer to gorillas on the “gene tree”. The intended take-away is that (obviously!) all three species must have descended from some precursor population having both the “A” and “a” variants. Either that, or (dare one suggest it?) nothing was ever lost, but just is. Chimpanzees simply have the “a” variable, and gorillas and humans both just happen to have the “A” variable—nothing to do with inheriting those particular variations from some common precursor.
Comparing only one gene locus, of course, proves little. That same set of alleles could be contrasted with any number of other genes which humans and gorillas share but chimps do not (or in any other combination). Just as a single gene shared by humans and gorillas doesn’t prove relatedness, a single gene difference doesn’t prove them to be unrelated. It’s the grand, over-generalized assumption of ancestral descent in question here, of which this particular study in variants is simply one example.
Ideally, of course, DNA analysis would compare entire genomes, not single gene loci. What’s most telling about DNA studies searching for gene similarities or divergences is that studies don’t typically compare genomes of entire organisms. So, the question is: What portion of a genome—and which portion—is being studied? Humans have ~20,000 protein-coding genes, which is only ~1.5% of DNA in the entire human genome—the 1.5% that common-descent studies are primarily focused on. Is there nothing to be considered in the remaining 98.5%? Nothing that might possibly give reason for pause before making bold conclusions about either similarities or relatedness? Not to mention that gene studies invariably rely on merely a sample of DNA and, therefore, provide an incomplete (often conflicting) picture.
A further well-known problem is that homology based on anatomical similarity does not always agree with DNA comparisons. Where differences exist, DNA comparisons usually trump anatomical similarities. (It is now believed, for example, that fungi are closer relatives to animals than they are to plants, and archaea are now considered different from bacteria.)
Yet, how can this be? If ancestral descent is the key to establishing distinct clades, there shouldn’t be any differences between placement on the “species tree” based on comparative anatomy and placement on the “gene tree” based on comparisons of DNA, RNA, and proteins. Merely consider the problematic “tree” divergence in the case of grasses, metazoan animals, lizards and turtles. What’s more, the fact that various molecular studies notoriously provide widely divergent “gene trees” argues against any clear evidence of verifiable descent.
The sophisticated DNA argument is little more than the classic argument from anatomical affinity. Do two species have similar features? Then, odds are, they’re ancestrally related! Do two genomes have sweeping correspondences (or perhaps similar differences in variants)? Then that correspondence must surely mean ancestral descent!
Comparable anatomies, comparable DNA, and comparable genomes, maybe. But, given the hard reality of unique, species-specific sex, any further comparison—including, most crucially, descent—comes to a screeching halt.
The Similarity of Dissimilarity
Might there be a transition from one unique form of sexual reproduction to another unique form of sexual reproduction, whether by gradual mutation or perhaps geographic isolation? Not likely. Even considering group populations associated with geographic isolation, asking for the simultaneous appearance of the first-ever sexually-compatible pair of any species is simply asking too much to permit the kind of descent required for any clade composed of sexually-disparate creatures. Sexually speaking, you can’t get there from here.
Which needn’t be all that alarming when it comes to clades and nomenclature. Are there similarities between whales and hippos? Of course. So, by all means highlight those similarities in the classification of creatures. But if it’s similarity we’re after, we mustn’t overlook the crucial similarity of sexual dissimilarity. Do whales have sex and reproduce like hippos have sex and reproduce? Obviously not. And the same would be true of any supposed intermediate creatures. Species which can’t possibly be the offspring of any species other than their own might well share any number of similarities (even genetic features), but not because of any actual relatedness by descent. While relatedness clearly provides a higher likelihood of similarity, it is not equally the case that similarity reflects relatedness.
Would anyone suggest, for instance, that moths and bats are related by ancestry simply because both have wings for flying? Whether moths and bats each acquired their wings by adaptation to the same habitat is the subject of much speculation. But as for any shared ancestry, there is no doubt, surely even among those who champion clades based on common ancestry. So, might there be design similarities? Clearly. Does that necessarily indicate relatedness? Only if you’re winging it!
A descent-based view of clades based on hypothetical relatedness may easily fit a consensus narrative, but it simply doesn’t fit the facts. Not that the descent-based view claims to be based on actual fact. As seen in this candid acknowledgement by Weinstein, it’s merely a necessary supposition required by the narrative:
This reasoning, and the precise language that it engenders, does not require us to know anything about the most recent common ancestor uniting a given clade; invariably we know little, if anything about it. The claim is only that such a creature must have existed. It is extremely unlikely that we will have a fossil, or any other physical evidence, in this respect. Patterns of shared characteristics among its descendants suggest what it was probably like. We cannot say more with any degree of confidence; fortunately, we do not need to.
Of course, “we don’t need to” if our approach is indifferent to factual reality. But we do need to if actual facts matter. Unique sex producing unique offspring is a fact-based observation that no rational person is likely to dispute. By contrast, a merely hypothetical ancestor provides scant assurance of reliability.
Indeed, it wouldn’t be merely one hypothetical ancestor we need, but a long line of ancestors spread over countless generations throughout the clade, having sex and reproducing in some unknown manner contrary to how we know with absolute certainty that species have sex and reproduce. Undoubtedly, these intermediate creatures “must have existed” for the same reason that the original progenitor “must have existed,” given that the existence of each and every one, whether real or merely assumed, is an indispensable narrative imperative. Begging the question: When formulating clades, which is to be preferred—a logical narrative fitting verifiable facts, or unverifiable facts being made to fit a mostly speculative narrative?
Having argued that the “most recent common ancestor” is a matter of speculation based on shared characteristics among its (always presumed) descendants, Weinstein nevertheless insists that “Defining a clade is never arbitrary because the claim is inherently factual. Clams, for example, are either descended from the most recent common ancestor of all snails, or they are not.” Who could disagree?
So, if facts ultimately do matter, then the facts are clear: Given the inherent sexual barriers along any purported line of descent one might wish to suggest, clams and snails are not, in actual fact, descendants of a common ancestor. Closely examine the sexual features and unique offspring of each and every presumed ancestor of, first clams, then snails (not merely spin-off varieties, but indisputably-distinct species). Where along the supposed line of descent are we to find a departure from the sui generis offspring of each assumed precursor? How far can we go before verifiable fact inexorably gives way to speculation—speculation which, of necessity, would be inconsistent with known facts about sex and progeny?
In the Big Picture
Speculation about shared ancestry simply needn’t come into play in order to compare and contrast differences among various creatures. For instance, when Weinstein broaches the matter of sex, he makes clear that traits like male and female can be compared functionally. “Male plants and male newts, for example,” says Weinstein, “do not behave alike because they have inherited male quirks from a distant shared male ancestor. Their behavior is similar because they face the same selective forces. For males of most species, offspring are cheap. There is little point in looking for anything beyond the obvious.”
Hear, hear! What, indeed, is the point in looking for anything beyond the obvious? When there are any number of sufficient methods capable of logically categorizing creatures, why insist on a method so heavily dependent on purely speculative ancestry that runs afoul of the known parameters of reproduction and descent?
Even more intriguing is the comparison made between male plants and male newts. If both plants and animals can be traced back even farther to a common ancestor, as is presumed, why should we rule out common male traits being inherited from a distant shared ancestor? Indeed, why are plants and newts not in the same clade? Hear again the definition: “To be a clade, a group of creatures must include an ancestor and all of its descendants.” Given consensus thinking, all creatures of every type have arisen from a single common ancestor. Even supposing there are “clades within clades,” or “nesting clades,” by cladistic definition all creatures that have ever existed must logically belong to a single, all-encompassing superclade.
Yet, including plants and animals within a universal superclade only highlights the stubbornly-persistent sex problem. On one hand, all types of sexually-reproducing creatures share the feature of sexual reproduction in common. On the other hand, by its very nature sexual reproduction (being absolutely unique and idiosyncratic) is not shared.
Never is that more true than in the sexual distinctions between the two great sub-clades of plants and animals. Are both sexual? Yes. Are there reproductive similarities? One could say so. Are both types of creatures the same reproductively? Hardly. Male and female they have in common, and also meiosis, but when it comes to their mating and reproductive systems, they couldn’t be more different, or more sexually exclusive. On this point, reasonable people can all agree: plants and animals won’t be mating and reproducing anytime soon.
Turn then to the subset grouping of animals inclusive of amphibians, reptiles, and mammals. Are all three sexual? Yes. Are there reproductive similarities? Again, one could say so. Are the three groups of creatures the same reproductively? By no means. What they each have in common is gender and reproduction by meiosis. Beyond that, everything else is up for grabs.
Merely consider the reproductive differences between amphibians and reptiles, described by Mark Jerome Walters:
When animals left the sea…they were deprived of the wet environment required by egg and sperm. So instead of depositing sperm and egg externally in the water, many land animals adopted a means of internal fertilization, depositing gametes in a sequestered internal environment. Reptiles encase the fertilized egg in the watery interior of a shell.
The move from sea to land had great implications for courtship. Whereas in the sea, partners merely had to be brought into proximity with each other, on land actual mating was usually required. This meant major changes not just in courtship patterns, but also in the physical design of the animals themselves. A simple vent for expulsion of egg or sperm would no longer do.
Should “actual mating” be determinative of a clade? How about the differences between eggs? As Michael Denton reminds us, “The amniotic egg of the reptile is vastly more complex and utterly different to that of an amphibian. There are hardly two eggs in the whole animal kingdom which differ more fundamentally.” Does this fact alone nix the idea of descent-determined clades?
When it comes to major intersections from one archetypal form of sexual reproduction like amphibian sex to a radically-different archetypal form of sexual reproduction like reptile sex, it’s difficult to maintain any pretense of clades being based on descent from a common ancestor. If nothing else, there’s no escaping the “switching problem.” Suppose, for example, that a male amphibian and a female amphibian acquired (simultaneously) the characteristics of a male reptile and a female reptile. At some snapshot moment they would have to turn off all their amphibious sex routines. No pair of amphibians could produce the first generation of reptiles while they remain essentially amphibious. Since disparate methods of sexual reproduction obviously can’t co-exist within a given species, any all-inclusive, universal superclade assuming the common descent of both amphibian and reptile creatures seems patently jury-rigged.
So, it’s back to the original question: Similar because related, or related because similar? As we’ve seen, relatedness generally manifests itself in similarities, but similarity doesn’t necessarily, or even typically, indicate relatedness. (Especially when the starting point of “similarity” is often more in the speculative eye of the beholder than in actual fact.)
Yet, perhaps the question, being binary, is too narrowly framed. If the problem of species-unique sexual reproduction prevents the descent required by clades in search of a common ancestor, then the question can’t be answered by either option. Without descent in actual fact, there’s no relatedness whatsoever for either part of the question. Similar because related? Where two organisms couldn’t possibly be related, any similarity must be accounted for on some other basis. Related because similar? Any perceived similarity obviously isn’t pointing to a relatedness that never could have existed.
Which is to say that clades are most safely populated when similarities are based on observable traits and features rather than on speculative ancestors that “must have existed” simply because of an assumed narrative—a narrative that runs counter to how descent actually works. Or, more troubling, doesn’t work.