I’m looking at different fish on Wikipedia and they often have a sidebar showing the “Scientific classification” of Kingdom, Phylum, Class, Order, Suborder, Superfamily, Family, Subfamily, Genus, Species.
I’m confused by how evolution fits into this because evolution happens at the level of the individual.
For a mutation to arise and be passed on, it has to happen to an individual(s) that is part of a breeding population, which to me means “species,” so that’s where I’m hung up.
At some point, some “things” started to develop a notochord and passed that along and it was good and so it survived and continued to evolve and those “things” started the whole Phylum Chordata?
If you looked at the “Scientific classification” for that “thing” would it say Kingdom: Animalia, Species: Thing… with no other classification possible?
And it gets even more confusing for me the further down the list you go. How does a Family arise from an Order? Is there a “founding species” for the whole Family?
The standard taxonomical classification system is an artifact of history and human convenience. There really is no absolute definition that separates an “order” from a “suborder”; it just comes down to what Linneaus and a few centuries worth of taxonomists thought made sense at the time.
The more general evolutionary term is “clade”, which refers to a set of all the things that share a single common ancestor. You, me, and all of our other (presumably) human relatives define a clade that’s been named “Homo sapiens”. Add an orangutan, and now you’ve defined a clade that also includes chimpanzees, gorillas, as well as all the other species that descended from a single common ancestor. We’ve named that clade “hominids”.
While a lot of the original taxonomical groups align with what we now know are clades, many groups are “paraphyletic”, lumping together multiple clades while excluding other species that should be included in a proper clade. “Fish” is a paraphyletic group, since the proper clade should be “vertebrates”, but in Linneaus’ original taxonomy “fish” excludes several groups which we now know form the “tetrapod” clade.
Modern taxonomists frequently try to reorganize classifications to better match evolution, such that each grouping at every level is a proper clade, but history and custom remains stubborn in several cases. That’s part of the reason for the proliferation of groupings between the classical Linnaean classification, because defining a superfamily (or multiple levels between the superfamily and the family) lets you identify and name proper clades.
No, mutations happen to individuals, but evolution happens to populations, as a mutation either spreads through that population through the reproductive success of individuals or it dies out. (In fact, a pretty common definition of “evolution” is a change in the frequency of alleles in a population.)
Yes, as I understand it.
If that “thing” existed alone, yes. As soon as there are other things with notochords, then you can start classifying them into various categories.
Remember, the whole classification system is a human attempt to bring order out of chaos, to classify and describe and organize the bewildering variety of life. There’s no inherent “it must be this way”; instead, classifications are an agreement as to how to describe the varying forms of life (and there are competing agreements, as for example phylogenetic nomenclature differs from traditional Linnaean taxonomy).
There was a population that was ancestral to the entire family, yes, and not ancestral to other families within the order.
This is simply an artifact of the historical development of classification. Classes, Orders, Families, and so on are simply pigeonholes created by Linnaeus in order to create order among the organisms he was familiar with.
The modern model for classification is cladistics, At present we are in a state in which we are trying to reconcile Linnaean and cladistic classification, which is not really possible and creates conflicts.
As said - there is no ‘physical’ thing as a species.
I often wonder how that plays out morally or ethically. We aspire to save and preserve species of animal, rather than individual animals.
So if you have, say, 2 grass-eating animals competing for the same resource (say, 2 different types of antelope on a limited grassland), if one species is common worldwide and one is rare, we might cull the common animal to preserve the rare.
But the only ‘entitlement’ that one animal has over the other is that one has a few more relatives, and we, as humans, see that as less ‘deserving’ than the poor lonely rare species.
I wonder if the common species think ‘What? You’re killing us because we are successful?’ Just because YOU think it’s important to preserve every single species on earth? What have we done wrong?"
It’s worth noting that there’s no limit to the number of layers of clades you could describe (well, there is, but it’s so exceedingly high that you’d never reach it). Traditional taxonomy recognized seven layers, named “kingdom”, “phylum”, “class”, “order”, “family”, “genus”, “species”, but it often becomes convenient to describe more layers than that. So, if you want to give those levels names, and want to keep the new names consistent with the old ones, you have to start introducing subclasses and superorders and so on, and even then you can run out of names.
Take us, for instance. Under the traditional system, our order is primates, our family is the apes, our genus is Homo, and our species is Homo sapiens. But there are a lot of other meaningful groups you can define in between there. For instance, some primates are monkeys, and all of the monkeys are more closely related to each other than any is to any non-monkey. And some of the monkeys are old-world monkeys, and all of the old-world monkeys are more closely related to each other than any of them are to any new-world monkeys. And some of the old-world monkeys are apes, which again are more closely related to each other (of particular note, an old-world tailed monkey is more closely related to an ape than it is to a new-world tailed monkey). Then, within the apes, you’ve got a meaningful group that includes the gorillas, chimps, and humans, but not the orangutan or gibbon. And within that group, you’ve got another one that includes the chimps and humans, but not the gorillas. And then within that group, you’ve got one that includes the Australopitheci, Proconsuls, and Homos, without the chimps, and so on.
Might I suggest you look at Richard Dawkins’s wonderful book, The Ancestor’s Tale: The Ancestor's Tale - Wikipedia where a lot of this explained clearly.
Right. Here’s a molecular phylogeny of primates. There are some clear groups, but what you choose to label them in terms of Linnaean categories is arbitrary. On the left are some subcategories like Infraorder and Parvorder that indicate intermediate levels between Order and Family.
It used to be common to classify marsupials as all belonging to a single Order, largely because classification was done by Europeans where no marsupials occurred. Now the consensus is that marsupials deserve at least half a dozen Ordersto recognize their diversity.
The meaning of categories also can vary greatly between different groups. The degree of morphological differentiation required to recognize a family in insects, for example, may be much greater than in vertebrates. I’ve heard it said that if birds were beetles, they would all be in the same family.
As the others said, taxonomy is an attempt to impose a man-made, neat-ant-tidy branched tree of relatedness on a domain that is the result of a branched tree of ancestry, but is not intrinsically neat and tidy.
Even the branching events are more fuzzy that we might like - in reality, what probably happens at each fork in the road, is that actually a broad continuum of inter-related species and subspecies (see Ring Species) gets broken in the middle - and the break persists long enough for the two sides never to rejoin - now they share features of their common ancestor, but are on separate evolutionary branches from one another.
Wait a moment, what’s Lagomorpha doing on that phylogeny? Isn’t that rabbits? And aren’t lagomorphs and rodents sister clades, meaning that any phylogeny that includes primates and lagomorphs should also include rodents?
It’s useful to include an arbitrary but obviously distantly related species as an “outgroup” when constructing phylogenies. That roots the phylogeny well enough to make some inferences about the relationships near the base of the tree (in this case, where do the tree shrews fit in?) The rest of the proper clade including Lagomorpha and Primates isn’t of interest to whoever made this particular phylogeny.
Right. In that tree “Lagomorpha” just represents “all mammals less closely related to Primates than Scandentia (Tree Shrews).” However, the clade of Lagomorpha + Rodentia is a sister group to the one comprised of Tree Shrews + Colugos + Primates. Together those clades make up one of the four Superorders of placental mammals, the Euarchontoglires or Supraprimates.
The problem is that you are looking at it in hindsight, based on what the descendants of that “thing” became later. Something like a notochord does not develop all at once due to a single mutation. At the time, the ancestor of vertebrates would have been just one of a group of species that all had some kind of strengthening rod developed to varying degrees, which if there had been taxonomists at the time would have been grouped in a single genus or family.
Today mammals are defined on the basis of a group of characteristics including producing milk, having hair, and a suite of skeletal characteristics. But these didn’t all develop at the same time. In fact, it is very difficult to classify some fossils as being mammalian or not, simply because they may have a mosaic of characters, some “mammalian” and some “reptilian.” For this reason, some cladists just define “Mammalia” as the group including the last common ancestor of monotremes, marsupials, and placentals, and all its descendants. Earlier related groups are called Mammaliaformes.
It is somewhat interesting, though, that among surviving animals, those characteristics are so correlated. Every animal that has three inner earbones also produces milk. Every animal that produces milk also has hair. Every animal that has hair also has three inner earbones. Almost all of the animals that have those three characteristics bear live young without eggshells. Most animals which bear live young without eggshells also have those other characteristics.
Those characteristics had to evolve one at a time. I’m not sure in what order, but for the sake of argument, let’s say that it was earbones, milk, hair, live birth. That means that there were some descendants of the first earbone animal that produced milk, and some that don’t, but that every single one of the milkless ones went extinct. And then of the ones with milk and earbones, some had hair and some didn’t, and every last one of the hairless ones went extinct. And then of those animals with milk, earbones, and hair, some laid eggs and some didn’t, and very nearly every last one of the egg-layers went extinct. And sure, you can posit that each of those changes was evolutionarily beneficial in some way, but yet, there are plenty of creatures without milk, fur, or inner earbones that somehow managed to avoid extinction.
Things are a lot messier, however, if you look at the other major surviving group of the Amniota, the Reptilia (which cladistically speaking includes birds). While they all have the same kind of jaw architecture and ear bones, some have scales, and some have feathers; some lay eggs, and others give live birth; some are ectotherms, and some are endotherms; some have three-chambered hearts, and some have four-chambered hearts; some have air-sac respiratory systems, and some don’t.
Oh, right, I forgot the four-chambered heart as yet another mammalian characteristic (…and the warm blood flows through the large four-chambered heart, maintaining the high metabolism rate they have…). And ectothermicity, while not unique to mammals, is also novel to them in the sense that the only other surviving ectotherms derived it independently of mammals. That’s an awful lot of novel traits that exist together, for seemingly no good reason. I’m guessing it’s related to mass extinctions? That is to say, that at one point there were proto-mammals with and without all of those traits, and then suddenly most of them went extinct, with the sole survivor being one that happened to have all of those traits?
The mass extinction at the end of the Permian knocked off many therapsids, the ancestors of the mammals which had previously been dominant. They were largely replaced by the archosaurs, including crocodiles and eventually dinosaurs. It is hypothesized that the surviving mammalian ancestors avoided competition with the archosaurs because they were small and nocturnal. In fact, several features you mention are not unrelated but are adaptations to a nocturnal life-style: endothermy to withstand cooler night temperatures, which required fur for insulation and a four-chambered heart; the modification of jaw bones into ear bones contributed to sharpened hearing when vision was less efficient. Mammary glands are also related to the evolution of hair, apparently originating from glands associated with hair follicles. So what appear to be unrelated characters are in fact connected to a particular evolutionary strategy.
I wonder, if something like the megaconus had survived until today, would the definition of “mammal” have been extended to cover it, or would it still be considered a “not quite mammal?” I’m thinking that it would be. (And if all the monotremes had been extinct and we somehow knew that they laid eggs, they never would have been concidered “true mammals”, either.)
Of course there are, as long as you are using those words in their ordinary senses. What you mean is those words have no taxonomic significance. Just because birds have been found to be Reptilia doesn’t mean that people are going to understand you if you say you are going reptile-watching.
To entomologists. That doesn’t make it incorrect to refer to a spider as a bug.
