Evolutionary biology questions...seems things have changed in 20 years

[Nerys Ghemor]

When I was younger, evolutionary biology, particularly to do with human evolution, was one of my absolute favorite things to read about, and I still like to try to keep up with new discoveries as they come out.

Unfortunately, though, it seems certain changes in the field (and in biology/evolution in general) seem to have caught me unawares.

So I have two questions, if you could please bear with me and explain in fairly simple terms, to get back up to date:

1) Why are we suddenly discussing "hominins" instead of "hominids"?

2) What is a clade? How does this differ from a family, phylum, etc.? Why is it important? This was not taught in biology when I was coming up. Does the whole Kingdom/Phylum/etc./etc./etc. thing not apply anymore?

[Venardhi]

Taxonomy is in a constant state of debate.

A Clade is essentially "everything descended from X."

'Homonin' refers to only our direct ancestors whereas 'Homonid' is a looser term that includes those ancestors and their closely related species. Since we aren't exactly sure which begat which in many cases, Homonin is something of an optimistic classification to create with only our current knowledge to define it.

[Robert Maxwell]

As Venardi said, taxonomy is always in flux. While the basic classifications are still kingdom, phylum, class, order, family, genus, and species, our research into the evolutionary history of both humans and other organisms has shown that those clear lines are a lot fuzzier than we'd like, so we have other designations like "paraphylum," "superfamily," "subfamily," "superorder," etc. It's also worth remembering that these designations are arbitrary, though scientists do try to apply them consistently. It's just difficult because evolution hasn't followed a very specific, predictable pattern--not even among humans and our ancestors.

[Christopher]

As I understand it, the change from "hominid" to "hominin" is a result of our understanding that humans are more closely related to other great apes than we used to think. We used to treat humans and great apes as separate taxonomic families, Hominidae and Pongidae respectively. But once we found out that chimpanzees were more closely related to humans than they were to gorillas and orangutans, we realized that was an invalid subdivision, that humans are actually a subset of the same family as the other great apes. So that family is now called Hominidae, which has two subfamilies, Homininae and Ponginae -- although there's still some debate about which one chimps and gorillas should be in.

http://en.wikipedia.org/wiki/Hominidae

[Deckerd]

I didn't realise evolution was predictable.

Christopher wrote:

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there's still some debate about which one chimps and gorillas should be in.

I watched a documentary a few years ago arguing that both should be classified as species of homo. The implications for hunting and the pet trade alone would be massive.

[Robert Maxwell]

Deckerd wrote:

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I didn't realise evolution was predictable.

It isn't. I was making the point that you can't even predict how one species led to another--whatever "missing links" might be in between. The gaps in the fossil record continue to confound scientists, though we are gradually filling in the missing pieces. It's a puzzle that will never be complete and there are still plenty of surprises to discover.

Maybe your issue is with the word "predict." Would you prefer "assume" or "speculate" or "conjecture"?

[PlixTixiplik]

The answers to the hominin/hominid question given above are correct. Hominin refers to the tribe Hominini (a tribe is the taxonomic level below subfamily), because most apes are now included in the family Hominidae.

To answer the question about a clade...

A clade is a monophyletic group - which means that it includes a single common ancestor and ALL of its descendants. Some of the traditional taxonomic groups are not clades (are not monophyletic), but are paraphyletic "grades" (paraphyletic means that it includes a common ancestor but only SOME of the descendants. For example, Pisces is not a clade because tetrapods (reptiles, birds, mammals, etc.), which evolved from one group of fish, are not included. As a result, Pisces is no longer used as a valid group, although many individual fish groups are clades.

Clades do not supersede the Kingdom/Class/... organization (they are still used), but it is preferable that a given Class or Family or Subfamily or whatever be a clade rather than a paraphyletic grade. Clades are like those Russian nesting dolls, in that (for example) the clade Hominini (a tribe) nests within the clade Homininae (a subfamily) that nests within the clade Hominidae (a family) that nests within the clade Hominoidea (a superfamily)...

The increasing use of clades reflects the increasing importance of cladistics in reconstructing phylogeny (taxonomic relationships). Cladistics is a quantitative method that classifies taxa based on the shared occurrence of newly-evolved, or derived, characters (called synapomorphies). When Linnaeus created Pisces it was basically defined (although not explicitly) on lacking the derived characters found in tetrapods, which is now viewed as a bad way to define groups.

[Christopher]

On the other hand, cladistics by itself can be too imprecise a way of defining things, because by a strict cladistic interpretation, humans and all other vertebrates would actually be fish.

[PlixTixiplik]

Christopher wrote:

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On the other hand, cladistics by itself can be too imprecise a way of defining things, because by a strict cladistic interpretation, humans and all other vertebrates would actually be fish.

That's partially true, in the manner of the Russian nesting dolls I described above, but doesn't have anything to with precision. It's also partially false because there is no such group as "fish." Humans belong to the clade Gnathostomata (jawed vertebrates), as do bony fish (Osteichthyes) and Cartilaginous fish (Chondrichthyes). Humans also belong to the Teleostomi, which includes bony fish and tetrapods. We also belong to Tetrapoda, which includes amphibians, reptiles, etc. We also belong to Amniota, Mammalia, Theria, Placentalia, etc.

[Christopher]

There's no such taxonomic category formally labelled "fish," but that doesn't mean there's no such group as "fish." Show people what's swimming in an aquarium and they'll call them fish, and they won't be wrong, because that's what the word means. Labels do not define reality, they just describe it. And there are different schemes for labeling things, some more formal than others, but more formal doesn't necessarily mean more real.

[PlixTixiplik]

It's true that there is a group of animals known to most people as "fish", and that naming serves a valid purpose, but you said that a "strict cladistic interpretation" would say that humans are fish, which is not true. Cladistics would say that humans and "fish" are vertebrates, that humans and most "fish" are jawed vertebrates, and that humans and a smaller group of "fish" are teleostomes. Agnathan, Chondrichthyan, and Osteichthyan fish are all monophyletic groups, none of which includes mammals. This is different from the "birds are dinosaurs" statement, which is true because Aves is a monophyletic clade nested within another monophyletic clade (Dinosauria).

You also said that that method would be imprecise, which is also not true. The hierarchical nature of phylogenetic classification (with clades nested within clades) allows for explicit hypotheses of evolutionary relationships. For example, Hominoidea = Hominidae+Hylobates (gibbons); Hominidae = Homininae+Pongo (orangutan); etc.

[Christopher]

Okay, okay, I was making a wry observation, not a formal dissertation. I was just trying to convey, in laypersons' terms, that it wasn't as simple as just defining everything by what it's descended from.

[gturner]

But in some usages, "fish" includes birds, such as when you go to Long John Silvers and get chicken strips, which actually taste like fish...

[Christopher]

^Except I've never gone to Long John Silver's.

[Silvercrest]

This must be the answer to "everything tastes like chicken, so what does chicken taste like?"