If you’re reluctant to drop $80 on a copy of Zimmer/Emlen’s Evolution: Making Sense of Life
Go download it and read it now. There will be a test later.
Pharyngula
Evolution, development, and random biological ejaculations from a godless liberal
If you’re reluctant to drop $80 on a copy of Zimmer/Emlen’s Evolution: Making Sense of Life
Go download it and read it now. There will be a test later.
PZ:
Awww, you miss grading tests that much?
Nah, I’m just reflexively cruel.
Saying the dinosaurs died out not because of extinction rates, but because of low origination rates seems like a very strange way to put things. It’s like saying passenger pigeons were wiped out not from dying – every bird always dies – but from lack of offspring. True, but that lack was still the result of them being killed, right?
Is it really usual to define extinction as including both species that died out with no followers and ones that gave way to some offshoot of themselves, or is that just a convenience for the statistical model? Unfortunately the sample cuts off before it revisits the topic, but it seems strange to me.
How much will this test count toward our grade?
It’s an interesting excerpt and I learned something about species diversity. Unfortunately the excerpt was not a full chapter, it cuts off in the middle of a sentence.
@3 Monad
I agree that it’s a weird claim about dinosaurs, for several reasons. First, it supposes that dinosaurs (other than birds) were declining before the K/T extinction, which is a controversial topic; there are papers arguing that they were in fact at a peak of diversity. Second, whatever their diversity, they were wiped out by an asteroid, and the reason they left no descendants is that they were all deceased; they were ex-dinosaurs; they had joined the choir invisible, etc. Third, it reifies the idea of species and genera while ignoring a variety of sampling biases. Finally, it’s a poor example to illustrate the concept they’re trying to explain, that diversity depends on both speciation and extinction rates.
I think you’re confusing a single species (a single species has one origination and one extinction) with an larger grouping of many species (which has rates of originations (alpha) and extinctions (omega)). Even if the extinction rate is the same over time, a lowering of the origination rates to below the extinction rate will cause the whole group to go extinct over time (omega > alpha). Lowering the standing diversity seems like it would impact a grouping’s “ability” to recover, but obviously the larger standing diversity didn’t just appear out of no where (i.e. there was a smaller standing diversity before it was larger where alpha > omega). Also, this section seems to be more about modeling what occurred than attempting to explain why it occurred. Such as: Why did the origination rate of dinosaurs decrease?
There is a movement among biologists to just drop the term macroevolution. I’m one. I’m also not the only one although I doubt there are very many. Most likely because few people care enough to even think about how useful a term it is.
It’s fuzzy, ill defined, and most importantly doesn’t add anything. All macroevoluiton is, almost always microevolution repeated enough times to yield…macroevolution. So where do you draw the line in a microevolution series and call it macroevolution. It’s pretty subjective.
If I never saw the word again, it wouldn’t be any loss at all.
Raven,
So you don’t believe there is such a thing as species selection? You think those paleobiologist are all on about nothing?
Quite the strawperson you invented here.
Do I accept that evolution can produce visible differences? Sure, I don’t look anything like a bacterium or a plant even though we have an ultimate common ancestor. That however wasn’t the point I made.
1. I had to look up species selection again. Contrary to your assertion, not all evolutionary biologists accept Gould’s ” macroevolutionary processes which shape evolution that are not driven by the microevolutionary mechanisms that are the basis of the Modern Synthesis.”
2. Clarify if you want. What are those macroevolutionary processes that are separate from microevolutionary mechanisms? We do know that mutations and natural selection produce evolutionary changes because we see it every day.
@changerofbits:
Actually, I guess what I’m questioning is if they all get one. There are no Homo ergaster any more, but if we suppose that is because they changed enough to become H. sapiens, H. neanderthalensis, and so on, does that count as them going extinct? Because it is very different from what happened to Ectopistes migratorius, which is what the word usually evokes.
Well, sure; and all the dinosaurs being killed in a global catastrophe would definitely drop the origination rate to zero. But again, I find it strange to describe it that way and explicitly not as a change of extinction rates. It makes it sound like extinctions continued unchanged even if what happened is they stopped being “extinctions” by changing into new species and started being “extinctions” by all dying. Is that truly standard use?
Raven:
I said nothing like what you attribute to me. We aren’t arguing about whether evolution occurs. Just about whether “macroevolution” is a useful term. If there are macroevolutionary processes that can’t be reduced to accumulated microevolution, then I suggest that it is. Species selection is the process most often suggested in that context. Nobody should argue about whether it happens, as it’s an inevitable consequence of differential speciation and extinction. What we can argue about is whether it’s of any real importance in the history of life. I myself think it’s of some importance, though not the main factor. As David Jablonski is fond of saying, “Species selection never built an eye”.
Monad:
The technical term for the transformation of an entire population to a new species is “pseudoextinction”. And that opens up a serious can of worms. How do you define a species, especially for two populations existing at very different times? How do you recognize species in the fossil record? There are no good answers.
1. Of which there is no agreement among evolutionary biologists as I pointed out above.
2. Well, OK. List them. What are these “macraevolutionary processes that can’t be reduced to accumulated microevolution?
The old alternative was saltation, sudden appearance of new forms. It happens, but it is rare and explainable by standard evolution. Tasmanian facial tumor. Sudden appearance of what some call a new phylum. Easily explained by plain old TOE.
3. I’ve read Gould although it was many years ago. His attempt to add a major change to the TOE got vague and lacked data when it came to molecular mechanisms. AFAIK, it hasn’t gotten better since.
1. I’d say rather that the disagreement is about whether any of those processes are important.
2. Species selection is the most prominent such process. No new mechanisms are required; it’s merely the analog of natural selection with species corresponding to individuals, speciation to reproduction, and extinction to death. Neither speciation nor extinction can be reduced to a change in allele frequencies within a population.
The K/T extinction event is a fine example of mass species selection. It didn’t strike at random, but preferentially killed off species with particular characteristics; large, terrestrial animals took the biggest hit, for example. When the selective regime changes rapidly enough, and there is no relevant selectable variation already present within populations, a major change in the biota and its evolutionary trajectories can occur quickly — according to some explanations of the extinction, in about 20 minutes. And all without any changes in allele frequency in any population (though this may have happened in species that survived).
Note: some wouldn’t call that species selection, reserving the term for selection entirely on species-level properties such as species range. But I think that’s weird, and anyway it’s still a macroevolutionary process if you want to call it something else.
1. That is debatable especially in the case of speciation. What makes a new species new is a slightly different genome.
2. I’ve seen the debates about species selection and group selection and don’t have a preference either way. I just don’t see why putting macroevolution in front of species selection adds anything. Especially since macroevolution has another more common meaning i.e. visible changes in a clade.
I don’t think we are going to resolve this so will have to agree to disagree. I’m not running late yet, but I will be if I don’t focus on some immediate tasks.
PS You could call the Chicxulub asteroid strike macroevolution. It certainly had macro effects. Somehow, I just want to call it an asteroid impact.
A mechanism of reproductive isolation isn’t usually going to be reducible to a change in allele frequencies.
No, none of that is debatable. If you attempt to debate it you will likely discover that. Yes, what makes species different from each other is difference in genomes. And changes in each population arise through microevolutionary processes. But speciation involves at least two populations. Further, differences in speciation rate are not contained with “changes in allele frequency”. Most importantly, the part you cast aside without comment, differences in extinction rate, most definitely do not involve changes in allele frequency.
“Visible changes in a clade” seems a vague and meaningless phrase and in such not in need of a name, while if there is such a thing as a macroevolutionary process (which I think we have established that there is), we need a name to distinguish it from microevolutionary processes.
No, the asteroid strike wasn’t macroevolution. Similarly, snow isn’t microevolution. But both of them create selective environments in which macroevolution (selective extinction of big animals) and microevolution (differential reproduction of mammals with white fur, small ears, etc.) happen.
You’re being entirely too flippant here, and I don’t believe you’re thinking very hard about what you’re saying.
Thank you, PZ.
@raven
I think you’re not thinking of speciation as a process.
A list of things we use to tell species apart can almost reduce to a list of allele differences, with the exception of maybe a new gene or two.
So
is true in a sense. But only in one sense of the word “make”. The following would actually be a more common use of “make” and still truthful, yet entirely contradictory to your assertion:
Does that bring you closer to PZ and Harshman?
It seems to me that for species selection to be analogous to natural selection, the factors that impact it need to be analogously heritable. In other words the species specific factors that promote speciation, the analogy to reproduction, have to be inherited by the daughter species too.
But I’m not aware of any mechanism that would allow a daughter species to reliably inherit things like range and reproductive isolation.
And all the things that ARE heritable by a daughter species from the parent species are the same genome-related things that are covered by regular microevolution.
Absent this mechanism for heredity, all we get are random environmental effects with no coherent directionality.
@amphiox:
This thread is on slow burn, so I’m going to take the risk and not quote your #19 – I’d have to do so in its entirety anyway, and that’s tedious unless the comment is significantly separated from the response.
But let’s talk about this. IANAB, so, y’know, take this with the grain of nucleotides it deserves. But “the factors that impact it” need to be heritable?
No.
Regardless of whether species selection is analogous to natural selection, there are factors that impact natural selection that are not heritable. Availability of food, and breadth of different kinds of food available, etc. are not heritable. Whether your prey run faster this generation than during your parents’ generation is not something your offspring inherit. The prey could have gotten faster during the change of the last generation, while not speeding up at all (or even slowing down) during the changeover to the next generation.
Genome + some epigenetics (correct me if I’m leaving anything out) makes the individual. The particular collection and distribution of genes and alleles of genes in a population makes the species.
But natural selection is more than just that genome + some epi stuff. It’s the totality of conditions that makes your particular genome advantageous or disadvantageous to have. To know that you have to know the genome (and epi stuff), sure. But that’s only the barest beginning of investigating natural selection. To say that this generation has more individuals with longer legs than the previous generation is not to say anything about whether this generation has a collective selection advantage. Longer legs can be an advantage or a disadvantage, depending on lots of different possible factors.
If species selection is analogous to natural selection, then, yes, we have know something about genes and alleles (and epi stuff), but the selecting is done by forces which make it advantageous or disadvantageous to have that population-wide distribution of genes and alleles of genes (and epi stuff).
That stuff isn’t inherited any more than the forces acting on individuals are inherited in the natural selection model. And it doesn’t have to be. It’s not supposed to be, if species selection is analogous to natural selection.
The difference in the models, as I imperfectly understand it, is that the level of analysis is the species, and therefore advantages are compared species to species, not individual, where advantages are compared individual to individual within a species.
Amphiox:
I have a much simpler response. Some people restrict the term “species selection” to those species-level characters. A case could be made that they are heritable, but I don’t choose to make it, because I don’t restrict the term. What I call species selection is called “species sorting” by some; I don’t care what term you use.
Anyway, I would say that most species selection (or sorting, if you like) does depend on genetic, and thus heritable, characters. It’s just that the populations in question have no selectable variation in those characters; that is, such intrapopulation variation as exists is irrelevant to the selective regime exerting species selection. This is particularly obvious when the time scale of application of the selection is much less than the time needed to accumulate and select variation, as in the immediate aftermath of the K/T impact. Big dinosaurs just died; individual size variation within a species was not relevant to the selective factor, which I would hypothesize to be heat radiation. That’s only one sort of species selection; it just happens to be the easiest one to describe. Anyway, no change in allele frequencies, big change in composition of the biota.
I think that this is good, but I think that some people aren’t understanding how it’s “selecting” since excessive heat sterilizes – it doesn’t merely kill certain organisms with susceptibility to heat.
And, yes, size variation isn’t relative to the selective factor. But this is only increasing confusion among your readers. If the characters of the species aren’t relevant to the “selective factor” then the “selective factor” isn’t selecting anything at all.
You need to identify characters that **are** relevant to your selective factors. If you want to talk about Chicxulub – or even a vast increase in wildfire during a 25 year period of z=+5 drought – identifying the characters a species might possess that would cause it to differentially survive is important to people understanding your argument.
Animals generally don’t have characteristics that allow them to survive environments of 500+ degrees. So to illustrate your point, you show that animals that were more likely to be aquatic and/or burrowing were more likely to survive.
And, indeed, that’s what we find at the K/Pg boundary. Dinos leave little evidence of which I’m aware of any burrowing behaviors. Dinos outside of Aves (or maybe Paraves, I’m not sure) show little evidence of water diving and other aquatic behaviors.
I’m sure lots of little birds got scorched to death, but some who could dive would survive because of that – if they were in the right location. I’m sure lots of mammals died, but burrowing mammals with omnivorous diets did well through the transition.
Other things played a role. I’m sure that if your species range was limited to the crater itself, it wouldn’t matter if you were a burrower or not.
It’s not surviving bone char that was a species-level characteristic that allowed groups to survive Chicxulub. It was any characteristic that allowed the species’ members to avoid being burned down to the bone in the first place that allowed individuals – and thus their groups – to survive.
Has no one read Raup’s Extinction: Bad Genes or Bad Luck? Because the answer is often bad luck, so talking about selective factors is irrelevant.
Selective factors aren’t always irrelevant. Take the K/T event: it didn’t cause extinctions at random. It selectively removed species with certain characteristics; in particular, all large, terrestrial animals went kaput. Size variation within species was irrelevant to survive, but variation between species was enough so that the bigger ones became extinct and the smaller ones sometimes didn’t. Size was an important factor, though not the only one. It’s hard to hide under something if you’re big.
The feature of the K/T event I’m talking about here was radiant heat. Animals out in the open got broiled. Those under even a few inches of soil, rock, or water didn’t. (Being under a tree was no good because the tree caught fire.) See this. There were of course other features that resulted in different sorts of selection, or in just random hits (the “field of bullets”).
@John Horshman:
I’m not sure size was a factor at all.
Time to sexual maturity, population density, and other factors (not incidentally reptiles and mammals who live amphibiously tend to smaller sizes, and all burrowing vertebrates are median-ish size vertebrates or smaller (unless you want to use polar bear’s snow-burrows as making polar bears a “burrowing mammal”) correlate to size but aren’t caused by it, and most of those are much more likely to be acted upon by the environment immediately post-chicxulub than size qua size.
@PZ:
I haven’t read it – thanks for the heads up. I’ll take a look.
In a discussion about species selection, though, I’ve been limiting myself to talking about when selection actually happens (those other cases, common or not, didn’t seem relevant).
I am pretty ignorant in this area, though, so I was planning on starting the Zimmer/Emlen chapter tonight, and if Raup isn’t that long, I’ll try to get to that within the week (so as to get it done before fall term starts here…). Maybe what I have to say will change after reading those sources.
I would suggest reading Steven Stanley’s book Macroevolution. Old, but still thought-provoking.
Size was certainly a factor, though of course it might only be strongly correlated with the true causal factor. I was not considering the environment post-Chicxulub, or not post- by more than a few hours. Just the initial impact and re-entry of debris that caused the major heat radiation. Nothing there about time to maturity or population density. There were subsequent effects of the impact that created other selective regimes; certainly the forams weren’t broiled.
I would argue that these kind of factors do NOT actually impact naturally, because they are going to affect all organisms equally, or at least randomly. The thing that DOES impact naturally selection is how the individual organisms deal with or adapt to or react to those factors like food availability, and that IS potentially heritable.
If those factors are specific to the K/T event, and not generalizable to extinctions in general, or even mass extinctions in general, then it IS random, because the K/T event itself was a random occurrence. So we just have a random event that happened to, this time, preferentially knock out the big species. The next random event might preferentially knock out species with some other characteristic. Maybe the End Ordovician preferentially knocked out species that happened to have larval stages that lived in shallow water (I don’t know if this is actually true, it’s just an example for the argument). But each of these is simply a specific detail of the singular event of that specific mass extinction. It still falls under the category of luck, and I don’t see a justification for elevating it to something that deserves a special name or consideration as a special, unique mechanism, like “species selection” that is generalizable to the larger phenomenon we’re trying to study.
re #27:
That should read “impact natural selection”
@Monad #1. Lack of offspring is precisely the cause of extinction of passenger pigeons. Disruption of colonial nesting meant that the age-distribution of the population shifted to a greater number of post reproductive individuals- i.e. no replacements. A comment I read many years ago comes to mind: ‘If there were really 4 billion passenger pigeons, why wasn’t the American landscape covered with lead shot?”