Week 8: Background
and Mass Extinctions: The Alternation of Macroevolutionary Regimes (1986)
Paper for Tuesday
Author of the Blurb:
Michael Foote, University
of Chicago
Foote is a macroevolutionary paleobiologist. With
publications like "Geographic ranges of genera and their constituent
species: structure, evolutionary dynamics, and extinction resistance" and
"Greenhouse-icehouse transition in the Late Ordovician marks a step change
in extinction regime in the marine plankton" you can tell why he was
selected to write the blurb for this paper. His bio on the University
of Chicago
Author of the Paper:
David Ira Jablonski
Jablonski got his Ph.D. in 1979 at Yale. He did postdoctoral
work at Berkeley University of Chicago University
of Chicago
Cliff notes of the
paper:
Incidentally, I think Nick and I both already read this
paper once for Dr. Myers' Invertebrate Paleontology course a year or so ago.
Jablonski's analysis is on Cretaceous mollusks. During the
Cretaceous a number of 'qualities' or characteristics which species and genera
possessed enhanced their survivorship - a broad range, high-dispersal spawning
of planktonic larva, high diversity. All of these things essentially became
meaningless during the mass extinction, instead, a genera which had a broad
range - heedless of the range of the component species - had enhanced
survivorship. In other words: during a mass extinction the 'instruments' of
extinction do not simply increase in magnitude - what the actual instruments of
extinction are change themselves.
Using a mathematical analysis Jablonski shows that direct developers
have higher rates of speciation and extinction during background times than planktotrophs.
During background times, planktotrophs have a median species lifespan of 6
million years, whereas direct developers had a median species lifespan of 2
million years. During the mass extinction however, planktotrophs were no more
or less likely to become extinct than direct developers. (Fig. 1)
Instead, during the mass extinction, diversity of the 'clade
level' (genera) was determined strongly by the range of the genera. Genera with
small ranges just get wrecked, while genera with a broad range are 'ok'. (Fig.
2)
Finally, Jablonski talks about other mass extinctions. There
are similar trends for bivalves, bryozoans, and ammonites throughout the Phanerozoic.
The essential takeaway: the selective pressures during a
mass extinction event are fundamentally different then those which exist during
the long intervening intervals of time. Species which are super well adapted for
the environment during a non mass extinction phase are doomed when mass
extinctions come around - conversely, certain species which kind of suck during
non mass extinction phases will be the cat's pajamas when mass extinction time
comes swinging back around.
During a large-scale perturbation of the environment, traits
which might otherwise be beneficial can be lost entirely.
Questions I had:
1) Overall I feel I understand the paper pretty well, but I
remain confused on one point. When he did the analysis of genera survivorship
during the mass extinction, how was Jablonski able to compare the rate of
genera survivorship during mass extinctions to the rate during background
times? Surely the size of the range of a genera changes a lot through time
during normal background processes. In fact, wouldn't you generally expect
genera to be most likely to go extinct when they're small during background
times too? Jablonski talks about it a bit, but I just don't get it. Ich bins
verwert!
2) A more dreamy question, is geographic range a 'property'
of a species/genera? Do certain physiological characteristics inherent to a
genera control their dispersal across the globe, or is it more of a random
thing? I'm not even quite sure which of those options Jablonski is trying to
advance in this paper.
3) Finally, Jablonski says that this research shows there
are hierarchies - tiers if you like - of evolution, with some evolutionary
processes like selection and drift acting at different foci. We've been talking
about this a lot in Macroevolution, so Nick and Agathe will know what I'm
talking about - we recently read a paper by Gould and Salthe that said
selection can only act on one genealogical
level: the organism. Who's right, can selection and drift act on a variety of
different "focal levels," as Jablonski implies, or can it only act on
one tier, as Gould and Salthe suggest?
As Lucius started to point out with his first question, I feel like some more detailed methodology on this paper would have been helpful to better interpret the results. Also, this paper concludes that “geographic range at a clade level appears to be the most favorable trait to survive mass extinction” but, can we generalize this pattern to other clades besides bivalves and gastropods (clade hygrophila)?
ReplyDeleteAlso, is the pattern that Jablonski described in 1986 still true today? Orzechowski et al. 2015 analyzes (through a meta-analysis) the impact of environmental conditions to the extinction selectivity for benthic marine bivalves and gastropods over the last 500 million years. This study concludes that there is a “remarkable consistency” between geographic range and extinction selectivity. Based on that, we could probably agree that Jablonski was in the right direction…
Lol, Orzechowski vs Jablonski. They should go skiing together sometime. But seriously, I scanned the Orzechowski paper for good measure, and I think he says that both species and genera with wide ranges are more likely to survive: "In contrast, the odds of taxa with broad geographic ranges surviving an extinction (>2500 km for genera, >500 km for species) are on average three times greater than narrow-ranging taxa (estimate of odds ratio: 2.8, 95% confidence interval = 2.3–3.5), regardless of the prevailing global environmental conditions. The environmental independence of geographic range size extinction selectivity emphasizes the critical role of geographic range size in setting conservation priorities." I believe he's only referring to background extinctions for this purpose (he talks about analyzing a 458 million year range) so I think and Jablonski would actually be in agreement, at least on the species survivorship thing. Not sure about genera though, and that relates back to one of my questions: I'm not sure how Jablonski measured that for genera during background times.
DeleteYour last question made me go back and look over the last part of Jablonski's paper again. Is he really suggesting clade-level selection, or is he just using his data to support an ecological and genealogical hierarchy? It seems to me like he's saying that the 'focal levels' at which selection is operating is just a result of the 'trickle up' or 'trickle down' mechanics of the hierarchy. "...this selectivity at the clade level cannot yet be inferred to indicate an ongoing process of evolution by clade selection." Not sure though, may have to go back and read again.
ReplyDeleteI also have a macroevolution question from the paper: Jablonski says that planktotrophs have greater larval dispersal which suppresses divergence of populations. How does this work? Is it because gene flow is dispersed over a broader range?
As it relates to that last bit, your supposition is just right: direct developers are born as 'smaller versions' of the adult form and generally crawl away from the egg mass. That results in very low dispersal. The planktotrophic organisms, on the other hand, spew a ton of eggs up into the water column and the little babies spend the first half of their lives living in the open ocean (plankto) eating plankton (trophic), after which they metamorphose into the adult form and become epifaunal or infaunal or somesuch. Spending their larval stage in the water column like that not only means they have high dispersal but also that its harder for them to become segregated and speciate allopatrically.
ReplyDeleteAs it relates to the first question, I'm still not sure. I'm sure Dr. Smith can explain it to me. Cheers!
I do not understand what the author means by larvael mode at all nor shell morphology. Does this mean that some larvae have smaller shells or don't have shells?
ReplyDeleteI think that geographic range can get pretty tricky when you correlate that with extinctions. It causes so many assumptions. For instance, how do they know that a species total range is the range that they occupied at one time? With time scales that are tens of million years, what if a species has a relatively small range but that range was shifting due to changing environmental conditions, but when the paleontologists collected the fossils, it appears that the animal inhabited a rather large range?
I think that a species that occupies larger ranges have local adaptations that allow them to survive better in their certain parts of their range... per your question #2 Lucius, the physiology of the genera absolutely do control the species range and a species with a lot of phenotypic plasticity are able to live in different habitats and that gives the genus a leg up with extinction events.
ReplyDeleteI think what isn't explicitly stated by Jablonski (but I think is a pretty strong inference from this paper and others) about wide-ranging species is that species who are wide-ranging are more likely to be able to occupy different types of ecosystems/environments. If they are able to do that, than they may be able to use one particular ecosystem if the other(s) get absolutely wrecked by the mass extinction. It would be interesting to test and see whether certain environments or ecosystems experienced larger rates of extinction than others during mass extinction vs background extinction.
ReplyDelete