One of the enduring puzzles of biology is the rapidity of speciation, which is associated with the famous paleontological challenge of finding fossil evidence of the “missing link” between species. It seems that species can stay stable for a long time, whereas the process of evolution from one species to another happens in a blink of evolutionary time.
Here’s the summary of the recent paper, Imbalanced speciation pulses sustain the radiation of mammals, by Ignacio Quintero, Nicolas Lartillot, and Hélène Morlon:
Mammals have drawn particular research attention because of their huge level of morphological variation. Until recently, most believed that the dinosaur decline at the end of the Cretaceous opened niches for mammals to fill, resulting in a burst of speciation. Quintero et al. developed a model that integrated phylogenetics and fossils to estimate diversification rates across all mammalian lineages and concluded that mammalian speciation rates were high well before the Cretaceous–Paleogene boundary. Rather than leading to a burst of diversification, the major environmental changes during the time filtered out more slowly speciating mammalian lineages, leading to domination of the group by those that speciated more rapidly.
In another paper, The diffused evolutionary dynamics of morphological novelty, Quintero writes:
Rates of evolution are fundamental to understand the processes that shaped the history of life. The predominant view holds that high rates of phenotypic evolution result from lineage transitions across peaks in an adaptive landscape, with subsequent slow-downs, but evidence remains debated. . . . I find that evolutionary rates do not conform to expectations from adaptive landscape theory, but rather have been stable, unaffected by the accumulation of phenotypic disparity. Long-term evolutionary trends, such as several net increases in clade-average body size, result both from sustained evolution at the lineage level and the sorting of species phenotypes and their underlying evolutionary rates at the clade level, sometimes acting in opposite directions. These findings substantiate an active role of species in shaping their environment that generate continuous novelty of life forms.
Interesting. There are some vaguely related issues in biology that are tied to political controversy, but here I’d just like to get some sort of general understanding of speciation, which seems a lot more interesting to me than clichés of the “selfish gene” or “two brothers or eight cousins” variety.
Isn’t this Gould and Eldredge’s Idea of punctuated equalibrium? https://en.wikipedia.org/wiki/Punctuated_equilibrium
I find some of these claims rather strange.
For example, Sewall Wright’s landmark theory about adaptive landscapes makes no claim about rates of speciation. One could certainly approach diversification rate from an adaptive landscape perspective, but the notion here that you can go the other way and interpret adaptive landscape theory based on diversification rates makes no sense to me.
In general, I don’t see how diversification rates tell us much about the continuing conundrum of speciation. A vampire finch is still a finch, and all the species that evolve from them will still be finches unless some second, truly novel innovation other than blood-drinking occurs. A vampire finch may appear on a different island with a blue head and red feet, and be named a new species, but that is hardly an evolutionary innovation. What we still don’t know is how a seed-eating bird made the leap to drink blood. Was it just one bird or was there a population dynamic, and either way, how did it get fixed in a population? Those are the questions we still struggle with.
There is also a general claim in the cited paper that fast-diversifying lineages are important while the slower-diversifying ones are less so, but this smooths over the difference mentioned above between minor and major innovations.
Plants are a far better laboratory for understanding speciation than mammals, primarily because plants can reproduce in the somatic line in various ways while animals are limited to the germ line (you can’t cut off your fingertip and grow a new you). In plants we clearly see clades that undergo rapid diversification, such as the Hesperolinon/Caulanthus clade in California, and plants that show genetic stabilization such as the lodgepole pine. In the cited paper, the H/C clade becomes very important due to rapid speciation while lodgepole pine fades into the background, yet lodgepole pine is a widespread and important component of ecosystems while the H/C clade – a group of small, mostly rare annuals that pioneer newly exposed landscapes – don’t seem to be important for anything except studying evolution. Which one is really winning the evolutionary race, the ubiquitous turtle or the rare hare? The lodgepole knows that when you evolve, what has evolved is no longer you, and the current you is very well suited to widespread success.
Some plants can also self-pollinate and thereby avoid the pitfalls of backcrossing that seem so problematical for true innovation.
There is a speciation paper that I consider to be a landmark in human thought that has been “relegated to the dustbin of history.” I think it is important because it captures a nascent species with chromosomal changes that really could lead to important innovations. The paper is “Rapid Evolution in Clarkia” by Harlan Lewis and Peter Raven. Unfortunately I cannot find a non-paywalled version.
It feels like temporal scale differences are relevant here. For example, presumably the mammalian examples in the paper are inferences from fossils (with everything that implies about nonrandom discoverability thinning of what actually happened), whereas examples like the H/C clade are shallower in time with (currently) most products of the radiation currently extant? A similar extreme example would be the apomictic genus Taraxacum (Dandelions) where a few thousand biological species (sensu Mayr) exist but “novelty” is limited to minor variations in soil niches and long-term inbreeding may ultimately doom them (and no chance of future paleobotanists distinguishing these microspecies from fossils).
As I repeatedly say, it is possible to learn a lot from this website. For example, “clades” and “cladistics” with the latter suspiciously sounding like a bad pun.
“a group of organisms that is composed of a common ancestor and all of its descendants. [2] Clades are the fundamental unit of cladistics.”
Now do “organism”.
Finding fossil evidence of the “missing link” between species sometimes seems easier than finding evidence of a “working link” between parentheses in fossilized scientific papers…
Reproductive isolation still seems to be accepted as a key contributor to speciation. Two difficulties in trying to understand speciation using the fossil record are (i) there is a good chance that speciation is actually taking place “elsewhere” (from where you are digging!) and (ii) that major genomic changes likely to be key to speciation processes can occur without much effect on morphology. Matt Skagg’s reference to Clarkia above seems to be an example of this. Even dramatic chromosomal rearrangements as in the Clarkia example may not have much effect on morphology but they have the advantage (speciation-wise) of causing an abrupt reproductive isolation. This differs from the more classical ideas about speciation occurring in geographically-isolated populations that may have large contributions from genetic drift.
Genome sequencing is uncovering many examples, e.g. the not-so-distant whole genome duplications (WGD) in vertebrates (some Carp and Salmon/Trout and Frog species; WGD events are quite common in plants). These might be expected to generate Goldschmidt-like “hopeful monsters” but it seems they can have little immediate apparent impact on morphology (a problem if the fossil record is your main dataset). Their influence occurs subsequently over what might be very long time periods through further genome rearrangements, mutations in or loss of, duplicate genes. Their definitely a driver of genomic and phenotypic complexity.
Don’t understand how this conclusion from the Quintero PNAS abstract arises from their analysis: “These findings substantiate an active role of species in shaping their environment that generate continuous novelty of life forms.”
Classical ideas of speciation have always accommodated the different possible spatial modes of allopatric v sympatric v peripatric speciation.
Yes, for sure – I don’t disagree.
Where do the major controversies in evolutionary theory lie now? 30 years ago Dawkins (adaptationist) vs Gould (contingent contributions to evolution) was a prominent source of interest – I preferred Gould to Dawkins’s proselytizing style, and time may have been rather favourable to Gould. It seems that Dawkins can’t let that controversy rest and doesn’t lose an opportunity to continue the “debate” in his (Dawkins) favour even though Gould passed away more that 20 years ago!
But things do seem to have calmed down controversy-wise. There is convincing evidence from genomics, cell biology and population genetics for non-adaptive contributions to evolution (Michael Lynch and also Ohta’s “nearly neutral” theory) which is quite impactful in its implications but it seems like the adaptationists haven’t felt like opposing this – perhaps the evidence is too strong. Adaptationists may say that if quite profound changes at the level of the genome or cell aren’t manifest as immediate changes in the phenotype then these don’t matter from the point of view of adaptation/natural selection, even if they may make important contributions to evolution/speciation and the generation of complexity. It seems to me that the explosion of genome sequence information and the abundant mutational variation in the human population (for example) tends to support Kimura’s and Ohta’s neutral and nearly neutral theories, but I haven’t seen much debate about this.
Otherwise controversy seems to lie in the status of junk DNA but some of that debate is a little contrived since junk DNA is anathema to creationist notions (and also a little problematic for strict adaptationists). Maybe also whether widespread transcription of sRNA’s is meaningful or “noise”. Denis Noble has been prominent in attacking Neo-Darwinism but his ideas haven’t made much of an impact and are probably not part of a serious controversy.
Maybe the deluge of genome sequence information in the post-genomic era has put a damper on evolutionary controversies – or it’s possible I just haven’t been paying attention in the right places.
That is a really good point about using morphology from the fossil record as a proxy for speciation. Chihuahuas and St. Bernards are both Canis familiaris.
I was also befuddled by the claim about species shaping their environment to generate continuous novelty. I bungled (confabulated) the clade I was referring to above, it should have been Steptanthus/Caulanthus, but it is a good example of a diverse and rapidly diversifying clade that is most certainly NOT shaping its environment.
A more pertinent observation for this thread is that some species in this clade appear to have (1) adapted to serpentine, (2) become reproductively isolated (including obligate self-pollinators with nonfunctional flowers!), and (3) developed unique morphological and functional traits. Trying to construct a hypothetical pathway in which these changes happen sequentially under Darwinian selection is really, really hard. Parsimony (sorry Andrew) suggests a much simpler pathway: they began as systemically mutated hopeful monsters (“founders”) unable to back-cross, found unlimited open habitat on serpentine, and underwent population increases with little or no selection being operative. That may or may not be right, but our understanding of speciation events will be improved by considering questions like this, not by looking at the rate of morphological changes in the fossil record.