Tuesday, May 27, 2014

A Survivor From The Namib Desert

This beautiful passage from Robert Krulwich's essay on the plant Welwitschia mirabilis, a survivor, the last representative of its genus, isolated and endemic to the Namib desert in South Africa:

Welwitschia, when you finally get to see one, sits apart. It's very alone. All its relatives, its cousins, nieces, nephews have died away. It is the last remaining plant in its genus, the last in its family, the last in its order. "No other organism on earth can lay such a claim to being 'one of its kind,' " writes biologist Richard Fortey. It comes from a community of plants that thrived more than 200 million years ago. All of them slowly vanished, except for Welwitschia. It has survived by doing very little, very, very slowly — sipping little wafts of dew in the early mornings, otherwise minding its own business, as the big, busy world goes by.

With much recent attention given to the question of de-extinction i.e. bringing back extinct species, a more urgent focus needs to be on species and populations which are alive today and hanging by a thread. Genetics will play a role here too along with old fashioned conservation of the ecology in which these creatures persist despite all odds.

Thursday, May 15, 2014

Sherlock Holmes And Long Term Evolutionary Patterns In Dinosaur Body Size

This week's (in Pune, India) Sherlock Holmes Elementary featured a murder mystery involving a smuggled dinosaur fossil. The fossil is believed by some palaeontologists to be entombed in rocks of the earliest Paleocene, making that dinosaur a survivor of the end Cretaceous mass extinction. Other palaeontologists strongly disagree with this survivor fauna scenario, which the drama turns into a motive for murder. There was a fair amount of geology and palaeontology in the episode and a reference to a term "dead clade walking".  This term is real life was coined by David Jablonski a palaeontologist from University of Chicago.  The term means that a clade or a lineage has survived a mass extinction but its fate has been sealed. Over a period of time say a few million years after the mass extinction that group does eventually go extinct.

Some dinosaur species may have survived the mass extinction but by early Palaeocene they were certainly all gone... except one lineage.. the Availea or birds. They prospered in the Cenozoic, radiating into a hundreds of species. Their success may have had deep roots and one important factor may have been the small size of their ancestors.

A paper in PLOS Biology explores the long term evolutionary patterns of dinosaurs and finds a positive relationship between high rates of evolution and small size-

Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage:

Author Summary

Animals display huge morphological and ecological diversity. One possible explanation of how this diversity evolved is the "niche filling" model of adaptive radiation—under which evolutionary rates are highest early in the evolution of a group, as lineages diversify to fill disparate ecological niches. We studied patterns of body size evolution in dinosaurs and birds to test this model, and to explore the links between modern day diversity and major extinct radiations. We found rapid evolutionary rates in early dinosaur evolution, beginning more than 200 million years ago, as dinosaur body sizes diversified rapidly to fill new ecological niches, including herbivory. High rates were maintained only on the evolutionary line leading to birds, which continued to produce new ecological diversity not seen in other dinosaurs. Small body size might have been key to maintaining evolutionary potential (evolvability) in birds, which broke the lower body size limit of about 1 kg seen in other dinosaurs. Our results suggest that the maintenance of evolvability in only some lineages explains the unbalanced distribution of morphological and ecological diversity seen among groups of animals, both extinct and extant. Important living groups such as birds might therefore result from sustained, rapid evolutionary rates over timescales of hundreds of millions of years.

In general the rapid-evolvers would be the smallest-bodied species -- the ones that reach reproductive age quickly and while they are still small. Rates of evolution depend on generational time. Which predicts that large-bodied/long-generation-time species would have more difficulty adapting to rapidly-changing extinction conditions.

The authors also mention that body size evolution in many non-avian dinosaur lineages seem to follow Cope's Rule, an increase in body size of descendant species over time. The earliest species in that lineage would be small and over time there would be a trend towards evolution of larger bodied species.

What could cause such a trend? Is larger body size advantageous and hence being favored by natural selection? Again the relationship to mass extinctions is intriguing. The authors mention that during the late Triassic mass extinction many branches of dinosaurs became extinct. If larger bodied dinosaur species were disproportionately killed off the survivor species of dinosaurs in early Jurassic would have been small bodied. This could explain Cope's Rule in a rather novel way (as explained by my adviser Anthony Arnold - who has worked on size evolution in foraminfera - in an email to me) - "since it means that rather than evolution favoring size increase, mass extinction selectively removes larger-bodied species, leaving behind the smaller guys as survivors. Since they don't have much room to get smaller the survivors could even speciate at random and the only direction in which the variance has room to expand is toward larger size.

Maybe!.. arm waving.."

So ecological crises may lead to species sorting based on size with smaller size being favored and then evolution of a trend towards larger size that reflects simply an inability to become any smaller!..

Monday, May 12, 2014

Book- John Tyler Bonner- Randomness In Evolution

This book is surely worth reading. Randomness in Evolution by John Tyler Bonner. I have loved Bonner's  previous books The Evolution of Complexity and Life Cycles. This promises to be interesting too.

In Current Science Raghavendra Gadagkar gives a positive review. The main premise of the book is that morphologies of unicellular eukaryotes like radiolarians and diatoms to give two examples are neutral phenotypes. That means that the variation of size, shape, ornamentation on skeletal material did not become common because it contributed to reproductive fitness. Rather for example different shapes were selectively neutral i.e. that is they neither gave the organism any advantage over another shape nor a disadvantage. One of the shapes became common just by chance, through random genetic drift.

Size according to Bonner is an important constraint on whether natural selection or drift becomes important in shaping morphology. Randomness is more important in small organisms with relatively simple genetic controls on morphology, while natural selection is more important in larger organisms with elaborate interlocking developmental steps.

I'll reserve comments until I have read the book but just would like to point out that even small unicellular eukaryotes have very sophisticated cellular machinery for processes like photosynthesis and regulating cellular functions. These would have evolved via natural selection.

Wednesday, May 7, 2014

Earthquakes Triggered By Fluid Injection Along Faults

A friend sent me this abstract published in the Seismological Society of America 2014 annual meeting-

Triggered Earthquakes Far From the Wellbore: Fluid Pressure Migration and the 2008-2014 Jones Swarm, Central Oklahoma

KERANEN, K. M., Cornell University, Ithaca, NY, USA, keranen@cornell.edu; WEINGARTEN, M., University of Colorado, Boulder, CO, USA, matthew.weingarten@gmail.com; BEKINS, B., USGS, Menlo Park, CA, USA, babekins@usgs.gov; GE, S., University of Colorado, Boulder, CO, USA, Ges@colorado.edu; ABERS, G. A., Lamont-Doherty Earth Observatory, Palisades, NY, USA, abers@ldeo.columbia.edu

Earthquake relocations and hydrogeologic modeling show that the Jones earthquake swarm, occurring near Oklahoma City since 2008, is linked to disposal wells injecting high volumes of water along the Nemaha Fault. Felt and recorded earthquakes in the Jones swarm began in 2008, approximately 15 km from four high-volume wastewater disposal wells. These wells dispose of ~2-3 million barrels per month (4-5 million barrels per month cumulatively) in two adjacent locations on the downthrown side of the Nemaha fault. Earthquakes are observed to migrate away from these high-volume disposal wells up the structural dip and down hydraulic gradient. Hydrogeologic modeling shows that the increase in subsurface pore pressure resulting from the fluid injection is of sufficient magnitude to trigger slip on pre-existing faults. The region of increased pore pressure grows outward through time with injection. The larger, mapped faults in the subsurface may act as conduits or guides to fluid flow, and may transmit fluid pressure into basement. Our results demonstrate that wastewater disposal can raise fluid pressure and trigger earthquakes at tens of kilometers from the wellbore on existing faults.

Fracking for shale gas by itself has not been shown to trigger biggish earthquakes but the wastewater disposal that follows fracking has.

How does increasing pore pressures increase the chances of slip along a fault? High pore pressures reduce the effective normal stresses acting perpendicular to faults. These normal stresses resist shear movements i.e. fault blocks from sliding past each other. With increased pore pressure the effective normal stresses decreases, allowing shear movements.  Geophyicist Mark Zoback explains in more detail about the risks and management of seismic risk posed by wastewater disposal.