Evolution Through Punctuated Equilibrium: History Of An Idea

Palaeontologist Niles Eldredge explains how one of the most famous papers on paleontology and evolution came to be published: 

Steve was determined to be a part of Tom’s plan to do a GSA symposium and publish a book of essays on this new-fangled concept of “paleobiology.” Tom had a list of topics and was shopping around for speakers to be assigned to each one. When Steve saw the list, he told me that he had first wanted “morphology”—but that was already assigned to Dave Raup. So he opted instead for “phylogeny”—but that had been grabbed up by Mike Ghiselin. That left only “speciation,” the last of the evolutionarily imbued topics on Tom’s list, as yet unassigned. Steve called me up, explained the situation, and said he had settled for speciation—but could not think of anything much to say about it beyond the manuscript I had written and recently submitted to Evolution—there of course being no Paleobiology as yet. “The Allopatric Model and Phylogeny in Paleozoic Invertebrates”—a distinctly un-Gouldian, plodding, if accurate, title (Eldredge 1971). Without Ralph Gordon Johnson in the editorial chair of Evolution at that time, I doubt that that early paper would have been accepted. As it was, it was likely to have gone relatively unnoticed—had not Tom come along, Steve grabbing “Speciation”—and Steve asking if we could coauthor the paper along the basic lines of my first effort. He was stuck with “speciation,” and couldn’t think of anything much to say beyond what I had said in the Allopatric Model manuscript.

This passage is from an article by Niles Eldredge titled Reflections On Punctuated Equilibria, published in a recent issue of Paleobiology. The 1972 paper he refers to, coauthored with Stephen Jay Gould, was, Punctuated equilibria: an alternative to phyletic gradualism. It marked the beginnings of a long debate on how to interpret the patterns of morphological change observed in the fossil record. Do species remain in stasis, showing little morphological change through much of their existence as Eldredge and Gould argued? Do periods of rapid morphological change coincide with the origin of new species (speciation)? Supporters hailed it as a revolutionary work. Critics called it 'evolution by jerks', a jibe aimed not just at the patterns of change.

Dr. Eldredge provides a very insightful look at the history of this idea including some fascinating snippets on Darwin's thinking about divergence and species origins. For Darwin, change accumulates incrementally over long passages of time. Divergence via natural selection can give rise to descendant varieties even without geographic isolation of a population. Later thinking has given more importance to exogenous factors like climate change in causing habitat fragmentation and reproductive isolation. Populations gets geographically isolated first, and then diverge from the ancestral species either through natural selection or random genetic drift. Eldredge and Gould applied this idea to the fossil record and emphasized that the sudden appearance of new fossil species is a manifestation of long periods of stability interrupted by episodes of isolation and geologically rapid shifts in morphology (allopatric speciation).

There is a lot to take in and think about the long term patterns of change preserved in the fossil record. But it is enriching reading. The article is open access.

Remotely India: Chittagong Tripura Fold Belt

Remotely India #13

Did you know that the easternmost part of the Bengal delta is being compressed into folded hill ranges? These go by the name Chittagong Tripura Fold Belt (CTFB), also referred to by geologists as the Outer Indo Burman (Myanmar) Ranges.

Take a look at the annotated satellite image below. The CTFB appears as a series of north south oriented ridges and valleys, extending from northern Tripura to south of Cox Bazaar in Bangladesh. 

Structurally they are made up of strata folded into anticlines (upwarps) and synclines (downwarps). To the east, they are separated from the inner Indo Burman (Myanmar) Ranges (IBR) by the north south trending Kaladan Fault. The Chittagong Coastal Fault marks the westernmost boundary of this fold belt, although the sedimentary pile below the sea bed of the Bay of Bengal to the west is also deforming. The 'deformation front' of this terrain is therefore further to the west of the Coastal Fault. 

As you might have guessed, these fold belts are a result of the Indian tectonic plate converging with Asia. But the nature of tectonic plate interaction is different from the plate collision that formed the Himalaya. In the case of the Himalaya, the continental crust of the Indian plate has collided with the continental crust of the Asia plate. The lower part of the Indian continental crust has slid under Tibet while thick slices of the Indian upper crust have been thrust up by faults to form the different geologic units of the Himalaya. 

Tracing the mountain arc southwards from its bend around Arunachal Pradesh, a different type of tectonic plate interaction is unfolding. In the Himalaya collision zone the more buoyant continental crust is sliding at a shallow angle underneath Tibet, a process known as underplating. In contrast, the Indian tectonic plate along this eastern convergence zone is made up of denser oceanic crust. As a result, along the zone of contact with Asia, this dense plate is subducting or taking a deep dive at a steeper angle into the mantle. 

Another difference apparent from the surface structure is the presence of both vertical and sideways movement of crustal blocks. This occurs because the Indian plate is pressing into Asia at an angle. Oblique convergence results in thrust faulting wherein rocks are moved up along east sloping fault planes. Collision at an angle also causes blocks to slide past each other along strike slip faults.  

The IBR is an older mountain chain formed by the subduction of the Tethyan oceanic crust underneath the Asia plate and the smaller Myanmar plate. This process, initiated in the Late Cretaceous around 100 million  years ago, eventually led to the formation of a complex fold belt by mid Miocene times (15-20 million years ago). 

This fold belt is made up of deep sea sediments and fragments of the Tethyan oceanic plate. These rocks were subjected to very high pressures during mountain building. Sheared and fractured rock units occur in a melange made up of dismembered blocks of varied rock types juxtaposed by faults. Heat and high pressure acting on rocks rich in aluminum, calcium, iron, titanium, and magnesium has resulted in the formation of deposits of exquisite gemstones such as jade, rubies, sapphires, spinel, and peridote. The IBR is studded with precious stones!

By Miocene (~20 million  years ago) the IBR had emerged above sea level as elevated ranges and had started eroding. Sediments shed from these hills were deposited in delta and shallow marine environments of the Bengal Basin to the west. During continued subduction of the Indian plate, between 2-4 million  years ago, this thin skin of the crust made up of about 5 km of sediment was scraped off, faulted, and crumpled up to form the CTFB. Geologists call these scraped off wedges of sediments that form along subduction zones as 'áccretionary prisms'. 

Further to the south, the Andaman Islands is also an accretionary prism formed along the plate junction between India and Asia.

The deformation of the CFTB diminishes from the east to the west. There are two distinct structural domains of this belt. To the east is a more tightly folded belt known as the Eastern Highly Compressed Fold Thrust Zone. Towards the west, is the more open Western Fold Thrust Zone. The emergent part of this fold belt is bounded to the west by the Chittagong Coastal Fault. However, geophysical studies show that the strata below the Bay of Bengal sea bed is also being warped and can be considered part of a westward growing CTFB.

The annotated satellite image below is a close up of the CTFB and the IBR. The black line is the Kaladan Fault separating the two, but even without my annotation, the two terrains have a distinctly different appearance. The older IBR have been more deeply dissected by streams. They have an etched faceted texture. To the west, the younger ranges of the CTFB have a more uniform even texture. 

Finally, I just wanted to put up a structural cross section of the CTFB. The folded and faulted nature of the sedimentary strata is apparent, as is the difference between the more tightly folded eastern zone compared with the more open western domain. Source: Md. Sakawat Hossian et.al. 2022: Lithosphere.

Scientists study terrains like the Chittagong Tripura Fold Belt to understand the mechanical response of the crust to different types of tectonic plate interactions. There is an economic incentive too. The IBR with its precious stone deposits has long been a target of exploration. In the CTFB natural gas seepage has been observed at many places. Geologists are interested in understanding the subsurface structure to target search for hydrocarbon accumulations.

As always, exploring Indian geology from satellite imagery is fun and a great learning experience for me. Stay tuned for more such stories!