SCIENCE: EXTINCTIONS AND EVOLUTION

EXTINCTIONS

The Sixth Extinction by Elizabeth Kolbert was the subject of an interview with her by Brad Plumer of the The Washington Post published in our local paper on March 2, 2014.  I have not read the book, but the interview was quite informative.  The increasing rates of extinction caused by various impacts human populations are having on the survival of other species may be leading to the sixth major extinction event.

Previous major extinction events presumably were caused by major volcanic eruption episodes, asteroid impact, and possibly widespread glaciation events; all likely associated with climatic changes.  Her discussion of the concept of "catastrophism" suggested by Cuvier was dismissed over a hundred years before the 1980's suggested in her interview.  Catastrophism was just one of several theories for the cause of evolution and/or change in the fossil record already known at the time.  Darwin's documentation of the theory of natural selection was rapidly accepted by many scientists, even before the role of the genes was clarified, as an explanation for the cause of evolution.

From the interview I suspect she does a good job of discussing the causes of increased extinction rates thought to be a harbinger of a presumed coming sixth extinction.  The fact that there have probably been many more major extinction events than the five indicated does not detract from the main thesis of her book of a possible coming sixth extinction.

In chapter eight, extinction's role in evolution, of an unpublished 2010 manuscript (Evolution Insights) I note that, after discussing the end of the dinosaurs, "Other mass extinction events have been documented marking the end of the Devonian, Permian, and other periods.  The transitions from the end of one unit of geological time to another usually show an unconformity of rocks, perhaps caused by the physical disruption due to a major collision.  The demise of species may be more severe after a prolonged period having no disruption.  The interlude may enable many to become so specialized to constant conditions that they cannot survive major disruption of those conditions."

"After the extinction event, the survivors again specialize into additional new species that fill the vacant jobs of top predators and many smaller special niches  The survivors that provided the ancestral material for new species were likely to have been generalists or specialized in some way that may have contributed to their survival.  As food supplies become diminished, small animals may be favored over large animals in finding adequate food to survive."

NUMBERS OF SPECIES

Once the terrestrial flora and fauna was well established in the Paleozoic, the numbers of species are thought to have peaked at somewhat comparable levels prior to major extinction events.  Once the approximate maximum was reached following an extinction event, the numbers of species may have hovered near the maximum as old species die off in competition with new and better adapted species.  Others have speculated, on limited available data, that the average species survives for about a million years, with great variability of extremes in both directions.  The fossil record would seem to support the idea that there have been hundreds of time more species that have lived on earth than the maximum at any one time.


THE ORIGIN OF SEX

A time of more frequent asteroid bombardment during Precambrian times played a crucial role in evolution.  The role the evolution of the Pogonophora from early annelid stock has been discussed earlier in this blog site.  The possible role of the same type of rigors had an even earlier role in evolution of major aspects of the origin of sex. Found in the manuscript mentioned above, are the following quotations from the chapter eight section titled, "The early roots of sex, fusion for survival".

"The starvation of protists during the early life extinction events meant degrowth to eventual fatal levels if they could not eat another protist or, better yet, fuse with another to make a viable mass.  Perhaps the reductions during degrowth resulted in a reduced genome lacking essential genes.  Or continued reduction after fusion made the survival of only the normal single gene genome an outcome that over time developed the needed stable genetic controls."

"The things that chance genetic changes did to help the process occur over millennia may have been rapidly selected as those not fusing died or were eaten before the resumption of hospitable conditions.  Eventually the process was fine tuned as it became the gamete union restoring the appropriate number of pairs of chromosomes as we see in sexual reproduction today.  The reduction division of meiosis may have been selected for maintaining the proper gene numbers in fusion of ancestors over a billion years ago."

Supporting evidence from simple organisms included fusion of amoeboid stages in cellular slime molds, suctorian ciliate's abilities to directly incorporate the protoplasm of prey species of ciliates into their own protoplasm without formation of food vacuoles, and a presumed lack of immune rejection in simple organisms, evidenced by the symbiotic origin of mitochondria and cilia.

Extinctions events may also have caused release of abundant nutrients from dead and decaying organisms.  The nutrients could be in solution as absorbable compounds most organisms could utilize, or as particulate matter available to filter-feeders.  Species able to utilize those mechanisms effectively may not have been the ones fusing in the early development of sex, the stage of the process discussed occurred a billion or more years ago.

EXTINCTION PEAKS

The most outstanding research I am familiar with about the history of prior extinctions is:
Raup, David M., and J. John Sepkoski, Jr.  1986. Periodic extinction of families and genera.  Science, 300:1734-1737.
I measured their graph of extinction rate peaks and found that a regression line of the peaks reached 100% for extinction rates at the end of the Precambrian.  [I know that percentages aren't appropriate in calculating regressions, but I thought the use of the number as a proxy for actual numbers was informative.]  Far greater species numbers presumably go to zero in each genus or family that goes extinct.  Their data only cover groups that fossilize and are found in rocks of known ages.


Joseph G. Engemann           March 6, 2014