Thursday, January 17, 2019

Variable Rates of Extinction and Evolution

A New Theory?

While composing the previous post about the status of survival/extinction of the Tasmanian tiger I thought of the impact of impending extinction on rates of evolution.  It may be discussed in literature I am not familiar with, but it was new to me; perhaps the first new idea  about evolution that I have had in ten years and certainly the first since I turned ninety last year.😊

The theory of "punctuated equilibrium" in rate of evolution proposed by Gould has largely been abandoned as variation caused by probable artifacts of sampling problems faced by paleontologists.  The assumption of a uniform rate underlying known variability around such a rate has been accepted by many molecular phylogeny researchers and caused the errors in our understanding of the relationship of major phyla to one another as shown by the sixth post on this blog site posted 5/31/2013.

The Tasmanian tiger

Maybe it is extinct, maybe it is surviving in remote areas of Tasmania and/or continental Australia.  But the bounty on them in Tasmania reduced them to such a small population that there has not been accepted evidence of them in the past few dozen years.  Could some have learned to avoid inhabited areas or developed excellent ways of avoiding human contact?  Not doing so could have eliminated those lacking such skills during the time the bounty was effective.

Genetic diversity

A reduced population makes it likely that some genetic diversity is lost and thus adaptability and the chance of surviving becomes more limited, possibly causing further loss of genetic diversity and ultimate extinction.  Extinction from such an event is not as dramatic as extinction from natural disasters such as asteroid impact, disease, introduced predators, climate change, and evolution of better competitors.

The ecology of extinction and expansion of range

There are many causes of both factors.  In a stable environment competition is a major factor as some balance is reached if the less well adapted species can specialize in utilizing a part of its environmental resource of food and housing more effectively or, alternatively, utilize a greater range of resources.  Migration and other factors may also be part of the equation.

Either being a specialist or a generalist can be an effective way of competing for survival,  The generalist probably will yield a greater diversity of new specialists following a major extinction event.  The best specialists may also do well and dominate in the same post extinction time.  If overspecialization occurs a species may become extinct earlier during a major extinction episode.

Species differ in their habitat requirements.  Large predators typically require large suitable areas.  Fragmented habitat may make life difficult for many species although connecting corridors of suitable habitat can help survival.  The prey species of predators also have their special requirements.  For the Tasmanian tiger, life may have been difficult to sustain because many of the medium sized marsupials of forests are adapted to tree-dwelling.  Kangaroos and larger wallabies are more likely be in more sparsely forested habitats.

Human activities would seem to be generally detrimental to survival for many species.  Besides our causing reduction of favorable habitat, pollution, introduction of pest species, and our general lack of awareness of the damage we do should put us in awe of the survivors.  The good that we do by providing some food at critical times can be canceled by the danger feeding stations provide for disease transmission as well as sites attracting predators.  The natural spacing of species and their survival is more likely to be enhanced by well-designed land conservancy programs.

In retrospect, I do not have a new theory, it is just awareness of the complexity and breadth of application of existing ones.

Joseph Engemann    Kalamazoo, Michigan     January 18, 2019 

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