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
Showing posts with label Tasmanian tiger. Show all posts
Showing posts with label Tasmanian tiger. Show all posts
Thursday, January 17, 2019
Tuesday, January 15, 2019
The Tasmanian Tiger, Extinct, or Not?
The uncertain state of affairs
The question of exinction of the Tasmanian tiger (the thylacine) was raised in an interesting article, "Paper Tiger" (Brooke Jarvis, The New Yorker, July 2, 2018, pp. 44-54). According to Jarvis, the last one in captivity died in a Hobart, Tasmania, zoo in 1936. That was twenty years before I arrived and was mentored by Dr. Eric Guiler, an expert on many Tasmanian zoological topics, during my year as a Fulbright scholar at the University of Tasmania.
On several field trips with Dr. Guiler (see endnote), and others by myself, many other animals or their sign, such as tracks or droppings were seen. But such lack of evidence is not very convincing because- such rarities as the Tasmanian devil and the platypus I only saw in zoos, Michigan mammals known to exist but seldom seen outside of zoos include badgers, bobcats, cougars, flying squirrels, mink, and otters.
It seems reasonable that with the bounty put on tigers, to reduce their predation on farm animals, their numbers may have been reduced beyond their ability to survive. But Tasmania has much uninhabited potential tiger habitat where few would have chances to observe them. Some suspect that some may still survive in wild country along the north coast of the Australian mainland. Both are plausible, but seem to require better documented evidence than is presently known.
A few potential factors
In favor of their non-extinction is the possibility those most adapted to avoiding humans may have left some survivors. Anecdotal reports would seem to support this view. But the ease for humans to see what they want to see has caused the shooting death of numerous hunters and a few cattle during the hunting season in Michigan; eye-witness testimony is of questionable reliability.
Extinction is perhaps more likely when numbers are greatly reduced. Habitat reduction is one cause. But small numbers may increase inbreeding and thus increase the likely of mortality due to deleterious genes. Small numbers can also interfere with opportunities for mating and consequent reproduction. Other factors may also operate if social learning is involved, although I have not heard of Tasmanian tigers forming packs.
Extinct or not, it is still an open question. Negative facts and hypotheses such as extinction are very difficult, if not impossible, for a scientist to prove.
endnote: Dr. Guiler is the one standing to the left in the picture in post number 51 -
https://evolutioninsights.blogspot.com/2014/06/the-baleen-whale-tooth.html
Joseph G. Engemann Emeritus Professor of Biology Western Michigan University, Kalamazoo, Michigan January 15, 2019
The question of exinction of the Tasmanian tiger (the thylacine) was raised in an interesting article, "Paper Tiger" (Brooke Jarvis, The New Yorker, July 2, 2018, pp. 44-54). According to Jarvis, the last one in captivity died in a Hobart, Tasmania, zoo in 1936. That was twenty years before I arrived and was mentored by Dr. Eric Guiler, an expert on many Tasmanian zoological topics, during my year as a Fulbright scholar at the University of Tasmania.
On several field trips with Dr. Guiler (see endnote), and others by myself, many other animals or their sign, such as tracks or droppings were seen. But such lack of evidence is not very convincing because- such rarities as the Tasmanian devil and the platypus I only saw in zoos, Michigan mammals known to exist but seldom seen outside of zoos include badgers, bobcats, cougars, flying squirrels, mink, and otters.
It seems reasonable that with the bounty put on tigers, to reduce their predation on farm animals, their numbers may have been reduced beyond their ability to survive. But Tasmania has much uninhabited potential tiger habitat where few would have chances to observe them. Some suspect that some may still survive in wild country along the north coast of the Australian mainland. Both are plausible, but seem to require better documented evidence than is presently known.
A few potential factors
In favor of their non-extinction is the possibility those most adapted to avoiding humans may have left some survivors. Anecdotal reports would seem to support this view. But the ease for humans to see what they want to see has caused the shooting death of numerous hunters and a few cattle during the hunting season in Michigan; eye-witness testimony is of questionable reliability.
Extinction is perhaps more likely when numbers are greatly reduced. Habitat reduction is one cause. But small numbers may increase inbreeding and thus increase the likely of mortality due to deleterious genes. Small numbers can also interfere with opportunities for mating and consequent reproduction. Other factors may also operate if social learning is involved, although I have not heard of Tasmanian tigers forming packs.
Extinct or not, it is still an open question. Negative facts and hypotheses such as extinction are very difficult, if not impossible, for a scientist to prove.
endnote: Dr. Guiler is the one standing to the left in the picture in post number 51 -
https://evolutioninsights.blogspot.com/2014/06/the-baleen-whale-tooth.html
Joseph G. Engemann Emeritus Professor of Biology Western Michigan University, Kalamazoo, Michigan January 15, 2019
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