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Extinction, macroevolution, and biodiversity conservation

posted Nov 26, 2017, 1:05 PM by Santiago Claramunt   [ updated Nov 26, 2017, 6:29 PM ]

A recent article in The Washington Post by R. A. Pyron entitled “We don’t need to save endangered species. Extinction is part of evolution” suggests that efforts to save endangered species are misguided because extinction is part of the evolutionary process. The article seems to draw from theory and patterns in evolutionary biology, and macroevolution in particular, that suggest that not only extinction has been pervasive through the history of life but also that extinction may have had an overall positive effect on biodiversity. However, this surprising conclusion is the result of a mix of a superficial handling of fundamental concepts and simplistic logic. Alarmingly, based on such flimsy conceptual grounds, the article concludes that we should not worry about human-induced species extinctions. Below, I quote some of the most misleading statements and explain the flaws contained in them, in particular, those that relate to extinction and macroevolution.



Mass extinctions periodically wipe out up to 95 percent of all species in one fell swoop

In reality, mass extinctions were very rare events in the history of life. There have been only five mass extinctions and there was nothing “periodical” about them. Different mass extinctions were triggered by different causes, many not well understood. Extinction may benefit some surviving lineages by eliminating ecological competitors, but suggesting an overall “positive” role of mass extinctions on biodiversity is erroneous. Without mass extinctions, the diversity of life forms on Earth may have been higher today.

 

Extinction is the engine of evolution, the mechanism by which natural selection prunes the poorly adapted and allows the hardiest to flourish.”

Here Pyron confounds two different levels or organization, two different meanings of “extinction”, and misrepresents how natural selection works. Natural selection works at the population level within species. Some genetic variants prosper and others disappear due to their correlation with characteristics that increase or decrease the organism’s genetic contribution of individuals to the next generations. Increased mortality of the “poorly adapted” is only one mechanism of natural selection but natural selection can also work through differential reproductive output without affecting mortality. A phenomenon at a different level is “species selection”, which is the result of differential rates of speciation and extinction in different lineages due to different species characteristics. Only at this level extinction plays a role in selecting species. However, only when extinction rates are lower than speciation rates, biodiversity flourishes. When extinction rates are higher than speciation rates, biodiversity dwindles. If extinction has a general role in evolution, it is a destructive one; clearly, it is not “the engine of evolution”, as claimed.

 

Species constantly go extinct,…

We know that species extinction is a regular phenomenon. The problem is the rate at which species become extinct. It has been demonstrated that human-induced extinction probabilities are much higher than normal background extinction rates that species experienced in the past. They are comparable to those of mass extinction events.


“…and every species that is alive today will one day follow suit.”

The idea that all species eventually go extinct is only partially true because there are two different meanings of “extinction.” In its most fundamental meaning, extinction is the disappearance of all individuals of a species, the irreversible end of a lineage. This kind of extinction is the one that we fear we are increasing among today’s species. Alternatively, when a species evolves into one or more new forms (a speciation event), the new forms may be called new species and the ancestral species may be declared “extinct”. This kind of extinction is called “pseudoextinction”. Pseudoextinciton is common in paleobiological datasets because ancestral-descendant relationships are difficult to infer, thus absolute extinction and pseudoextinction are hard to distinguish. This problem results in inflated estimates of extinction rates from fossils. Only considering pseudoextinction it is true that all species become “extinct” relatively quickly in geological times. However, this is not the kind of extinction that it is increasing nowadays due to human activities. Pyron confounds these two different meanings of “extinction”, leading to the fallacious and dangerous implication that human-induced extinction is not bad for biodiversity.

In reality, today’s biodiversity is the result of the survival of lineages, lineages that coalesce in common ancestors that can be traced back to the origin of life. In other words, today’s biodiversity is the result of the absence of extinction (in its fundamental meaning) along those lineages over the entire history of life on Earth.

 

There is no such thing as an “endangered species,” except for all species.”

This statement is obviously flawed. First, extinction risks are vastly different for different species. All species are “endangered” only in the trivial sense that all have a probability of extinction greater than zero. But in reality, as explained above, some lineages experience true extinction and other persist for a geologically long time.


The only reason we should conserve biodiversity is for ourselves, to create a stable future for human beings.

This is a personal opinion. There are many other valid reasons to conserve biodiversity.


Conserving a species we have helped to kill off, but on which we are not directly dependent, serves to discharge our own guilt, but little else.”

This statement seems to contradict the previous one. Biosphere “stability” is the result of a myriad of indirect interactions and effects. If we only conserve species on which we are directly dependent, we probably won’t get the desired “stable future.”


…, extinction does not carry moral significance, even when we have caused it.

This is another personal opinion. Many others, including me, believe that driving species to extinction is morally incorrect. With ups and downs, the history of human civilization has seen an ever-increasing awareness of each other and of other living creatures, and with that, the establishment of somewhat universal values and morality (e.g. the declaration of human rights, humane treatment of domestic animals). I believe that it is a matter of time that the irreversible destruction of nature will be universally seen as immoral. I hope it will happen before we do much more damage to our natural world.

 

Yet we are obsessed with reviving the status quo ante.”

The changes that we are imposing on our biosphere are so dramatic and rapid that an obsession with keeping things as before is a way of trying to slow down the accelerated change. Nobody is proposing enforcing a static nature, with no speciation nor extinction, or a static climate forever.


Twenty-one thousand years ago, Boston was under an ice sheet a kilometer thick.

And that is one of the reasons that biodiversity in neighboring forests is much lower than in tropical forests.


“whatever effort we make to maintain the current climate will eventually be overrun by the inexorable forces of space and geology. Our concern, in other words, should not be protecting the animal kingdom, which will be just fine.” 

Except that most of the animal kingdom will face the next glaciation cycle with extremely fragmented habitats and very low population levels, if not extinct already.


This is how evolution proceeds: through extinction. The inevitability of death is the only constant in life, and 99.9 percent of all species that have ever lived, as many as 50 billion, have already gone extinct.”

Again, by confounding levels of organizations and treating different kinds of “extinction” as the same, Pyron implies that there is nothing wrong with high levels of extinction. Again, today’s biodiversity is the product of lineages that experienced zero extinction, not, somehow, the product of high levels of extinction.

The rest of the article just keeps falling into misconceptions, simple-minded reasoning, and statements that are at odds with basic ecological and evolutionary principles. I just wanted to point out some of the most obvious misconceptions related to evolutionary biology and to emphasize the idea that extinction has been a destructive force during the history of life on Earth.

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