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Natural selection is a central tenet of evolution. However, most biologists seem determined to prevent the extinction of the species that have been selected against. Why is this? Preservation of genetic diversity?

I thought evolution was supposed to create diversity not destroy it. What am I missing?

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We're not trying to prevent extinction - species go extinct all the time. We're trying to prevent extinction caused by man. One of the big things that sets us apart from other species is how massively we manipulate the environment we live in. Some of those manipulations have a high destructive potential, and we do not understand ecosystems well enough to confidently say that destruction of species caused by us would go without lasting effects that we'd rather avoid. –  Armatus May 14 '13 at 20:47
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Evolution by natural selection is not progress but adjustment toward a local optimum; the local optimum changes with environment (which includes the biological systems that change to seek a local optimum). One might also ask why librarians and scholars seek to preserve publications that seem to present outdated ideas; existing biological systems contain historical information and formerly isolated systems present convenient laboratories for improving understanding (much of the anti-extinction effort is compensating for artificially increases mixing). –  Paul A. Clayton May 14 '13 at 22:15
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Yes, hence why they're trying to contain invasive species, or at least protect those species they're threatening. Same reason goes for containment of genetically engineered organisms as well. –  Armatus May 14 '13 at 22:20
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"Survival of the fittest" is not a central tenet. Scientists strongly prefer the term "natural selection". Evolution is a very stochastic and opportunistic process, and much of it is not necessarily "progress" in the common sense. Are cows evolutionarily superior to elephants because we breed cows for food but slaughter elephants for their ivory? Or some island species that gets wiped out by a random meteor? Plenty more about it here: en.wikipedia.org/wiki/Survival_of_the_fittest –  Alexander D. Scouras May 15 '13 at 12:11
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@GoodGravy True, but as I said I think the relevant difference is the extent of manipulation, which is vast for humans. Aside from that, I think it boils down to fear of change. –  Armatus May 15 '13 at 19:12
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3 Answers

First, a shorthand definition of Evolution is heritable change and not 'progress' (which is a rather loaded word). Also, evolution is not 'supposed' to do anything, and can lead to both an increase or a decrease in diversity, since (local) diversity is simply the net result of speciation rate and extinction rate (if we exclude migration of new species from other areas). Therefore, I think the question is framed in the wrong way.

Second, there are many reasons why biologists might want to halt species extinctions, as hinted in the other comments and answers. Most obviously, conservation biologists want to prevent man-made extinctions which are due to how we have influenced/modified/destroyed the natural environments. I don't think conservation biologists in general are determined to halt all 'natural' extinction. However, most of the time it is really hard to pinpoint what would be a 'natural' extinction. Either way, the current rates of extinction, which have been estimated to lie 100-1000 times (IUCN, 2007) above background rates (some say it's closer to 10000 times higher), indicate that most extinctions are man-made.

The rationales people use for preserving biodiversity can however differ. Three distinct categories are:

1) For human survival and/or economy, e.g. through ecosystem services or resources we rely on.

2) For human welfare, e.g. recreation or esthetical reasons.

3) Ethical considerations, e.g. inherent value of species or to preserve the unique evolutionary history of the system.

Another way to structure these is to label 1) & 2) as instrumental values and 3) as inherent values. The instrumental values can then be subcategorized as services, goods/resources, information (e.g. genetic to be used in medical research), and esthetical/spiritual values.

Since the direct value of individual species are often (usually) uncertain (irrespectivly of the direct rationales used) the precautionary principle is often invoked to justify species conservation. Also, numerous examples exist of how aspects of biodiversity are related to human values (some recent examples are Garibaldi et al. 2013 and Gamfeldt et al. 2013), such as production or services, which can also be used to justify both conserving biodiversity or individual species. However, there are also ideas of functional diversity which focus more on preserving functional groups of species instead of 'general' biodiversity, i.e. individual species are less important.

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In the first paragraph you portray extinction due to invasive species as illegitimate. In the second paragraph you say there is nothing wrong with 'natural' extinction. It would seem that you are re-defining extinction to exclude extinction caused by invasive species, which is what humans are most like. Then you say that "current rates of extinction" are orders of magnitude larger than "background rates." It would seem that by background rates you mean "without invasive species." So instead of answering the question you have re-defined extinction to avoid the messy parts. –  JoeHobbit May 27 '13 at 16:30
    
I think you have misunderstood some parts of my answer and I have tried to clarify my writing. The first paragraph is only dealing with (in my mind) the rather loaded statements on evolution ('progress' and what evolution is 'supposed' to do). Nothing about invasive species in implied. 'Background rates' is a standard concept referring to "normal" extinction rates pre-modern humans. Nowhere am I redefining extinction, or are implying that invasive species are unimportant or should be excluded (on islands they are one of the main reasons for extinction of native flora/fauna). –  fileunderwater May 27 '13 at 18:00
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Evolution by natural selection requires heritable change but evolution is simply the change in the allele frequency of a population over time. –  KennyPeanuts May 29 '13 at 15:00
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A change in allele frequency is inherited to the next generation, therefore it is heritable. So what is you point? For natural selection to lead to evolution the trait must be heritable, but that is another question. With genetic drift the allele frequency can also change. Either way, the result is a change in allele frequencies which is inherited to the next generation. –  fileunderwater May 29 '13 at 15:15
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@fileunderwater I agree. I misinterpreted what was being said on my first reading. –  KennyPeanuts May 30 '13 at 11:20
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Evolution is simply a change in the allele frequency of a population over time. It is not progressive nor does it have to create diversity. Some of the mechanisms of evolution such as genetic drift and stabilizing selection tend to reduce the genetic diversity of populations.

Furthermore evolution and extinction are two independent processes. Extinction is the complete loss of a unit of genetic diversity (e.g., a species) from the planet. Evolution and extinction are linked because extinction eliminates a particular evolutionary history that cannot be recovered, but the two processes are affected by different factors within the environment.

Clarifying these definitions we can see that both evolution and extinction are value neutral from the perspective of the biosphere.

Humans tend to ascribe value to biodiversity based on aesthetics, ecosystem services, ethics, etc... and thus we may strive to preserve it for these reasons but this is unrelated to the biological function of evolution and extinction.

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I agree with your last two paragraphs, but not the first two. Calling genetic drift a "mechanism of evolution" is inaccurate at best since no new genes are produced. If evolutionary history cannot be recovered, as you say, then how have various traits "re-evolved" multiple times throughout history? It is only the "inferior" combinations of those traits that (hopefully) will never re-emerge. This is where I fundamentally disagree with evolution. Each creature is majestic in its complexity and to merely call it a "mistake" and "inferior" needing to go extinct for life to march forward is mad. –  JoeHobbit May 29 '13 at 21:03
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Genetic drift is a mechanism of evolution - that is a fact. Creation of new alleles and genes is another thing. Also, that traits "re-evole" is generally due to convergent evolution, where similar selective pressures or niche spaces can produce very similar evolutionary outcomes. However, the genetic basis for the reevolved (convergent) traits in different species are (in general) completely different. Therefore, it is not in any way a case of "recovering evolutionary history". –  fileunderwater May 30 '13 at 8:02
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Good question. Evolution will and does continue. Mass extinctions have happened in the past and in fact we are the product and beneficiaries of that process; when niches open up from extinctions, other species just move in and take over, eventually they will adapt to take the place of those who have left. Of course that's a useful perspective since it will happen. A large number of animals and plants are endangered and while the impact overall can't be estimated since we have never counted all the animals and plants, much less microscopic species, an example of our dire straits can be gleaned from the fact that perhaps half of amphibians are endangered,

There are various arguments that there is lost knowledge - when these animals and plants we have never even cataloged disappear we will lose hundreds of millions of years of biological 'experience', of lost knowledge as well as an aesthetic/ethical argument that this is clearly wrong somehow; like a mass murder of animals and plants.

I think that most people who have never seen these animals and plants, understood how wonderful and awe-inspiring they are, how many odd abilities that evolution has gifted them. It seems like a nearly universal experience that those who have seen this are pretty strongly in favor of efforts to save them.

But that's not the question at hand; really won't the ecosphere heal and eventually be okay? Now that we have discovered life so far under the ocean and in the depths of the earth that even nuclear holocaust would probably not touch them. The answer is yes. The ecosphere and life will go on. There are two other useful questions though:

1) will we go on with it? Mass extinctions are the result of changes in the environment - all living things in that environment will sink or swim as it were and humans should not exclude ourselves. Will the future human race deal with loss of oxygen content, the loss of fresh water and the misery of population explosion? If the environment just keeps getting worse, will we do well? We're all assuming that we will figure out how to feed ourselves, find drinking water when both food and water are not really growing to meet our growing population, not to mention the desertification of arable land and the rise in temperature that will impact human geography and economics over the next 100 years or so. If the environment were to stabilize at least we would not be standing on shifting ground. If you are familiar with Malthus' argument, when there is no food or land left to exploit nature will take its course with us - war, famine etc etc. That is also a natural state, which we've managed to slowly eliminate. We may be bringing it back in.

2) How long do we want to wait for the ecosystem to heal? After the mass extinctions of the past, hundreds of millions of years were needed for the species around to re-establish themselves as they are now. Is that really a good trade off for a few decades of industrial-economic expansion?

Until then, the survivors are not necessarily the ones we'd prefer.

I volunteered at a survey of shellfish in the San Franscisco Bay. At one time the shellfish here were so plentiful that the natives piled giant shellmounds the size of small hills from their regular diets. Now because of fertilizer runoff and pollution these shellfish are nearly gone - we didn't see one the entire day looking around the survey area. Instead they nesting shells were covered with a slimy bunch of sea squirts - plant-like animals which can grow quickly enough to absorb the nutrients and robust enough to ignore the pollution and chemicals. I'd probably rather have a bay scallops on my plate rather than a salad of seq squirts. The Bay is swimmable, but is plenty slimy.

Got to sign off now, but that's the outline of an answer...

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There are contradictions in this answer: First we "benefit" from the process of mass extinction, then we "loose knowledge." Some creatures benefit from the extinction of others. However the "loss of knowledge" argument seems hocus pocus since various traits have been shown to have evolved multiple times in different species. While I agree that organisms are majestic etc. I don't see how evolution provides any precedent for preserving diversity. Like you said; the extinction of one species provides room for a superior species to speciate. Human speciation would seem to be the ultimate good. –  JoeHobbit May 27 '13 at 16:41
    
Your comment about nuclear holocaust is hilarious in its reference to morality for which evolution provides no basis. Your questioning human survival is even more comical since you previously said that humans are "too fit" and must be restrained from dominating other species. Your final joke about waiting for the ecosystem to heal is choice. Human speciation would "heal" the ecosystem, no? Wait! Humans aren't speciating! They are becoming inbreds! New question: how to reconcile speciation with inbreeding? –  JoeHobbit May 27 '13 at 16:52
    
I can't find the word 'loose' in my writeup...? Evolution doesn't preserve diversity - but people try to learn from it - if human values mean anything here, which I allow it might not. Still, what goes away, never comes back. Superiority is a different question - the superiority of one species over another is similar to questions of the value of diversity - it means nothing in absolute terms, overall, if we hold the mass extinction as fine, then we must also allow our own extinction or changes to our species as similarly valueless - our actions will change us, probably not for the better. –  shigeta May 28 '13 at 4:01
    
You could just as easily have said that "our actions will change us, probably for the better." –  JoeHobbit May 29 '13 at 20:34
    
Not really. Not all changes are improve human happiness or well being - global warming is like everyone moving south a few hundred miles or more, unless they own a house at low altitudes above sea level - then your whole country might disappear. I see it more as a case of short sighted personal consumption trumping long term planning and rational thought. Not human beings at their best in my opinion. Using relativisitic values arguments are probably not going to change my reasoning in this matter. Sorry. –  shigeta May 29 '13 at 21:02
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