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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?
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 colonization 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, other aspects of biodiversity are functional diversity, which focus on preserving functional groups of species, or phylogenetic diversity, focusing on taxonomic coverage and uniqueness (evolutionary history), instead of 'overall' biodiversity (see e.g. Flynn et al, 2011 for a discussion of these concepts). Under these perspectives less focus is placed on individual species.
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.
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...
Simply put, evolution through natural selection is a process which occurs naturally. It will occur to any self-duplicating organism with: variations, hereditary traits and threats to propagation (e.g. all life forms we are familiar with). Evolutionary biologists study this, the same way astrophysicists study space, and studying black holes or life forms doesn't indicate anything other then knowing how it works.
Actually, considering that most people in a certain field take interest in it (because logic), it is to be expected that biologists as a whole would rather see life doesn't go extinct then it does - again saying nothing about whether extinctions have or have not happened in the past.
