We'll Still Be Killing Species After We've Died Out
By Branden Holmes
We humans are guilty of catastrophically modifying the planet. And what is worse is that often it backfires. For example, leaving deforested areas barren and unproductive after a soil's nutrients have been used up by binge agriculture and erosion has set in. Unlike most other species we do not instinctively regulate our own behaviour in order to preserve resources for the future. We simply plunder and pillage, both the land and the sea, and then move to a new area, repeating both the process and our previous mistakes. We might reproduce like any other sexual species, but uniquely we have sophisticated medicine which takes a blanket approach, healing not just the healthy but also the sick and dying. Thereby defying natural selection and fueling our global growth out of all proportion to what is sustainable. Thus we have an insatiable thirst for resources which has driven us to exploit the planet to the maximal degree. And we have now affected it to such an extent that we have caused global climate change.
Beyond climate we have also destroyed much of the biodiversity that existed when our species was in its infancy. A situation that constitutes life's greatest 'geologically sudden depletion since the time of the terrible lizards (i.e. dinosaurs). We have greatly reduced the global population of species. We have hacked away at the Tree of Life, lopping off leaves, twigs and even branches; whole groups of organisms simply gone. Millions of years worth of evolutionary knowledge have disappeared down the drain of extinction. And it's all our fault. Cloning is too little, too late.
There are three key ways in which we reduce biodiversity:
1. We reduce species
The first way in which we reduce biodiversity is by driving species (and subspecies) extinct. This is the most obvious and direct way in which we adversely affect biodiversity, and with which the greater portion of this website is concerned. So I shall say very little else about it here, except to note one point in particular. The loss of subspecies is generally treated with less of a concern than full species. The only exceptions being situations where all recent subspecies of a species go extinct (e.g. Xenicus longipes, the bush wren or mātuhituhi). Or, the subspecies was from a beloved species. While this differential concern is justified to some degree (viz. greater genetic loss), it is also arbitrary in two important ways: (recent) monotypy is accidental, and subspecies are species-in-waiting.
2. We reduce genetic diversity
Natural selection acts upon 'visible' genetic differences between individuals, which by virtue of heritibility bestow differential survival capacity and fecundity upon their biological benefactors. With less genetic diversity, individuals within a population will tend to be more similar, including in their susceptibilities and weaknesses (i.e. their reproductive fate). And individuals in highly inbred populations are basically clones of each other. A disease epidemic which kills one individual is likely to kill most if not all of the others too.
Species which have fallen below the viability threshold may still reproduce, but such efforts are in vain. Their species has been earmarked for obliteration. All that is left is for them to actualise their extinction. A lack of genetic diversity clearly hinders adaptation (see below), but in high levels can affect the individual's ability to survive at all, let alone reproduce. Individuals with severe deformities, one of the common consequences of inbreeding, are almost guaranteed to die without ever having left any descendents. Except, perhaps, members of our own unique genus. Modern research is uncovering just how sophisticated Neanderthal medicine was.
Most loss of genetic diversity isn't anywhere near as immediately consequential for a population as inbreeding since inbreeding is at the extreme end the loss spectrum. With the greater part of the spectrum allowing for the population to stay above the viability threshold. But with the loss of every gene (technically, allele) the population loses adaptive potential. Certain genes may enhance the survivability of individuals, and if they are expunged from the gene pool then the species becomes more likely to go extinct. However, environmental changes will continue long into the future (including human-caused climate change), and hence the loss of genetic diversity now can affect the ability of species to adapt in the future after we've gone.
3. We reduce individuals (and erect reproductive barriers)
A smaller population can have more genetic diversity than a larger population. This is because there is only a weak to strong correlation between the number of (a)sexually reproducing individuals in a population and genetic diversity within that population. In order to make the crucial distinction between individuals and genetic diversity, consider four possible scenarios that cause a genetically diverse population to head towards extincion:
- The individuals are geographically (or otherwise reproductively) isolated from one another.
- There is a seriously skewed male:female sex ratio. At the extreme end, the population can represent only one sex.
- The individuals are well past their sexual prime.
- The individuals are sterile.
Moreover, a population can be afflicted by multiple of these worst case scenarios.
Losing the Knowledge to Survive
Life on Earth has existed for roughly four billion years. During that time it has acquired the knowledge to survive. Knowledge which is now being slowly eroded away by humans as we reduce biodiversity. Life as a whole is incredibly resilient, and may actually have originated in outerspace (panspermia), travelling to Earth only once it had evolved, seeding the planet with DNA. It flowered after the evolutionary innovation that was the eukaryotic cell, and has to date produced a wide variety of bauplans. A series of major body plans to which virtually all species can be ascribed.
Life has suffered several mass extinctions, during which time the Tree of Life was ravaged, like trees after a storm. But life has managed to survive all of these, and in fact has bounced back stronger than previously. Life has accrued the knowledge to survive even the harshest of conditions, due to genetic diversity, which can be seen as an investment in the future of the population. Almost pre-empting catastrophe.
The different genomes (that is, the sum total of the genes) of individuals render them differentially capable of surviving, which is why some individuals leave no descendents while others leave a multitude of viable offspring. In the short term this is a problem for the species as the reason many individuals don't survive and reproduce is because of mutations, many of which are not conducive to reproductive success. Mutations which could have been advantageous to survival, and if inherited would have increased survival rates and hence a population increase (if correlated to greater reproductive success).
But in the long term with a dynamic environment the goal posts change. An individual which would have been considered (relatively) unfit a few thousand generations ago may now find itself the fittest of the lot. What is beneficial is relative to the environment in which the individual has to survive. And with the environment ever-changing one cannot predict which characteristics will give individuals the greatest reproductive advantage. It is lucky that with individuals so different to each other that at least some of them will probably have the genetic makeup to survive a given environmental change, and be able to go on and found a new population.
The term 'environmental change' is a rather ambiguous one though. To take one aspect, the length of time over which the change occurs can be orders of magnitude different. A hailstorm can be considered an environmental change if it is particularly severe. And one which by virtue of it's nature can have the effect of killing individuals randomly in terms of their fitness level. A fitter individual is not necessarily at any greater advantage to be able to survive the hailstorm than other individuals. It can simply be a matter of circumstance. Which individuals get caught out in it and which were already safe or were able to get to safety.
When we reduce the size of a population's gene pool we create the situation whereby the individuals in that population are more similar to each other than previously. Their individual fate is more likely to be similar to that of other individuals. Instead of there being some individual rabbits which by pure coincidence just happened to have genomes which made them resistant to myxamatosis, if we had of reduced their numbers drastically prior to releasing the virus here in Australia in 1950, then with less genetic "options" actualized there would have been a much less chance that some of the individuals would have been able to survive the disease.
The advantages of lost genetic diversity
The loss of genetic diversity isn't all one way, however. In fact there are circumstances under which it is good for a species to lose genetic diversity. The loss of alleles (gene variants) means less competition for any advantageous mutations which may occur, facilitating their spread through the population. Although such scenarios will be extremely rare and not detract significantly from the general rule that the loss of genetic variation in a population is invariably bad. Especially since such advantages can really only be diagnosed in hindsight, after the loss of the diversity. It remains the case that humans are generally bad for biodiversity even if we do on occasion help remedy some tiny proportion of the problem we caused in the first place.
Evolution on a post-human planet
The arising of speculative evolution (or speculative biology) has led to an increased interest in what life will be like in the future, especially in the absence of the restrictive effects of humanity and its destructive habits. Unfortunately, the movement tends to place more emphasis on the artistic/imagination side of things, with biological reality taking a back seat. In actuality no species is capable of an infinite amount of change in a given direction, and certainly not in any direction whatsoever. There is a such thing as phylogenetic constraint.
'Design space', the sum total of combinations of physically possible life forms, is traversed only by the thinnest of thin webs which represent what the philosopher Daniel Dennett has called biologically possible life forms. Life forms which could not merely exist as isolated individuals (what is physically possible), but could actually survive in an environment surrounded by other life forms, competing for resources, and capable of reproduction (whether sexual or asexual).
It seems to me that the greatest consideration that speculative evolutionists need to take into account is the enduring effect that we humans will have on the rest of the planet even long after we are gone. The failure of many species to adapt to future changes to their environment will be the result of humans, caused by lost genetic diversity suffered thousands, if not millions, of years in the past (in the future). Evolution cannot be predicted, but knowing that many ecological niches will be vacated because of us, and the relatively high extinction rate (compared to the background extinction rate), the relative selective pressures upon populations will be consituted quite differently in the actual future than what would have been the case if we had never of evolved in the first place, or at least not become as evolutionarily successful as we did.