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Survival of the fittest?

Photo: cosmosmagazine.com

Hang on a minute, let’s dissect that often misquoted statement. Coined by Herbert Spencer in 1864 it often causes confusion when talking about one process of evolution: Natural selection.

To be reproductively successful an organism doesn’t necessarily have to be brilliant at surviving, the term survival of the fittest wrongly implies “the survival of those best adapted for surviving” or that “only the fittest individuals will prevail” which unfortunately has been used by countless governments over the years as a pseudoscientific guise to justify faulty social policies in favour of profit for the fat cats or other vested interests which would profit from social injustice and environmental degradation.

A more accurate term if you wish would be: “Persistence of the fitter traits in a population”. This is more correct because we are talking about evolutionary fitness, what I mean by this is the number of offspring an organism produces.

Selection acts upon genes within individuals who act as survival machines for those genes, genes which result in their respective survival machines producing larger amounts of offspring and therefore greater replicates of the genes they possess persist in the population. So in this respect genes are selfish, selection acts to promote genes which boost fitness and remove genes which reduce fitness; what is good for the gene is not always good for the organism.

A study of wild Soay sheep on St Kilda showed that individuals with a stronger immune system may have been capable of living for up to 15 years but produced fewer offspring than those with weaker immune defences and only lived for 3-4 years.  This is not a conscious decision by the sheep or the forces of evolution but the trait for a weaker immune system is in fact fitter than that for a strong immune system.  Mutation is the source of variation within a population; they occur randomly, most are either inert or detrimental however under the correct environmental conditions they can be favourable.  In the Soay sheep population a mutation has arisen to suppress the immune system, under the conditions present on St Kilda this has increased how many offspring a sheep can have meaning that the relative frequency of the immunosuppressant gene increases in the population. In this example the trait is thought to be favoured by the increasingly mild winters caused by climate change however disease or some other random event could reverse the trend for smaller sheep with weaker immune systems.

Selection is one of four evolutionary forces at work the other three are genetic drift, gene flow and mutation. Genetic drift describes random events in a randomly mating population which affect proportional frequency of traits within a population in the absence of selection alleles will drift to fixation or extinction. These events include natural disasters or failed reproduction in the form of one generation producing all males in the next generation. In small populations genetic drift is a potent force and acts to reduce diversity, our species is thought to have experienced a ‘bottleneck event’ where the population and therefore genetic diversity is severely reduced by a dramatic environmental change. New traits arise through mutation, these traits are then acted upon by selection, those that are favourable are maintained in the population and those which aren’t are driven to extinction. Gene flow however mixes things up a bit, gene flow is simply the movement of genes within a population. If it is high it can override the effects of selection and genetic drift and maintain diversity within a population. Where gene flow is restricted inbreeding depression results, drift causes deleterious mutations to accumulate within the population and selection drives the population to extinction.

The natural world is currently in crisis, due to our actions species are becoming extinct at a rate 1000 times faster than normal under average conditions. Earth is losing species like a burst artery loses blood. As the human population grows (expected to hit 7 billion sometime in 2012) we are leaving less and less space for nature. This means natural populations of plants, animals and fungi are being fragmented into smaller isolated populations in pockets of habitat unable to support them in the long term. With isolation comes restricted gene flow leading to inbreeding depression and eventual extinction. There are countless species with such small populations that they are likely to go extinct without intensive breeding programmes, the most famous of which being the panda. Small populations with less variation are less able to deal with changing conditions so there is strength in higher genetic diversity. Conservationists main aim is to preserve enough genetic diversity to allow natural populations to respond to changing environments through evolution. The biggest single threat to biodiversity is habitat destruction and fragmentation.

So why should we protect biodiversity? Well on a logical level, if we’re not careful, if we allow enough species to go extinct the ecosystems we take for granted to provide us with freshwater, clean air, pollination, food and a stable climate among many other services (which we take for granted) will collapse leaving us to clean up the mess or face a population crash ourselves. On a moral level I think Colin Tudge puts it quite nicely in his book the variety of life: ‘For my part I want simply to suggest that it is a privilege to be conscious in this universe, to live on this planet and to share it with so many goodly creatures. we can destroy them easily enough but with somewhat greater effort we could save them. It has to be worth doing. I cannot demonstrate that it has to be done and neither can anyone else. But it is hard to think of anything more worthwhile’.

Should we lead ourselves to possible extinction, taking countless other species with us, simply because we could not be bothered to make space for natural systems to function properly?

For more info go to www.iucn.org, to help out on a local level contact project V for volunteering placements, Dr Martyn Stenning in the lifesciences department or come along to the environment society meetings on Wednesdays at 1pm in room 126 Falmer house.

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