4.4 Genetic diversity and mass extinctions
It is for this reason that there are now international agreements on the need to work together to retain genetic diversity in all species and, more generally, biological diversity (species and habitat diversity).
From a practical point of view, can you suggest some species where retaining genetic diversity is of particular importance to humankind?
Any species that we exploit for food or other materials is of practical importance to us.
When we cultivate a plant or domesticate an animal, we tend to breed and select for desirable features: heavy yield, fast maturing or low fat to meat ratio, for example. Because only a few ‘good’ plants are selected to be parents of the next generation, this tends to reduce the genetic variability within the species. So work is currently in progress to try and ensure that we do not lose the potential for reintroducing variability. This is done through schemes such as the rare breeds societies and seed banks like the one in Nicaragua for tropical hardwood seeds, and the Kew Gardens Millennium Seed Appeal in the UK.
We have already said that the blame for loss of species from particular habitats cannot always be laid at our door. An example here is the loss of the elm through Dutch elm disease. Regrettably, there are well-documented cases such as the dodo where we, or our ancestors, are clearly to blame. Despite the fact that, as far as we can tell from the fossil record, extinctions are a normal part of evolution, the current rate of extinction is the cause of our concern. The fossil record shows that five major extinction episodes have taken place over the past 500 million years, but the fear is that
… humanity has initiated the sixth great extinction spasm, rushing to eternity a large fraction of our fellow species in a single generation.
(E. O. Wilson, 1992)
You may feel that this is alarmist, given that the Earth recovered from the other five episodes, but consider the time-scales. Previous mass extinctions (the most recent of which put paid to the dinosaurs some 66 million years ago), are presumed to have taken place over several million years, with the recovery to similar levels of diversity taking up to 20 million years. At the present time, extinctions are occurring most rapidly in the tropics, yet, even in Britain it is estimated that around 6 per cent of our species have become extinct during the 20th century.
Why should we worry about this?
Once a species becomes extinct it has gone for ever. We depend on other species for our livelihoods and our health.