Of course periodic extinction of species has happened throughout the biological lifetime of the planet, it’s nothing new and is to be expected when conditions such as the climate change, or when catastrophic events occur such as a large meteorite impact. An example of this is the “Permian-Triassic extinction” 252 million years ago when 95% of marine species and 70% of land species were wiped out. Even species that were common and had a long history of success such as the Trilobites were not insured for this event.
They had existed for about 270 million years, but whatever happened 252 million years ago was not survivable for any of the Trilobites. It makes picking a winner species in the game of life more like a lottery. Looked at in the long view these events are not without benefit since the new ecological niches available after the departure of their previous inhabitants can encourage adaptation to incomers who in turn can evolve independently and form new species. In this extreme case, the “Permian-Triassic extinction”, recovery was slow possibly because so much biodiversity was lost.
Rocks from a time when diversity was in fashion: the Pillow Lavas of Anglesey mark the end of the Pre-Cambrian and the start of the Cambrian ‘Explosion’ in Biodiversity'
It’s not so simple for us though, we are “knowing” animals and live for a brief time in relation to the lifetime of the planet. Our cultures have often developed our sense of responsibility towards other species, but it is obviously not possible for us to prevent natural events of the death and birth of species although we feel intimately bound in the process, a good example being the Passenger Pigeon. This animal was endemic to North America and existed in immense numbers, a flock seen in 1866 took 14 hours to pass overhead, being 300 miles long and 1 mile wide and this was just one flock! By 1914 the bird was extinct. The usual suspects seem to have done for the species: its habitat had changed through increased competition and predation had increased, both brought about by our own species.
It is obviously not possible is to conserve everything so decisions on what to conserve are important. Such decisions often are based on subjective criteria, Pandas come before the Poison Dart Frog and the Florida perforate reindeer lichen can happily go bust. It is therefore important to identify criteria for conservation which are as objective as possible, this in turn poses the question of how do we select the criteria themselves. One possible criterion for conservation would be the use to humans. This has historical pedigree, for instance trees for construction and wheat for food, a plant which probably would not survive without cultivation and human aided distribution because of large size of its seeds.
There are other uses of course, medicines: willow bark as a source of aspirin has been used for centuries, but new drugs are being found in extracts from plants such as the Madagascar periwinkle (Catharanthus roseus) used to treat childhood leukaemia and Hodgkin’s disease. Then there is conserving as an insurance policy, preserve species so that if conditions on earth alter we will have suitable species, or at least their DNA, to help deal with the new conditions.
We may need to decide on how unique a particular species is before attempting its conservation. Looking at the morphology does not yield the answer, Peacocks look pretty amazing, but in reality are one species of many related birds in the Galliformes order which contains the Turkey and the Chicken. There are methods available to determine how unique and therefore valuable certain species are, such methods rely on cladistic relationships between species as determined by the similarity of their DNA. What this means is drawing a special graph (called a Cladogram) to display the species, the lines between the species indicate evolutionary relationship and the length of the line indicates the amount of genetic change:
The choice of preserving D, for example is poor because it shows little genetic uniqueness from E. The more unique species is B, so given the choice of preserving B or D, B must win.
Now try these questions
1. Summarise the advantages and disadvantages of major extinction events.
2. Argue the case that we may be witnessing a major extinction event, what are the driving factors for extinctions, should we do anything about it; is it feasible to resist the changes?
3. What effect does a major extinction have at the individual, species and phylum levels?
4. What can we learn about evolution from studying very ancient ecosystems such as the Burgess Shale?
5. Summarise the use of cladistics in the study of evolution. How can this study be combined with analysis of DNA sequences to decide on the uniqueness of species?
6. Why should organisms be conserved? What additional problems are there, ecological, political and social which make the preservation of species particularly difficult?
7. Make the case for the conservation of an organism of your choice.
Reading around the subject
The “Cambrian Explosion”, that is in diversity:
http://en.wikipedia.org/wiki/Cambrian_explosion
The importance of biological data to land developers and planners http://link.springer.com/article/10.1023/A%3A1008165311026
Some examples of syllabus content for which the above exercise is relevant:
John Giles
