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The Basic Mechanisms of evolutionary change





There are many current misunderstandings about the process of evolution. Darwin’s often quoted phrase, ‘survival of the fittest’, is regularly interpreted as something to do with the health and vigour of an animal; seen this way, it does not really explain much about the process of evolution.


Evolutionary change mainly occurs in response to a constraint (pressure) or set of constraints. Constraints such as overpopulation or dwindling food resources, for example, may force some members of a species population to change their habitat. This change, in turn, brings about a new set of constraints as the new habitat or food resource would not be, perhaps, ideally suited for the pioneering species. To exploit the new habitat or food resources, significant adaptations may need to take place. For example, the digestive system may need to be adapted to process the new diet, or perhaps new defence tactics may be needed for a different set of predators.


So natural selection acts on those pioneering species and characteristics are selected which better suit the new environment. Those individuals which are incrementally better suited for the new habitat are more likely to survive over others which are not - and would go on to produce offspring with those traits. Ultimately, the greater the pioneering change in the habitat - the more profound the biological change takes place and a new species or even a new type of animal eventually emerges.


For those non-pioneering populations, while conditions remain adequate, no genetic change takes place and they remain the same for tens, or even hundreds of millions of years. This is evolutionary stasis. In this way, the process of evolution obeys Newton’s First Law of Motion; an object in motion or standstill remains in that state unless acted on by a net force.


There will be an inevitable ‘period of discomfort’ for all pioneering species until successful adaptation takes place and this may take hundreds or even thousands of generations.


Adaptation can only really occur in the new pioneering population if there is some kind of barrier which prevents them from mingling with the original population. This is because any useful adaptations gained for the new environment would become diluted by the larger gene pool of the non-pioneering species.


Barriers which allow new species to adapt separately are often between two contrasting ecosystems such as rainforest/savannah or savannah/desert. However, an effective barrier could be simply the difference between tree dwelling and ground dwelling within a forest. The barrier could be a temperature one - as for example - between lowland steamy rainforest and the colder high ground of mountain ranges or volcanoes. Another barrier could be between dry land and rivers/estuaries. In this way, a predatory type of species may specialise in hunting for fish in rivers and may become separated from the original species that hunted on the adjacent savannahs.


So it can be seen that each habitat presents its own set of conditions and demands. Cold, glacial habitats often demand an increase in size of the species, or thickening of insulating skin or fur. Species which pioneer water environments eventually (after thousands of years of selection) take on more fish-like features. Mammals which pioneer tree canopy habitats tend to take on monkey-like features, after considerable adaptation.


We will look at these environmental demands in more detail later and learn that the end product type of a pioneering species, given a habitat type, is predictable.



Incremental Selection and Slow Evolution



This type of evolution is brought about by the incremental change to those attributes which better suit the new habitat - and this process takes place in successive generations of the pioneering species.


This is how giraffes got their long necks. The original pioneering species of the acacia scrub-land was perhaps something like the okapi (Okapia) with a normal neck length. Okapis live around the fringes of rainforests. A group of okapi pioneering the acacia bush-land would have to adapt to compete with other grazers like gazelle and buck. Those with longer legs and longer necks would have the advantage over others in that they would be able to reach for leaves that are above the reach of the other animals. The umbrella tree, Acacia tortilis) is a common tree on the savannahs and its main characteristic is that the foliage is high above the ground.  



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