The workings of Evolution

In the natural world, organisms compete with one another to survive. Birds compete for the best nesting spots, males fight for the right to mate, and females defend their territories for raising young. In all these competitions and countless others, the winners are the ones best suited for the existing conditions. Whether it is the first bird to get to the nesting grounds or the most aggressive bird, the largest, strongest male or the sneakiest one, the female with the shortest temper or the one who produce the largest litters, there will be some more fit than other to survive. It is this survival of the fittest that drives evolution on the individual level. Evolution means change but not within individuals rather evolution works at the population level. It is the population that changes over time until the individuals making up the new population are different enough from the original ones to be considered members of a different species.

Environmental changes are some of the main forces that put pressure on populations causing evolution to occur. Continents drift causing climates to change, new species enter an environment bringing changes, existing species go extinct leaving open niches and effecting countless other life cycles, and geological forces bring down mountains and fill in seas. Those populations that can adapt to these changes will thrive while those unable to evolve and take advantage of the changes will die out. We can currently see major environmental changes occurring in the polar regions as the ice sheets shrink and withdraw. For some species this means a loss of habitat, while other will see an increase in the areas available to them. As cold hardy species are pushed into a smaller area their populations will fall due to competition for the available space and food supply. The pressure put on any one of these populations may drive some individuals to seek out new food sources. If there are some individuals in the population that are better adapted to taking advantage of this new food source, then those individuals will survive to reproduce and pass on those new adaptations. Given enough time the population makeup will change and a new species will appear.

Individuals do not evolve populations do, as less fit individuals die out in the competition to survive and their gene lines die with them. While evolution effects species at the population level, it is at the cellular level, in the genes, that changes occur leading to evolution. Say we have a population of small cat sized, plant eating, mammals living on a continent where the climate is a warm, moist, tropical one. The area is covered with tropical forests with plenty of soft, ground level vegetation. There are scattered open areas but hawks hunt there, that can carry off the small individuals. One pressure on the population is that of securing mating rights which in this case means the males fight for the females. The animals show a variation in sizes, from larger individuals that easily win fights over females but who are less able to hide from forest preditors, to smaller specimens who can hide better, but who rarely get mates after fighting. The genetic makeup of the population remains stable because while the larger males will pass on more of their large body size genes, there will be fewer of them around as they are more likely to be preyed upon. What happens when change is introduced to the system? Say continental drift causes a mountain range to be lifted up along the southern edge of the landmass. Gradually over millions of years the warm southery winds are cut off causing the interior of the continent to both dry out and become colder. The tropical forest is replaced with more open grassland and scattered trees. Now the larger animals will be better able to survive as they can exploit the expanding open areas without being attacked by hawks due to their size. As more of the larger individuals survive they will pass on more of their genes until large size is the norm.

The genetic makeup of the individuals did not change because environmental changes took place. The genes for a larger body size already existed before the climate change took place. Genetic variation occurs in all healthy populations. It is the survival pressures placed on the population that determines which of the existing variations will survive and be passed on to mold the next generation and so forth. Man has taken advantage of the pre-existing genetic variations present in the animals he has domesticated to mold them into forms he finds useful or simply pleasant. Through selective breeding we made ourselves the deciding factor as to which genes were picked to survive and which were not, giving rise to animals as diverse as miniature chihuahuas and irish wolf hounds. If two equally diverse animals were found living on different islands they would be considered seperate species even if they could interbreed, just as the different varieties of tortoises from the Galapago's islands have been so classified today. It is best to remember also, that the categorizing of organisms into species is a man-made artifical endeavor with meaning only for our primate brains.

One way to think of evolution is that it is like a manufacturing plant. The same basic raw materials (the genes) are used to make a variety of end products (species) that are offered for sell to the public (the environment). Depending on the raw materials used, different products are produced with different uses and abilities. Which of the products the manufacturers decide to keep producing depends on several factors (environmental pressures) such as cost of production, ease of distribution, consumer sales, and consumer satisfaction after the sale. Just like in the natural world, change through evolution is inevitable.