. . . e v o l u t i o n e m . . .

 

         taking a fresh look at evolution

The Basic Mechanisms of Evolution

The vestigial muscles around our ears. Click to enlarge

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 anything about the process of evolution.

 

Evolutionary change only occurs in response to a constraint (pressure) or set of constraints. A constraint such as overpopulation or dwindling food resources, for example, may force a portion of a species population to change its habitat. This change, in turn, brings about a new set of constraints as the new habitat or food resource would, perhaps, not be ideally suited for the pioneering species. To exploit the new habitat or food resources, significant adaptation may need to take place. 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 in the new territory.

 

So natural selection acts on those pioneering species and characteristics which better suit the new environment are selected. Those individuals which are incrementally better suited for the new habitat would survive over others and go on to produce offspring with these traits. Ultimately, the greater the pioneering change in the habitat - the more profound the biological change and a new species or even 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, in what I call  the process of 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 is 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 frequently mingling with the original population. This is because any useful adaptations would be dissipated in the larger gene pool. The pioneering population will have a limited gene pool of its own.

 

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 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 predator which may dedicate itself to hunting for fish in the rivers may separate from the same species that hunts 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 the selection, incrementally, for those attributes slightly more advantageous to the ‘new’ habitat - over thousands of successive generations of a 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 an average neck. Okapis live generally in 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 acacia leaves that are above the reach of 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.  

 

 

Rapid Change Evolution through the process of Reversion.

 

In the chromosomes of all animals and plants there must be gene clusters which are dormant and these are made up of old models and formats from ancestral species. From time to time these dormant gene clusters are accidentally activated during the development of the embryo. Possible causes for this re-activation could be pathogenic insult (bacterial or viral corruption), toxins from poisonous plants or even natural ionising radiation.

 

In another section we will discuss how embryological development replays the evolutionary process (a phenomenon first observed by Haeckel). During this complex sequence of gene expression - as the organism develops - it is quite feasible that occasionally old sections of the sequence could ‘lock in’ during the process. This could result in an alternative tissue type for example.

 

In other instances, the sequence may be arrested at the penultimate point in embryo development. This could explain the ‘throwback’ features in some humans which seem to resemble those of apes. An example is the full body pelage (hair covering) which occurs in about one in five billion humans. Many other ‘recent’ throwback features will be discussed in a different section. This premature arrestment of development is an effective way of divesting derived characteristics.

 

A bird is a very derived creature and all its features are configured for a lightweight build and the mechanics of flight. If a bird were to enter a niche were flight was never necessary, it is conceivable that these features could be an impediment, and that those of a reptilian ancestor would be more appropriate. So junk genes keep options open.

 

So profound change in the anatomy or physiology of an animal species can take place within the space of a single generation. A whole new dentition pattern, for instance, could arise - say from homodonty (even toothed) to heterodonty (different toothed), capturing an earlier (ancestral) configuration. Teeth could re-appear in a toothless species. Fibrous scales could appear on an otherwise furry species. Paddles could appear in place of hands or feet.

 

This type of evolutionary adaptation has none of the incremental intermediate stages between one form and another that occur in the other type of adaptation already discussed. Accordingly, this explains the lack of fossil intermediates, a matter which has been puzzling biologists for many years.

 

Evolutionary Stasis

 

There is a common misunderstanding that animals are on some kind of evolutionary time clock where there is a gradual change towards greater complexity by some mutational process. I have even been asked the question, “why haven’t all monkeys evolved into apes?”

If there are no constraints on a species population, then no evolution takes place and the animal or plant will stay the same indefinitely. Therefore many species of animals have remained unaltered for tens, or even hundreds of millions of years. Some bacteria in fact have been around in their current form for billions of years.

 

Mutation only plays a small part in the evolutionary process as it most often causes deleterious effects. Gene cluster damage can often be repaired through out-breeding.

 

Oceans have provided the most constant environments as they have always been in existence and extensive in their range. Therefore one would expect that fewer constraints would have acted on the creatures of the sea. The evolution of marine animals has, indeed, not progressed to the degree of complexity and intelligence as land animals. In fact not much has really happened in the oceans, in terms of evolution, over the last 400 million years. The only really intelligent animals in the oceans are derived from land animals which had become evolutionary advanced on land before entering the oceans. These are the whales and dolphins.

 

Over the last 180 million years the land masses have been drifting over the surface of the planet, changing their latitude and climatic conditions. Land animals have been constantly challenged and these challenges have required intelligent opportunism. Many of those animals which were unable to adapt to changing conditions became extinct. Arduous conditions often naturally select for intelligent behavioural repertoires. Large rivers and deep lakes are perhaps the most consistent habitats on land and this could account for the evolutionary stasis of the crocodile. Crocodiles have always been able to conceal themselves in rivers and avoid predation by predators like Tyrannosaurus rex  and a river habitat offers a constant source of prey animals which need to either cross the river in migration or just drink from it. The crocodile’s ability to ambush animals, drag them under water and drown them, ensures a constant supply of food. They can endure long periods of starvation by living on fat reserves stored in their tails. Crocodiles have been in their present form for at least 180 million years and the break up of the Super-continent Gondwanaland and the drift of its fragments, has dispersed these animals as far afield as Australia on one side of the globe and America on the other.

 

Evolutionary Trimming and Vestigial-ism

 

Each species organism is constantly being ‘tuned’ for survivability. When a species pioneers a new habitat - certain attributes required in the old habitat may be surplus to requirements in the new one.

 

Fish which have chosen to live in dark caves, for example, no longer need functional eyes and so these are reduced to a minimal presence. They become what we call vestigial. Why bother with a pair of eyes when organs are, from an energy point of view, expensive to run?

 

Snakes have vestigial pectoral and pelvic girdles. We have sinus tubes which appear to be the vestiges of a very elaborate olfactory system. We, like other apes have vestigial tails . We also have vestigial third eyelids (nicticating membranes). Functioning third eyelids are found in many birds and reptiles. We also have many vestigial muscles systems like those that can move the scalp or move the ears. We also have an organ in our digestive system known as the appendix and this is the vestige of a large caecum found in other animals. Basically, vestigial organs are comprised of cells which are constantly suppressed in their development.

 

One may ask why do mammals have tails anyway? Tails do serve many purposes. In rats and mice, for example, they are used for counterpoise when walking along and reaching out from narrow branches. Kangaroos use them for balance when hopping along on their back legs. Beavers, otters and platypuses use their tails as paddles when swimming. Some monkey tails are prehensile while others are used for balance or braking when swinging through the trees. Many animals use their tails as whisks for removing flies from their rear quarters.