A Sample Essay From 2009 On Evolutionary Constraint

Evolution is not a forward thinking process, it is a series of continuous and infinitesimal changes to form in response to current environmental stimuli, what provides the selective advantage today, may be an unwanted anachronism in the environment of tomorrow. In this respect animals are greatly functionally constrained by their past, and must make do with alterations to an imperfect system in order to be successful, rather than a purposeful "designed" solution to current selection pressures.

The anachronisms constraining animals can vary greatly in magnitude, from minor inconveniences to massive shifts in body plan. For example, in humans the testicles descend from near the neck, a relic from bygone days swimming around as an ancestral fish-like organism. When the testicles descend to the position occupied in humans they pass through the muscular wall of the abdomen and over the ureter tubules from the kidney. The first issue is a prime cause of hernias, obviously not a desired and purposeful effect, and the second is an obviously inefficient pathway taking the ducts from the testes a long and roundabout route to their destination. While an ungraceful solution this is not a massive constraint on human form.

The above is only one example of a prime cause of constraint. This relates to the difference in morphology between complex tetrapod vertebrates, such as the higher mammals, and the ancestral fish-like organisms that saw the development of circulatory systems. The circulatory needs of a fish like organism are vastly different to those of a land-dwelling tetrapod. This manifests itself in several problems that have presented evolutionary hurdles in the development of terrestriality.

The first problem encountered is one of development. The evolution of viviparity has led to a complex embryonic circulatory system, the whole organism must be able to function long before it can breathe. Because of this the organism is constrained by its developmental pathways from any great alterations from the current system, and also that the adult circulatory system must follow on from the embryonic one. An example of this is the ductus arteriosus in humans which directs blood away from the lungs in the embryo but in the adult is a small vestigial ligamentum arteriosum.

While primitive evolutionary theories had that the developing embryo followed out its own evolutionary pathway within the womb we now know this not to be the case. This does not mean that such claims were baseless however, and Haeckel`s assertion that "ontogeny recapitulates phylogeny" does ring true in many cases, especially where evolutionary constraint is concerned. As such, the chronological timeline of development within the womb sometimes leads to unfortunate and inefficient oddities which are non-beneficial to later stages. Examples of this include the supply of blood to the yolk sac in mammals, the presence of six ancestral aortic arches at the beginning of development , looping over the pharynx. (see Fig. 1). This problem is illustrated well by the case of the recurrent laryngeal nerve, which passes down into the thorax before looping up to innervate the pharynx, this occurs due to a mismatch of developmental progress stemming from an ancestral state where the nerve did indeed take the most direct route to its target, before the evolution of long neck like structures. We can assume that this does not pose too much constraint on form however, citing the giraffe as a prime example.

(Fig 1)

Another example of evolutionary constraint is in regard to the permeability of the capillaries. Because larger animals require a high blood pressure to force flow to the extremeties of the body, and because the whole system is connected, tissues near the heart, and everywhere under high pressure see a drain of capillary fluid into the tissues. This is a direct result of the need for high pressure flow. This problem has been circumvented by the independent evolution of a lymphatic system which can drain the tissues and return the fluid to the blood stream. This is an evolutionary constraint that results from several factors: having a single circulatory system, having only one pressure source in the system, and having large capillary pores due to use of the blood as a transport system for larger molecules as well as oxygen. While this system is passive and valve driven in mammals and birds many organisms have a suite of lymphatic hearts throughout the body for active function: this is a sizable adaptation to overcome a considerable constraint.

The most classic example of this constraint is partially solved by lungfish. The dilemma in question relates to the joint usage of lungs and gills, as the lungs are derived from non gill tissue the blood would mix with the deoxygenated blood from the other tissues, greatly reducing the ppO2, which in turn greatly reduces the efficiency of the circulatory system. In the lungfish a septum has evolved, partially separating the oxygenated and deoxygenated blood across the heart. This solution has been left by the wayside in amphibians, and far from being a limitation, has no effect. As amphibians also gain oxygen from their integument, both systemic and pulmonary veins have a high ppO2. This shows that there are multiple ways around evolutionary constraint, some more unexpected than others. It is important to recognise each solution as equally valid, and not be tempted to see our own adaptations as superior due to anthropocentric beliefs, as this will cloud judgement of the processes and selection pressures involved. Both birds (and crocodiles) and mammals have extended this septum to provide a four chambered heart: the dual circulatory system. The multiple occurrence of this adaptation shows its necessity in the maintenance of high partial pressure of oxygen to heavily respiring tissue.

In conclusion it can be said that while animals are ultimately totally constrained by their past, they have much with which to work. Developmental plasticity can lead to the re-purposing of entire structures over time, and often adaptations will occur that overcome ancestral limitations, this is exemplified by the development of the simple linear system of circulation in integument-only gas exchangers to the complex double circulatory system in mammals, that must also cope with the different demands of embryonic development.