Monday, March 29, 2010

Why Devolution?

On page 51, Coyne makes observations on the 'devolution' of whales; the process in which a potential land dweller returned to an aquatic habitat. At that point, however, it was highly likely that the land animals had sustained a reliable food source and was able to survive effectively. Even with the dying off of aquatic predators, why would natural selection favor the return to the water? Also, how would this 'devolution' progress? Air breathing evolved from lack of oxygen in water and mutations to be able to extract oxygen from the air. What are some adaptations that allow a land dweller that moved into the water to be able to conserve oxygen from air while underwater?


  1. Coyne describes the evolution of mammals to aquatic organisms as "remarkably fast": 10 million years, roughly the time that it took for us to diverge from chimpanzees. What may have prompted this was the opening of a new environment relatively free from predators after the extinction of many aquatic reptilian carnivores around the same time as the extinction of the dinosaurs. Those mammals that roamed the oceans, if only for a brief time, found many sources of energy and nutrition in the relatively predator and competitor free environment of the ocean, increasing their chances of survival and reproduction compared to the terrestrial environment rife with competitors and potential predators. These early organisms may have had a similar mechanism as our modern mousedeer, diving into the ocean at the sight of a bird of prey. Thus, what originally started as a simple increase in survival rate among those individuals who spent more time in the water may have gradually led to more and more skills and traits more suited for long periods of time spent underwater. The mammalian diving reflex, in which an organism greatly decreases blood flow to peripheral appendages and more toward vital organs, for example, allows for more efficient gas exchange and an increased inner pressure in the thoracic cavity to enhance a mammal's survivability in getting away from predators by diving to deeper depths. Traits or reflexes such as these may have gradually built up over time, eventually culminating in what we see today as our modern whale.
    Another factor may have been an attempt at better temperature regulation. In Africa, hippopotami often spend their lives submerged in tropical rivers to cool down from the harsh rays of the sun. Perhaps the ancestors of the modern whale faced a similar predicament in which an extremely hot environment caused organisms to seek ways of releasing excess heat.

  2. As Brian said, moving to water could have provided an environment without predation so that an organism could easily survive and find food if it was capable of spending time underwater. Also, like he said, the mammalian diving reflex provides evidence that land mammals could have occasionally moved to water for short periods of time for several purposes ("Effects of varying thermal and apneic conditions on the human diving reflex"). There are other adaptations on humans that could also provide evidence that hominids were able to spend time underwater, so much so that biologists came up with the aquatic ape hypothesis in 1942 ( These features include little hair, similar to other aquatic mammals (Morgan, Elaine (1997). The Aquatic Ape Hypothesis); the philtrum could possibly be used to block out water(; sebaceous glands water-proof the hair and skin; voluntary breath control to allow diving; and vestigial webbing between the fingers of humans ("Wetlands or aquatic ape? Availability of food resources").Adaptations like these could have arisen as a result of the selection for traits of organisms who could travel in water for food. This 'devolution' likely progressed first as a mammal who had some adaptation that allowed it to move to water was able to survive and reproduce better as a result; as more adaptations built up, this species might have been able to occupy an environmental niche in the aquatic environment or been out-competed in its terrestrial environment; either condition would have favored that species' movement from land to water.

  3. Cetaceans are decendents of land living mammals who live in water. Cetaceans are omnivores, meaning they require consumption of other animals. Having a large body, early ancestors of cetaceans required large consumption of food. And since there were more animals living in the ocean than the terrestrial land, there was a adaptational pull to the ocean. This allowed the ancestors of the cetaceans to evolve in a new environment. Being land mammals, they required oxygen consumption in the air. This allowed the selective advantage of voluntary breathing. That is why most of the cetaceans are voluntary breathers. Also, as it was mentioned by two previous posts, oceans give a larger area for survival. And since most of the terrestrial predators cannot chase after the cetacean ancestors through water, it allowed for more pull towards the ocean. However, it is hard to call this a de-evolution because the cetaceans retain their mammalistic nature. This includes the mammalian diving reflex, as mentioned above. Furthermore, the internal development of the cetaceans, similar to other mammalian development, allows the high chance of offspring. Also there is another evolution that the cetacean adapted. There are dolphins that shows high intelligence for more effective food gathering. Whales with teeth show an aggressive behavior to fight off sharks. And other whales have large body size to fend off the predators.

  4. At the time when devolution began in whales, the land was very populated as evolution from aquatic animals to terrestrial animals was the norm. Since the land was very populated predation was a problem, and as animals moved from water to land, the water would appear a much safer environment. This would be a selective advantage for some animals then to avoid predation. The large area of the ocean also helps the organisms in the sense that they are not limited by boundaries and can escape predation by continuous swimming. Food was another major factor in devolution. With so many animals on land, it would become more beneficial to take advantage of the greater food supply (untapped by the evolved organisms). This devolution process is seen in whales as they may have evolved from animals such as hippo who spend much of the time in the water. One adaptation that may have influenced that final devolution would be the mammalian diving reflex which allows animals to submerge for long lengths of time without returning to the surface for more air. This saves energy because the animal does not have to go up and down as much to replenish its air supply. Such adaptations such as this would aid animals as they evolved into aquatic animals again if evolutionary pressures provide that move as a selective advantage.

    Why Evolution is True