Monday, March 8, 2010

Land Mammals Migrating to the Water

On pages 51-52, Jerry Coyne talks about the evolution of whales from land mammals such as Indohyus, Pakicetus, Rodhocetus, and Dorudon, with each new species spending more and more of its lifetime in the water. He discusses one possible reason for why these animals went back to the water in the first place, which was the lack of competition for food from dinosaurs. This way, they could live in an environment that had abundant food and was predator-free. What other explanations are there for why land mammals moved back to the water? Provide examples and use external sources as evidence.


  1. They are not completely sure why some land mammals reverse migrated through some mutations to become aquatic animals once again. However, there are a couple of theories that have been put out there. First, is the one that Jerry Coyne discusses that the predators of the sea went extinct such as the fish-eating mosasaurs, ichthyosaurs, and plesiosaurs. With these animals extinct aquatic animals would not have competition for food sources or many dangerous predators to attack them, (page 51-52). A reason that the mutations might have begun to occur is that the water could have been a safe haven to get away from the predators on land. Creating the beginning of adaptation of the land animals as they tried to take refuge in the water, moving into being aquatic mammals. Dugongs, whales, and dolphins are all animals that moved from sea to land then back to the sea. They may have gone back completely because the sea at the time was open, not heavily occupied, and made it very easy to get water unlike when living on land when water was another thing that was necessary to find to survive (Richard Dawkins). Another things is that Dugongs travel in groups and they are large peaceful animals that do not seem to want much interaction with other species but need more room in order to live in groups, reproduce, and find a good amount of food. The ocean is the perfect place for that since at the time it was not heavily occupied and it takes up much more of the earth's surface then land, providing more room to make into a habitat ( There are also animals like seals, and sea lions who were land animals and went back to the water only at an intermediate level in order to gain the benefits from the water and the land at the same time (Richard Dawkins). Overall the land mammals seemed to move back to the water because it was an opportunity that came along at the time to have an easier life and up their chances at surviving and reproducing.,

  2. The evolution of whales, dolphins and porpoises from terrestrial mammals is an important example of Darwinian macroevolution. Although abundant fossil evidence has elucidated the backwards transition from land to water, causes remain obscure. Jerry Coyne suggests that “one possibility involves the disappearance of dinosaurs… [that would] have competed with aquatic mammals for food” (Coyne, 51).
    Certainly biotic factors such as predation and competition for resources may have limited the habitat selection of terrestrial mammals. Thus, the sudden disappearance of aquatic dinosaurs would likely influence dispersal of terrestrial mammals, that is “the movement of individuals away from their area of origin or high population density”. (C & R, 1152)
    Such natural expansion and adaptation must however have been contingent upon terrestrial conditions. Fossils of ancestral species of whales and dolphins demonstrate rapid anatomical modification and radical changes in habitat and niche selection, changes that took place in about “10 million years” according to Coyne. Fossil evidence of the diversification of cetaceans and whale evolution demonstrates the influence of ‘structure and function’ on natural selection.
    The “evolution of nostrils atop of the skull” (“Transitional forms”), the development of stream-lined bodies “that do not have fur” and an “exceptional hearing…because of a shell-like bone called a melon organ” are examples of such anatomical modification that made the ancestral whale more adapted to the demands of an aquatic environment. The fur-less, stream-lined bodies and the modification of ear bones particularly facilitated movement and prey localization. (“The Evolution of the Whale”)
    The incredibly rapid evolution of the whale from terrestrial mammals suggests that although the absence of competition would have allowed for considerable diversification, the availability of food may not have alone spurred whale evolution since early cetaceans “already lived in shallow freshwater” (Thewissen, 287)
    Some theorists instead suggest that refuge from predators may have influenced whale evolution, as, according to Thewissen, “raoellids used the water as a refuge against danger.” The rapid “descent with modification” suggests that aquatic conditions were considerably more favourable than those of the terrestrial environment in which remained many non-dinosaur predators. Similarly, the gigantesque nature of modern and ancestral whales and the Eocene evolution of whale hearing may reflect the development of anatomical defenses against predation. It is also possible that the size of the whale ancestor may have developed as a result of the surplus food source.


    Why Evolution is True. Jerry Coyne

    Campbell, Neil A. Reece, Jane B.. Biology, AP Edition 8e. 2008.

    "Transitional forms"

    Thewissen, JGM. "From Land to water: the origin of Whales, Dolphins and Porpoises". 2009

    "The Evolution of the Whale".