June 7, 2009

• Convergent Evolution

  Evolution, if you will recall, is the adaption of a creature to it’s environment. If this is true, it would be assumed that different animals in similar enviroments would develop similar traits. This is exactly what we see, and is known as convergent evolution. It is a strong piece of evidence towards evolutionary theory. For example:

  
  All four of these species evolved in seclusion to each other, in areas with very large social insect mounds (Isoptera or Hymenoptera – termites or ants, which in themselves have converged on the trait of a eusocial structure). (Note: the Giant Armadillo is now extinct.) All of these animals have some very distinct traits that they all share: long, tubelike snout; long, sticky, retractible tongue to lap up insects; large curved claws for breaking into mounds. These shared traits are called analogous structures. But despite these similarities, there is substantial evidence to show that they all came from different ancestors. For example, Spiny Anteaters, or Echidnas, are monotremes, meaning they are egg-laying mammals (like platipi). South American anteaters are most closely related to sloths. Aardvarks, which are not shown above, but which share all these features as well, have been shown using molecular evidence that they are most closely related to elephants and sea cows. They all also appear on seperated continents.

  Cetaceans and fish both have fusiform (torpedo-like) shapes, with a tail fluke for locomotion.
  
  There are key differences in the form… the fluke of fishes is held vertically, while cetaceans (whales and dolphins) hold theirs horizontally. But there are plenty of striking differences that make it obvious that the dolphin is more closely related to cats and dogs than it is to the shark that it resembles so much. For example, the dolphin must surface to breath air through lungs, and cannot stay under to take in oxygen dissolved in the water via gills like the shark. The dolphin is endothermic (is warm-blooded), feeds its young via mammary glands, and has hair (albeit very little).

  Bats, birds, pterosaurs and insects all can (or could, in the case of pterosaurs) fly. But all have quite different ways of doing it, and all have extremely different ancestors.
  
  As you can see in this pterosaur wing, the skin is stretched along the first digit of the arm, with the other digits staying very small.
  
  This bird wing shows that the wing is made up of all the bones of the arm, not just the pointer finger.
  
  This bat wing shows that the wing is made up of skin stretched along the arm, between all the digits of the arm, and connecting to the leg.
  
  An insect wing, as I’m sure you all know, has no bones (just like the insect it is attached to). It is merely a membrane stretched between veins.
  And obviously, all these animals are not very closely related. Flying insects (known as pterygotes) are descendants of apterygotes (non-flying insects) like collembolans (springtails). Pterosaurs are descendants of archosaurs (the common ancestors of the dinosaurs as well). Birds are descendants of therapod dinosaurs (dinos that stood on their hind feet). Bats are a bit of a mystery, but it is believed that they are most closely related to lemurs.

  Convergance is seen as a support for evolutionary theory because if we look at intelligent design the same way that humans design, this pattern just doesn’t make sense. If life were designed by humans, we would similar functions to be performed by similar features, much like the GPS used in boats, cars, cell phones and planes. Instead, we see the same task being served by multiple structures.

  Random Biology Fact: Elephants are the only mammals that cannot jump.

  • 9:58 pm
  • 20 Comments