For centuries scientists have noticed that different organisms may share remarkable similarities. For example: a butterfly have wings for flying just like bats, but the 2 animals are very different in the way they are constructed. Another kind of similarity is structural: The pattern of bones in a bat's wing is similar to that in a porpoise's flipper, though the wing is used for flying and the flipper is used for swimming.
the 1840s British anatomist Richard Owen called the first kind of similarity "analogy," and the second kind "homology". Analogy suggests independent adaptations to external conditions, while homology suggests deeper structural affinities.
Homology was considered a more reliable guide in grouping organisms together in families, orders, classes and phyla. classic examples of homologous structures are the forelimbs of vertebrates (animals with backbones).
Although a bat has wings for flying, a porpoise has flippers for swimming, a horse has legs for running, and a human has hands for grasping, the bone patterns in their forelimbs are similar. Such skeletal similarities, along with other internal affinities such as warm-bloodedness and milk production, justify classifying all these creatures as mammals despite their external differences.
In The Origin of Species Darwin argued that the best explanation for homology is descent with modification.
"If we suppose that an early progenitor-the archetype as it may be called-of all mammals, birds and reptiles, had its limbs constructed on the existing pattern,"
then "the similar framework of bones in the hand of a man, wing of a bat, fin of the porpoise, and leg of the horse... at once explain themselves on the theory of descent with slow and slight modifications."
Darwin considered homology important evidence for evolution, listing it among the facts which "proclaim so plainly, that the innumerable species, genera and families, with which this world is peopled, are all descended, each within its own class or group, from common parents." The link between homology and common descent was so central to Darwin's theory that his followers actually re-defined homology to mean features inherited from a common ancestor. Even after homology was re-defined, however, the Darwinian account remained incomplete without a mechanism to explain why homologous features were so similar in such different organisms. When neo-Darwinism arose in the 1930s and 1940s, it seemed to have a solution to this problem:
Homologous features were attributed to similar genes inherited from a common ancestor.
Modern Darwinists continue to use homology as evidence for their theory. In fact, next to the Darwinian tree of life, homology in vertebrate limbs is probably the most common icon of evolution in biology textbooks.
But the icon conceals two serious problems:
First, if homology is defined as similarity due to common descent, then it is circular reasoning to use it as evidence for common descent.
Second, biologists have known for decades that homologous features are not due to similar genes, so the mechanism that produces them remains unknown.
Next week we will be looking at these 2 problems.
Research the evidence, find the TRUTH and remember.....
Have an Intelligent Faith!!
Parts taken from
Jonathan Wells. Icons of Evolution: Science or Myth? Why Much of What We Teach About Evolution Is Wrong