Analogous structures are structures in organisms that are similar in appearance, structure, and/or function but did not originate in a common ancestor. This is in contrast to a homologous structure, which evolutionists claim is the result of descent from a common ancestor with that feature, such as the pentadactyl terminus of the forearm in vertebrates. An example of an analogous structure, agreed on by both creationists and evolutionists, is the wing.
- Creationists assert that since bats, birds, and pterosaurs do not share a common ancestor, their wings must have been created separately for each of them;
- Evolutionists assert that since bats, birds, and pterosaurs do share a common ancestor, but that ancestor did not have wings, then each line developed the structures independently — a hypothetical process known as convergent evolution.
Another extraordinary example of analogous structures are the parallels between placental and marsupial mammals. There exist (or existed) marsupials that are virtually indistinguishable from rabbits, mice, moles, badgers, squirrels, wolves, sabertooth tigers, and other species — except for their radically different reproductive system.
- Evolutionary biologists speculate that placentals and marsupials diverged long before they diverged into the animals listed above — so they assert that marsupials and placentals underwent parallel diversification into ecological niches that demanded similar structures;
- Because creationists believe that placentals and marsupials were designed separately and therefore do not have a common ancestor, they explain the analogous structures simply as similar designs reapplied in different species.
Implications of analogous structures
Analogous structures multiply the difficulties in the common descent model. The extraordinary series of random mutations and survival pressures had to occur not only once -- but independently, in a very similar way, three separate times in the case of wings, and a dozen times in the case of marsupials. In other words, if the development of wings required 100 fortuitous events at a probably of 1/1M each, the development of convergent requires that same highly improbable series of events to reoccur independently and repeatedly.
The explanation is ad hoc in the sense that it was invented in order to explain away a difficulty in a theory, and is not itself supported by experimental evidence. Ad hoc reasoning is widely criticized among philosophers of science -- particularly Thomas Kuhn and Karl Popper -- and similar reasoning has been used to prop up many failing theories, including the Ptolemaic system.