In biology, homology is a similarity between different creatures supposedly due to common descent (evolution). Similarities due to other reasons are known as convergent evolution. Homology is one of the main pieces of evidence used for evolution. However, such similarities can also usually be explained due to the different creatures having the same Designer.
On the basis of evolutionary theory, one can predict that creatures descended from a common ancestor would share characteristics that they have inherited from that common ancestor. In fact it is this similarity which was much of the basis for Darwin's views on evolution. So evidence of widespread similarity which cannot be explained by common descent would not be predicted, and would be inconsistent with the theory.
Similarity from design
However, evidence that is consistent with more than one theory cannot be used to favour one theory over another. The creationary view is that the Creator would re-use the same design in different creatures. There are several reasons for this, and these can be illustrated by an analogy with the design of cars (US: automobiles).
An analogy from cars
Similarity due to similar purpose
All three vehicles will have some things in common due to them all having the same purpose, that of conveying people quickly between two locations. That is, they will all have wheels, a motor, seats, and equipment to control the car, such as a steering mechanism, accelerator, and brakes.
So the three cars have these things in common, even though they didn't all have the same designer.
Similarity due to similar environment and natural selection
There is nothing inherent about the cars' purpose that requires them to use similar fuel. Indeed, cars have been developed that ran on steam and others that run on electricity. However, cars require a network of refuelling points, and there already exists a network of places to refill a car with petrol (US: gasoline). A car using a different fuel will not be readily selected by motorists because of the difficulty of refuelling.
Again, the three cars have this in common, even though they didn't all have the same designer.
Cars have also been developed that run on ethanol. These could have a similar problem, but in Brazil a network of ethanol-refuelling points has been established. Therefore, in this environment (Brazil), ethanol-powered cars are selected, but this is less likely in other environments.
Similarity due to a common designer
There is also nothing inherent about a car's purpose that requires them to all have the engine in the same place, or for the engine to be cooled in the same way. The Corolla, like most cars, has the engine in the front, and the engine is cooled by liquid. The Beetle and the early Porsche have the engine in the rear, and the engines are cooled by air.
In this case, the similarity is due to both the Beetle and the Porsche having a common designer (Ferdinand Porsche).
Analogy to living things
Similar to the cars described above, living things can be similar for several different reasons that have nothing to do with common descent (evolution).
Similarity of design in living things can be due to them having a similar purpose, due to a similar environment and natural selection, and due to them having a common designer.
It is therefore clear that although evolution predicts similarity of design, and this prediction is borne out in many cases, this in itself is not conclusive, or even strong, evidence for evolution.
Additionally, similarity that is not due to common descent is more consistent with a common designer than with evolution.
The pentadactyl limb
An example of this is the pentadactyl limb of frogs and humans. The pentadactyl (five-fingered) limb is, for evolution, a classic example of homology. Yet it can be shown that this is not likely due to common descent.
If the five-digit hand of humans and frogs was due to being inherited from a common ancestor, that similarity must be in the form of common inherited genetic information. Yet the genetic information is not the same. This is clear from the fact that the five-fingered hand develops in the embryo in completely different ways. In frogs, the fingers are formed by buds which grow outward, whilst in humans the fingers are formed by programmed cell death dividing the tip of the limb into five regions that become the fingers.
This dissimilarity of formation producing a similarity of structure is entirely consistent with a designer using a different design to achieve a similar result, but is inconsistent with common descent.
- ↑ Anatomical Homology (NCSE) "Homology is similarity in structure and position that occurs because a trait occurred in a common ancestor. If the similarity is not due to common ancestry, the structures would not be homologous."
- ↑ Similarities and differences: understanding homology and analogy (University of California, Berkely) "homology: similarity inherited from a common ancestor. analogy: similarity due to convergent evolution"