Thinking about evolution. They say that who has the benificial mutation passes it on to the next generation, right? We have arms, right? The arm didn’t sprout all at once, right? So, here’s my question. How could an animal that had an extra nub, or a partial arm or a leg, be more likely to survive then it’s competition?
The “partial” part would have served a different purpose.
What JKB said.
EG a species of fish feeds on things found in the mud on the oceans’ floor. They dog in the mud somewhat. This uncovers more food. A 5% stronger fin will get you %5 more food. Eventually, you end up with fish whose fins are strong enough to support them on land for brief periods. They use this ability to climb out of the water and spawn in a place where the eggs will be safer from predators. Eventually, you end up with an amphibious creature with legs.
The planerian is a fine example of "half an appendage". The planarian does not have eyes. It has 2 "eye spots". These are just bundles of light-sensitive nerves on the skin. These eye spots have no lenses of any kind.
For more on evolution, read Stephen Jay Gould. IIRC one of the essays in Ever Since Darwin deals with just the question you raised.
‘Climbing Mount Improbable’ by Richard Dawkins is good for that sort of question too. He also wrote ‘The Blind Watchmaker’ which is more famous.
If you had partial arms or legs and all of your competition had none, don’t you think that’d be superior?
‘irreducible complexity’ is the term, I believe.
It is one of the key debating points of the creationist camp, but relies largely on the arguer not being able (or willing) to posit a scenario in which the system being examined could have developed by small increments.
TalkOrigins has a good article on it here.
Half an arm is incredibly useful when compared to having no arm.
I agree with the previous answers, but let me present it a different way…
There is no such thing as “half an appendage” as you present it. Each species is “complete” in its own right. Since evolution has no goal or end, there is no half-way point. Each species’ anatomy is suited to its current needs and environment. As environment/needs change, natural selection tends to select certain variations on existing structures.
Should giraffes think that humans have “half a neck”?
A flying squirrel’s flying ability is laughable to a hawk, but it save’s the squirrel’s behind when fleeing a non-gliding predator.
Another point to keep in mind is that one structure (arm or whatever) can serve more than one function. Human hands may be best suited for manipulating objects, but they can also be used to aid in swimming (even though we’re land animals) or for communication. If in the far future, humans move to the seas, then our hands may modify accordingly. But our current half-finned hands are not useless.
A mudskipper can’t do much on land…but it can do enough to suit its purposes.