I'm not an expert by any stretch of the imagination, but I believe the key to evolution is coming to grips with the massive number of generations involved as well the selective pressures of natural selection.
The points that you raise about a fish evoloving legs would make it a 'weaker' fish are completely true. Let's switch it up a bit though and change our viewpoint.
Long long ago, the oceans were full of fish of all varieties. Let's assume for arguments sake that one of these species of fish evoloved into a 'better' predator fish - maybe it had slightly longer fins to make it swim faster or more manuverable. Maybe it had better cardiovascular traits making it a more effective hunter. Maybe it had a slight coloration change that made it easire for it to stalk it's prey.
Each of these changes is a very small and subtle thing that would give it a slight advantage over the others in the area. So what would happen to the other species that could no longer compete effectively?
They would be driven out looking for other sources of food.
So now lets move onto one of these exiled species. They were forced due to their inability to compete in their native environment to move to the shallows where hunting the 'big prey' was no longer an option so they were forced to eke out a living eating shallow water insects that lived both on and around the water.
In addition, they had to still compete with the the more 'effective' fish that would prey on their young making survival difficult. So what do they do? They would start moving to shallower and shallower water to escape to make reproducing more successful. Some of these fish would even start using the tidal pools right on the shoreline to lay their eggs.
Time passes and the waters slowly receed. As they do, these fish which are now ingrained with laying their eggs in these tidal pool continue to try to return to them to lay their eggs. Similarly to salmon jumping waterfalls to get back upstream, these fish would flop their way into the pools to lay their eggs.
Initially it would just be a very short distance - a foot or two - but as the eons passed this distance got further and further. The fish that had stronger fins were able to make the trip easier ensuring that future generation would have stronger fins.
More eons pass. The fish now are able to 'crawl' the distance on their evolved fins and we have the beginning of land walking creatures. This process took literally millions of years and billions if not trillions of fish to accomplish.
It's all about the pressures of your surroundings that drive you to evolove in slow, methodical ways making it slightly easier for you over over the others around you.
As I said, I'm no expert but this scenario is one that I could definately accept as having happened.
Edited by Codegate, : No reason given.
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