[quote][quote]Alright, here's the thing about natural selection: The fish isn't going to develop limbs and a broader body if environmental factors favor the slender, streamlined body type.[/quote]Hence, the organism will not evolve to more complicated biology.[/quote] It will if environmental factors favor such a change. [quote][quote]If a fish evolves to the point where it has developed primitive limbs, then environmental factors must be such that these limbs are either beneficial or make no difference.[/quote]Whether you like it or not, limbs in water WILL slow you down and MAKE THE CREATURE SUSCEPTIBLE to be preyed on. [/quote] Key words "in water". I explained this below. [quote][quote]Limbs are going to make a pretty big difference, so they must have some benefit. [/quote]Not always. Have you tried swimming out to open sea?[/quote] It's easier to swim with limbs instead of fins than it is to walk instead of limbs. I've explained this already, the limb-fish would not have evolved in open sea. [quote][quote]So what factors would favor limbs? It's likely that the fish population had to spend brief amounts of time on wet land or in shallow water.[/quote]Slowly or instantly? And if slowly, it would have to be heriditary, but any physical gains for any organism does NOT give the offspring the same physical capabilities. Example? If you married a world-renown weight lifter as you yourself are, your children will not inherit your strength.[/quote] Some organisms are stronger than others simply based on genetics. That strength is hereditary. A champion weight lifter likely has genes that make him good at lifting in addition to his intense training. [quote][quote]This could...[/quote]Now we're being hypothetical?[/quote] We've been hypothetical from the start. Your entire fish analogy is hypothetical. [quote][quote]...be to get between two very close bodies of water that had been separated by drought or some other geographical factor.[/quote]That would no longer be present because it's consistency would have to remain constant throughout the entire life of an entire organism's generation.[/quote] Climate change tends to be very gradual... [quote][quote]It could also be that there was a certain predator that could not reach them in those areas, so the fish would wait in shallow or no water while the creature moved away.[/quote]For it's entire life? It's food is obviously not in the shallows; hence, the predator waits.[/quote] Or, the predator goes after easier prey that can't escape onto land. Thus, the trait of being able to wait on land is selected for. [quote][quote]These fish now need to be able to stay out of the water for certain periods of time without suffocating or dehydrating, and they would need to be able to propel themselves in little to no water.[/quote]Now natural selection has to occur for a malnourished organism among its species. Keep in mind that you are implying that the experience will somehow become heriditary.[/quote] Where did malnourishment come in? I explained in another comment thread that experience can be passed from parent to child through observation. Also, instinct is hereditary. For example, I'm guessing nobody ever taught you to breathe or blink. You just knew how. [quote][quote]In this case, the squat, flat underbelly body type would have been better...[/quote]Which developed by?[/quote] Mutation can make changes. Sexual reproduction can also make changes, as the child organism's genes are changed even if all the parent DNAwas copied perfectly, which wouldn't happen. [quote][quote]...as they could have remain upright while the streamlined fish would fall on their sides. Stronger fins and tails would also have been favored, because the fish would have needed to drag themselves in and out of the water, and their simple back and forth motion wouldn't have been enough. So, the limbs would have been an advantage to this particular population.[/quote]Hypothetically.[/quote] Again, this is a hypothetical discussion. Your fish example is purely hypothetical. Explain how limbs could not be an advantage to a terrestrial or partially terrestrial population. Here's a hint: saying snakes and worms don't have limbs isn't a good example. I'd love to see the worm that can move faster than me.