Wednesday, April 7, 2010

Why Do Fish Still Exist?

Coyne frequently references organisms evolving to better their chances of survival. A prime example of this is the evolution of fish into reptiles and reptiles into mammals, but if environmental stress variables forced water-based species to evolve to a more land-based life, why do fish even exist anymore? Wouldn't they have all evolved and followed a similar path of evolution as their reptilian relatives? In addition, reptiles coexist with fish in many areas where environmental stress variables once forced fish to evolve into reptiles to survive. If, going by the definition of evolution, a species' inability to evolve will lead to its death, then why is it that fish managed to survive stress variables in the same location that some of them actually evolved into reptiles?


  1. Some may say that evolution "directs" species toward better survival, but evolution is a mindless driving force. Organisms only adapt to fit new environments when the environment itself changes. Whereas in fish, a variation in population met had a better chance of survival when it could either breathe air, live in freshwater, and eventually move onto a vast ecosystem where the only predator or threat was the lack of water. Evolution isn't a linear path, where fish become amphibians, then reptiles, then mammals. Evolution is adaptation to survive in a given environment, so what may look like a step back (humans cultivating flowers for "our" use, or the presence of whales) is just the species surviving the best way that nature can provide. While life on land was a significant advantage to move there, it doesn't necessarily mean that ALL fish would evolve to live on land. Even if it did occur, the lack of predators and danger in the water would lead to an incentive for these creatures to move back to the water. For example, mammals similar to deer evolved to become whales due to advantageous environment provided too them. Another example would be the gigantic mammals that appeared after the extinction of the dinosaurs. Without the threat of dinosaurs, mammals had an evolutionary advantage to grow much bigger to fit their environment. So, knowing this, even if the entire population of fish species were to evolve to land due to overwhelming survival advantage of land, and disadvantage of water, eventually animals would take over the hostile environment if they eventually overcame that single obstacle. Just like how pyrococcus furiosus didn't initially evolve there, it moved into an environment that it was able to survive in due to lack of predators. Even today, scientists are still astounded to find living creatures miles beneath the earth, in the hottest geysers, and in the coldest regions of Antarctica. These organisms may not have developed there, but they moved there b/c of the evolutionary advantage of survival. Looking back, this question is merely another iteration of the common "intelligent" design argument of "why are there still apes if humans evolved from them?". Well, even if this were true (humans evolved from our ape-like ancestors, which we were closely related to), variation in population or separation of a population due to environment would lead to the species adapting to different needs, such as hunting animals on a savanna instead of foraging fruit in a tropical forest.


    Biology book

  2. As Jerry Coyne says somewhere in his book, a species does not HAVE to evolve. Oftentimes, species are very suited to their environments over long periods of time and do not evolve at all.
    There are several reasons for why fish are still around. First of all, fish in deep seas or environments that rarely change did not have pressures put on them to adapt and evolve. Those fish had no reason to become amphibians or reptiles and therefore didn't.
    Also, "evolution...need not always proceed smoothly, or at an even pace" (p 30). It is likely that many fish experienced microevolution to small changes in their environments. They did not live on coastal environments, in which amphibians' qualities would have been advantageous, and therefore the pressures on them did not drive them to evolve into amphibians and reptiles.
    In short, then, all fish haven't evolved because they were suited enough to their habitats. Only the fish in shallow/coastal waters had selective advantages in amphibian characteristics.

    There are several reasons why fish can coexist with reptiles.
    Although they live in the same place, they don't necessarily have the same ecological niche - "all of the physical, chemical and biological conditions required by a species for survival, growth and reproduction" ( In an area that has both reptiles and fish, such as a pond, the reptile does not have the same conditions for survival as the fish. The fish is able to feed, osmoregulate, and reproduce in the water while the reptile does so on land.
    The reptile/amphibian could have evolved there in the first place because there were too many fish, and it was becoming very difficult to occupy the same ecological niche. So in order to find nutrients, fish that were able to venture outside of the water (or break through the limits of the fishes' niche in any other way) would have had a selective advantage and would have evolved. Their coexistence can be the product of evolution. Even if the pond's circumstances originally could have fostered the evolution of amphibians and the death of fish, over time the fish adapted and evolved to be able to occupy the same habitat again.

  3. Simply because reptiles evolved from fish does not mean that fish would not exist today. Because evolution is more of a "branching" process, this would mean that only one out of the several branches of the different types of fishes may have evolved into present day reptiles. However, the other branches of fish are still alive today. According to Campbell, there are some fish, the Dipnoi that are freshwater fish with both lungs and gills. This seems to be one of the fish that may be a good representation of how reptiles evolved from fish. One cause for evolution may have been an environmental pressure that jeopardized the fish's ability to respire. From such an environmental pressure, the fish had to adapt, and those who did not have gills with adequate surface area, or an operculum, did not survive and reproduce. The operculum is a cover over the gills that may have formed in order to increase the water that pumps through the fish's mouth for gas exchange. The operculum also allows the fish to save energy by allowing it to continue to ventilate even when it is not swimming. Sharks on the other hand, do not have this operculum and thus must continue swimming all day in order to breath. Different fish have slightly different forms of gills. Crayfish and lobsters have appendages, similar to an operculum, over their gills that help push water over them in otrder to allow for increased gas exchange. They may have developed these appendages due to an environmental pressure. While this was their adaptation, other fishes, such as Dipnoi, may have developed lungs so that they could respire both on land and under water, eventually leading to presence of amphibians. The gills of the sea star are different from others in that they are small tube-like projections on their skin. Reptiles on the other hand, have lungs, similar to the lungfish, that allow them to breathe. Some reptiles have more permeable skin that allow for more efficient gas exchange. All of the different kinds of gills seen in fish make it easier to visualize the branching formation of all the different types of fish that led to the appearance of reptiles.

    Campbell Book

  4. Murad does pose his question politely and intelligently, but, as Tianyu deftly notes, he is merely parroting the same old Intelligent Design memes of appeal to common sense and argument from ignorance. Science specifically addresses phenomena which challenge our intuition about the natural world. It's easy to take facile jibes at a complex subject like Evolution. And it's sure to draw applause from the peanut gallery of popular consensus. Do Creationists really derive satisfaction from such shallow one dimensional reasoning? Kudos Tianyu, Nanna and Julia in your succinct evisceration of this thought-terminating cliché.