The idea of evolutionary progress is a slippery slope in the philosophy of biology. Not only is there controversy concerning whether or not there is progress at all in evolution, but if there is in fact progress, what form does it take? Generally, if a biologist is to accept the idea of progress in evolution they must do so on the basis of instrumental ideals. That is, they must base their idea of progress only on factors relevant to evolution (such as fecundity, adaptability, survivability, etc), and not on such noninstrumental factors such has intrinsic value or morality. That being said, biologists must, at the outset, deny any hitherto ideas about human dignity and intrinsic worth. Such ideas are taboo if progress is to be attributed to evolution. Moving on, there are several competing ideas for what instrumental progress in evolution might be defined as. The first most common sense suggestion seems to be adaptedness. That is, progression in evolution consists in an increasing adaptedness to the environment. Indeed, it seem that organisms later on in evolutionary history should be more progressive in that they are better adapted (because they survived). However, a myriad of problems faces this assertion. First off, organisms are not the only things to have changed over time. The environments that organisms live in have changed as well. This immediately raises an issue as to how fitness in an early environment is to be translated into fitness in a later environment. That is, how do we determine how on organism in one environment compares in adaptation to another organism living in a completely different later environment? Similarly, two organisms living in the same environment may be adapted to that environment in different ways. Again, how do we choose who is the better adapted?
    One way to resolve this problem is to introduce the idea of progress as the accumulation of traits. That is, progress in evolution could involve the retention of early adaptations in organisms in so far as each subsequent evolutionary species will be adapted to a wider range of environments. But this is not limited to just the retention of earlier adaptations. As previously stated, this idea of progress would also include development of new adaptations to later environments. Thus, in a thought experiment in which a primitive species of snail switches environments with a modern snail, the modern snail would be expected to survive with greater probability in so far as it has retained any adaptations to the primitive environment through evolutionary generations. However this is not without its shortcomings. The environments inhabited by early and subsequent snails (prior to the modern snail) could have changed in such a way as to reverse any previously advantageous adaptations. For example, a decrease in oxygen levels in the atmosphere could have caused snail species to shrink in body size over time. Since early snails were bigger than their modern day equivalents, we would expect the modern day snail to be out-performed because of its mal-adaptedness.
    Some more proposals of progress include intelligence, complexity, and body size. Intelligence certainly seems plausible, and would play out well for humans. It seems any organism that is better suited to analyze its environment and act accordingly would have a higher survival rate. However, it is possible for this trend to reverse. For example, organisms living in steady, easy to predict environments would be better adapted to not waste so much energy on intelligence. Complexity as a proposal for progress in evolution has one major downside. In order for complexity to count as sufficient for evolutionary progress it must be defined. There are far too many levels of complexity (genic, physiological, metabolic, nervous, etc.). Furthermore, there does not seem to be any objective way to qualify one level of complexity as being more adaptive than another. Lastly, body size seems a poor choice for complexity. While there would be an obvious upside concerning the ability to evade and defend against predation, an obvious pitfall of large body size is mobility. An organism that increases in size would decrease in mobility thereby limiting its resource options. This is not advantageous to adaptation.
    It seems clear that one of two options is available to biologists. Either stray away from using “progress” in theories of evolution, or come to consensus on what is to count as sufficient grounds for progress. While the latter seems unlikely, the former is just as unattractive to many biologists.