This phenomenon contradicts the notion that evolution is continuously striving towards a specific direction. It was explained further above that, in terms of reproductive fitness, the evolutionary trend is always upwards.
Because the environment changes, this “progress” always remains relative – progress relative to the adaptedness to an environment. In case we want to metaphorically talk about a “goal” of evolution, then the goal is simply the copying success of genes.
However, evolution never “goes down” in fitness, because organisms with a lower fitness (by definition) will be more likely to lose the race for reproduction, being outcompeted by their conspecifics. Conversely, there is also no mechanism that prevents short-sighted improvements that will turn out to carry a design disadvantage in the long run.
Natural selection has no foresight. In each round, the individuals that die early or are comparatively bad at reproducing are weeded out.
Evolutionary biology is difficult to grasp because it strongly violates some of our basic intuitions about how things work. We have the tendency to favor teleological (“goal-directed”) explanations over causal explanations (Kelemen & Rosset, 2009) even though with regard to nature, teleological explanations are wrong.
Evolution being blind implies that lineages only ever strive towards local optima in regard to the fitness landscape of all possible gene combinations. Global optima, where reaching them would require foresight or planning , are thus not to be expected as a result of evolution.
Evolutionary changes in a population happen when the relative frequency of alleles in the genepool is changing. In large populations where no migration happens and where the environment remains constant, it is well possible that very few such changes happen and that evolution thus remains static for long periods of time.
Not all their basic designs for visual organs had the same potential for sight improvements in the organism’s evolutionary future. Because evolution lacks foresight, all kinds of designs evolved, as long as there was a continuous pathway of beneficial mutations leading up to them.
The notion that evolution has a goal/purpose is a widespread misconception that comes in several variations. Many people falsely believe that evolution was working towards humans as a final product. But from a biological perspective, there is really no reason to single out the human species specifically. Evolution produced a “tree of life”, ...
It did not produce a “ladder of life” where humans would throne at the top. Homo sapiens is one species out of many, and all currently existing organisms have an evolutionary history of the same length, leading back to the same origin most likely.
This should not be confused with claims about complexity, size or abilities having to go “upwards” as well : Evolutionary history is full of examples where lineages became smaller. One example would be the common sparrow, whose ancestors were predatory dinosaurs.