I got asked a question yesterday on Facebook on whether evolution in bacteria and primates differs. The short answer is no.
The long answer is that the process of evolution itself is so simple in a way that it really doesn’t matter what its object is, i.e. what it operates on. It’s easier to understand this if you think about atoms. The forces governing their behaviour are equal properties in different measures. Mass is mass, whether it’s zero or one hundred units. The slight differences in these properties can lead to huge noticeable differences in final products, just look around.
Natural selection isn’t something that can change depending on what it acts on, be it bacteria or primates. Natural selection is there, and any difference in products is just that. Bacteria and primates are different because of the different evolutionary paths their ancestors took, the different environments and selection pressures that led to their present day evolution.
Another question was whether increasing complexity of species was an inevitable result of evolution. I suppose this is a bit like the question “Can you return a fried egg back to a raw egg?” According to our current knowledge, the answer is no, we can’t undo the burning of an egg or piece of paper. It’s hard to imagine evolution working backwards, because it’s hard to imagine undoing evolution. The matter of fried egg can’t work its way to become a raw egg, it’s an irreversible reaction. In a sense, ancient species had the resources to evolve in certain ways, but that doesn’t mean the subsequent versions of themselves necessarily have the potential to reverse it. If anything, any further evolution that leads to a similarity to previous species would not be a reversal, but merely a continuation of the same evolutionary pathway. Strictly speaking, there is no reason why natural selection on its own couldn’t lead to future species looking more like ancient species rather than more recent past species on the same evolutionary pathway.
An example that springs to mind, although not on an interspecific level, but on an intraspecific level (that is, within one species), is one insect. This insect has switched its wings on and off repeatedly over a long period of time, depending on its environment. Would you call that backwards evolution? I wouldn’t. Evolution evolves one way, regardless of similarity between past and future species. A bit like time.