Given that the mechanism of natural selection is sensitive, how do species survive in an evolutionary midway? The ability to breath in air requires complex protein systems. An animal that is developing these organs would not be able to utilize its function until it fully evolves. While it doesn't, wouldn't the wasted resources used in growing half-developed lungs be.. wasted? Or is all evolution continuous?

  • $\begingroup$ I have this very question in my mind as I take H2 biology. It is not from homework. $\endgroup$
    – Liu Tianyi
    Commented Nov 6, 2016 at 5:55

1 Answer 1


It is a common question

Statements of the kind "Evolution is impossible because of intermediate state" are common from religious extremists literature. There is therefore a lot of resource online that already debunk this type of statement but I guess, I'll just quickly write one more! Of course, I am not suggesting that you are taking an opinion-based position, you might well be genuily seeking for an understanding of the world.

Quickly speaking there are a series of concepts that are necessary to understand in order to answer this question.

Fitness landscape and fitness valley

The term "evolutionary midways" is not used in biology but one can rephrase it with the concept of fitness landscape and fitness valley.

  • Fitness landscape

    • The fitness landscape is the function that maps fitness (generally represented in y-axis) to phenotypic trait value (typically multi-dimensional but often represented on a single axis for simplicity). Fitness landscape may also refer to the mapping from genotype to fitness but we will not be concerned with that for this answer.
  • Fitness valley

    • A fitness valley refers to a region of low fitness on the fitness landscape. The movement from one locally high fitness state to another locally high fitness peak might well get through a region of low fitness. This is what you call fitness valley and on the fitness landscape and this is what you are interested in.

Is there always a fitness valley?

Let's first consider reasons for which a population might not experience such fitness valley

  • Logical Fallacy: Argument to ignorance
    • It is common for a layman to assume that intermediate states ought to be associated with low fitness. In reality the intermediate states might well be neutral or only slightly deleterious (and therefore be mainly affect by genetic drift) or might even be directly beneficial. It appears to be a mistake to assume such fitness valley are more common than they are in reality just because you fail to imagine how a trait may have evolved. In philosophy of logic, this is called an argument to ignorance. Of course, fitness valleys do exist. So, when they exist, do populations ever manage to get over them?

Assuming there is a fitness valley (for a given trait), can populations just "jump over it"?

  • Do populations need to get through an intermediate state?
    • Darwin framed his theory into the idea that evolution is a very gradual process. As we know today, this is far from being true. Evolution can actually occur through a big steps. There are single mutations that double the amount of genetic material, there are single mutations that completely affect the physiology or anatomy of an individual. There are cases of horizontal gene transfer (transfer of genetic material between species) as well that can cause sudden important change without having to get through intermediate stages. It is sometimes wrong to assume that there must be intermediate steps.

If there is a fitness valley and the population will not "jump over it"...

Let's assume a fitness valley (for a given trait, for a given population) and let's assume that the population will not "jump over it" thanks to a mutation of big effect.

  • Shifting Balance Theory

    • There are a number of different forces that affect evolutionary processes. Layman often think of natural selection as being the only force causing evolution but this is wrong. Genetic drift is another very important process that affect evolutionary process. Genetic drift refers to the random sampling of individual at one generation to build up the successive generation. The relative importance of drift to natural selection in driving evolution of a population depends on the effective population size. Have a look at Why is the strength of genetic drift inversely proportional to the population size?. Genetic drift can typically randomly yield a population toward a fitness valley, from which natural selection will yield the population to either come back or start climbing the opposite fitness peak. This concept is called shifting balance theory. There has been a lot of work on shifting balance theory suggesting that it well be much more common than previously thought.
  • Phenotypic flexibility

    • One may wrongly assumed that if a mutation occurs to make, say, a bone a big longer, then one organism would simultaneously need mutations that would make muscles, nerves and other tissues longer accordingly. In reality our developmental process is very flexible to an environmental change or to a mutational change. This flexibility is very much used in medicine and agriculture. If a bone gets a bit longer, muscle might just stretch a little more and end up developing in accordance to the new mutation. It would therefore be wrong to assume that because we are made of a complex interaction of traits, that a mutation cannot bring anything that would not be beneficial. If you are interested in the evolution of complex system you might want to have a look at the post How did the cardiovascular system evolve? for example.

Source of information

I would recommend that you have a look at a very short and introductory course to evolutionary biology such as Understanding Evolution by UC Berkeley for example and you will be able to understand much more about what evolution is and how it occurs.

  • $\begingroup$ en.wikipedia.org/wiki/Irreducible_complexity Seems to be what I was writing about. (Just found this out). Thanks for your answer too. $\endgroup$
    – Liu Tianyi
    Commented Oct 28, 2016 at 16:44
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    $\begingroup$ No, I am not at all writing about Irreducible Complexity. Irreducible Complexity has nothing to do with science. It is a fallacy (or set of fallacies) build by creationists. As the wikipedia article you link says: Irreducible complexity [..] is rejected by the scientific community, which regards intelligent design as pseudoscience. If your aim is to learn about evolution, I would recommend reading from the popular science literature (and not pseudo-science and other sources of unjustified arguments). There are many good sources of knowledge, one of them being linked in the post. $\endgroup$
    – Remi.b
    Commented Oct 28, 2016 at 16:52
  • 1
    $\begingroup$ Yes you weren't writing about it but I was asking about it. I had thought that complex systems are irreducible, therefore midways are impossible. But I was wrong. $\endgroup$
    – Liu Tianyi
    Commented Oct 29, 2016 at 5:10

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