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Is there a term for the valid assumption that a sufficiently complex and refined organ must be the result of natural selection on a large time scale?

Example: A biologist exists in a world where nothing has eyes. They find a creature with an eye as developed as a modern mammal's in our reality. Assuming this biologist understood what the eye was able to do as far as limiting and focusing incoming light and sending visual information to the brain, etc - it would go without saying that this was a gradually evolved system.

It would be impossible for a parent from a completely eyeless ancestry to spawn a child that had the equivalent of a fully developed modern mammalian eye, both because of the statistically improbability of that happening, and possibly even because of limitations in the way dna works.

Is there a word for this assumption? e.g. "As per the Axiom of Reliable Intuitive Ancestry Determination for Sufficiently Complex and Refined Traits, we will start our genetic research confident that the eye must be the result of iterative natural selection on a large scale of time."

Preferably a term that acknowledges the validity or reliability of this assumption when it comes to very complex and refined organs or systems, such as the eye, the brain, or the circulatory system.

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  • $\begingroup$ It's clear to me what you are asking. How can a biologist that exists in a world without eyes know what an eye is? That biologist must not also have eyes. $\endgroup$
    – kmm
    Commented Aug 23, 2016 at 1:51
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    $\begingroup$ This is a clunky question. As for as I'm aware, whilst it fluctuates, phenotype changes via natural selection are necessarily gradual (unless this has a technical meaning I'm unaware of). Are you looking for a word to describe the difference between "Eyes were selected for via natural selection" and "Eyes were selected for via laboratory/agricultural intervention"? $\endgroup$
    – James
    Commented Aug 23, 2016 at 4:49
  • $\begingroup$ @James - it's more like: "how do you know those bumps on the face weren't a recent addition to this creatures genome? Did you do any testing?" "No, I didn't have to. These bumps actually are actually very complex beneath the surface of the skin and perform a vital and well adapted function. The complexity and refinement alone are enough to know these must be the result of long gradual selection, even in the absence of other evidence." $\endgroup$
    – nsfnotthrowingaway
    Commented Aug 23, 2016 at 15:20
  • $\begingroup$ I think I've almost grasped the question. Could you say "What is the word for assuming that the phenotype has a genetic underpinning?" $\endgroup$
    – James
    Commented Aug 24, 2016 at 4:52
  • $\begingroup$ Unless I misunderstand, wouldn't all phenotypes have a genetic underpinning? From my previous comment, one might say "We can conclude that the bump in the forehead is not just an oddball family trait that neutrally came into play a couple of generations ago." - "Do you have genetic or fossil evidence?" - "No, but we were able to determine some complex and well developed functions that this bump provides." - A way to shorten this to "do you know this through genetic evidence, fossil evidence, or [blank]?" [Blank] being a word or phrase that means the complexity itself is evidence enough. $\endgroup$
    – nsfnotthrowingaway
    Commented Aug 24, 2016 at 13:14

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The 'term' you are looking for is actually gradual change or gradual evolution. If you want to make clear that this is an assumption or a theoretical consideration, you could say assumption of gradual change or theory of gradual change, replacing change with evolution if you like.

The idea comes from Darwin directly and was challenged in the 1970's by biologists like Stephen Jay Gould that suggested that evolution is not driven by gradual accumulation of changes but rather by morphological equilibria disturbed by many rapid adaptations forming new species, a concept called punctuated equilibrium.

It speaks for itself that these concepts are opposing but not mutually exclusive. They are rather two extreme states in a continuum.

Edit: As pointed out in a comment by Daniel Weissmann gradual changes also occur in neutral evolution; this is what we call accumulation of mutations - in fact, it also applies to (at least slightly) deleterious variants, especially in populations with small effective population sizes. This means, that the concept of gradual evolution can be generalised to neutral evolutionary processes. However, it does not mean, that the process involving natural selection cannot also be described by gradual evolution. You just have to specify what you are actually talking about. When talking about obviously complex adaptations, you would assume that they have not evolved neutrally.

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  • $\begingroup$ Say this new species not only has a complex organ like the eye, but also a large lump on it's forehead. We think the forehead lump may be there to keep dirt off of it's nose. However, without a close examination to other factors, like parents, environment, dna, etc, we cannot assume gradual change. It may be very well be an anomalous mutation in this individual or incidental to adaptations elsewhere, or DNA evidence might show it in fact has been gradually evolved. With the eye we can make this determination with no evidence aside from how well adapted and complex it is. $\endgroup$
    – nsfnotthrowingaway
    Commented Aug 22, 2016 at 11:23
  • $\begingroup$ I don't think this is what OP is looking for. The assumption of gradual change doesn't really say anything about natural selection -- it's generally taken to apply equally to neutral evolution. $\endgroup$
    – Daniel Weissman
    Commented Aug 23, 2016 at 2:16
  • $\begingroup$ @DanielWeissman: Thanks for your comment. I edited my answer to account for your argument. $\endgroup$
    – AlexDeLarge
    Commented Aug 23, 2016 at 10:43

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