What is the ultimate measure of evolutionary success?

There is only one measure of "evolutionary success": having more offspring. A "useful" trait gets conserved and propagated by the simple virtue of there being more next-generation individuals carrying it and particular genetic feature "encoding" it. That's all there is to it.

One can view this as genes "wishing" to create phenotypic features that would propagate them (as in "Selfish Gene"), or as competition between individuals, or groups, or populations. But those are all metaphors making it easier to understand the same underlying phenomenon: random change and environmental pressure which makes the carrier more or less successful at reproduction.

You will sometimes hear the term "evolutionary successful species" applied to one that spread out of its original niche, or "evolutionary successful adaptation" for one that spread quickly through population (like us or our lactase persistence mutation), but, again, that's the same thing.

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