One thing to remember about evolution and how traits came to be is that it’s all past tense. The trait worked for some time and so the descendants share the trait. The same traits may or may not work well today or tomorrow. Evolution doesn’t plan.
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… and another important question is: how did holometabolism, or larval stages in general, develop during evolution? Is it a modififcation of the embryonal stage which other taxa perform inside the egg? Why is there “similarity” between some molluscs (e.g. Bivalvia), amphibians, cnidarians and insects in having a larval stage, and other related taxa haven’t?
Holometaboly isn’t a “trait” that appears randomly though, it’s an entire system of evolutionary specializations that developed because of strong pressure towards evolving that way.
Hemimetaboly is the default, less derived state, and it persists because it is no disadvantage to insects that still use it (and, of course, is also likely an advantage to those with certain ecological roles). Holometaboly is the derived state that developed because it had a clear and distinct advantage to the ancestors of groups that do use it.
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“Trait” is the wrong word. What I’m trying to get at is that at some point something changed to allow the development of holometablolism. Whatever changed was (is?) inherent in all insects, unless insects are polyphyletic and there have always been two types of development (something I don’t really believe). Whatever started that path may have been a random thing, rather than something inevitable.
I agree completely with paragraph 2.
I do wonder how it worked for those Devonian-epoch amphibians like Eryops and Seymouria, which seem like they would have had ecological niches akin to today’s crocodilians. Could they really have had a tadpole-like stage? Or did amphibian metamorphosis come in later?
There are fossils showing that giant temnospondyl amphibians had larvae like modern amphibians.
I imagine that amphibian metamorphosis occurs because their eggs are best suited to being laid in water and it’s easier for something large to crawl out of the water and avoid dehydration or predation, so they probably evolved to only go on land in later life stages from the start.
I looked that up last night. It looks like the temnospodyl larvae were more like salamander larvae than frog tadpoles. So that suggests that the tadpole form represents evolution toward more extreme differences between adult and larva.
One of the hypotheses for the pathways that may have allowed holometabolism to occur is the first thing you said; done via the prolongation of the inhibition of juvenile hormone. This hormone is responsible for controlling development towards the imaginal (next instar) form by inhibiting it. In hemimetabolous (and paurometabolous, etc) insects, there is a pronymphal stage in the egg that develops into a nymph before hatching, and then continues to grow after hatching until it finally reaches the adult stage. In holometabolous insects, a larva and a pupal stage replace the nymph stages. The larva can be thought of as the pronymphal stage before hatching that remained in its form largely unchanged in structure even after hatching. This may be because the juvenile hormone following the pronymphal stage bypassed inhibition through the action of one or more mutations throughout several generations. The pupal form more or less then equates to a single nymph stage before adulthood.
This source has neat diagrams: DOI: 10.1038/46737
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