> Global changes have since driven many other giant animals to extinction, but anacondas grow just as big today.

But why? Why have anacondas - and sharks? - been immune to evolving? Why hasn’t a significant predator evolved - or invaded? - to feed on them? Why hasn’t 12 million years made the species fragile?

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icegreentea2 4 days ago | parent | next [-]

We can't observe these ancient snakes or sharks actually living, so we can only make assumptions based on their morphology. And morphology certainly constraints and suggests behavior, and so we can make some assumptions here.

That being said if the question is "why have we not seen significant morphological changes" - there are a few ways to think about this.

First is that we would be blind to many types of morphological evolution. For example, if an isolated sub population of snakes or sharks started shrinking due to isolated environmental pressures, we would be unlikely to see this, but also if we fixate on the "largest anaconda", then we would filter out all "smaller snakes".

Second, the way we talk about "not evolving", especially for sharks is probably misleading. When we say sharks haven't changed, we mean to say that the shark body plan hasn't significantly changed. And this makes sense - they have a very efficient body plan for being a hunter in the see. We have "proof" of the suitable-ness since dolphins and other whales have converged onto a very similar body plan. Conversely, there are plenty of extinct sharks with body features that seem totally bizarre (https://www.fieldmuseum.org/blog/four-fossil-sharks-are-cool...).

Finally, especially in the context of "the largest" - the largest animals that can exist in a given environment is.... environmentally constrained, especially for land animals. The largest anaconda is likely near the largest sizes that the local environment to support, and so something larger appear is unlikely, without drastic environmental changes.

	▲	chiefalchemist 4 days ago \| parent [-]
		Why hasn’t anything evolved to prey on them? Given all the calories they could provide.

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meindnoch 4 days ago | parent | prev | next [-]

They've reached a local maximum.

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adolph 4 days ago | parent | prev [-]

This comment seems to be generating downvotes, but I find the questions fascinating. It is a stretch to say the fossil record rules out any evolutionary changes to anacondas. However, if anacondas represent another form of so-called "living fossils" [0], it is interesting to think about what makes them resistant to the change that seems to occur as a matter of course in many other organisms?

0. https://en.wikipedia.org/wiki/Living_fossil

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chiefalchemist 4 days ago | parent [-]

It was my comment. Thanks, cause I see plenty of NH treads that go completely off topic and there are too few down votes for that.

In any case, 12M years a long time for a species to survive, let alone survive “as is”. It makes me think of the creature in Alien and how it evolved into deadly perfection. But these creatures aren’t fictional.

p.s. Aren’t octopuses another species of little to no change? But they’re weird anyway so it’s not a surprise?

	▲	adolph 3 days ago \| parent [-]
		Octopuses are all soft tissue (except for their "beak"), so it would be very hard to determine from the fossil record how much change has occurred. Perhaps as a result, they are not mentioned in the "living fossil" article above. An additional complication is that some cephalopods have a relatively unique ability to change gene expression in response to environmental factors [0]. As a result, even if one were to see physiological change or change in ecological niche, it might not be as a result of speciation. Hopefuly soon techniques of analyzing ancient DNA [1] will be more broadly used to understand the stories of long surviving species. 0. https://www.nsf.gov/news/masters-acclimation-octopuses-adjus... 1. https://en.wikipedia.org/wiki/Ancient_DNA