New research in parallel evolution - Not all crabs are closely related.

[shunyadragon]

Research on crabs reveal parallel evolution of five unrelated groups, An example of the environment is the cause for evolution. Also described here is parallel evolution in birds.

Source: https://theconversation.com/crabs-have-evolved-five-separate-times-why-do-the-same-forms-keep-appearing-in-nature-195739

Crabs have evolved five separate times – why do the same forms keep appearing in nature?


Published: December 6, 2022 6.08am ESTAuthor

Professor of Evolutionary Palaeobiology at the Milner Centre for Evolution, University of BathDisclosure statement



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Charles Darwin believed evolution created “endless forms most beautiful”. It’s a nice sentiment but it doesn’t explain why evolution keeps making crabs.

Scientists have long wondered whether there are limits to what evolution can do or if Darwin had the right idea. The truth may lie somewhere between the two.

While there doesn’t seem to be a ceiling on the number of species that might evolve, there may be restraints on how many fundamental forms those species can evolve into. The evolution of crab-like creatures may be one of the best examples of this, since they have evolved not just once but at least five times.

Crabs belong to a group of crustaceans called decapods – literally “ten footed”, since they have five pairs of walking legs. Some decapods, like lobsters and shrimp, have a thick, muscular abdomen, which is the bulk of the animal that we eat. With a quick flick of their abdomen lobsters can shoot off backwards and escape predators.
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Crabs, by contrast, have a compressed abdomen, tucked away under a flattened but widened thorax and shell. This allows them to scuttle into rock crevices for protection. Evolution repeatedly hit upon this solution because it works well under similar sets of circumstances.
Five groups of “crabs”

King crabs evolved from lobster-like ancestors within the Anomura. By CSIRO, CC BY 3.0, CC BYRedeye sponge crabs carry sponges with them for camouflage. Porco_Rosso/Shutterstock
The largest crab group are the Brachyura (true crabs) including the edible crab and Atlantic blue crab. They had an ancestor that was also crab shaped. Some species have evolved “backwards” and straightened out their abdomens again. The other large group are the Anomura (false crabs), with an ancestor that looked more like a lobster.
Porcelain crabs superficially resemble true crabs. volkan yenel/Shutterstock
However, at least four groups of Anomura – sponge crabs, porcelain crabs, king crabs and the Australian hairy stone crab – have independently evolved into a crab-like form in much the same way as the true crabs. Like the true crabs, their compact bodies are more defensive, and can move sideways faster.

This means “crabs” aren’t a real biological group. They are a collection of branches in the decapod tree that evolved to look the same.
Hairy stone crab (Lomis hirta) Tim Binns/Wikimedia, CC BY-SA
But crabs aren’t the exception.

Something similar happened in the evolution of birds from feathered dinosaurs. Feathers may have first evolved for insulation, to attract mates, for protecting eggs and possibly also as “nets” for catching prey. Millions of years later, feathers elongated and streamlined for flying.

Palaeontologists disagree about the details, but all modern birds (Neoaves) evolved from ground-dwelling ancestors just after the mass extinction that wiped out the other dinosaurs. However, feathered wings and flight also evolved earlier in other groups of dinosaurs, including troodontids and dromaeosaurs. Some of these, like Microraptor, had four wings.

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[rogue06]

Cool article, but it seems that they don't consider the possibility that "crabs" might be a man-made construct much like how "race" is, and that different organisms can develop crab-like features if the ecological niche they occupy favors crab-like creatures.

[tabibito]

Cool article, but it seems that they don't consider the possibility that "crabs" might be a man-made construct much like how "race" is, and that different organisms can develop crab-like features if the ecological niche they occupy favors crab-like creatures.

Similar environments give rise to similar creatures. Why; it's almost as if evolution has a set of "laws" that might be likened to those of gravity or momentum.

[shunyadragon]

Cool article, but it seems that they don't consider the possibility that "crabs" might be a man-made construct much like how "race" is, and that different organisms can develop crab-like features if the ecological niche they occupy favors crab-like creatures.

I believe the article describes the crabs and birds as more distantly related than what would be called a 'race' or even subspecies, but I will read further. For example parallel evolution in birds originate from distantly related feathered dinosaurs.

[rogue06]

Similar environments give rise to similar creatures. Why; it's almost as if evolution has a set of "laws" that might be likened to those of gravity or momentum.

Convergent evolution.

Like how whether it be reptile, mammal or fish, some predatory marine creatures take on very similar features.

A similar thing can be observed in placental mammals and marsupial mammals living in your neck of the woods

3-s2.0-B0122268652001334-gr5.gif

[Cow Poke]

Cool article, but it seems that they don't consider the possibility that "crabs" might be a man-made construct much like how "race" is, and that different organisms can develop crab-like features if the ecological niche they occupy favors crab-like creatures.

You're picking on Aunt Tilly again, aren't you?

[rogue06]

You're picking on Aunt Tilly again, aren't you?

The cheek pincher who I broke of her habit

[Cow Poke]

The cheek pincher who I broke of her habit

That's why she had one big claw and one little one?

[rogue06]

That's why she had one big claw and one little one?

why she quit pinching (and digging in while twisting) cheeks.

I thought she died years ago, but in a note last year from a cousin, she mentioned that her oldest had moved back home with her because of illness and passed shortly after.

[shunyadragon]

That's why she had one big claw and one little one?

Right or left handed crabs?

[Cow Poke]

Right or left handed crabs?

Yes.

[tabibito]

Convergent evolution.

A similar thing can be observed in placental mammals and marsupial mammals living in your neck of the woods

? I thought there were no placental mammals evolved here. The only placental mammals (before white settlement) were dingoes and some rodents - and they seem to have been brought in by the first settlers (Aboriginals). CHECKING I find also bats, but they seem to have flown in by themselves, and something called a Tingamarra porterorum dating to the Eocene: not to be confused with its contemporary, Tingamarra Alamitophis (for which Gondwanaland seems to be responsible).

3-s2.0-B0122268652001334-gr5.gif

I'm still rather impressed by the Aldabran flightless rails.

[rogue06]

? I thought there were no placental mammals evolved here. The only placental mammals (before white settlement) were dingoes and some rodents - and they seem to have been brought in by the first settlers (Aboriginals). CHECKING I find also bats, but they seem to have flown in by themselves, and something called a Tingamarra porterorum dating to the Eocene: not to be confused with its contemporary, Tingamarra Alamitophis (for which Gondwanaland seems to be responsible).



I'm still rather impressed by the Aldabran flightless rails.

Placentals evolved (diversified and adapted) there after arriving. Placentals didn't originate there (in that sense of "evolved there"), having arisen in what today are the Americas.

[TheLurch]

One of the more intriguing articles I saw on this was one that pondered whether this might mean that we should expect to find crab-like forms if we ever discover life on an exoplanet.

https://arstechnica.com/science/2021...true-in-space/

[rogue06]

One of the more intriguing articles I saw on this was one that pondered whether this might mean that we should expect to find crab-like forms if we ever discover life on an exoplanet.

https://arstechnica.com/science/2021...true-in-space/

I thought I had seen something about that but couldn't find any articles.