Colonising spiders evolve the same three forms

A central truth of evolution is that it is a contingent process – one in which chance plays a decisive role. This means, of course, that evolution is not predictable. How then, do we account for new research published in the journal Cell Press that reports a genus of tiny, blind and beautiful Hawaiian spiders that seem to re-evolve the same three forms every time they colonise a new island?

The spiders, of the genus Ariamnes, live in the high forests of most of the archipelago’s islands. They are nocturnal species and nearly blind. Despite this, they come in three distinctive and colourful forms, each of which is an adaptive camouflage to certain aspects of their environment. 

“You’ve got this dark one that lives in rocks or in bark, a shiny and reflective gold one that lives under leaves, and this one that’s a matte white, completely white, that lives on lichen,” says lead author Rosemary Gillespie, an evolutionary biologist at the University of California, Berkeley, US. 

This camouflage helps protect them against their major predator, the Hawaiian honeycreeper (a member of the finch subfamily Carduelinae).

Remarkably, Gillespie and her international team, using a mixture of genetic analysis and software modelling, have discovered that the stick spiders constantly re-evolve these same three forms, known as ecomorphs, in a rare example of repeated and predictable convergent evolution.

The curious geological history of the Hawaiian archipelago has helped the team to understand the evolution of the several different species within the Ariamnes genus. The Hawaiian Islands are the most isolated landmass in the world, with the oldest island of Kauai at the western end of the chain and the newest island of Hawaii at the eastern. This chronological sequence also reflects the sequence of colonisation of the islands by various organisms, including the stick spiders, over the past five million years.

As, say, a dark species moved to a newer island, it would quickly evolve into three new species, each displaying one of the three ecomorphs. As other Ariamnes species arrived they too would evolve into the three ecomorphs, leaving a pattern of bewildering relations between them. Species that look the same may well be distantly related, while species exhibiting different forms might be close evolutionary kin.

Importantly, these three forms remain constant. 

“They don’t evolve to be orange or striped. There isn’t any additional diversification,” says Gillespie. 

This is incredibly rare, only occurring in a lizard species from the Caribbean and, interestingly, another spider species from Hawaii.

The researchers think a unique set of circumstances bring about this evolutionary déjà vu – a lack of diversity in predators, a limited set of genetics with a possible preprogramed DNA switch to facilitate the emergence of the three ecomorphs, a free-living lifestyle, and an environment that rewards only certain types of camouflage, all seem to contribute.

While the exact reasons remain unclear, Gillespie and her team hope that further research into the similarities within this select group of organisms that evolve the same forms over and again will “provide insight into what elements of evolution are predictable, and under which circumstances we expect evolution to be predictable and under which we do not”.

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