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Beetles are incredibly diverse.
We’re talking somewhere around 400,000 individual species — from weird-looking weevils to
giant goliaths.
They’re the most diverse group of complex organisms on Earth, and they make up over
20% of all named animal species.
That’s right. One in five species on this planet is...a beetle.
...But, why though?
For a single group to be that massive, there must be an explanation.
They’ve been around a long time, but so have crocodiles and there’s only like 20
species of those guys.
One established idea comes from a report published in Science back in 1998.
It suggests that beetles get their diversity thanks to a relationship they formed way back
in the Cretaceous period with a brand-new food source — flowering
plants.
Even though flowering plants are common today, they’re actually newcomers in terms of evolution.
They first appeared around 130 million years ago, during the early Cretaceous.
Now, they weren’t responsible for the original evolution of beetles;
the first beetles evolved way further back. But flowers did give them a huge opportunity.
For millions of years, non-flowering plants like conifers and cycads had been the dominant
flora on Earth,
but about 125 million years ago there was a huge shift in the plant world as flowers
took over.
The flower takeover was probably a rough time for animals
that had adapted to the way things were, but beetles may have had a special advantage.
For instance, they may have been some of the first flower pollinators,
since they’d already been busy pollinating those older plant species.
And bees and butterflies weren’t around yet to compete with them.
But beetles also probably ate these new plants.
The 1998 study suggests that, overall,
flowering plants would have opened tons of new niches
and likely supported a huge burst of diversification.
This idea was based on a scientific technique called phylogenetic analysis.
It’s a technique scientists use to estimate evolutionary relationships
by comparing the traits or genetic sequences of a group of organisms.
This can help reveal things like how closely two species are related,
where certain traits were gained or lost,
or how far back two groups diverged.
But even though this analysis shows a connection between the rise of flowering plants
and the diversification of beetles,
there are still questions— like just how closely these events actually
lined up, or why beetles, of all things, could take
advantage of this new resource.
And it seems like there may be more to the story.
A new analysis from 2019 hints that the beetles got a helping hand
from a new set of digestive enzymes, which made it easier for them to digest this
new food source.
The authors of the study were trying to figure out when and how
beetles got certain enzymes that let them digest tough, complex compounds
found in plant cell walls, like cellulose.
The research was also based on phylogeny,
just like the 1998 report.
And based on their analysis,
the scientists think that these digestive enzyme genes
first showed up in beetle genomes
around the same time flowering plants evolved.
This may help explain why beetles were able to thrive
alongside flowering plants when they took over:
they would have been able to digest these new food sources
and take advantage of completely new niches,
like wood-boring.
But what’s really cool is where these enzymes came from.
Because the fact that beetles make enzymes
specifically for digesting cellulose is pretty unusual.
Other animals, like termites, generally count on microbes
in their gut to digest cellulose—
and some beetles do that too.
But, in many cases, beetles have the instructions
for these digestive enzymes in their own genome.
These special genes look like they’re the result of horizontal gene transfer
— a strange thing that happens where genetic material from one species
gets into the genome of another species, sometimes through a virus.
The scientists think the genes for these super-digesting enzymes
may have started out in some ancient bacteria or fungus
before the beetles picked them up.
That tells us that beetle diversity probably comes down to
a combination of factors—partly their evolution
alongside flowers, and partly these super-digesting enzymes.
But there may be one more thing beetles have going for them
— that’s the fact that they tend to not go extinct...
A 2007 analysis looked at the largest group of beetle species,
which includes families like scarab beetles and stag beetles,
and it found that they seem really good at avoiding extinction.
Many families may even have weathered the event
that killed the dinosaurs fairly well.
Which is pretty amazing!
In general, biological groups go extinct
because they lose their niche,
either to competitors or changing environments.
But beetle families seem to be able to roll with the changes.
Research focused on the last 25 million years
suggests they’re mobile enough to change their home ranges
in response to external pressures
like changes in climate, for example.
And the fact that they’re so good at fitting into
different niches might help them survive as a whole.
While certain species might get out-competed or die out
if their specific niche disappears,
beetles inhabit enough diverse niches that
the family as a whole can keep going—
even if some species are lost.
Overall, this suggests that if we want to know
what makes beetles special,
we shouldn’t just study what makes them diverse,
but also what keeps them around.
The fact that plain-old beetles can reveal
so much about evolution just goes to show you
how much science is right in front of you
in your everyday world.
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