Sudden evolutionary change
Scientists describe an elusive example of abrupt evolution happening in columbines

Date:
February 16, 2022
Source:
University of California - Santa Barbara
Summary:
Researchers have identified a case of a sudden evolutionary
change. In a new study, the scientists describe a population of
columbines that have lost their petals, including the characteristic
nectar spurs. The finding adds weight to the idea that adaptation
can occur in large jumps, rather than merely plodding along over
extended timespans.



FULL STORY ==========================================================================
When Charles Darwin first codified the theory of evolution by means
of natural selection, he thought of it as a gradual process. "We see
nothing of these slow changes in progress, until the hand of time has
marked the long lapse of ages," he wrote in his seminal work, "On the
Origin of Species."

==========================================================================
But Darwin didn't have the full picture. "Evolution doesn't necessarily
take all these small changes like Darwin proposed," said Scott Hodges,
a professor in UC Santa Barbara's Department of Ecology, Evolution,
and Marine Biology.

Hodges, doctoral student Zachary Cabin and their colleagues just have identified a case of a sudden evolutionary change. In the journal Current Biology, the scientists describe a population of columbines that have
lost their petals, including the characteristic nectar spurs. A drastic
change caused by a mutation in a single gene. The finding adds weight
to the idea that adaptation can occur in large jumps, rather than merely plodding along over extended timespans.

Ever since the theory of evolution was put forward, biologists
have debated whether it always occurs in small, gradual steps over
long timespans or sometimes as an equilibrium punctuated by abrupt
changes. Often, large morphological changes appear within short geologic timescales where intermediate forms may not have fossilized. The
question then remains whether many small changes occurred in a short
period of time, or perhaps whether single large-scale mutation might
be responsible. So, researchers really have to catch the development
in action if they hope to build a case that sudden changes can drive
evolution.

Enter the Colorado blue columbine. In one population, a mutation has
caused many of the plants to lose their petals with the iconic nectar
spurs. While not an uncommon occurrence in columbines, spurlessness
seems to have stuck around in this area: About a quarter of the plants
lack the distinctive feature.

A single gene The team plumbed the plant's genome to find the source
of the unusual morphology. They considered a gene, APETALA3-3, known
to affect spur development. They found that this single gene controlled
the entire development of the flower's spurs and nectaries.



==========================================================================
"The gene is either on or off, so it's about as simple of a change you
can get," said lead author Zachary Cabin. "But that simple difference
causes a radical change in morphology." A single broken gene causes
mutant plants to develop flowers with no petals or nectar spurs.

If these flowers were preserved in the fossil record, scientist could
well sort them into two wholly different genera. And there would also
be a puzzling gap: no intermediate form documenting a transition from
one morphology to the other.

"This finding shows that evolution can occur in a big jump if the right
kind of gene is involved," Hodges said. APETALA3-3 tells the developing
organ to become a petal. "When it's broken, those instructions aren't
there anymore, and that causes it to develop into a completely different
organ, a sepal," he explained.

APETALA3-3 is a type of homeotic gene, one that specifies the development
of an entire organ. A mutation in one of these genes can have a drastic
effect on an organism's morphology. For instance, one homeotic mutation
causes a fly to develop legs where it should have antennae. "Most of the mutations of this nature are going to be like that, just awful," Hodges continued. "The animal won't have any chance of surviving. Biologist
Richard Goldschmidt called them 'hopeless monsters.'" But once in a very
long while, one of these radical changes might provide a beneficial trait
in a particular environment, creating a "hopeful monster." And a hopeful monster would show that evolution can proceed in single, large jumps, supporting the punctuated equilibrium hypothesis.



==========================================================================
"We did not have a good example of a hopeful monster due to a single
genetic change," said Hodges, "until now." Researchers have to catch
these abrupt changes as they're happening, otherwise they disappear into
an organism's genome. For example, other relatives of columbines have
lost their petals and nectaries in the past, but it's now impossible to
tell if these events occurred in one fell swoop. The fact that it is
actively happening in the Colorado blue columbine enabled the team to
confirm their status as a hopeful monster.

"There's definitely some luck involved with us being around at the right
time to capture this," Cabin said.

Surprising selection Catching the change in action offers another benefit
as well: the opportunity to study the genetics and selective pressures
at work.

The team discovered five versions, or alleles, of APETALA3-3, only one
of which codes for a petal with a functional nectar spur. The other four
were broken, as Hodges put it. They also determined that spurlessness is
a recessive trait. The flower will develop normally as long as the plant
has one copy of the functional allele. But any two of the mutant alleles together will prevent this. "You can mix and match them," Cabin explained.

About a quarter of Colorado blue columbines in this area display the
recessive trait of spurlesness, more than can be attributed to mere
chance.

Across all species of columbines it's possible to find rare individuals
that develop flowers without nectar spurs. But with a quarter of the
Colorado population missing the feature, Cabin and Hodges knew this was
more than a chance occurrence. "To get that many of this mutant type
really suggests that there's selection favoring it somehow," Hodges said,
which he finds odd, since the spur produces nectar that attracts the
plant's pollinators.

Hodges is deeply familiar with columbines, and all of his previous
research suggests that nectar spurs are important to the group. Even
slight changes to the structure have driven speciation and diversification
in the genus. "So, how the heck can you lose your spurs and still be
favored?" he asked.

Attracting pollinators is only one factor contributing to reproductive
success.

It turned out the mutant plants actually produced more seeds than their counterparts, much to the team's surprise. They began combing through
their observations, searching for an explanation.

"The first time we really realized the pattern was at the airport on the
way home," Cabin recalled. He was reading off data as Hodges entered it
into the computer. "Scott could see the pattern developing, because he
had all the data in front of him, and was getting more and more excited."
The team had recorded herbivory from caterpillars, aphids and deer on
the different morphs. Damage from caterpillars and aphids can hamper
seed production, Cabin explained, while deer can devastate an entire
plant. And as the data built up, a clear trend emerged: Deer and aphids preferred flowers with nectar spurs.

Shifts in floral morphology are usually driven by pollinators, but
spurlessness seems to be driven by herbivory. "Natural selection can
come from very surprising sources," Hodges said. "It's not always what
you'd expect it to be." Timing it right Now that they've identified
their hopeful monster, Cabin and Hodges plan to investigate the DNA
around APETALA3-3to build a timeline of when the mutations may have
occurred. When the gene first mutated, only one of the plant's chromosomes
was affected. That means that every descendant with that mutation would
have the same genetic code around APETALA3-3 for many generations,
Hodges explained.

However, chromosomes do swap alleles occasionally in a process
called recombination. By tracking the amount of recombination that has accumulated around the different versions of APETALA3-3, the scientists
can estimate how long ago each mutation occurred. More variation requires
more time to accumulate. And the closer this variation is to APETALA3-3
itself, the more recombination events there have been since a mutation
first appeared.

The researchers also want to track how spurlessness is spreading
through the population. The different morphs do interbreed, but
genetic evidence suggests that there's less mating between the
two groups. The Colorado blue columbine may be diverging into
separate species, especially since the two types seem to rely on
different pollinators. "That splitting process would be slow,"
Cabin said, "but there is evidence that it could be on its way." ========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Barbara. Original written by Harrison
Tasoff. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Zachary Cabin, Nathan J. Derieg, Alexandra Garton, Timothy Ngo,
Ashley
Quezada, Constantine Gasseholm, Mark Simon, Scott A. Hodges. Non-
pollinator selection for a floral homeotic mutant conferring loss
of nectar reward in Aquilegia coerulea. Current Biology, 2022;
DOI: 10.1016/ j.cub.2022.01.066 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220216112300.htm

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