Nav: Home

Size matters -- To livebearer fish, big fins are a big deal

January 17, 2019

To female molly and Limia fish, nothing is hotter than a male with a large dorsal fin. But these fins aren't just decorations to attract females.

Males also use them to fight or intimidate rivals. For scientists who study evolution, the fins present a chicken-and-egg dilemma. Which came first -- ornamental fins for courtship displays, or fighting fins only later used in displays?

In a new paper, biologists from the University of California, Riverside, studied the evolution of 40 molly and Limia species, and concluded dorsal fin displays arose first for males to compete with other males, only later being used in courtship displays to females. These changes in fin function went hand in hand with enlargement of the male dorsal fin. The fins reached extreme sizes in a few species and appear to be associated with rapid evolution, especially in mollies.

When Charles Darwin proposed his theory of evolution, he noted that some features arise not from natural selection by outside pressures like predators and food sources, but from competition within a species to find mates and reproduce. He called this "sexual selection," and scientists have used it to explain why males from many species have elaborate ornamentation and are larger than females.

Simply put, the ornament can evolve either because females prefer to mate with ornamented males, or because the ornament helps males defeat rivals. The male peacock's tail, for example, probably evolved through female choice. Deer antlers, on the other hand, probably evolved to compete with other males.

But whether male ornamentation arises through female choice or male-male aggression also has implications for how a group of animals diverges from an ancestral population and evolves into a separate species. In existing species, male-male aggression and covert or "forced" mating occurs much more often than courtship displays and female choice, leading some biologists to predict that, in general, male ornamentation probably evolved first for aggression.

"This is something that biologists have argued about for over a century, whether it's all in male-male competition for females or how important is female choice of a male," said Daniel Goldberg, the paper's first author, who conducted the research for his master's thesis. "The problem is, even though it was predicted in the 1990s that male ornamentation first evolved for aggression in many species of animals, studies that have been able to prove this for a particular group of animals have been few and far between."

Senior author David Reznick, a professor of biology at UC Riverside, has been working to unravel this problem in guppies. Guppies, like mollies and Limia, give birth to live young instead of laying eggs. The physiology of live birth comes in two main forms for these fish. In some species, the female produces a fully yolked egg, which is simply retained within her body, with no further nourishment after fertilization. In other species, the females have something like a placenta that nourishes eggs throughout development.

"The first step, which came before Goldberg's work, was to show that the male traits were tied to the type of female reproduction. You only get this kind of ornament in the ones that don't have placentas," explained Reznick. In live-bearing nonplacental fish, the female makes all her investment before the egg is fertilized, and the only thing left to ensure a quality offspring is to choose a high-quality male to fertilize it. The ornamentation could signal male quality.

Mollies are found along the Gulf Coast of North America and throughout Central America, while Limia are native to islands of the Greater Antilles in the Caribbean. Both are popular aquarium pets. Goldberg took comprehensive measurements of various body dimensions of male and female molly and Limia species, along with detailed observational notes, to develop an ornamentation index that measures dorsal fin size relative to body size. He then used this information from the living species to follow the evolutionary tree back through time to figure out what the original ancestors would have looked like.

"If you look outside and you see a tree, where we have two branches that emanate from a single branch, that corresponds on an evolutionary tree to a speciation event," explained co-author Mark Springer, a professor of biology at UC Riverside who worked on the evolutionary sequence. "We're interested in trying to reconstruct what the ancestral state was like at each one of those branching points in the tree."

"We know what the branch tips are like, we know what each species is, and so we use that information to go back through time and envision their common ancestor," added Reznick.

As predicted, the researchers found that in both mollies and Limia, males evolved larger dorsal fins first for fighting other males, only later using them in courtship displays. At that point, it was only a matter of time until they reached huge sizes in a few exceptionally ornamented modern species: three species of aptly-named sailfin mollies, and one species of Limia: the humpback Limia.

The evolution of ornamentation really took off with the evolution of courtship.

"Mollies and Limia have a lot of different species, and the evolution of elaborate courtship displays seems to go hand in hand with rapid speciation," said Goldberg, who is now pursuing a doctorate in biology at Illinois State University. Reznick and Springer are studying the speciation rate in a separate project.

The findings may be broadly applicable to other species and may help biologists solve the chicken-and-egg riddle of male ornamentation.
-end-
The paper, In Love and War: The Morphometric and Phylogenetic Basis of Ornamentation, and the Evolution of Male Display Behavior, in the Livebearer Genus Poecilia, was published Jan. 17 in Evolution.

University of California - Riverside

Related Evolution Articles:

Artificial evolution of an industry
A research team has taken a deep dive into the newly emerging domain of 'forward-looking' business strategies that show firms have far more ability to actively influence the future of their markets than once thought.
Paleontology: Experiments in evolution
A new find from Patagonia sheds light on the evolution of large predatory dinosaurs.
A window into evolution
The C4 cycle supercharges photosynthesis and evolved independently more than 62 times.
Is evolution predictable?
An international team of scientists working with Heliconius butterflies at the Smithsonian Tropical Research Institute (STRI) in Panama was faced with a mystery: how do pairs of unrelated butterflies from Peru to Costa Rica evolve nearly the same wing-color patterns over and over again?
Predicting evolution
A new method of 're-barcoding' DNA allows scientists to track rapid evolution in yeast.
Insect evolution: Insect evolution
Scientists at Ludwig-Maximilians-Universitaet (LMU) in Munich have shown that the incidence of midge and fly larvae in amber is far higher than previously thought.
Evolution of aesthetic dentistry
One of the main goals of dental treatment is to mimic teeth and design smiles in the most natural and aesthetic manner, based on the individual and specific needs of the patient.
An evolution in the understanding of evolution
In an open-source research paper, a UVA Engineering professor and her former Ph.D. student share a new, more accurate method for modeling evolutionary change.
Chemical evolution -- One-pot wonder
Before life, there was RNA: Scientists at Ludwig-Maximilians-Universitaet (LMU) in Munich show how the four different letters of this genetic alphabet could be created from simple precursor molecules on early Earth -- under the same environmental conditions.
Catching evolution in the act
Researchers have produced some of the first evidence that shows that artificial selection and natural selection act on the same genes, a hypothesis predicted by Charles Darwin in 1859.
More Evolution News and Evolution Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Climate Mindset
In the past few months, human beings have come together to fight a global threat. This hour, TED speakers explore how our response can be the catalyst to fight another global crisis: climate change. Guests include political strategist Tom Rivett-Carnac, diplomat Christiana Figueres, climate justice activist Xiye Bastida, and writer, illustrator, and artist Oliver Jeffers.
Now Playing: Science for the People

#562 Superbug to Bedside
By now we're all good and scared about antibiotic resistance, one of the many things coming to get us all. But there's good news, sort of. News antibiotics are coming out! How do they get tested? What does that kind of a trial look like and how does it happen? Host Bethany Brookeshire talks with Matt McCarthy, author of "Superbugs: The Race to Stop an Epidemic", about the ins and outs of testing a new antibiotic in the hospital.
Now Playing: Radiolab

Speedy Beet
There are few musical moments more well-worn than the first four notes of Beethoven's Fifth Symphony. But in this short, we find out that Beethoven might have made a last-ditch effort to keep his music from ever feeling familiar, to keep pushing his listeners to a kind of psychological limit. Big thanks to our Brooklyn Philharmonic musicians: Deborah Buck and Suzy Perelman on violin, Arash Amini on cello, and Ah Ling Neu on viola. And check out The First Four Notes, Matthew Guerrieri's book on Beethoven's Fifth. Support Radiolab today at Radiolab.org/donate.