Category Archives: evolution

Cuckholding crows don’t necessarily have healthier chicks

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ResearchBlogging.orgBirds are bad at monogamy. There are a number of good evolutionary reasons to cheat on your mate, and it’s not clear which one is the most likely explanation. A new study of American crows, however, suggests that, for females, cheating isn’t necessarily the best choice [$-a].

Avian infidelity isn’t obvious, because many birds are socially monogamous, forming couples for one or more breeding seasons to raise chicks. However, DNA-based paternity testing has overturned this intuition — a 2002 review of such studies [PDF] estimated that “cheating” occurs in 90% of bird species, and an average of 11% of chicks are “illegitimate.”

The biological term for this non-monogamy is “extrapair copulation,” often abbreviated to EPC. Evolutionary reasons for EPC behavior break down by which parent benefits from the cuckoldry: Females benefit if EPC means their chicks will be less inbred, which can make them less prone to disease or recessive genetic disorders. Males benefit if EPC means they will have more chicks than they would otherwise. Perhaps more importantly, EPC might impose real costs on females, if it leads mated males to invest less in caring for the chicks in their nests because they can’t be sure the chicks are theirs [PDF].


Crows in flight. Photo by wolfpix.

In the new study, Townsend et al. evaluated the costs and benefits of EPC for female American crows, which have a social structure that adds a twist to the cost-benefit analysis. Mated pairs of crows live in larger family groups, which include “auxiliary,” unmated males who may help feed and protect chicks — perhaps especially if those chicks are the result of their own EPC. Females also engaged in EPC with males from outside the family group, who should be less closely related than within-group males, and whose chicks would be more genetically healthy than those sired by any within-group male, mated or not.

Townsend et al. observed several such family groups over four years, using DNA fingerprinting methods to identify the parents of chicks as they were born, and tracking the chicks’ health and survival as well as how frequently mated crows and auxiliary males tended them. Contrary to what might have been expected, chicks produced by EPC were more, not less, inbred; they didn’t grow faster or have a higher probability of survival than chicks produced by mated parents. On the other hand, cuckholded males tended chicks sired by others as often as they did their own.

The most telling result is that broods containing chicks produced by EPC were more frequently tended by auxiliary males — but only when the EPC was with a within-group male. This suggests that EPC mainly benefits male crows, not females. From a mated female’s perspective, EPC produces chicks that are less genetically fit, and no more or less likely to survive, than chicks sired by her mate. On the other hand, an unmated male can only have offspring through EPC, and if he does, it makes sense for him to give them extra assistance. Males from outside the family group don’t stick around to offer that help, but auxiliary males from within the group can, and do.

References

Arnqvist, G., & Kirkpatrick, M. (2005). The evolution of infidelity in socially monogamous passerines: The strength of direct and indirect selection on extrapair copulation behavior in females. The American Naturalist, 165 (s5) DOI: 10.1086/429350

Griffith, S.C., Owens, I.P.F., & Thuman, K.A. (2002). Extrapair paternity in birds: A review of interspecific variation and adaptive function. Molecular Ecology (11), 2195-212 : 10.1046/j.1365-294X.2002.01613.x

Townsend, A., Clark, A., & McGowan, K. (2010). Direct benefits and genetic costs of extrapair paternity for female American Crows (Corvus brachyrhynchos). The American Naturalist, 175 (1) DOI: 10.1086/648553

Evolving antibacterial therapies

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On Slate, Brian Palmer says we need better tactics, not better antibiotics, to combat drug-resistant bacteria. But the new “tactics” he describes are, basically, new drugs:

In vitro studies have shown that chemicals like ascorbic acid shut down the movement of antibiotic resistance between cells. (Right now it’s effective only at concentrations that a person couldn’t tolerate, but it’s a start.) Because almost all antibiotic resistance relies on genetic transfer, this technique might be the solution we’ve been seeking since the very first colony of bacteria solved penicillin in 1944.

Drugs that combat gene transfer between bacteria probably would slow the spread of new antibiotic-resistance genes. Until bacteria evolve ways to transfer genes in spite of anti-transfer drugs, that is.

A genuinely new approach to circumvent antibiotic resistance will require actually thinking about the evolutionary consequences of therapy — and creating natural selection that eliminates the damage done by bacteria without also creating a fitness advantage for resistance to the therapy. That’s tricky, to say the least, but it’s not impossible. Such an approach has been outlined to control disease-carrying mosquitoes, for instance.

“The Origin,” 150 years old today

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Charles Darwin’s groundbreaking work, The Origin of Species, was published 24 November, 1859, 150 years ago today. This makes a rather neat bookend to the Darwin Bicentenary, the year of events commemorating the 200th anniversary of Darwin’s birth on 12 February, 1809. I’m going to be lazy and simply link to everything I wrote back concerning that earlier anniversary.

Oh, and serendipitously, today is also the anniversary of the discovery of Lucy in 1974. I saw her in person (behind glass) on a trip to Seattle during last year’s fall break, which was pretty cool.


Photo by CharlesFred.

Cost of killing nest-mates offset by benefits of killing nest-mates

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ResearchBlogging.orgAmong birds, brood parasites are the ultimate freeloaders — species like the common cuckoo and the brown-headed cowbird lay their eggs in other birds’ nests, leaving the host to raise the parasite chicks at the expense of its own. But while brood parasitism is easy on the parents, it isn’t so easy on their chicks, as a study recently published in PLoS ONE suggests.


A reed warbler feeds a common cuckoo chick. Photo from WikiMedia Commons.

A brood parasitic chick faces two challenges. The first is to avoid being recognized by its adoptive parents and ejected from the nest; the second is to win parental attention in competition with their adoptive nest-mates. The first challenge may be partially met by the evolution of eggshells that match host eggshells; and brood parasite parents may also help by keeping watch on the host nest so they can punsish hosts who eject introduced eggs. (This punishment behavior has been described as an “avian mafia [$-a].”)

In competition with their adoptive nest-mates, though, parasitic chicks are on their own. If the host’s own eggs hatch, the host has more mouths to feed and less time to spend on the parasitic chick. On the other hand, a brood parasitic mother can’t kick out the host’s eggs at the time she leaves her own egg with the host, because the host may abandon a nest that contains only a single unfamiliar-looking egg. This leaves it to freshly-hatched brood parasite chicks to do the heavy lifting involved in ejecting their host’s eggs themselves.


A common cuckoo chick pushes one of its host’s eggs out of the nest. Detail of figure 1 from Anderson et al. (2009).

Egg eviction looks like hard work — the chicks attempt it while they’re not much bigger than the eggs. Anderson et al. investigated the cost of all this adoptive-siblicidal effort by manipulating reed warbler nests that had been parasitized by common cuckoos,* taking away the hosts’ eggs in experimental nests, and comparing the growth of cuckoo chicks in those nests to that of chicks in unmanipulated nests, who had to do the evicting themselves.

They found that there is a cost to eviction effort: during the period of development when they would be doing all they could to push eggs out of the nest, cuckoo chicks grew faster when they didn’t have eggs to push. But they didn’t grow much faster, and by the time they were ready to leave the nest, the advantage had disappeared. Anderson et al. take this to mean that the cost of eviction is “recoverable” through the benefits of increased parental attention later on. I would add that it points out how important your choice of time frame can be when investigating how traits or behaviors affect organisms’ evolutionary fitness — sometimes a cost paid at one point in development is an investment toward later benefits.

——–
*The common cuckoo is the species first known to parasitize other birds’ nests, and its name is the linguistic source of the term “cuckold.”

References

Anderson, M., Moskát, C., Bán, M., Grim, T., Cassey, P., & Hauber, M. (2009). Egg eviction imposes a recoverable cost of virulence in chicks of a brood parasite. PLoS ONE, 4 (11) DOI: 10.1371/journal.pone.0007725

Hoover, J., & Robinson, S. (2007). Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs. Proc. Nat. Acad. Sci. USA, 104 (11), 4479-83 DOI: 10.1073/pnas.0609710104

Lahti, D. (2005). Evolution of bird eggs in the absence of cuckoo parasitism. Proceedings of the National Academy of Sciences, 102 (50), 18057-62 DOI: 10.1073/pnas.0508930102

Soler, M., Soler, J., Martinez, J., & Moller, A. (1995). Magpie host manipulation by great spotted cuckoos: Evidence for an avian mafia? Evolution, 49 (4), 770-5 DOI: 10.2307/2410329

Pollination before flowers

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ResearchBlogging.orgWhich came first, the pollinator or the pollinated? An article in this week’s Science suggests that a diverse group of insects may have been drinking nectar and pollinating plants millions of years before the appearance of modern flowering plants [$-a].



Panorpis communis, a modern scorpionfly species, and a sketch of ancient, pollinating scorpionflies. Photo by JR Guillaumin; sketch from Ollerton and Coulthard (2009).

Prior to the origins of modern flowering plants, or angiosperms, in the early-middle Cretaceous period, most of the diversity of land plants were gymnosperms. These plants are characterized by “naked seeds” — reproductive organs exposed to the air, where the wind can catch pollen and carry it from one plant to fertilize the ovules of another. In a world dominated by gymnosperms, the thinking used to be, animal pollinators were mostly unnecessary.

The new paper by Ren et al. challenges this idea with the description of a set of fossilized scorpionflies, all of which have strikingly long probosces that are clearly suited to sucking up liquid. The earliest of these fossils are from the Jurassic, tens of millions of years before the flowering plants began to diversify. In modern insects, sucking mouthparts like the ones described are associated with two kinds of feeding: drinking pollen, and drinking blood. To determine which was most likely in this case, Ren et al. performed energy-dispersive spectroscopy on the best-preserved fossil, and found no sign of the elevated levels of iron in the proboscis that would result from the residue of blood meals. This suggests that the scorpionflies were drinking nectar, or something like it.

Nectar has one major function in plants: to attract insects. Ant-protected plants reward their ants with nectar, and flowering plants use nectar to lure animal pollinators close enough to pick up or drop off pollen. If these ancient scorpionflies were, in fact, living on nectar, Ren et al. reason they were probably pollinating contemporary plants, which were all gymnosperms. The authors identify a diverse list of candidate host plants, including seed ferns and a relative of the modern ginkgo, whose reproductive structures were (1) too well-sheltered for efficient wind pollination or (2) included tubular structures similar to those that modern plants use to guide nectar-feeding pollinators. Finally, the authors point out, many modern gymnosperms produce “ovular secretions” that are very similar to the nectar produced by angiosperms.

As a neontologist, I’m often amazed how much can be told from million-years-old fossils — who knew there was a way to test for residual blood in a fossilized proboscis? At the same time, Ren et al. connect some mighty scattered dots to build their hypothesis. The real clincher is that it seems mighty unlikely that animal pollination would be rare in a world that already has both flying insects and pollen-producing plants. Animal pollination is much more efficient than wind pollination, and if there’s one constant in evolutionary history, it’s that living things rarely miss an opportunity like that.

References

Ollerton, J., & Coulthard, E. (2009). Evolution of animal pollination. Science, 326 (5954), 808-9 DOI: 10.1126/science.1181154

Ren, D., Labandeira, C., Santiago-Blay, J., Rasnitsyn, A., Shih, C., Bashkuev, A., Logan, M., Hotton, C., & Dilcher, D. (2009). A probable pollination mode before angiosperms: Eurasian, long-proboscid scorpionflies. Science, 326 (5954), 840-7 DOI: 10.1126/science.1178338

How to synchronize flowering without really trying

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This post was chosen as an Editor's Selection for ResearchBlogging.orgOne way plants can gain an advantage in their dealings with pollinators, seed dispersers, or herbivores is to act collectively. For instance, when oak trees husband their resources for an extra-big crop of acorns every few years instead of spreading them out, acorn-eating rodents are overwhelmed by the bumper crop, and more likely to miss some, or even forget some of the nuts they cache. These benefits of synchronized mass seed production, or “masting,” are straightforward, but how it happens is less clear. A paper in the latest issue of Ecology Letters has an answer — synchronization happens accidentally [$-a].


Bumper acorn crops ensure that squirrels miss a few. Photo by douglas.earl.

When Dan Janzen first described masting as an adaptation in plants’ coevolution with seed predators, he proposed that “an internal physiological system” [$-a] acted as a timer between masting events, with masting ultimately triggered by weather conditions. However, mathematical models have suggested a different possibility, the “resource-budget hypothesis:” that masting synchronization arises through an interaction of resource and pollen limitation [$-a].

Resource limitation works in concert with pollen limitation by catching plants at two stages of the seed-production process. First, if the resources required for seed production are more than can be accumulated in a single year, or if the availability of resources varies from year to year, then some years will be spent building up reserves instead of producing flowers. When reserves are built up, seed production is limited by the availability of pollen to fertilize flowers. Plants that flower when most of the rest of the population doesn’t will fail to set much seed, so they’ll have reserves to make seeds in the next year. This doesn’t require Janzen’s “internal physiological system” for the plants to synchronize, although such a system might evolve to reduce the likelihood of wasting resources by flowering out of synch.

The new paper tests this model in populations of a western U.S. wildflower, Astralagus scaphoides, which flowers at high frequency every alternate year. The authors prevented seed production in the plants by removing their flowers, either in a “press” of three years in a row or in a single “pulse” during one high-flowering year. The plants’ response to these treatments would reveal the role of resource and pollen limitation in synchronizing seed production.

If resource depletion after fruit set prevents reproduction in successive years, we predicted that ‘press’ plants would flower more than control plants every year, as they were never allowed to set fruit. We predicted that ‘pulse’ plants would flower again in 2006, but not set fruit due to density-dependent pollen limitation in a low-flowering year.

The authors also measured the sugars stored in the roots of plants collected before and after flowering in a high-flowering year.


Seed predator in action. Photo by tombream07.

The resource-budget hypothesis worked. Plants prevented from setting seed were forced out of synch with the rest of the population. “Pulse” plants flowered the year after treatment, but because few other plants did, they received little pollen and set little seed. They then had resources to flower yet another year, with the rest of the population this time, and set much more seed, depleting their reserves and bringing them back into synch. “Press” plants continued to flower at high rates each year, as long as they were prevented from setting any seed. Sugar levels built up in the tested roots during non-flowering years, and dropped after high-flowering years.

So masting arises as an emergent result of two limitations acting on plants — the resources needed to make seed, and good access to pollen. A couple of simple rules lead, undirected, to an ordered system that affects entire natural communities.

References

Crone, E., Miller, E., & Sala, A. (2009). How do plants know when other plants are flowering? Resource depletion, pollen limitation and mast-seeding in a perennial wildflower. Ecology Letters, 12 (11), 1119-26 DOI: 10.1111/j.1461-0248.2009.01365.x

Janzen, D. (1971). Seed predation by animals Ann. Rev. Ecol. Syst., 2 (1), 465-92 DOI: 10.1146/annurev.es.02.110171.002341

Janzen, D. (1976). Why bamboos wait so long to flower Ann. Rev. Ecol. Syst., 7 (1), 347-91 DOI: 10.1146/annurev.es.07.110176.002023

Satake, A., & Iwasa, Y. (2000). Pollen coupling of forest trees: Forming synchronized and periodic reproduction out of chaos. J. Theoretical Biol., 203 (2), 63-84 DOI: 10.1006/jtbi.1999.1066

First step to mutualism doesn’t look so friendly

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This post was chosen as an Editor's Selection for ResearchBlogging.orgAnt-plant protection mutualism is a widespread and elegant species interaction. How do species strike bargain like this, requiring specialized behaviors and structures in each partner, in the first place? A new report in The American Naturalist suggests an answer: maybe ants took the initiative [$-a].

In exchange for protection from herbivores and competitors [big PDF], “myrmecophytic” host plants grow hollow structures called domatia and often produce nectar to shelter and feed a colony of ants. This mutualism is really a sort of negotiated settlement between the partners; both ants and plants do what they can to get the most out of the interaction. We have evidence in some cases that host plants cut back support for ants if there aren’t any herbivores around; and, in other cases, that ants prune their host plants to prompt the growth of more domatia.



Domatium diversity: Ant domatia on Acacia (above) and Cordia nodosa (below). Photos by Alastair Rae and Russian_in_Brazil.

So it isn’t entirely surprising that there might be cases where that bargain hasn’t been established yet, and that’s what the new paper reports. The observation turned up in connection with one of the most interesting forms of the ant-plant mutualism: the “devil’s gardens” of the Amazonian rainforest. Devil’s gardens are created by colonies of the ant Myrmelachista schumanni, which attacks possible competitors to its preferred host [$-a], Duroia hirsuta, leaving patches where nothing but D. hirsuta grows.

Clued in by native research assistants, the group studying the devil’s garden interaction discovered that trees growing at the edge of a garden are often afflicted with swollen, distorted trunks. Cutting into the swellings, they found them riddled with passages and populated by M. schumanni. The trees in question are not known as myrmecophytes, and it’s not clear that they receive any benefit from hosting ants. In fact, the authors report that ant-occupied trunks are weakened, and prone to breakage under their own weight or under heavy wind.

The paper doesn’t present direct evidence that the ants create the galls, but as the authors explain, this seems likely — M. schumanni kills its hosts’ competition by injecting them with formic acid, which parallels the irritants other gall-making insects inject into their host plants. It make sense that gall-making might have started as ants’ attempts to kill off trees that are too big to succumb to formic acid outright, but respond to it by growing galls like scar tissue. Furthermore — and this is pure speculation, of course — this looks like a first evolutionary step toward true ant-plant mutualism. Domatia may have originally evolved to redirect ants from more damaging gall-making, and since ants are naturally territorial about their nests, it might not take much behavioral change before they end up protecting their host.

References

Edwards, D., Frederickson, M., Shepard, G., & Yu, D. (2009). A plant needs ants like a dog needs fleas: Myrmelachista schumanni ants gall many tree species to create housing. The American Naturalist, 174 (5), 734-40 DOI: 10.1086/606022

Frederickson, M., Greene, M., & Gordon, D. (2005). “Devil’s gardens” bedevilled by ants Nature, 437 (7058), 495-6 DOI: 10.1038/437495a

Janzen, D. (1966). Coevolution of mutualism between ants and acacias in Central America Evolution, 20 (3), 249-75 DOI: 10.2307/2406628

Aiming at a moving target with a shaky pistol: Evolution in a random, changing world

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ResearchBlogging.orgBiologists can become distinctly cranky when we hear evolution described as “random.” This is because evolution isn’t random — it’s undirected. Although it acts on mutations that turn up randomly, natural selection is highly nonrandom, in that (all else being equal) traits that help their owners make more babies are always the ones that spread through a population.

However, even if natural selection predictably aims for the same target, that target is not necessarily fixed. The most obvious case of this is in the coevolution of interacting species, where adaptation by one forces adaptation in the other. This is a field of study in its own right; but one recent innovation is a theory paper by Gandon and Day, which tracks changes in the “fitness landscape” resulting from adaptations and counter-adaptations [$-a]. (For more detail on the paper, see Coevolvers.)


Ground finches (Geospiza fortis) with big beaks might be favored this year, but what about next? Photo by kookr.

Empirical studies have shown that selection’s target can move in unexpected ways, too. One of the best examples of this turned up in the course of the ongoing, decades-long study of finches on the Galapagos Island Daphne Major. As rainfall on the island fluctuated from year to year, the mix of available seeds changed as well, and the finches’ beaks — the size of which determines what seeds are easily cracked and eaten — evolved to keep up [$-a]. The resulting evolutionary path looks like a drunkard’s walk, and the study’s authors, Peter and Rosemary Grant, put the word unpredictable right in the title.

Making things still more complicated, there is actually a random component to the effects of natural selection. That is, in the real world, advantageous traits may not automatically result in greater fitness — they result in greater expected fitness. Last year, Sean Rice published a mathematical model of evolution in which fitness is a random variable. He found that greater variation around the expected fitness can increase the strength of natural selection; that is, more uncertainty about the relationship between fitness and a given trait may actually make that trait adapt more rapidly. In a just-published extension of this work, Rice and Anthony Papadopolous examined the effect of random migration among different populations on adaptive evolution in each population, and found that greater variation in migration rates can reduce the effect of migration on local evolution.

Introducing all this randomness into our view of evolution doesn’t necessarily make evolution unpredictable. As an excellent recent Radiolab episode discusses, there are patterns to be extracted from randomness. It takes more work — larger sample sizes, longer-term studies — for these patterns to become apparent. Yet it’s clear that this is work we’ll have to do in order to understand biological systems.

References

Gandon, S., & Day, T. (2009). Evolutionary epidemiology and the dynamics of adaptation Evolution, 63 (4), 826-38 DOI: 10.1111/j.1558-5646.2009.00609.x

Grant, P., & Grant, R. (2002). Unpredictable evolution in a 30-year study of Darwin’s finches Science, 296 (5568), 707-11 DOI: 10.1126/science.1070315

Rice, S. (2008). A stochastic version of the Price equation reveals the interplay of deterministic and stochastic processes in evolution BMC Evolutionary Biology, 8 (1) DOI: 10.1186/1471-2148-8-262

Rice, S., & Papadopoulos, A. (2009). Evolution with stochastic fitness and stochastic migration PLoS ONE, 4 (10) DOI: 10.1371/journal.pone.0007130

A radical idea

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Responding to Nature‘s review [$-a] of his new book about evolution, The Tangled Bank, Carl Zimmer objects to the reviewer’s justaposition of his work with the more, shall we say, combative book Richard Dawkins has just released. Zimmer has the audacity to assume that his readers aren’t hostile:

I envisioned my potential readers as curious people who didn’t know much about evolution–what the idea actually is and how scientists study it. I envisioned people who might be interested in learning the nuts and bolts of processes like selection and drift, and who might be intrigued by sexually deceptive wasps, whales with legs, the viruses that dominate our genome, and other features of life that evolution allows us to understand.

With all due respect for those who want to take the fight to the wingnuts in the war on science — I enjoy Pharyngula as much as the next grad student — this seems so much more, well, hopeful. Ultimately, it might even be more productive.

Growing up in a science-friendly household surrounded by creationists, I didn’t come to the conclusion that evolution was true because I read a diatribe about the idiocy of biblical literalism. I came to that conclusion because I thought dinosaurs were pretty cool, and it turned out that you could learn a lot more about dinosaurs in the context of their evolutionary history than if you just assumed they all died in Noah’s flood. I think that people in a similar state — “curious people who didn’t know much about evolution” are much more likely converts to the cause of science than the wingnuts. Certainly there must be a lot of them out there; otherwise who’s keeping the Discovery Channel afloat?

The omnivores’ solution: Tadpoles independently solve a common problem the same way

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ResearchBlogging.orgOne of the key observations in support of evolutionary theory is that similar lifestyles can lead distantly-related living things to evolve strikingly similar traits. Compare an echidna and a hedgehog, distantly related mammals with very similar lifestyles. This kind of convergence can occur on much smaller scales of time and space, too, as a new paper just released online by Proceedings of the Royal Society shows. Its authors demonstrate that populations of spadefoot toads have independently evolved the same response to competition from another toad species [$-a].


Spea multiplicata, the New Mexico spadefoot toad. Photo by J.N. Stuart.

As tadpoles, spadefoot toads (Spea multiplicata) have two feeding strategies: they can be omnivores, feeding on organic debris in the water around them; or carnivores, feeding on aquatic crustaceans and sometimes other tadpoles. The two strategies are linked to a developmental switch — tadpoles that start eating crustaceans develop larger heads, the better to eat their fellows, presumably. This switch is handy in minimizing competition for food with another, related toad species, S. bombifrons. In ponds where S. multiplicata and S. bombifrons occur together, S. multiplicata tadpoles are much more likely to become omnivores, and S. bombibfrons are more likely to become carnivores, than is the case for either species when they’re the only ones in the pond.

This solution to competition might have evolved two ways: it may have turned up once, in a single population of S. multiplicata, which was then able to colonize other ponds containing the competitor; or it may have emerged independently in multiple populations experiencing similar natural selection from competition. The new study’s authors show that the second scenario is more likely by comparing the genetic similarity of multiple S. multiplicata populations to the frequency of their competitors, and showing that competition strength, not genetic relatedness, is the better predictor of how likely S. multiplicata tadpoles are to become omnivores.

References

Pfennig, D., & Frankino, W. (1997). Kin-mediated morphogenesis in facultatively cannibalistic tadpoles Evolution, 51 (6) DOI: 10.2307/2411019

Rice, A., Leichty, A., & Pfennig, D. (2009). Parallel evolution and ecological selection: replicated character displacement in spadefoot toads Proc. R. Soc. B DOI: 10.1098/rspb.2009.1337