Genome complexity: a review of Reznick's guppies (Introduction)

by David Turell , Tuesday, May 07, 2024, 19:00 (12 days ago) @ David Turell

Just posted:

https://evolutionnews.org/2024/05/are-guppies-examples-of-darwinian-macroevolution/

"...a series of beautiful experiments led by professor David N. Reznick, who captured guppies from the downstream pools (where predators are prevalent) and moved them upstream (where predators are rare). The experiments were designed to answer the following question: “How, why, and how fast does adaptive evolution happen in the real world?” (Reznick and Travis 2019) What Reznick’s team observed is that the guppies from the downstream pools underwent rapid transformations when placed into the new upstream environment. The transplants took longer to reach sexual maturity and got larger. But the rate at which this happened was surprisingly fast. When the team calculated the rate of evolution for these genetic changes, using a unit called the Darwin, they reported the guppies changed at a rate of 3,700 to 45,000 Darwins while most of the rates found in fossils are only 0.1 to 1.0 Darwins.

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"According to the predation-driven selection hypothesis, the guppies grew larger when predation was relaxed. This suggests that in the downstream pools where predation is higher, the larger adults must be more at risk of being eaten than the smaller guppies. They came up with a way to mark individual guppies that would allow them to be recaptured, which facilitated Reznick’s team being able to calculate the death rates for adult and juvenile guppies. However, after collecting this data, they found that death rates were similar for these different sizes of guppies. This discovery showed that the cause of change could not be direct predation. So, what was causing the changes?

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"Instead of direct predation affecting the guppies’ life history, the guppies exhibited density dependent selection which means the population density was affecting life history traits. In the upstream pools where there was lower predation, the populations grew larger which meant population density increased. This might also have a selection effect. To directly quote from their abstract: “We have shown that the agent of selection on the life history, behavior, and physiology in low-predation communities is high population density and the cascade of ecological effects that stems from it.” (Reznick and Travis 2019) In other words the purported mechanism is that when the population of guppies is dense, certain individuals harboring specific alleles have a reproductive advantage, leading to allele frequency changes in the population.

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"This means the same allele frequency changes from one group of guppies from stream A were observed in a different group of guppies from stream B. This is not expected if such changes are due to random mutation (RM). (van der Zee et al. 2022) After all, what are the chances that RM would make the exact same changes over and over again?

"No new mutations were observed. Instead, the pre-existing allele frequency in the population shifted and this is hypothesized to be due to natural selection (NS) based on standing genetic variation. They say, “the rapid and repeatable evolution of life histories in six introduced populations means that this evolution was fueled by standing genetic variation rather than by new mutations.” This means the adaptive capacity was built into the population of the organism itself. There was no RM that did something new and helpful, that was then picked by the “agent” of NS.

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"Reznick’s work shows that standing genetic variation was already baked in within the guppy population, leaving no role for random mutation. Because the variation was baked in, this places novelty-generation farther back in history, into a setting where there is less direct access to the environmental pressures that the variation is responding to. But wait, aren’t RM/NS both required for something to be considered evolution and an example of macroevolution?

"Identifying the origin of novelty-generation as random mutation is pivotal to providing an authentic instance of Darwinian macroevolution. Instead, what we have here is an example of how populations rapidly adapt using preexisting genetic variation. Thus, these results do not provide favorable evidence for Darwinian macroevolution. Instead, they demonstrate that a previously touted example of “evolution happening before our eyes” is merely an example of population dynamics. This is where preservation of genetic diversity amongst the population means that individuals within the population represent different optimizations for unique environments.

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"This leaves us at a place where some questions have been resolved, but the most important persist. Random mutation (RM) has been eliminated as the cause for the rapid parallel changes in the guppies. Predatory-driven selection, a form of natural selection, has also been eliminated as a possibility. However, the outstanding question of where the standing genetic variation ultimately came from remains."

Comment: this paper shows us that the ability to make large variations is built into the DNA of a species. Certainly not macroevolution, but a reasonable explanation as to how species accomplish necessary changes without using new mutations