Are Guppies Examples of Darwinian Macroevolution?
In 1961, in the steep mountain streams of Trinidad where cascading waterfalls create barriers which predatory fish can’t overcome, a biologist named Caryl Haskins noticed and documented the variety and population structure of communities of the small freshwater fish we call guppies. (Haskins et al. 1961)
This launched 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.
The Darwin is a unit of evolutionary change set by J. B. S. Haldane in 1949. It is mostly used in paleontology to compare macroevolutionary changes in fossils. Since the obvious major difference between the upstream and downstream environments was a lack of predators upstream, the phenotypic observation — that the transplanted guppies had delayed time to sexual maturity and got larger — was interpreted to be the direct effect of no predation by the larger fish. This was initially interpreted as evidence that “evolution in nature could be far more rapid, by several orders of magnitude, than had been inferred from the fossil record.” (Science News Staff 1997) Indeed, Science News reported in 1997:
Because the guppies evolved at rates so much faster than those estimated from the fossil record, Reznick suggests that selection on such short time scales is powerful enough to be behind major evolutionary changes. He argues that the study demonstrates it’s “possible to reconcile large-scale evolutionary phenomena … with what we can see in our lifetimes.” (Science News Staff 1997)
The Humble Guppy
Since this announcement, the humble guppy has been touted not only as an example of evolution happening before our eyes, but also as an example demonstrating that rapid macroevolutionary change is possible. But there were lots of assumptions in these early analyses. For example, everyone (myself included) assumed that predation was directly responsible for the guppy phenotypic differences. This is exemplified in Kenneth Miller’s summary of the experiments in his book Finding Darwin’s God: A Scientist’s Search for Common Ground Between God and Evolution. Miller, who is a critic of intelligent design and a biologist at Brown University, explains that natural selection (specifically predator-based selection) would give a big reward to the upstream transplants for a longer period of growth since larger females mean more eggs.
Once the guppies were transplanted upstream to a predator-free environment, natural selection would give any tendency towards a longer period of growth before sexual maturity a big reward — the chance to produce more offspring, as the number of eggs a female can produce goes up with an increase in body size. (Miller 2007)
But Reznick and his group are careful scientists and they realized that a variety of hypotheses could be responsible for the changes they had observed when high-predation downstream guppies were transplanted to the upstream low-predation environment. Luckily, Reznick and his group were up to the challenge of designing and carrying out experiments to test these hypotheses.
The Risk of Being Eaten
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?
Let’s fast forward to when Reznick et al. completed many more experiments and published a review of their results. (Reznick and Travis 2019) They reported:
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.
Independent populations of guppies from different streams and pools exhibited parallel allele frequency changes after they were moved from downstream to upstream. From the abstract: “This gradient is repeated in many rivers; in each one, we see the same divergence between guppy populations in life history, behavior, morphology, and physiology.” (Reznick and Travis 2019) 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.
What Kind of “Evolution” Is This Then?
Typically, these repeatable, parallel changes are considered an incredible display of natural selection, with many scientists thinking that the standing genetic variation ultimately arose from random mutation as well as the mechanism that is now maintaining it. Thus, RM/NS are identified as the ultimate cause under both the direct and indirect hypotheses for how predators shape guppy evolution. Is this fair? 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?
The Source of Adaptive Information Is Unknown
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. The essential question that many people are curious about remains: What is the source of the standing variation or genetic diversity that enables adaptation?
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. In my next post I will explore this question further.
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