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Natural selection for newt toxicity
To determine if newt toxicity and snake resistance had coevolved, Butch had to go back to the basics of natural selection. By this time (more than thirty years after he started the investigation as a graduate student), Butch had become Dr. Brodie and was investigating the snakes and newts with the help of his son, Dr. Edmund D. Brodie, III, as well as many other researchers and students.
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| Butch with his son, Edmund D. Brodie, III, out in the field in 1966 (left), and Butch, his son, and grandson out in the field in 2006 (right). |
Their first job was to show that natural selection could operate on both newt toxicity and snake resistance. And in order to do that, they had to demonstrate that the three requirements of natural selection — variation, heritability, and differential reproductive success (i.e., selection) — was true of both the newts and the snakes. For example, if some newts produced more toxin than others (variation), and if more toxic newts escaped their predators and survived longer to produce more offspring (differential reproductive success or selection) and if those offspring inherited their parents' level of toxicity (heritability), over time the newts would evolve to be toxic through natural selection. But in order for natural selection to operate in that situation all three requirements must be met. Did the newts meet those requirements? Click on the "test it" links below to find out.
Can natural selection operate on
newt toxicity and snake resistance?
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variation |
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heritability |
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differential
reproductive
success/selection |
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TTX production in newts |
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TTX resistance in snakes |
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Do TTX levels in newts vary?
Japanese researchers developed a technique to measure TTX levels. When this method was applied to the newts by Brodie student Charles Hanifin, as expected, the amount of TTX varied greatly between individual newts in the same population, and even more so between populations.
The verdict:
There is variability in TTX production in newts.
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Is TTX production in newts heritable?
One way to find out if a trait is heritable is to see if parents and offspring resemble one another for that trait by capturing mother newts and raising their offspring. Unfortunately, newt eggs take too long to grow up in the lab, making it difficult to measure heritability this way. Instead, the Brodies and student Charles Hanifin approached the problem from another direction. Other toxic organisms whose toxicity is not genetically encoded, like poison dart frogs, get their toxicity from the insects they eat. Taking their cue from the frogs, the scientists set out to determine whether the newts acquire their toxicity from chemicals in their diet. They found that newts fed a diet of non-toxic crickets for a year in the lab actually became more toxic, not less! So newts' toxicity does not seem to come from their diets — leaving open the possibility that their toxicity is genetically encoded, and hence, heritable.
The verdict:
The newts aren't getting toxins from what they eat, so it seems likely that they are making the toxins themselves, which would be a heritable trait.
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Does TTX production in newts affect reproductive success?
Is toxicity advantageous — in other words, do newts with enough TTX to deter predators leave more offspring for future generations than those with insufficient TTX? Some simple observations were enough to answer this question. TTX serves as such a powerful defense that newts mistakenly eaten by non-resistant predators have survived! Non-resistant predators that attack and bite newts become intoxicated by the TTX, so toxic newts are able to survive the encounter by simply wiggling free. Dr. Brodie even watched a newt crawl unharmed out of the mouth of a bullfrog 15 minutes after being eaten! Newts with a lot of TTX escape predation and live to produce offspring — so TTX does seem to be advantageous.
The verdict:
TTX production is advantageous and improves reproductive success.
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Do levels of resistance in snakes vary?
Garter snakes crawl slowly and awkwardly when intoxicated with TTX — but some snakes handle the toxin better than others. The Brodies designed a special racetrack with infrared sensors to measure how fast snakes crawl. After injecting snakes with the same amount of TTX, very resistant snakes crawl nearly as fast as before, but less resistant snakes crawl much more slowly. This showed that garter snakes vary in their level of resistance to TTX.
The verdict:
There is variability in TTX resistance in snakes.
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Is resistance in snakes heritable?
To determine the heritability of resistance, the Brodies compared the TTX resistance levels of lab-reared snake siblings. If a trait is heritable, then the siblings, who share many of the same genes, should resemble each other. And indeed, garter snake siblings strongly resembled each other in their level of resistance to TTX, so the trait does seem to be heritable.
The verdict:
TTX resistance in snakes is heritable.
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Does resistance in snakes affect reproductive success?
Is resistance advantageous — in other words, do TTX resistant snakes leave more offspring for future generations than non-resistant snakes? The answer was pretty clear to Brodie student Becky Williams. High resistance snakes eat newts without any trouble, but snakes with lower resistance became very intoxicated and can't crawl for hours after eating a newt. These intoxicated snakes might be eaten by predators like hawks in the wild. So as long as they are trying to eat toxic newts, snakes with high TTX resistance will have an advantage over snakes with low or no resistance.
The verdict:
TTX resistance is advantageous and improves reproductive success.
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Through all of those experiments and observations, the Brodies showed that natural selection could operate on both newt toxicity and snake resistance — but they had one final hurdle yet to leap before they could satisfy themselves and the other scientists... |
