Tuesday, April 7, 2009
Ultimate Arms Race
Who will win in an "evolutionary 'arms race'"? The Oregon rough-skinned newt (Taricha granulosa) or the common garter snake (Thamnophis sirtalis)? (pg. 166). Will there be some form of "compromise" in the end? Why? Explain your reasoning. Relate back to ecology unit. It's up to you to decide....dun dun dun....
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The evolutionary arms race between the rough skinned newt and the common garter snake is a classic example of coevolution. The skinned newt applies natural selection pressures on the garter snake when it becomes more toxic and the garter snake's counter-adaptation to the more toxic newt applies a selection pressure on the newt. As we see in most coevolution models, there is no winner or loser since selections pressures are constantly applied on both organisms. However there could be a limit to which the newts can increase there toxicity. If the production of toxins becomes so energy costly that it significantly compromises the newt’s ability to garner enough food to survive, selection pressures could force newts from evolving into more toxic variants. At this point if the garner snakes are still resistant to the newts’ toxin, then the garner snake would “win” the arms race. However it is worth restating that most coevolution cases show continuous evolution from both creatures. For example, bacteria have continually developed resistance to many “counter-adapting” antibiotics.
ReplyDeleteBy the way, just so everyone can understand my pain, I just accidentally clicked the "Newer Post" link instead of "Post Comment" so I just lost my whole response that I spent so much time writing. Now I'm gunna rewrite the whole thing and its not gunna be as good, and I'm a little upset right now, but here I go.
ReplyDeleteIts hard to know who will win the "arms race," but the rough-skinned newt has to be the winner if it wants to survive. Since the newt really has no other mechanisms for defense, without the TTX, it would be easy prey, and probably would have died off and become extinct by now. The garter snake does not depend on its resistance to TTX for survival because it is a predator, and can find other prey. The newts are highly toxic because they depend on their toxicity for survival. Through natural selection, newts that are more toxic will survive and reproduce, and newts that are less toxic will be eaten and not survive and reproduce. The newts that do survive will pass on their "toxicity genes" and the baby newts will also be toxic, being able to survive and reproduce. However, at the same time, garter snakes that are resistant to TTX will have plenty of easy prey, because the newts have no other defense other than their toxin. Those snakes will then survive and reproduce, while the newts will less toxicity will die off, thus only the newts with higher toxicity will survive. This is the process of coevolution. In order to survive, the newts have to have a strong toxin to defend against predators, while the toxin needs to be strong enough to still affect the resistant garter snakes. And the garter snakes with higher resistance to the toxin will have a selective advantage over the other snakes because it can eat the toxic newts, so the more highly resistant garter snakes will then survive and reproduce. Vik, I don't quite agree when you said, "However there could be a limit to which the newts can increase there toxicity." The newts themselves have no control over their toxicity. Their toxicity is based on chance and their parents. If a newt is born with genes that code for a stronger toxin, then that newt will survive and reproduce, passing on its toxicity genes, while newts born with genes that code for less toxicity will be eaten, thus the rough-skinned newt has evolved with the highly toxic TTX and has successfully survived and reproduced.
This example of coevolution described as an "arms race" has been occurring for a very long time, and it is highly important to determine the extent of the selective pressures on each organism. In the garter snake's case, its resistance to TTX is probably less important to survival than the newt's toxicity. The newt is only one of many different prey for the garter snake, so the selective pressure that causes less resistant garter snakes to die when eating newts isn't as heavy as the newt's selective pressure of being toxic. This is due to the fact that the newt's toxicity is its only defense against every single one of its predators. Without the toxicity, the newt would be eaten and not be able to reproduce and pass on its genes into the newt gene pool. On the other hand, it is quite possible for a non-resistant garter snake to live its entire life without eating a single toxic newt and pass on its genes by reproducing. It is much more likely for a newt to cross paths with one of many predators than it is for a garter snake to cross paths with a single one of its prey. Therefore, on average, the newt will have to defend against more hungry predators than a garter snake has to defend against toxic newts. Overall, this shows that the selective pressure to become more toxic on newts is much more than the selective pressure to become more toxic resistant on garter snakes. Due to this, I believe that the rough-skinned newt will always be slightly "ahead" in the "evolutionary arms race" while the garter snake will slightly lag behind in TTX resistance because natural selection is working more heavily on the the newts.
ReplyDeleteThe relationship between the rough-skinned newt and the common garter snake is an example of co-evolution. The predator and prey relationship between the newt and the snake makes them the selection agents for the other species. Genetic variation within a population of newt would result in newts with varying levels of toxins. The snake would act as a selection agents of the newt because only the newts with the highest level of toxin would escape predation. The newt produces TTX, a tertrodotoxin that interfere the sodium channels in the snake’s nerve cells. Sodium channels are crucial for nerve cells cell to generate the membrane potential needed to send a nerve impulse. Thus, TTX is a neurotoxin that impairs the nervous system. This is a direction selection for the newts because low level toxin individuals will be eliminated from the gene pool until all individuals in the population produces high levels of toxin. The newt also acts as a selection agent for the snakes. Genetic variation within the snake population causes some individuals to be highly tolerant to toxin while others are less tolerant to the newt’s toxin. Directional selection would occur for the snakes because the snakes who cannot tolerate the newt’s toxin will die from the eating the newt and only the snakes that can tolerate the newt’s toxin will be able to survive and reproduce. Only toxin resistant genes would remain in the snake’s gene pool. Thus direction selection will push for an “arms race in which selection favors increasingly resistant snakes and increasingly toxic newts” (Carroll 166).
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I agree with Vik’s assertion (sorry Paul) that “a limit to which the newts can increase there toxicity”. Since the relationship between the snake and newt is a predator and prey relationship, the prey would always have to stay one step ahead of the predator in this arms race. If the newt’s toxin ever lags behind the snake’s resistance, then the newt population would be easily eliminated. On the other hand, if the newt becomes too toxic for the snake to handle, the snake could always find other prey to eat. Eventually, manufacturing toxins would be too costly to the newt and newts can no longer stay ahead of this arms race. Thus, the snake’s can freely feed on newts. Compromise can never be made between prey and predators species.
I believe that the only way to determine who would win the evolutionary arms race is based on the evolutionary rate, which is related to the number of generation per year. This rate is probably the same for both species, because they are both R-selected species so neither species will ever completely triumph over the other. Both species of newt and snake will continue to coevolve until the environment changes significantly enough to destroy both populations completely. The reason for this that over time, the ecosystem has developed a balance between all of the species, such that if one population gets too large, a negative feedback loop will regulate the population. For example, if the newt population gets too large, the snake population will bloom, thus bringing the ecosytem back into a state of equilibrium. In any case, the survival of both species depends on neither winning the arms race, because if the newts become too numerous (due to better anti-snake adaptations), they will deplete their own food source and become their own demise.
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