Question

Thamnophis sirtalis garter snakes eat poisonous newts, Taricha granulosa, containing tetrodotoxin. Resistance to tetrodotoxin is highly heritable in these snakes. Garter snakes must also escape predators by moving quickly, and crawling speed is also highly heritable. Selection on both resistance to tetrodotoxin and crawling speed are strong. Nonetheless, neither trait increases very much each generation if both are artificially selected in a lab population. Explain this result.

Answer 1

We are disscussing two traits of Thamnophis sirtalis garter snakes.

1. Resistance to tetrodotoxin, present in  Taricha granulosa (poisonous newts- pray of garter snake). This trait is highly heritable

2. High crawling speed, necessary to quickly move away from predators. This trait is highly heritable.

Also, selection on both traits is strong.

However, in lab population with artificial selection, in every generation both traits do not increase. This could be due to

1. In lab population of snakes, it is challenging to feed snakes with poisonous newts, which is a normal pray for these snakes in wild. Also, in lab sufficient food is provided to snakes, hence resistance to tetrodotoxin is not an essential trait for survival in lab as compared to natural selection in wild, which ensures resistance to tetrodotoxin as favorable trait.

2. In protected environment of lab, population is not under the selection pressure due to predation, which will be present in case of wild population. Thus, high crawling speed for survival is not an essential trait in lab population.

3. In absence of selection pressure, we can expect that both traits will not increase over generations in lab population, even if these are critical for wild population.

4. As both these traits have high heritability, these traits will remain in population at same proportion even after elimination of selection pressure. Natural selection ensures survival of fittest and snakes with out these traits will be naturally eliminated (by death) in wild population. Thus increasing frequency of both traits in population.

Artificial selection in lab, will not be similar to the natural selection. (In fact, snakes with high crawling speed are difficult to catch). So in future generation we can even expect selection pressure as low crawling speed.

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