Great work tackling this scientific paper. Reading and comprehending these sorts of papers is always a challenge to start with, and gets easier with practice. Remember that the authors anticipate their audience to be experts in the field, so that is why it may seem hard to read if you are just starting to learn about this stuff!
I really enjoyed reading your answers and you seem to be very much thinking in the right way. I tell you what I was thinking about these discussion questions.
Discussion Questions.
3) In this experiment, the authors discovered that smaller male beetles were substantially better at finding females than larger male beetles, particularly at cool temperatures. However, finding a mate is just one part of this beetle's life. Do you think it is always better to be smaller for these male beetles? Can you think of other parts of a beetles life where it might be advantageous to be larger? Do you think it is better for females to be smaller as well?
My Answer: Certainly there could be many life stages or situations where it is not advantageous to be smaller. Yessenia brought up a nice point about bigger females being able to have more offspring. This is called fecundity selection- often larger females can have more (or bigger) offspring, which may increase their fitness. Also, many of you recognized no predators were in this experiment, and predators may be able to attack smaller beetles better. We don't actually know if this is the case, but could probably design an experiment to try and figure out. Also, these beetles have something called nuptial gifts (the authors wrote about this in the beginning). Males offer females a gift at the time of mating, and females like bigger gifts, and bigger males can make bigger gifts. And do you know what the gift is? Beyond just giving the female sperm at mating, the male ejaculate contains a nutrient rich substance that the female eats!!!! Yep, females like to eat the male's large ejaculates, and it actually helps them nourish their offspring! Pretty awesome, huh!
Could spiders (or beetles) ever get this big??? Thanks to evolutionary constraints, they probably cannot! As Mr. Reedy knows, spiders are not my favorite animals, so I am particularly grateful to these constraints for this reason. He can probably tell you some good stories about my Arachnophobia...
4) Suppose that we went and found some beetles in their natural habitat, studied them for a year, and found that in every situation, it was better to be larger. What do you think would happen over a very long time (many thousands of generations)? Would we find beetles as big as a human some day?
You guys came up with some pretty good answers to this one too! If there is strong selection for beetles to be larger, they will evolve to be larger. In fact, the authors had already artificially produced larger and smaller strains. However, just because during one year, it was better to be larger, doesn't mean that it would consistently be better to be larger. Maybe the climate in the year of the study favored larger beetles. Or maybe the optimal beetle size is just slightly larger than they are now. Then we would expect selection to favor larger ones until they reach this size, but then when they get bigger than the optimal size, selection would favor smaller beetles. Julia brought up a great point, too. She thought that the structure of the beetles exoskeleton might limit how large the beetle can get, and she is probably right. You never find spiders or beetles, or many other invertebrates over a certain size, because of what we call constraints. For example, beetles have very different respiratory and circulatory systems than mammals do, and wouldn't likely be able to support such a large body size with the limitations imposed by theses systems.
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