In Does the mosquito’s hum alternate between D and F?, Doug says that different species of mosquitos that live in the same area would have different pitches to avoid inter-species mating. Obviously mosquitos wouldn’t want to waste time on mating if the offspring would be a dud (either wouldn’t be concieved, viable or fertile).
However, what is so bad about hybridization? Doesn’t it still mean that the individual’s genes get passed on? Is this one of those descent of species vs. descent of individual things?
Nothing necessarily wrong with it. Certain critters make a habit of it - Some fungi for example, certain genera of lizards ( whose hybrid offspring are very interesting indeed - the hybridizations cause a proliferation of “little” speciation events ), etc.
The problem is A) as your last sentence in your first paragraph notes, many hybrids are not viable. Consequently they die without passing their genes and whatever little behavioral quirk made them hybridize may just die out because of it and B) in certain species with very tight niche requirements even a viable hybrid may be less than optimal and not survive - Again not passing on the genes coding for behavior that may lead to hybridization. I don’t know which category mosquitos fall into, but whatever it is there is obviously strong selection pressure against that sort of thing.
There of course some critters that will mate with anything that moves ( like your hand )and quite a few things that don’t. In those cases what you are probably seeing a combination of low energetic costs to mating ( so why not keep on doing it in hopes of getting lucky? ) and/or a critter that has a specific mating season and mates in aggregate - i.e. in the right season they are in an environment with tons of potential mates and lots of compeitition in the immediate area, so you might as mate 'til you drop while you can and hope you score. Damn hormones .
It might also be caused (at least in part) by the Doppler effect. A mosquito flying towards you will sound a slightly higher pitch than one flying away from you. It would take a speed of 400 mph to double the pitch. (The speed required to double the pitch is about 0.6 the speed of sound:
Of course, 400 mph is too fast for an insect, but change of just one note should be possible.)
For even more details, see the paper “Synthesis of Virtual Motion in 3D Auditory Space” by Jenison, Neelon, Reale, and Brugge of the Psychology Department of U of Wisconsin, Madison.
They apply doppler-shifting of the recorded hum of a mosquito! Figure three in this paper shows the drop in pitch displayed by a spectrogram of the Fourier analysis of the sound. It looks like a drop of about one note to me. Maybe not enough for an F to a D, but a note change nonetheless.
)and quite a few things that don’t. In those cases what you are probably seeing a combination of low energetic costs to mating ( so why not keep on doing it in hopes of getting lucky? ) and/or a critter that has a specific mating season and mates in aggregate - i.e. in the right season they are in an environment with tons of potential mates and lots of compeitition in the immediate area, so you might as mate 'til you drop while you can and hope you score. Damn hormones 
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