North American cicada male of the genus Magicicada sing depending on the species at different times of the day and attract the females of the corresponding species. The species are very similar, but when crossing they have sterile (infertile) offspring. enter image description here In the species M. tredecim the time of the song is determined by the gene locus EARLYBIRD in an intermediate inheritance: The allele EARLYBIRD leads in the homozygous state to morning singing, the allele earlybird leads in the homozygous state to evening singing, heterozygotes sing at noon.

How does the frequency of the earlybird allele in the species M. tredecim change when the number of females of M. tredecassini increases? Does it increase, decrease, or does it remain the same?

I think it would not change the frequency of the earlybird allele, because they don't have a evolutionary advantage. But maybe it could also decrease the frequency, because infertile descendants sounds like a disadvantage? But I'm totally not sure and can't provide a real 'biological' reason since my thoughts are more like intuition.

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    $\begingroup$ Homework questions are off-topic on Biology unless you have shown your attempt at an answer. For more information see our homework policy. 2 $\endgroup$ – Remi.b Jan 20 '18 at 18:10
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    $\begingroup$ @Remi.b : Thanks for the hint. I have written down my own thoughts about this question. $\endgroup$ – ScientiaEtVeritas Jan 20 '18 at 18:16
  • $\begingroup$ Good. I am retracting my close vote. +1 $\endgroup$ – Remi.b Jan 20 '18 at 20:53

Infertile descendants indeed sound like a disadvantage. Individual must invest a fair amount of resources into making an offspring. Making sterile offspring is just a waste of resources as those offspring aren't going to pass on any gene further to any grandchildren.

If there is an increase of the tredecdssini females, then male tredecim that sing quite late are going to mate with many tredecdssini leading to the production of sterile offspring. There is therefore selection against the allele 'earlybird' leading to an increase in frequency of the 'EARLYBIRD' allele.

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