Okay, I didn't find exactly what I was looking for, but I did find this:
Organelles, those "critters" which live in our cells and keep them running, get left behind when the sperm enters the egg. All your organelles come from your mother, which means that males are a genetic dead-end for organelles. Organelles that "want" to pass their genes on "don't want to" end up inside a male.
This means that hermaphrodites are in a constant battle against rebellious organelles trying to destroy their male parts. Male-killer genes have been found in 140 species of plant, diverting resources away from the anthers and into the seed. You can tell they're at work because you end up with no males, just hermaphrodites and females. Plants with males and hermaphrodites only are almost unknown. The hermaphrodite plants are in a continual race against their organelles to stay male-fertile.
This doesn't apply to most animals. Imagine a race of hermaphrodite mice. There arrives in it's midst a mutation, which happens to kill male gonads. It spreads because females who have the gene do quite well; they have twice as many babies because they put no effort into making sperm. Soon the population is females with the male-organ-killing gene and hermaphrodites.
The hermaphrodites are now at a premium, because they're the only ones selling sperm to the females. It doesn't pay to have a male-organ-killing gene anymore; rather, it would pay to have a female-organ-killing gene so that the hermaphrodite could become male and concentrate solely on selling sperm to the females. If the female-organ-killing gene appears, hermaphrodites are no longer at a premium, because they're now competing with pure males and pure females, specialised at doing either task better than a hermaphrodite. Most of the sperm on offer comes with female-organ-killing genes, and most of the eggs on offer come with male-organ-killing genes, so the offspring are constantly forced to specialise.
The theory goes further, into how organelles, having lost that battle, attempt to kill off every male in a population, but that's enough to prove my point. I just wish I could find the "hermaphrodites and males and females in a cave" example I was looking for, but it's not in the book I thought it was...
Organelles, those "critters" which live in our cells and keep them running, get left behind when the sperm enters the egg. All your organelles come from your mother, which means that males are a genetic dead-end for organelles. Organelles that "want" to pass their genes on "don't want to" end up inside a male.
This means that hermaphrodites are in a constant battle against rebellious organelles trying to destroy their male parts. Male-killer genes have been found in 140 species of plant, diverting resources away from the anthers and into the seed. You can tell they're at work because you end up with no males, just hermaphrodites and females. Plants with males and hermaphrodites only are almost unknown. The hermaphrodite plants are in a continual race against their organelles to stay male-fertile.
This doesn't apply to most animals. Imagine a race of hermaphrodite mice. There arrives in it's midst a mutation, which happens to kill male gonads. It spreads because females who have the gene do quite well; they have twice as many babies because they put no effort into making sperm. Soon the population is females with the male-organ-killing gene and hermaphrodites.
The hermaphrodites are now at a premium, because they're the only ones selling sperm to the females. It doesn't pay to have a male-organ-killing gene anymore; rather, it would pay to have a female-organ-killing gene so that the hermaphrodite could become male and concentrate solely on selling sperm to the females. If the female-organ-killing gene appears, hermaphrodites are no longer at a premium, because they're now competing with pure males and pure females, specialised at doing either task better than a hermaphrodite. Most of the sperm on offer comes with female-organ-killing genes, and most of the eggs on offer come with male-organ-killing genes, so the offspring are constantly forced to specialise.
The theory goes further, into how organelles, having lost that battle, attempt to kill off every male in a population, but that's enough to prove my point. I just wish I could find the "hermaphrodites and males and females in a cave" example I was looking for, but it's not in the book I thought it was...
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