Sometimes a good idea just keeps getting rediscovered. The eye has evolved many times independently, and parasitism has evolved for no less than 60 times. This is called convergent evolution. And of course, it has its evil twin... divergent evolution.
Consider this picture below, of the complex shapes of closely related species of Bombus (bumblebees). These species are so similar that the main way to distinguish them is by looking at their penises...
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| Source: The Evolution of Primary Sexual Characters in Animals, Figure 4.1 |
Why are penises so diversely shaped? It's divergent evolution driven by a father-mother arms race. Not just penises, which are visible to the naked eye, this arms race goes all the way down to the proteins related to sex and fertilization. This is one reason why most hybrids, even between closely related species, can't even get pass the fertilization stage.
For more on how can quickly drive divergent evolution, creating two species that cannot interbreed, see Rapid evolution of reproductive barriers driven by sexual conflict (2000).
And of course, if penises have divergent evolution, so would sperms, and semen fluid.
Sexual selection and genital evolution (2013):
Male genitalia show patterns of divergent evolution, and sexual selection is recognised as being responsible for this taxonomically widespread phenomenon.
Sperm wars and the evolution of male fertility (2012)
... mating with multiple males (polyandry) is a taxonomically widespread female reproductive strategy... While females may gain benefits from mating with multiple males, the consequence of polyandry for males is that their sperm may face competition from the sperm of other males to fertilize a limited supply of ova.
There is good evidence that sfps are subject to selection from sperm competition, and recent work is pointing to an ability of males to adjust their seminal fluid chemistry in response to sperm competition from rival males.
It even has a section titled "Seminal fluid proteins: the neglected weaponary".
... the main adaptive value of changes in sperm design under sperm competition is that they increase sperm swimming velocity. Since a number of studies have shown that sperm swimming velocity is a main determinant of fertilization success, this explains why it is targeted by sexual selection so efficiently.. This study presents phylogenetically-robust evidence that sperm competition in mammals favours an increase in the size of all sperm components and an elongation of the head, which result in faster swimming speeds.
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| Figure 14.17 of Thorp and Covich’s Freshwater Invertebrates |
And similarly, divergent evolution would shape vagina
A nice news report to read is BBC's The Twisted World of Sexual Organs (2014)
The diversity is so vast, there is even a species of cave insects that has male vagina and female barbed penis, which vacuums out the nutritious semen. The females mate aggressively, apparently because food is scarce, so semen is a good food. (Female Penis, Male Vagina, and Their Correlated Evolution in a Cave Insect, 2014)
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| The graphical abstract. Graphical, certainly. |
The matings last 40-70 hours, and cannot be interrupted. When the researchers attempted to pull a male and female apart, the male's abdomen was ripped from the thorax without breaking the genital coupling.
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| The female is on top. |
Ahem... we found succubus.
Talking about the dangers of sex, there is one case where sperms are deadly. In one experiment, biologists mated two closely related nematodes: male Caenorhabditis nigoni, when mated with hermaphroditic Caenorhabditis briggsae, caused sterilization and even death of the herm, because the briggsae are self-fertilizers, and thus don't have to have competitive sperms, while the nigoni has only males and females, and so they have competitive sperms. When nigoni sperms are inside briggsae, they get so rowdy that they would seriously hurt the internal organs of briggsae.
From New Scientist:
When Haag examined the C. briggsae worms, he saw C. nigoni sperm in the body cavity, even in the worm’s head. The sperm had broken out of the worm’s uterus, destroyed the ovaries and then rampaged through the body, causing severe tissue damage.
“The two species are very close in evolutionary terms, yet when they mate all hell breaks loose,” says Haag.
(Intense Sperm-Mediated Sexual Conflict Promotes Reproductive Isolation in Caenorhabditis Nematodes, 2014)
Other instances of evolutionary divergence
Why are mammalian placenta so strangely shaped? It's the result of fetus-mother arms race. I wrote a full post all about the placenta and how it illustrates the arms race.
Why sexual reproduction at all? It's the result of parasite-host arms race.
Think about sex from the point of view of a selfish gene in a female turkey. There are two kinds of turkeys: male and female. The female turkeys can in fact clone themselves. The choice for you, is then to decide whether to clone yourself or have sex with a male.
If the turkey clones itself, then you have 100% chance of getting into the child. Else, 50%. Since the cost on the mother's body is the same in either case, why would any selfish gene prefer sexual reproduction? And yet, sex is everywhere in the tree of life, and stable over millions of years.
One compelling reason for sex is the Red Queen parasite hypothesis: Parasites and hosts are in a permanent war of control. If the host is too static, the parasite can evolve strategies to exploit the host, and vice versa. Think about it like a war of cryptography: if one side keep using the same codes, the other side would figure it out.
As such, both sides must keep changing. Sexual reproduction, by recombination, keeps each generation fresh and not predictable for the other side.
Think about sex from the point of view of a selfish gene in a female turkey. There are two kinds of turkeys: male and female. The female turkeys can in fact clone themselves. The choice for you, is then to decide whether to clone yourself or have sex with a male.
If the turkey clones itself, then you have 100% chance of getting into the child. Else, 50%. Since the cost on the mother's body is the same in either case, why would any selfish gene prefer sexual reproduction? And yet, sex is everywhere in the tree of life, and stable over millions of years.
One compelling reason for sex is the Red Queen parasite hypothesis: Parasites and hosts are in a permanent war of control. If the host is too static, the parasite can evolve strategies to exploit the host, and vice versa. Think about it like a war of cryptography: if one side keep using the same codes, the other side would figure it out.
As such, both sides must keep changing. Sexual reproduction, by recombination, keeps each generation fresh and not predictable for the other side.
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