Hello, everyone. What I have to present is contrary to most of modern biological thought, but from it, it would seem that universal common descent is a paradox that is not possible under natural conditions. The vast majority of readers might prejudge it as wrong immediately, but I respectfully ask that people give it a chance without presupposition and with an open mind. Even if you disagree, I certainly appreciate anyone who takes the time to read this and consider the thought experiment. By the end of it, you will probably, at the very least, be struggling to come up with a reconciliation.
Let's begin by examining feline penises. They all have a common attribute, and that is the fact that they are all barbed with numerous, small spikes that are of a similar substance to a fingernail. The barbs on the penis provide the required physical stimulation for ovulation in cat females. It is also thought by some that the penis-barbs also serve the purpose of scraping out competing sperm.
Natural selection would explain that, while in early development, a certain primitive feline of the male sex received a genetic mutation which resulted in a barbed penis. That male feline had sex with multiple females, and the barbs on the penis scraped out competing sperm, causing the cat to have more offspring than other males. The children retained the barbed-penis gene and afterward the males matured and had sex with other females, passing on their genes more than males of their generation without barbs, and the process continued with exponential growth until all male felines had barbed penises.
However, there's a problem with that. The female cat's reproductive system relies upon the spines of the male's penis in the first place. Unlike most mammals, cats have a specific physical trigger for ovulation, and that trigger is the spines. That ultimately means that, not only did the initial male cat have to randomly receive the mutation of the barbed penis, but a female in the same general area in a similar generation would also have to randomly received the genetic mutation that makes them precisely receptive to barbs of the other cat's general size and composure in order to ovulate. That by itself is almost impossible in terms of probability, but that's not all. The exponential notion of how the barbed penis would have spread goes out the window given this, because only the one female who happened to have the mutation would be receptive to the barbed penis. That means that only the couple's offspring would have the male and female reproductive qualities of felines as we know them today, and they would have had to have literally be in a bizarre situation such as hiding underground while all other felines on the planet are destroyed by an asteroid, and repopulated their species through extensive incest. This entire set of criteria is so unlikely to occur naturally, it is far within reason to call it practically impossible.
Yet, we're presented with the compelling image of a barbed penis which scrapes out competing sperm as a testament to how virtually all traits common to a species are here because of natural selection. It makes sense unless you factor in a detail like the female's reproductive system. This type of logic can be explained by the diagram below:
The claim positions itself to support two known facts, then uses itself to support a new, wider conclusion. Note that this particular claim, as explained above, does not hold up to reason. While the facts in black are observably true, the claims in red make sense given the facts presented
, but are proven wrong by examining them in relationship to other factors
There is really no way for the barbed penis and the female reproductive system that relies entirely upon the barbed penis to have developed separately and have overtaken the population of early felines who did not have barb-receptive reproductive systems exponentially through natural selection. Mathematically, the only reasonable conclusion is that felines were, at one time, in an initial state in which both sexes had barb-compatible reproductive systems. Extending the line of logic of pairing barbed and non-barbed reproductive systems, how could a species of animal with barbed penises have ultimately evolved from a previous ancestor that did not have barbed penises? Conversely, how could a species with non-barbed penises have evolved from a species with barbed penises?
In a hypothetical situation in which an "ancestor" species has a non-barb-receptive reproductive system, one individual would have to have either the barbed or barbed-receptive mutation, depending on sex, but the odds of another member of that species being nearby and having the mutation that would compliment theirs are so ridiculous, they are beyond practically impossible.
This is the Keyhole Paradox, because an oversimplified way of explaining it is to imagine that, by random chance, both a key and the exact-fitting lock for the key would have to randomly appear through mutation, and besides the incredible unlikelihood for this mutation to occur, all other animals of the initial species would be incompatible with either the "key" or the "lock," save for the offspring of this unlikely pair.
This logically disproves the claim that felines resulted from an ancestor that did not have barbed penises. Consequently, this also logically disproves the idea of universal common descent, regardless of species. Because of this keyhole paradox and how sexual reproduction works, it's practically impossible for a species with a barb-oriented sexual reproduction system to have evolved from a species whose sexual reproduction method is not barb-oriented through natural selection.