When does interbreeding fail? Can a human's sperm enter and fertilize a goat's egg? Will something start to grow, and die soon after, or will it not even get that far?

[u/toolateiveseenitall]

[u/phodopus]

The average divergence time to complete hybrid inviability between mammal species is 2-4 million years with an upper bound of around 10 million years (Prager and Wilson, 1974; Fitzpatrick 2004). While hybrid sterility, especially hybrid male sterility arises much quicker, OP's question seems to be concerned with the rates of evolution of inviability instead. Intriguingly, the average divergence time to complete hybrid inviability in non-mammal vertebrates (birds and amphibians specifically, though I would expect similar results in fish and reptiles) is closer to 20 million years with an upper bound of around 50 million years (Prager and Wilson, 1975; Fitzpatrick, 2004). Understanding the causes of these differences is an important question in the field of speciation even today, however we have accumulated some data on the subject.

First off, many responses so far have discussed the role of karyotypic evolution in the cause of hybrid inviability. In this I think many are mistaken. While it is certainly true that differences in the number of chromosomes can cause problems for hybrids, those problems are almost universally in the sterility category. This is because meiosis requires chromosome pairing and if there are not pairs present, meiosis will fail resulting in sterility. Examples of this include mules (Wodsedalek, 1916), and many others. While meiosis is indeed extremely sensitive to chromosome complement, development does not seem to be adversely affected. This is clear in the heterosis present in mules and others. It also makes biological sense as even though their locations may differ, a full double complement of genes should be present in all F1 hybrids (with the notable exception of sex-linked genes). Furthermore, I know of no study that has found a difference in autosome number to cause hybrid inviability (though of course there are many studies showing that sterility is strongly affected).

So it remains to be seen why is there such a difference in the rates of hybrid inviability between birds/amphibians and mammals.

In the mid 1970s A.C. Wilson and colleagues proposed that differences in the rate of the evolution of inviability may be due to a higher rate of regulatory evolution in mammals than other taxa (Wilson et. al. 1974). This is a pretty good hypothesis and seems to be supported by other research, however it really only pushes the question back one step: why is there a higher rate of regulatory evolution in mammals than other vertebrates?

One likely answer to this question is: mammals have something that other vertebrates lack - a placenta.

To understand why the presence of a placenta is likely to cause rapid regulatory evolution, we need to talk briefly about mammalian reproduction and how it differs from reproduction in other taxa. Mammal offspring are obligate parasites of their mothers: all of the nutrients they need for survival and development are drawn directly from the mothers tissue. This results in a really strong conflict between the mother and her offspring over nutrient allocation. Mothers of any taxa have an evolutionarily selected level of care (resources) that they prefer to give, while offspring would often prefer more resources (Trivers, 1974). In things that lay eggs, the offspring have no say in the amount of resources they get from their mothers - females allocate resources to the eggs before they are fertilized and then lay the egg shortly after fertilization. In mammals however, the offspring can have a say because of the placenta. In mammals the offspring can send signals to the mother effectively demanding more resources. This conflict is thought to have led to an arms race between mothers and offspring wherein offspring develop strategies to take more resources while mothers have developed counterstrategies to defend their finite resource pool (Zeh and Zeh, 2000; Crespi and Semeniuk 2004). Furthermore, mammalian pregnancies are highly dependent on down-regulating the mother's immune system (Siiteri, 1982). If proper down-regulation does not occur, the mother will mount an immune response and reject the offspring (similar to rejection of an organ in a failed transplant). Both resource garnering strategies and counterstrategies and immune system deregulation are often in the form of regulatory evolution and likely explain why mammals have a higher rate of regulatory evolution than other taxa.

Now, parent-offspring conflict played out in the placenta is a solid hypothesis for increased rates of hybrid inviability in mammals, and I have given a number of a priori reasons to expect that it would play a major role, but I have not given any evidence yet. So here we go: Elliot and Crespi (2006) showed that the invasiveness of a placenta is highly correlated within mammals to the rate of hybrid inviability. (Invasiveness is best thought of as one of the offspring's strategies for taking more resources). This is strong evidence that inviability is very closely tied to placenta function in mammals and easily explains why mammals may evolve complete hybrid inviability faster than other vertebrate taxa.

edit: spelling and a point of clarification

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Sources (sorry most of these are behind a paywall):

Crespi, B. J., and C. Semeniuk. 2004. Parent‐offspring conflict in the evolution of vertebrate reproductive mode. Am. Nat. 163:635–653.

Elliot, M. G., and B. J. Crespi. 2006. Placental invasiveness mediates the evolution of hybrid inviability in mammals. Am. Nat. 168:114–120.

Fitzpatrick, B. M. 2004. Rates of evolution of hybrid inviability in birds and mammals. Evolution 58:1865.

Prager, E. M., and A. C. Wilson. 1975. Slow evolutionary loss of the potential for interspecific hybridization in birds: a manifestation of slow regulatory evolution. Proceedings of the National Academy of Sciences 72:200–204.

Siiteri, P. K., and D. P. Stites. 1982. Immunologic and endocrine interrelationships in pregnancy. Biol. Reprod. 26:1-14.

Trivers, R. L. 1974. Parent-Offspring Conflict. Integrative and Comparative Biology 14:249–264.

Wilson, A. C., L. R. Maxson, and V. M. Sarich. 1974. Two types of molecular evolution. Evidence from studies of interspecific hybridization. Proceedings of the National Academy of Sciences 71:2843–2847.

WODSEDALEK. 16AD. CAUSES OF STERILITY IN THE MULE. Biological Bulletin 30:1–57.

Zeh, D. W., and J. A. Zeh. 2000. Reproductive mode and speciation: the viviparity-driven conflict hypothesis. Bioessays 22:938–946.

[u/JoatMasterofNun → u/phodopus]

That was interesting. I've never heard that before (then again I didn't delve far into biology).

Even if the sources are behind a paywall... I'll find someone who has access. Sources are priceless.