The simultaneous changes in many characters, induced by the temperature treatment in Kammerer’s experiments, involve a large number of genes. A mechanism that would reactivate or silence these genes when temperatures rise above 25 °C would disintegrate by random mutations or selection against it if it was not used during millions of years. If midwife toads would have used the switch and reverted to water breeding without parental care when temperatures exceeded 25 °C, most of the eggs would have perished to infections by Oomyceta (see next chapter) and young larvae would have suffered from high predation rates. The adaptive value of male parental care on land in Alytes (Figure 1) is that it reduces egg mortality and predation risk. The increase in fecundity of water-breeding midwife toads reported by Kammerer (1909) would not have been enough to compensate for the increased mortality and natural selection would have acted against the preservation of the switching mechanism.
Figure 1. Male midwife toad (Alytes obstetricans) carrying a batch of developing embryos. The drawing is by Bas Blankevoort @ Naturalis Biodiversity Center. Excellent footage on the breeding behaviour of the midwife toad can be seen at https://vimeo.com/86142455.
Midwife toads have an extended breeding season, lasting from spring throughout summer. A moderate increase in temperature to 25 - 30 °C, as in Kammerer’s experiments, is within the variation of an Iberian summer. A warm spell would, in Kammerer’s view, have caused the midwife toads to breed in water, but such warm spells are too short to last more than one breeding attempt. As adaptation to higher temperatures, according to Kammerer, occurs only gradually and after a number of generations, the switch to water would have reduced their reproductive success to almost zero, making that natural selection would have favoured animals that continued to breed on land. The genetic architecture proposed by Vargas et al. (2016), involving epigenetic switches on many different loci, would thus also have been selected against.