Episode 9: Chapter IX - Hybridism

We dedicate this episode of the podcast to Chapter 9 - Hybridism in Darwin's Origin of Species (OoS). We all agree that this chapter is one of the most challenging chapters to read in OoS. In this chapter Darwin force marches the reader through example after example of hybrids while simultaneously admitting his ignorance on why these creatures should exist. It is not inherently obvious to the reader why they are being subjected to this catalog of biological anomaly and minutiae concerning pollination of orchids and other flowers but Darwin is adamant to share his knowledge, like a proud new parent inflicting others to look at numerous photos of their newborn. Darwin , early in Chapter I of OoS, argued against the notion that new species are formed through hybridization of existing species and we expected him to make that point in this chapter, but he never really does. It seems that by the end of the chapter we are exhausted from Darwin's recounting various animal and plant hybrids and admitting his ignorance of sterility in hybrids but that does not dissuade Darwin from concluding he was right all along - “…the facts given in this chapter do not seem to me opposed to the belief that species aboriginally [emphasis added] existed as varieties”. Darwin stays on point. We started off our discussion identifying our favorite hybrids. Josh first suggested mermaids as his favorite hybrid but James required he offer up a "real" hybrid. Mermaids James grew up with in FloridaJosh introduced us to the Liger, a hybrid between a male lion and female tiger both species have 38 chromosomes which also allows for reciprocal mating. Male tigers and female lions create tigons. The hybrid nomenclature is a portmanteau word derived from using the male species as the prefix and the female as the ending of the name hence a liger and tigon being separate types of hybrids. 900 lb Hercules Sarah mentioned the Zedonk, a hybrid between a zebra and a donkey. This is another one of the hybrids that are created artificially in captivity. Few people realize that there are actually three different species of zebras - Grevy's, Plains and Mountain zebras - and each species has their own number of chromosomes. Grevy's zebras zebras have 46 chromosomes, Plains zebras have 44 and the Mountain zebra has 32 chromosomes whereas the donkey has 62. Image from Carole Coleman James suggested the Grolar, a hybrid between a grizzly and polar bear, was his favorite hybrid. A grizzly bear with her grolar cubsUnlike the liger and zedonk, the grolar occur in nature when the two species of bears interact. Historically grolar bears were less common since Grizzly bear distribution was further south than Polar bears but the warming of the climate has caused both bears to extend their range, polar bears southward and grolar bears northward, such they now more commonly overlap during the breeding season. Research suggests Polar bears "recently" diverged from brown bear ancestors with the speciation event occurring only 343,000-479,000 years ago. A mere blink of time in terms of the geological record. We discussed the most famous of all hybrids, the mule and how it is formed from the male donkey breeding with a female horse. Pictured below is a horse (left) and a mule (right) showing how the mule is often much larger than either of its parents, an example of Hybrid Vigor. picture by Merle We noted that the mule hybrid can only be formed from male donkeys and female horses and rarely rarely formed from a male horse and female donkey. Sarah suggested that the reason the mule can only be formed from a male donkey and female horse has to do with the constraints of difference in gestation times between horses and donkeys. It turns out that horse gestation is 11-12 months whereas donkey gestation period is 11-14 months. There is a lot of overlap in gestation time between the two animals but it is an intriguing idea to consider it may influence non-reciprocal successful mating.  We in