THE VARIATION HURDLE.
This is a “stop press” hurdle, discovered just before going to press. The following quote is from the book Darwin And His Flowers, by Mea Allan (In 1967 she was awarded the Leverhulme Research Scholarship to write about the botanists William Hooker and Joseph Dalton Hooker.), published by Faber and Faber, 1977, pages 122 to 123:- The author quotes Charles Darwin – “Does not Lyell (ie:- geologist Charles Lyell, Professor of Geology at King's College London in the 1830s.) give some argument about varieties being difficult to keep on account of pollen from other plants.” - - - - The author then tells us:- “The conclusion Darwin reached - - - was that transmutation of species had taken place when populations were isolated - - - and no longer able to prevent THE VARIATION NORMALLY KEPT IN CHECK BY BREEDING THROUGHOUT THE POULATION.” (My capitals.)
(My comment:- What this boils down to is that any variation in a species will be “kept in check” or removed because the animal possessing the variation is likely to breed with an animal that does NOT possess the variation, in which case the variation will become “diluted”, and over a few generations of “dilution” finally lost altogether. If a variation is to pass down the generations, then the animal possessing the variation must breed with another animal who also possesses that same variation, which is unlikely to happen. Then the progeny must also breed with animals possessing the same variation. A dog breeder can “enforce” selective breeding to maintain a particular variation, resulting in breeds of dogs that never appear in the wild state, because the variation would soon be “bred out” in the wild state. (In fact, when pedigree dogs are released into the wild, after a few generations, they have “bred out” the “variations”, and look more and more like wolves as the generations proceed.) The notion that this hurdle can be surmounted by small isolated populations encounters the following problem:- In a large population there is a good chance that more than one example of a particular variation exists in a cohort. In a small population, the chance of there being more than one example of a particular variation in a cohort is drastically reduced. I considered combining this “hurdle” with The Evolutionary Stasis Hurdle, because it helps to explain why Evolutionary Stasis is a constant feature of the geological record. However, I decided that it deserves a place as a separate hurdle.