That's what makes it hard to determine when evolution via genes is occurring vs purely environmental factors winnowing a current population.
To an evolutionary biologist, that statement doesn't make sense. What, exactly, is the distinction between "evolution via genes" and "purely environmental factors winnowing a population"? "Environmental factors winnowing a population" is natural selection, and that drives "evolution via genes". If the small-footed lizards drop off the trees and fail to reproduce, the frequency of alleles in the lizard population changes -- the alleles that favor large feet are now more common. This is "evolution via genes". Sure, some small-footed lizards might remain in the population, or smaller feet could become dominant if the selective pressures change, but that has nothing at all to do with whether or not "evolution via genes" is occurring.
The passing of genes to the next generation is a separate process that still reshuffles the genes via sex relentlessly regardless of environment.
I don't understand this, either. Selection (e.g., "environmental factors winnowing a population") causes the allelic frequencies for some genes in a population to change over time. "Relentless reshuffling" during recombination and sexual reproduction doesn't somehow negate this. For a simplistic analogy, think of a deck of cards. If you remove half of the red cards (i.e., some of the small-footed lizards die), it doesn't matter how many times you shuffle the deck -- the relative frequencies of red and black cards in "the population" aren't going to change.