From Andrew Porterfield at the Genetic Literacy Project:
idea called “genetic load” was developed in the 1930s by famed biologist J.B.S. Haldane, referring to any genome that had increasing numbers of deleterious mutations. The more mutations in a population, the more likely that members of that population couldn’t survive, ultimately threatening the fitness of that population. With enough mutations, a group couldn’t adapt as well to environments, and members would die off. Thus, there’s a limit to natural selection. Now, for some evolutionary biologists, the concept of genetic load has resurfaced as a genuine concern. …
He doesn’t think it’s that big a concern.
One problem is that modern sequencing has produced much more data, and shown us so many more ways that genes can vary (and be manipulated), and we need to get a better idea of what we’re looking at. Phenotypes can be neutral, and a vast amount of evolution is because of drift (earlobes, nose shape, and slight variations in eye color). And many variations are not conserved. More.
At one time, the Big Extinction was supposed to be World War III. Now that the genome has been mapped and the Big One put on hold, ppular worry shifts to genetics.
See also: Sixth mass extinction, but no news on defining “species”?
The Big One, as we remember it: