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For nearly a century, evolutionary biologists have observed chromosomes that cause lethality when made homozygous persisting at surprisingly high frequencies (>25%) in natural populations of many species. The evolutionary forces responsible for the maintenance of such detrimental mutations have been heavily debated—are some lethal mutations under balancing selection? We suggest that mutation–selection balance alone cannot explain lethal variation in nature and the possibility that other forces play a role. We review the potential that linked selection in particular may drive maintenance of lethal alleles through associative overdominance or linkage to beneficial mutations or by reducing effective population size. Over the past five decades, investigation into this mystery has tapered. During this time, key scientific advances have provided the ability to collect more accurate data and analyze them in new ways, making the underlying genetic bases and evolutionary forces of lethal alleles timely for study once more. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 11 is February 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates. 

When students answer test questions incorrectly, we often assume they don't understand the content; instead, they may struggle with certain cognitive skills or with how questions are asked. Our goal was to look beyond content to understand what makes assessment questions most challenging. On the basis of more than 76,000 answers to multiple-choice questions in a large, introductory biology course, we examined three question components—cognitive skills, procedural knowledge, and question forms—and their interactions. We found that the most challenging questions require the students to organize information and make meaning from it—skills that are essential in science. For example, some of the most challenging questions are presented as unstructured word problems and require interpretation; to answer correctly, the students must identify and extract the important information and construct their understanding from it. Our results highlight the importance of teaching students to organize and make meaning from the content we teach.