Chapter 20 - Evolution of Genes, Traits and Species

• Differential gene duplication and rapid sequence evolution due to genetic conflict within species are common mechanisms of intrinsic reproductive incompatibilities between species.
• Changes in regulatory sequences play a critical role in the evolution of body form.
• The same principles (variation, heredity, selection, and drift) that lead to the evolution of genes, genomes, and traits within species also contribute to the evolution of new species.
• Before the ability to study evolution at the molecular level, there was no inkling that much of evolution was in fact a result of genetic drift and not natural selection.
• New protein functions often arise through the duplication of genes and subsequent mutation.
• The number of amino acid differences between two different species in some molecule is directly proportional to the number of generations since their divergence from a common ancestor in the evolutionary past.
• Evolution can repeat itself by altering the very same nucleotide in independently arising sickle-cell mutations that lead to adaptive resistance to malaria.
• The selection of differences is not dependent on particular changes in the environment.
• Most genes in a genome are evolving under purifying selection to maintain sequence conservation, but some genes show evidence of positive selection for adaptive sequence changes.
• The fate of duplicate genes depends a great deal on the nature of mutations acquired after duplication.
• Among the evidence for the prevalence of neutral evolution is that the number of amino acid differences between two different species in some molecule—for example, hemoglobin—is directly proportional to the number of generations since their divergence from a common ancestor in the evolutionary past.
• Genes involved in genetic conflicts within species often contribute to hybrid incompatibilities between species.
• New DNA may arise by duplication of the entire genome (polyploidy), a frequent occurrence in plants, or by various mechanisms that produce duplicates of individual genes or sets of genes.
• The mutational processes that generate variation within the genome act at random, but the selective process that sorts out the advantageous and disadvantageous variants is not random.
• Differences that are not heritable cannot evolve, because the reproductive advantage of a variant will not carry across generational lines.
• The ability to study evolution at the level of DNA and proteins has transformed our understanding of the evolutionary process.
• Variation must be influenced by genetic differences for evolution to occur.
• In some F hybrid males, Overdrive is no longer suppressed, leading to sterility.
• A great deal of molecular evolution seems to be the replacement of one protein sequence by another one of equivalent function.
• The theory of evolution by natural selection explains the changes that take place in populations of organisms as the result of changes in the relative frequencies of different variants in the population.
• Molecular genetics has also begun to reveal the genetic changes that underlie the evolution of reproductive isolation between species.
• If there is no variation within a species for some trait, there can be no evolution.
• Pleiotropic effects of coding mutations can be circumvented by mutations in noncoding regulatory sequences.
• An inactivating mutation may occur in the coding region of either duplicate.
• Mutations may occur that alter the regulation of one duplicate or the activity of one encoded protein.
• One large-scale process for the expansion of gene number is the formation of polyploids, individuals with more than two chromosome sets.
• Gene duplication is a second mechanism that can increase gene number.
• These alleles may then become subject to positive selection and acquire a new function (neofunctionalization).
• To preserve the ancestral function, natural selection will maintain the integrity of both gene-coding regions.
• Modifications to coding and regulatory sequences are common means to evolutionary change within species.
• Many animal genomes harbor retroviral-like genetic elements that encode reverse-transcriptase activity.
• Occasionally, a host genome mRNA transcript is reverse transcribed into cDNA and inserted back in the genome, producing an intronless gene duplicate.
• A fourth mechanism that can expand gene number is retrotransposition.
• The fate of duplicated genes was once thought that because the ancestral function is provided by the original gene, duplicate genes are essentially spare genetic elements that are free to evolve new functions.
• Polyploids result from the duplication of the entire genome.
• In cases where the ancestral gene has more than one function and more than one regulatory element, as for most “toolkit” genes involved in development, a third possible outcome is that initial mutations inactivate or alter one regulatory element in each duplicate.
• The inactivated paralog is called a pseudogene and will generally be invisible to natural selection.
• The detailed analysis of genomes and population-genetic considerations has led to a better understanding of the alternative fates of new gene duplicates, with the evolution of new function being just one pathway.
• Misreplication of DNA during meiosis can cause segments of DNA to be duplicated.
• Detailed analyses of variation in the human genome has revealed that individual humans commonly carry small duplications that result in variation in gene-copy number.