Evolution of Evolution — Text-only | Flash Special Report
From Darwin's Gemmules to Evolutionary Genomics
Study of inheritance improves understanding of hybrid organisms and species formation
By Mohamed Noor
One of Charles Darwin's problems in presenting the theory of evolution by natural selection was that he lacked a mechanism for how variants, or slight differences within a species, passed from parents to offspring. The resemblance between relatives was obvious, illustrating that one's "essence" passes to children, but how such genetic information is transferred only became widely appreciated later. Darwin's challenge was understanding how variation persisted if inheritance involved serial blending: if one goes to a paint store and combines all paints by pairs over many iterations, eventually everything will be the same color.
To address this deficiency, he proposed the idea of "gemmules," supposed particles of inheritance secreted by cells that ultimately find their way into the germline where they mix with gemmules from the offspring’s other parent. These gemmules could acquire characteristics directly from the tissues that produced them, consistent with the idea popularized by French naturalist Jean-Baptiste Lamarck’s inheritance of acquired characters theory. More importantly for Darwin, gemmules could explain how heritable variation might persist, upon which natural selection could act to produce evolutionary change.
Since Darwin's time, the ability to study inheritance and heritable variation in an evolutionary context has grown exponentially. Progress began in the early 1900's with the rediscovery of Augustinian priest Gregor Mendel’s concept of allelic inheritance, or paired gene copies. Mendel’s theory that gene pairs, or a series of pairs, determine the characteristics of an organism gave birth to modern genetics. It was applied to evolution soon thereafter with the suggestion that "continuous" characters like human height may be explained by variations in the coding sequence.
The last 40 years have been equally transformative, if not more so. Evolutionary biologists capitalized on advances in genetics to understand the processes of adaptation and species formation. Protein variation documented in the 1960s led scientists to infer the operation of natural selection at the molecular level using the “neutral theory of molecular evolution.” Albeit imperfect, the theory served as a null or statistical hypothesis against which to compare patterns of variation within a species to random changes that could happen without the presence of natural selection. Funded by the National Science Foundation, experimentalists surveyed protein variation, DNA sequence variation at focal genes and most recently, genome-wide DNA sequence variation. Repeatedly, they found observed patterns were inconsistent with the null hypothesis, suggesting that natural selection plays a large and ubiquitous role shaping most genomes.
Progress is somewhat slower in understanding species formation, but recent genetic approaches are having an impact. For example, large-scale surveys show that many "good" species exchange genes with other species, creating hybrids and suggesting that reproductive isolation—the lack of genetic exchange that separates species—may be leaky initially. Recent work by my group and others suggests that certain regions of the genome may allow such leaky hybrid species to form constantly, unlike the paint example previously mentioned. Distinct hybrid types persist because parts of the genome in which genetic exchange is rare or absent never really mix. This hypothesis also explains the observation that closely-related, co-occurring species often differ by chromosomal rearrangements, but the rearranged genomic regions fail to blend in hybrids and thus allow the distinct parent species to continue unaltered. Support for this model of species persistence comes from genetic mapping studies and, as already mentioned, examination of patterns of DNA sequence variation within and between species.
Overall, genetic studies of adaptation and species formation are flourishing, as evidenced by recent studies identifying some of the genes responsible. Interestingly, a few examples of inheritance have been described, under the broad umbrella of "epigenetics," in which the experience of an individual alters features of its offspring for one or more generations. This idea is similar to Darwin's gemmule concept. If such inheritance proves common, then future theory and experimentation will integrate these ideas to test their evolutionary effects. Once again, we'll be running in a direction initially signaled by Darwin.
Mohamed A. F. Noor is a professor and associate chair of biology at Duke University in Durham, N.C. He is a recipient of the 2008 Darwin-Wallace medal by the Linnean Society of London, an honor given for "major advances in evolutionary biology" once every 50 years. He has been active in the evolution community, recently serving as editor of Evolution, and currently as a regular member in the National Institutes of Health Genetic Variation and Evolution study section, a council member for the Society for the Study of Evolution and American Genetics Association, and as associate editor for multiple journals. The National Science Foundation supports his research on the role of chromosomal rearrangements in the persistence of species.
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