Evolving Icons of Evolution
Peter and Rosemary Grant, the husband and wife pair from Princeton University, have spent 40 years studying four species of ground finch on one uninhabited island of the Galápagos archipelago. They are to be commended for their dedication, and for details that they have published over the years about the birds made famous by Darwin. (Design advocates appreciate the Grants for their findings that undermine neo-Darwinism, although they would not see it that way.)
In a new Perspective piece in Science Magazine, “Watching Speciation in Action,” they show that they are not the only ones who have witnessed the origin of species. Beginning with the Darwin quote about “grandeur in this view of life” that evolves, they describe a number of studies like theirs that illustrate organisms that have varied and diversified from parent stock. Let’s begin by listing the examples and what is known about them, both genetically and phenotypically. These can be considered their finest “icons of evolution” for 2017.
- Darwin’s finches. The Grants witnessed variations due to five beak genes, two transcription factor genes, hybridization, climate forcing, and reproductive isolation.
- Peppered moths. The black morph comes from one transposon, which “suggests that transposable elements may play a more important role in generating variation among species in ecologically important traits than is currently realized.”
- Pentstemons and morning glories. Asymmetric loss-of-function mutations and inversions tend to change pollinators from bees to hummingbirds, but not the reverse.
- Ruff, a wading bird “with an unusual mating system in which three male forms, differing in plumage and behavior, compete for females on a courting arena.” An inversion 3.8 million years ago, they say, was followed by mutations that produced this result.
- Deer mice in Nebraska’s sand hills. The lighter coat colors, adaptive for the sandy environment, “were the result of selection on not one but multiple mutations in the Agouti”
- Three-spined sticklebacks. Marine fish that have colonized freshwater environments repeatedly evolved into surface dwellers and bottom dwellers. They also lost the pelvic apparatus and armor through similar genetic pathways (involving a transcription factor and a signaling protein).
- High-altitude birds. Hemoglobin adaptation to high altitude has been shown to involve “repeated use of the same genetic pathway.” It “may be the case generally” that “closely related species use similar genes, whereas more distantly related species use different genetic pathways that depend on their genetic backgrounds.”
- Heliconius butterflies. The genes involved in Mullerian mimicry are shown to be due to introgressive hybridization, “that is, gene exchange between species as a result of hybrids backcrossing to a parental species.” This process is also seen in the Darwin finch study.
- Mosquitos and mice. Cases of insecticide resistance have been “transmitted between populations” by introgressive hybridization.
- Asian longhorn beetles. These eukaryotes, in a case “even more remarkable, and similar to introgressive hybridization,” obtained genes for digesting plant cell walls via horizontal gene transfer from bacteria and fungi.
- High-altitude humans. They say that Tibetans may have obtained adaptation to high altitude via interbreeding with Denisovans.
- Sunflowers. Members of Helianthus colonized salt marshes via “transgressive segregation,” a form of hybridization whereby individuals can “colonize novel habitats where neither parental lineage can survive.”
- Blind cave fish. Mexican fish lost vision via “release of hidden [cryptic] variation.” In their case, the heat shock protein HSP90 masks variation for eye size in surface waters. “This variation is exposed in the altered conductivity of cave water and becomes available for selection.”
It’s an impressive list. The Grants have thrown down the gauntlet to Darwin skeptics, providing a baker’s dozen of clear examples of variation leading to populations with phenotypic change. But before we bow before these icons, we should ask some important questions…
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