A Toothy Finding. In 2007, University of Helsinki evolutionary biologist Kathryn Kavanagh showed that molars emerge from front to back, with each tooth smaller than its precedent. Fodder for geeked-out dentists? Far from it: Her model predicted tooth development of rodents with different diets — a perfect confluence of a small mechanical observation and observed evolutionary trajectories.

Image: Kathryn Kavanagh / Nature

The Beginnings of Bones. Neural crest cells originate in the spinal cord before diffusing through our developing bodies, forming face and neck bones as well as sense organs and skin. The fossil record, nearly bereft of embryos, provides little direct insight into these critically important stages. But technologies that let researchers track cells during embryo development finally allowed them to watch the neural crest's development, culminating in the attachment of head to the body at its front, while the back attachment springs from the mesoderm tissue layer. With that established, scientists can decipher shared evolutionary histories from muscle attachments: the cleithrum, for example, a bony girdle found in fishes, lives on in humans as the shoulder blade.

Image: Wolfson Institute for Biomedical Research / Nature

Natural Selection in Speciation. That differing selection pressures will cleave one species into two is a simple principle expressed in complex ways. One of these is reproductive isolation — when, for example, one species of stickleback fish live in freshwater streams, and the other goes to sea. Scientists found that stream-bound sticklebacks prefer larger mates, and genetic analysis confirmed that their populations are indeed diverging.

Image: Ellen Edmondson and Hugh Chrisp / New York State Department of Environmental Conservation

Lizard Games.Take an island in the Bahamas, add a predatory lizard called Leiocephalus carinatus, and the results are immediate. Males among the lizard's favorite prey, Anolis sagrei, soon became longer-legged, so as to better flee after drawing predatory attention during mating displays. In contrast, more sedentary females became larger, making them harder to ingest — a neat display of sex-specific selection pressures.

Image: WikiMedia Commons

An Evolutionary Arms Race, Frozen in Time.Predator and prey evolve together; the adaptations of one driving adaptations in the other. But how can one study this over time, in detail? Biologists from Belgium's Catholic University of Leuven used water fleas and parasitic mites that had been preserved in the mud of a lake's bottom. The sediments were precisely dated and their inhabitants revived, allowing researchers tomix species from different eras and directly measure their developing capacity for infection and escape.

Image: University of Indiana / Daphnia Genomics Consortium

Gene Flow, With Purpose.If dispersed by random animal migration, genes flowing across a region ought to dilute local pockets of genetic adaptation. But migration isn't as random as it seems: As seen in a population of great tits (the bird!) tracked in Oxfordshire, England since 1970, genes flow along channels of opportunity. Individual birds picking nesting spots best-suited to their particular traits, producing local adaptations in tiny parts of the same small forest. (These birds, incidentally, belonged to the same population that have shifted breeding times to match a changing climate.)

Image: University of Oxford / Science

Selection Finds Its Own Level. Since natural selection favors traits that increase fitness, it seems that populations should eventually become genetically homogeneous. But evolution isn't so one-dimensional: When researchers adjusted the color frequencies of wild guppy populations in Trinidad, they found that unusual variants — regardless of color — had higher survival rates. This is called frequency-dependent survival: selection favoring the rare and disfavoring the common, preventing a long-term homogeneity that — no matter how beneficial in the short term — might someday prove disastrous.

Image: Kimberly Hughes / Nature

Making Do. Though so often elegant, evolution can also be jury-rigged and provisional. Witness the Moray eel, whose body is so long and narrow that — unlike other fish — the suction created when it opens its mouth is too weak to catch prey. The solution: a second set of jaws and teeth that sprout from the skeleton around its gills. It's not pretty, but it works.

Video: Rita Mehta / sciencetranslator/YouTube

The Genes of the Finches. The Galapagos finches whose beak adaptations were described by Darwin — and later tracked, over decades, by Peter and Rosemary Grant — are poster animals for evolution. In 2006, researchers found a genetic unit underlying their oft-described progress: calmodulin, whose expression during embryonic development changes beak shape.

Image: Peter Grant and Arkhat Abzhanov