What are guppies? Guppies (Poecilia reticulata) are small tropical fish, native to the coastal streams of northeast South America. They owe their name to Robert John Lechmere Guppy who introduced them into the aquarium trade. Guppies have since become very popular aquarium fish, known for colorful males and live-bearing females. But guppies are also one of the premier model systems for the study of ecology, evolution, genetics, and sexual selection. The guppy is now poised to become an important model for functional genomics of adaptive characters. What is so special about them? Guppies are extremely variable both phenotypically and genetically. Sexually mature males exhibit an amazing array of differently colored spots and stripes, such that every male almost seems unique (Figure 1), making the guppy one of the most polymorphic vertebrates known. Although the females do not show such coloration, they vary in terms of their preferred mates both within and between populations, making the guppy a powerful system for studying sexual selection. Like other members of the family Poeciliidae, such as swordtails and mollies, female guppies are live-bearing. Males fertilize the eggs using a stick-like modified anal fin, the so-called gonopodium. Guppies are ovoviviparous, i.e. the eggs develop inside the mother. However, also viviparity with placental nutrition has evolved several times within the Poecilliidae. What have guppies taught us about evolution? The guppy evolves quickly and the selective factors that shape this variability are well-studied. On the island of Trinidad, lowland areas of streams have a rich community of predatory fish, while further upstream, barrier waterfalls have allowed guppy populations to evolve without large predatory fish. Through various approaches, it has been shown that behaviors, life history and morphology all respond evolutionarily to the strength of predation. For instance, males tend to be brighter and more conspicuous in areas of low predation. Thus, the guppy is one of the few systems in which the dominant selective force can be so clearly identified. Manipulative experiments within and between streams have shown that guppy populations can evolve extremely rapidly, at speeds similar to those observed in artificial selection experiments in the lab. What about guppy genetics? The genetics underlying these characters are also unique. Beginning in the 1920s, Winge and colleagues showed that over 40 loci contribute to male coloration. Intriguingly, all of these loci are linked to the sex chromosomes, with about half of the alleles recombining between the X chromosome and the Y chromosome in a pseudoautosomal fashion, and the other half remaining on a non-recombining Y. And this Y chromosome, similar in size to the X, is itself variable between male lineages. In parallel with the amplification of male coloration genes, genes encoding photoreceptive opsin proteins have been duplicated and exhibit a high degree of adaptive diversification. What is the guppy's potential for biomedical research? With their fast generation times and ease of maintenance, guppies are a valuable resource for biomedical research. For instance, David Reznick and colleagues are exploiting population differences in life history as a model for understanding the forces shaping variation in aging, and in our lab we are studying the guppy mutant curveback as a so far unique model for familial idiopathic scoliosis. Guppy genomics? Detlef Weigel, Christine Dreyer and colleagues in Tübingen, Germany, are currently constructing a genetic linkage map to associate variable phenotypic traits with genes of known function. The mapping crosses will include highly differentiated and long-studied natural populations. Our lab has been using microsatellite markers developed in the closely related sword-tail Xiphophorus maculatus, a known model for the genetics of melanoma, to quickly build a set of linked markers in the guppy. Thus, the large group of biologists studying the family Poeciliidae is joining efforts in establishing genomic resources for this valuable group of species. What is in store for the guppy? Variation is essential to understanding the genetics underlying biological processes. The guppy provides a huge amount of natural variation that can be studied genetically. This variation has not been produced by mutagenesis screens, but instead has been shaped by natural selection. Thus, combined with emerging genomic resources, the guppy is an ideal organism for understanding the evolutionary genetics and molecular basis of adaptation.