Hannah These processes have been studied among three-spine stickleback fish in western Canada, where retreating glaciers formed several lakes at the end of the last ice age, 11,000 years ago. The sticklebacks entered the lakes from the sea, and were then isolated from their original marine habitat. In each of the six lakes, the fish subsequently followed similar evolutionary paths. Some evolved slender bodies for fast swimming to hunt small invertebrates in the open water, while others became bulky and remained near the lake floor in shallow waters. Reproductive isolation between the two groups has not yet occurred, and about one per cent of the members of each new generation are hybrids. At least, that was the case until the mid-1990s, and still is in most of the lakes. However, in one lake, Enos on Victoria Island, the two species suddenly seemed to converge again, as the number of hybrids increased rapidly. These fish have more intermediate body shapes and a mixture of previously distinct genetic markers. According to Eric Taylor, Associate Director of the Biodiversity Research Centre at the University of British Columbia in Vancouver, Canada, this might have been caused by the introduction of crayfish into the lake, which encouraged reverse speciation among the sticklebacks in three possible ways: by destroying nest materials and/or nests, thus reducing differences in nest-building preferences; by increasing the turbidity and, therefore, cloudiness of the water, making it harder for fish to select mates with their own shape; or by predation on eggs or the fish themselves, reducing the numbers of both species and, therefore, mate choice.

Lucas For species that rely on sound to communicate or execute mating strategies, noise pollution presents a problem. Extensive studies on songbirds show how many species have adapted by adjusting various aspects of their song to overcome residual noise. “We know that several successful city species are able to adjust their spectral frequency...[in response] to traffic noise conditions,” said Hans Slabbekoorn, an assistant professor in urban ecology and bird speciation at Leiden University in the Netherlands. “Great tits have a repertoire of song types covering a wide frequency range, are able to copy from neighbours after dispersal to their breeding territory, and seem to be able to adjust to local noise conditions.” But some less successful species are notable by their absence from cities. “Those species lacking the plasticity of post-dispersal learning, without a wide or high frequency range in terms of song use, may be at a disadvantage in noisy neighbourhoods,” said Slabbekoorn.

A - J Until recently, there was no direct evidence of an evolutionary response to urbanization that involved genetic adaptation. One of the first widely cited examples is the dark-eyed junco, a common North American species of sparrow whose adaptation to San Diego, California, has been accompanied by a significant decrease in the amount of white in its tail feathers (Yeh & Price, 2004). The precise selective mechanism is unclear, but it is likely that white tails evolved in the wild through sexual selection as a way of standing out among mates. The advantage of white tails for sexual signalling would then be diminished in an urban environment, where there is less competition for mates. Studies in mixed urban and wild populations has shown that there is little gene flow between members of the two populations.

Emma One of the primary impacts of many human activities is habitat fragmentation; that is, human use of the landscape creates habitat ‘‘islands’’, and the species within them often have little or no genetic contact with conspecific populations inhabiting other such islands. As an example, consider the eastern collared lizard (Crotaphytus collaris collaris), a species restricted to glades in the portion of its range located in the Ozarks. Desert-adapted plants and animals (such as scorpions, tarantulas, cacti, and collared lizards) invaded the Ozarks during the Xerothermic maximum about 8,000 years ago (the period of maximum warmth in our current interglacial period) and were cut off from their southwestern ancestral range at the end of the Xerothermic about 4,000 years ago (7). After that time and until European settlement, the fragmented Ozark glades were mostly separated by savannas Ozark savannas were a firemaintained community, and before European settlement, fires occurred frequently in the Ozarks However, with European settlement, clear cutting occurred throughout most of the Ozarks, often followed by cutting of second-growth forest as well. The present forest grew during a time in which fires were suppressed, particularly from about 1950 to the present. This new forest is an oak–hickory forest with a dense understory. As a consequence, many glades have been reduced in size, and some have disappeared completely (14). On the basis of microsatellite loci, the present populations have extreme population subdivision (FST 5 0.40, which measures the proportion of the genetic variation in the Total population that exists as differences between Subpopulations), with little genetic diversity within any single glade population but many fixed genetic differences among even nearby glade populations (15, 16). This pattern indicates a combination of small local population sizes and little to no gene flow.

Jasmine The Central European blackcap The Central European blackcap spends its summers in Germany and Austria and, until the 1960s, had spent its winters in balmy Spain. About 50 years ago, however, backyard bird feeding became popular in Britain. With a ready supply of food waiting for them in Britain, blackcaps that happened to carry genes that caused them to migrate northwest, instead of southwest to Spain, were able to survive and return to their summer breeding grounds in central Europe. Over time, the proportion of the population carrying northwest-migrating genes has increased. Today, about 10% of the population winters in Britain instead of Spain. This change in migration pattern has led to a shift in mate availability. The northwest route is shorter than the southwest route, so the northwest-migrating birds get back to Germany sooner each summer. Since blackcaps choose a mate for the season when they arrive at the breeding grounds, the birds tend to mate with others that follow the same migration route. In December of 2009, researchers from Germany and Canada confirmed that these migration and mating shifts have led to subtle differences between the two parts of the population. The splinter group has evolved rounder wings and narrower, longer beaks than their southward-flying brethren. The researchers hypothesize that both of these traits evolved via natural selection. Pointier wings are favored in birds that must travel longer distances, and rounder wings, which increase maneuverability, are favored when distance is less of an issue — as it is for the northwest migrators. Changes in beak size may be related to the food available to each sub-population: fruit for birds wintering in Spain and seeds and suet from garden feeders for birds wintering in Britain. The northwest migrators' narrower, longer beaks may allow them to better take advantage of all the different sorts of foods they wind up eating in the course of a year. These differences have evolved in just 30 generations and could signify the beginning of a speciation event.