Speciation in Animals

Polyploidy is frequent in animal species of arctic environments that have been recolonized since the last glacial retreat. Polyploid species of the cladocerans Daphnia and Bosmina are confined to the arctic, where they reproduce parthenogenetically (Beeton and Hebert 1988). During periods of maximum glacial advance, populations of these species may have been confined to isolated refugia south of the glacial front. As these populations expanded during glacial retreat, secondary contacts between them may have led to allopolyploidy. Polyploidy may be advantageous in the arctic because of developmental advantages (Adamowicz et al. 2002). The polyploid forms of these species, many of which are likely of recent origin, are genetically isolated from their diploid ancestors.

Recent speciation has been documented for animals, as well, including many instances involving alien species. Native animals may undergo sympatric speciation when alien plants provide suitable resources and a mechanism for reproductive separation of portions of a parental population (Via 2001). Often this appears to occur because the alien plants have a different reproductive phenology than that of native plants. Perhaps the best-documented example involves the maggot flies (Rhagoletis spp.) described in chapter 13. Biotypes of these flies on introduced apple and cherry trees are almost completely isolated from their ancestral forms on native trees, thus representing incipient species. Two-spotted spider mites (Tetranychus urticae) also tend to form genetically distinct host races adapted to the secondary chemistry or defensive morphology of the host plants (Agrawal et al. 2002b), as we noted in chapter 9.Adaptation to different hosts often involves fitness tradeoffs, that is, adaptation to a new host results in reduced fitness on the old host.

Another well-documented example of this sort involves the pea aphid

(Acyrthosiphon pisum), which was introduced to North America from Europe in the late 1800s (Via 1999). In North America, the pea aphid is considered a pest of alfalfa (Medicago sativa) and red clover (Trifolium pratense).Via (1999) showed that populations of aphids on these two forage plants are genetically divergent and that gene flow between them is reduced. She concluded that incipient barriers to successful interbreeding exist and that alfalfa and red clover biotypes of pea aphids are on the road to becoming reproductively isolated species.

Divergence of host-related populations of crop insect pests is probably not unusual. In France, for example, the European corn borer (Ostrinia nubilalis) has evolved host races on mugwort (Artemisia vulgaris), a wild plant, and maize (Zea mays), a crop plant native to the Americas (Martel et al. 2003). The frequency of interbreeding between the two forms appears to be less than 1%. Several factors apparently contribute to the strong genetic isolation of the two forms (Thomas et al. 2003). Populations on mugwort complete their larval development and emerge about 10 days earlier than those on maize, so the chances of mating between the two forms are greatly reduced. Sex pheromones produced by the two races also differ, reinforcing the tendency for mating of individuals of the same race. Selection for populations on maize also appears to be related to an enemy-free environment. A parasitoid wasp usually kills more than 50% of corn borer larvae overwintering on mugwort but does not attack larvae overwintering on maize plants.

Over the longer term, good evidence exists for full speciation by these sorts of processes. When Polynesian colonists reached Hawaii in about 400 CE, they brought with them a variety of food plants, including banana (Musa paradisiaca) and coconut palm (Cocos nucifera). Zimmerman (1960) noted that since the arrival of these plants, five species of moths of the genus Hedylepta have evolved to use banana and one to use coconut palm. This genus of moths includes 23 species endemic to Hawaii; the remaining 17 species feed primarily on various monocot plants. The ancestral form of Hedylepta apparently used a native Hawaiian palm of the genus Pritchardia.

Animals occupying insular environments of relatively recent origin or recent colonization demonstrate that speciation can easily occur within a few thousand years. Many examples of this process have been documented for fish in lakes and coastal lagoons in regions covered by the last Pleistocene glaciers (Schluter 1997, 2000). Many of these lakes contain pairs or trios of closely related species that show character displacement in feeding morphology (Schluter and McPhail 1992,1993). Many sets of species differ in the number of gill rakers; those with the larger number are plankton feeders and those with the smaller number are benthic feeders. Recent invasions of freshwater rivers, lakes, and impoundments by many marine organisms have set the stage for evolutionary adaptation and speciation (Lee and Bell 1999).

In Tasmania, Australia, populations of Galaxias truttaceus have become isolated in lakes that have been separated from streams in the last 3,000-7,000 years (Ovenden and White 1990).The stream Galaxias are anadromous, spawning in autumn and spending 3 months at sea before returning to streams. The lake Galaxias spawn in spring and are thus isolated in space and time from their stream relatives.Although not the result of recent invasion of new environments in the strict sense, silverside fish (Odontesthes spp.) in coastal waters of southern Brazil show similar patterns of rapid speciation (Beheregaray et al. 2002).

The threespine stickleback (Gasterosteus aculeatus),a widespread marine and anadromous fish of the northern hemisphere, has invaded postglacial freshwater environments in many locations. Many of these invasions have given rise to incipient species that have differentiated in sympatry or para-patry (McKinnon and Rundle 2002). For example, Schluter and McPhail (1992) examined the pattern of distribution and divergence of marine and freshwater sticklebacks in coastal British Columbia, Canada. The marine form (Gasterosteus aculeatus) occurs in estuarine habitats. Since the end of Pleistocene glaciation, about 13,000 yr ago, this marine form has colonized many rivers and lakes. Many of the lakes became separated from the ocean about 12,500 yr ago, but some may have been reconnected for a period about 11,000 yr ago. In five lakes on islands in the Strait of Georgia, two sympatric forms of sticklebacks exist, apparently reflecting invasions dating from these two periods.These forms are best regarded as separate species, as they are strongly reproductively isolated, the incidence of hybrid individuals being only about 1%. In addition, they are divergent in morphology and feeding ecology, with one form being specialized for planktonic and one for benthic feeding.

Anadromous and freshwater forms of the threespine stickleback occur in many, perhaps hundreds, of streams and rivers that have become available only in postglacial times (McKinnon and Rundle 2002). In some river systems, differentiated lake and stream forms exist.The differentiated forms of these fish typically differ in multiple traits and appear to be the result of divergent selection, rather than of genetic drift or founder effects.

Most also show some degree of hybridization as well as a strong degree of assortative mating because of differences in habitat preferences or habitat-related courtship traits.

Incipient speciation is likely in several other taxa of northern fish (Schluter 1996). In the case of rainbow smelt (Osmerus mordax) in Lake Saint-Jean, Ontario, Canada, dwarf and normal forms coexist and use the same spawning areas in two inflowing rivers (Saint-Laurent et al. 2003). Smelt populations in Lake Saint-Jean probably date from between 8,700 and 10,300 yr ago. Genetic analyses revealed that fish spawning in the two inflowing rivers were distinct and that within each river population dwarf and normal forms existed. The dwarf forms grow more slowly, reach maturity more quickly, and reproduce once at maturity.The normal form grows faster, matures more slowly, and reproduces repeatedly. Dwarf fish possess more closely spaced gill rakers, suggesting a greater tendency to feed on plankton in open water. No evidence was found that these forms resulted from separate invasions of Lake Saint-Jean by smelt. Thus, divergent selection, strong enough to outweigh gene flow between dwarf and normal forms, was apparently responsible for differentiation of the two forms.

The development of reproductive isolation between different populations of colonizing species can begin very soon after colonization. Hendry et al. (2000) documented incipient speciation between populations of sockeye salmon (Oncorhynchus nerka) breeding in different locations within the Lake Washington watershed, Washington State. Sockeye salmon were extirpated in this watershed by anthropogenic impacts in the early 1900s. Successful reintroductions were made between 1937 and 1945 using hatchery-propagated juveniles originally obtained from Baker Lake, Washington. These introductions resulted in a large population that breeds in the Cedar River, which flows into the southern end of Lake Washington. In 1957, a small population breeding at Pleasure Point, on the eastern shore of Lake Washington, was discovered. The Cedar River sockeyes spawn in gravel beds in flowing water that varies substantially in temperature from fall through spring, whereas the Pleasure Point fish spawn in quiet-water gravel beds in which little change in temperature occurs seasonally.

Comparisons of the fish were made at these two breeding sites in 1992, a maximum of 56 yr or 13 fish generations after the original introductions to the Lake Washington watershed (Hendry et al. 2000; Hendry 2001). By examining otoliths, calcareous structures of the inner ear, the site at which individual fish were hatched could be determined because the incubation temperature creates distinctive otolith patterns. By this method, fish resident at the two breeding sites could be distinguished from new immigrants from the opposite site. Resident fish at the two breeding sites differed significantly in morphology in ways that correlated with characteristics of the two habitats. Males had shallower bodies in the river population, as expected for fish adapted for maximum swimming efficiency. Females in the river population were larger bodied, which enables them to spawn at greater depths in the river gravel, reducing the chance that the eggs will be lost by scouring by the flowing water. Differences were also found in features adapting the fry to the different temperature regimes of the two sites. All of these differences are known to have a strong genetic basis in salmon. Furthermore, the differentiation of the Pleasure Point population occurred in spite of an immigration rate of about 39%, indicating that gene flow into the Pleasure Point population was much less.Thus, although full reproductive isolation had not yet been achieved, selection was favoring reduced interbreeding.

Animals that have colonized oceanic islands or other islands of recent origin also show many instances of incipient speciation. On the island of Madeira, located in the Atlantic 600 km west of North Africa, the house mouse (Mus musculus) was introduced by humans, either via early Portuguese ships that carried them to the islands or by Viking visitors as early as the ninth century (Gündüz et al. 2001). In either case, within a few hundred years, six distinct chromosomal races have evolved in coastal valleys separated by mountain barriers that reach from the interior highlands to the coast (Britton-Davidian et al. 2000). Owing to chromosomal fusions, these races possess reduced chromosome numbers relative to mainland European mice. Formation of fertile hybrids among these races appears to be impossible, so they are reproductively isolated and constitute sibling species. Mitochondrial DNA analyses suggest that the Madeiran mice are most similar to present-day populations in northern Europe, an observation consistent with an introduction by Viking ships (Gündüz et al. 2001).

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