Examples Of Introgression

Introgression as an important evolutionary force is more than just a theory: Geneticists know of many cases in which it has definitely occurred. Most of the examples that are well understood involve domesticated animals and plants, mainly because there are practical economic reasons for undertaking close genetic studies of domesticated species.

Introgression is hardly rare; in fact, it is ubiquitous among domesticated plants. For example, the wheat that produces our daily bread is derived from three different wild grasses. There's evidence of introgression in alfalfa, barley, chili peppers, lettuce, maize (corn), potatoes, rice, rye, sorghum, and soybeans—and that's just a partial list. But since plants are better than animals at tolerating complex genetic events such as changes in chromosome number, introgression in domesticated animals may be a better analogy.

Cows were domesticated at least twice: in the Middle East (humpless taurine cattle) and in India (humped, droopy-eared zebu cattle), and possibly a third time in North Africa. The wild ancestors of taurine and zebu cattle were separated for several hundred thousand years, yet those breeds are interfertile. Zebu genes have been spreading among taurine cattle in Africa and western Asia for the past 4,000 years. It appears that some zebu genes increase tolerance of aridity and heat as well as resistance to rinderpest, a virulent bovine disease. This is very similar to the pattern of introgression we believe must have occurred among moderns and archaics such as Neanderthals.

Evidence of adaptive introgression in wild populations was once rare, but it has become easier to discover and document it

Texas longhorn

in recent years thanks to improved DNA-sequencing techniques. We now have genetic evidence for adaptive introgression in wild organisms such as damselflies, mosquitoes, lake trout, and European hares. One of the most interesting cases of the phenomenon in a (partly) wild population is the recent evolution of honey bees, which is particularly interesting because of some close parallels with human evolution.

Honey bees originated in eastern tropical Africa several million years ago and later expanded into Eurasia in two different migrations. One of these led to Western European honey bees and the other to Asian honey bees. Bees living in temperate climates faced fundamentally new problems: more than anything else, cold winters. To a large extent, their adaptation to those new climates was mediated by changes in social behavior.

They needed to choose nest sites that would protect them from the weather, store much more honey, and form a winter cluster—that is, a tightly packed clump of bees that conserves heat. In a 2008 study, Amos Zayed and Charles Whitfield concluded that approximately 10 percent of all protein-coding genes in bees underwent positive selection in that process of adaptation.15 The history of honeybees parallels the history of humans in an interesting way—both involved expansion into a new environment with a drastically different climate followed by strong selection and adaptation.

The parallels don't stop there. Soon after the initial colonization of the Americas, Europeans introduced honey bees, where they did well, swarming many times a year and outrunning colonists. However, they did not do as well in the neotrop-ics, the part of the New World most unlike Europe. Warwick Kerr, a twentieth-century Brazilian geneticist and bee breeder, attempted to develop a strain of bees that would be more productive in the tropics. Using hybridization techniques, he bred Western European bees with African bees. In 1956, twenty-six of his Tanzanian queens escaped and began colonies, and their hybrid descendants have since spread over much of North and South America. These Africanized bees produce more honey than European bees in warm climates, but they are very aggressive and often attack people and animals that come too close to their hives. Once aroused, they may chase an enemy a mile or more. This high level of aggression is adaptive in Africa, where bees have not been domesticated. There, bee colonies are attacked by honey badgers and other predators, and humans raid beehives rather than keeping bees.

Almost all Africanized bee colonies have African mtDNA, but a significant fraction of their nuclear genome is European. That fraction is significantly higher in coding regions of the genome than in noncoding DNA, which indicates that Africanized bees have succeeded in picking up adaptive alleles from European bees while retaining those African alleles that are adaptive in this situation (that is, most of them). The noncoding regions, presumably neutral or close to neutral, are incorporated at the rate 1 /(2N), while favorable coding genes are incorporated at the 2s rate, as discussed above. It has been suggested that there may be genetic incompatibilities between European mtDNA and the African nuclear genome, which would explain why we find so very few Africanized bees with European mtDNA. The Zayed-Whitfield study provided evidence that "invasive populations can exploit hybridization in an adaptive fashion," which is only reasonable. Just as the Africanized bees have incorporated advantageous genes from the local indigenous bee populations, modern humans, we believe, incorporated advantageous genes from their archaic human precursors, especially from the Neanderthals.

Many instances of adaptive introgression—those, for example, that involve biochemical changes that do not affect appearance—are cryptic and were effectively undetectable before the development of modern molecular research methods. This is worth remembering when we look at the fossil record: The majority of adaptive genetic events do not have noticeable skeletal signs. Some cases of adaptive introgression, though, have readily visible effects, as when genes that increased drought tolerance spread from Utah cliffrose to bitterbrush. The intro-gressed bitterbrush looks more like cliffrose and can survive in places where ordinary bitterbrush cannot.16 In this case, the population with introgressed genes reflects that introgression in its external appearance, but more often the effects of introgression are not readily apparent in the gross anatomy of an organism.

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