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ScienceWeek
EVOLUTION: HYBRIDS AND SPECIATION
The following points are made by Richard J. Abbott (Science 2003 301:1189):
1) Why sex evolved and is maintained in most living organisms remains a key question in evolutionary biology (1). What is indisputable, however, is that sexual reproduction generates new gene combinations, some of which may render the organism better adapted to new environments. The range of different genotypes among offspring increases with the level of genetic divergence between parents. Therefore, matings between different species (that is, interspecific hybridization could potentially generate a vast range of different offspring genotypes, provided that the resulting hybrid zygotes develop and exhibit some fertility. For example, Rieseberg et al (2) describe how hybridization between two sunflower species generated offspring genotypes that are adapted to habitats very different from those occupied by the parents. This resulted in three diploid hybrid sunflower species that are ecologically isolated from each other and their progenitors. The Rieseberg et al findings provide proof that interspecific hybridization can be adaptive.
2) In many plant groups, hybridization between different species is prevented by prezygotic barriers. Such barriers may arise when species have their own specific pollinator, occupy a habitat different from other species, or are spatially separate from other species. Under these conditions, postzygotic barriers manifested in the form of embryo abortion or low hybrid viability may be absent or weak. However, prezygotic barriers can be "leaky", especially when habitats are disturbed in some way, so hybrids are sometimes produced that are often sterile or exhibit reduced fertility. Such problems of low fertility can be overcome, either by chromosome doubling (allopolyploidy) or recombination (3), to produce a stable fertile hybrid that is reproductively isolated from its parents by a strong postzygotic barrier (4,5), and which is therefore regarded as a new species.
3) Although postzygotic barriers are effective mechanisms of reproductive isolation, they present major obstacles to the establishment of a new hybrid species in the wild. Hybrids are born into populations comprising one or both parent species and will initially be represented as a minority component. Consequently, most matings by a fertile hybrid will be with a parent rather than another hybrid, and will result in the production of no offspring or sterile offspring. The hybrid therefore suffers from what is termed a "minority type disadvantage" (4). It can escape from this predicament by evolving a prezygotic barrier that prevents it from mating with its parents. This can be achieved through uniparental reproduction (asexual reproduction or selfing), by flowering earlier or later, by attracting a different pollinator, by occupying a different habitat (ecological isolation), or through spatial isolation due to geographical separation after dispersal (3). Ecological or spatial isolation will also enable a hybrid to avoid any adverse effects of interspecific competition with a parent.
References (abridged):
1. S. A. West et al., J. Evol. Biol. 12, 1003 (1999)
2. L. H. Rieseberg et al. Science 301, 1211 (2003)
3. V. Grant, Plant Speciation (Columbia Univ. Press, New York, ed. 2, 1981)
4. D. A. Levin, The Role of Chromosomal Change in Plant Evolution (Oxford Univ. Press, Oxford, 2002)
5. J. F. Gutierrez-Marcos et al., Philos. Trans. R. Soc. London Ser. B 358, 1105 (2003)
Science http://www.sciencemag.org
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EVOLUTIONARY BIOLOGY: ON THE RED CANARY AND HYBRIDS
Hans Duncker (1881-1961) was to birds what Gregor Mendel (1822-1884) was to peas. By combining his understanding of inheritance with the hidden knowledge of generations of bird keepers, Duncker was able to go one better than either Darwin or Mendel and start to build his own organism -- a genetically engineered canary. He also precipitated a fervid worldwide contest among bird keepers to create a red canary...
When Hans Duncker began the quest to breed a red canary in the 1920s, a huge amount was known about hybrids. Bird fanciers across Europe had been generating hybrids for more than two hundred years and had written extensively about their obsession. Furthermore, biologists like Darwin and Mendel had also written many pages on the subject. Duncker was therefore well aware that to create his red canary he had to contend with biological issues. The first was probably a bonus, but the other two were serious obstacles. First, he knew that any mules he produced would be tough, long-lived birds -- a phenomenon called "hybrid vigor" that was well-known to the ancient Egyptians, who were the first to cross horses and donkeys. Second, as was common knowledge among bird breeders, mules are always more difficult to breed than canaries. The third and most significant hurdle was the fact that mules and hybrids are almost always sterile.
The true mule, the hybrid offspring of a horse and a donkey, is unbeatable as a beast of burden, combining the strength of a horse and the endurance and surefootedness of a donkey --unequivocal hybrid vigor. Bird breeders recognized the same phenomenon, and in his 44-volume /Histoire Naturelle/ written in the mid 1700s, the great but eccentric Georges de Buffon (1707-1788) reported that the offspring from the "cini (serin), siskin and goldfinch with the hen canary are stronger than canaries, sing longer, and their notes are fuller and more sonorous." Even today breeders often comment on the longer lives of their mules compared with parent species.
The inherent vigor of hybrids arises because they literally consist of the best of both parents. Breeding two distinct species together is the ultimate form of outbreeding, and if it works, it works very well indeed. All of us, and indeed all sexually reproducing organisms, get one set of genetic material from each of our parents, which at fertilization align themselves with their opposite numbers, gene for gene. Only one from each pair of alleles is ever turned on, and the secret of the hybrids success is that the genes that are turned on are those that best complement the rest of that individual's genetic makeup -- hence their greater overall vigor.
Strictly speaking, "mule" refers only to an animal whose father is a donkey and whose mother is a mare. When the parentage is the other way round -- a more difficult feat -- the offspring is a "hinny". In the past some people claimed they could tell mules from hinnies because their front end resembled the sire and the back end the dam -- reinforcing the already prevalent view of male superiority in reproduction. Some fanciers imagined they could see something similar in birds. Buffon related how hybrids assumed the father's appearance in their head, legs, and tail, and the mothers in the middle...
Darwin's notebooks from the 1850s are full of anecdotes, snip-pets of information, and curious facts about hybrids. He believed them to hold the secret to the origin of species. By looking at which species would hybridize and produce fertile offspring and which wouldn't, Darwin thought he could establish boundaries between species, He knew, for example, that the more similar two species were, the more likely they were to hybridize, and that two finch species were more likely to breed together than a finch and crow. But there were so many exceptions to this pattern that by the time he put pen to paper twenty years later, Darwin was unable to infer much about the origin of species from the study of hybrids. Normally a rapid writer, he took three months to write the difficult chapter on hybrids in the /Origin of Species/, which he summed up as follows: "Crosses between forms sufficiently distinct to be ranked as species, and their hybrids, are very generally, but not universally, sterile. The sterility is of all degrees... and... does not strictly follow systematic affinity, but is governed by several curious and complex laws."
The evolutionary biologist Ernst Mayr later confirmed that the only pattern is a rather messy one. First, it is true that the less related two species are, the less likely they are to hybridize. On the other hand, not all closely related species hybridize easily. Whether hybridization is possible depends to some extent on whether the two species naturally occur in the same habitat. If they do, then they may have evolved different courtship displays specifically to avoid hybridization. Two closely related European warblers, the willow warbler and the chiffchaff, occur in the same areas of woodland and look very similar but have completely different songs, which enable them to unambiguously recognize each other and avoid mispairing. They are sexually isolated by song. Such differences evolve because hybrid offspring are generally less fertile than purebred offspring, so natural selection favors individuals that breed with their own species.
Adapted from: Tim Birkhead: A Brand-New Bird: How Two Amateur Scientists Created the First Genetically Engineered Animal. Basic Books 2003, p.xiii,143,146. More information at: http://www.amazon.com/exec/obidos/ASIN/0465006655/scienceweek
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ON HYBRIDIZATION AND EXTINCTION
The following points are made by Donald A. Levin (American Scientist 2002 90:254):
1) The well-defined sets of attributes one finds populating the natural world make up the fundamental units of biodiversity: species. Investigators have now described nearly two million species (millions more await attention) and placed them within an elaborate taxonomic hierarchy. But even the naturalists of antiquity realized that some organisms resemble one another so much that they ought to be classified in the same general group or genus. Only much later did Charles Darwin and Alfred Russell Wallace realize that species within the same genus share many traits because they evolved from a common ancestor. That is, what was once one type of plant or animal split into two or more species.
2) Despite their overall similarity, different species in the same genera do not normally interbreed. They may be prevented from doing so because they have widely separated home ranges or different reproductive seasons. Indeed, that they do not freely exchange genes, for whatever reason, defines them as separate species. Yet in some circumstances separate species will mate, and if such a liaison is successful, a hybrid results. Although such hybridization never takes place in the vast majority of genera, it is quite common in some. Botanists believe that hybridization between species happens in 6 to 16 percent of plant genera. Crossing between species is less common in animals, although it is not infrequent in some groups. For example, 9 percent of all bird species hybridize. Such a blurring of taxonomic lines also takes place within primate genera, including lemurs, gibbons and baboons. Anthropologists have even speculated that humans and Neanderthals may have once interbred.
3) With hybridization so rampant, one wonders how species ever maintain their distinctness. They do, in part, because the production of hybrids does not necessarily shift genetic material between species. For genes to traffic in this way, hybrids must cross with at least one of the parent species. In many instances that just doesn't happen. Why? As Darwin had observed, most hybrids are inferior to their parents. Some abort as embryos, others die as juveniles, and others still grow to adulthood but cannot reproduce. Mules, for example, are vigorous but sterile: If you want to produce a mule, as people have been doing for more than 2,000 years, you have to mate a female horse and a male donkey. Getting it backward will result in a hinny, which is also sterile but less robust. Hence many hybrids are unable to pass their genes back to members of their parent species (1-5).
References (abridged):
1. Anderson, E. 1949. Introgressive Hybridization. New York: John Wiley & Sons.
2. Arnold, M. 1997. Natural Hybridization and Evolution. New York: Oxford University Press.
3. Ellstrand, N. C, H. C. Prentice and }. F. Hancock. 1999. Gene flow and introgression from domesticated plants into their wild relatives. Annual Review of Ecology and Systematics 30:539-563.
4. Ellstrand, N. C., R. Whitkus and L. H. Rieseberg. 1996. Distribution of spontaneous plant hybrids. Proceeding of the National Academy of Sciences of the USA. 93:5090-5093.
5. Grant, P. R., and B. R. Grant. 1992. Hybridization of bird species. Science 258:193-197.
American Scientist http://www.americanscientist.org
ScienceWeek http://scienceweek.com
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