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Download Lecture #14 -Isolation and the Origin of Species Fall 2001
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Transcript
Lecture #14 -Isolation and the Origin of Species Fall 2001 ** Isolating Mechanisms - are phenomena that prevent inbreeding of closely related taxa (plants or animals). They are based on the following factors: (a) spatial distance, (b) environmental factors and (c) reproductive biology. Reproductive Isolating Mechanisms are usually placed in one of two categories: ** Prezygotic Mechanisms & ** Postzygotic Mechanisms Prezygotic mechanisms include: (a) geographical isolation = spatial distance prevents inbreeding - taxa of plants do not occur in the same area ... e.g., Plantanus occidentalis and P. orientalis occur in the northeastern U.S. and the Mediterranean respectively ... and are distinct. However, when you put them together, they produce fully fertile hybrids (remember allopatric ... when ranges do not overlap). Often geographical isolation is promoted by barriers such as mountains, rivers, oceans, deserts, roads, etc.. (b) ecological isolation - plants live in the same region but grow in distinctly different habitats. Vernonia (ironweed) meadows one species lives in sunny, moist environments along roadsides and in the other in shady, well-drained mature hardwood forests. Sympatrically, they rarely form hybrids; however, in the greenhouse they cross. (c) seasonal (temporal) - plants occur in the same region but flower at different times. Some ironweeds and sunflowers bloom in midsummer and others in late summer and fall. (d) ethological - plants are isolated from one another by pollinator behavior patterns. Certain hummingbirds, bees and moths are species specific with regard to the flower they visit. (e) mechanical - pollination is prevented by structural differences in flower parts. In the Asclepiadaceae, the proper pollinia must be inserted by the pollinator into the slits in the gynoecium (similar phenomenon in orchids). Postzygotic Mechanisms (mechanisms that kick in after fertilization) include: (a) hybrid inviability - the hybrid zygote dies, the endosperm disintegrates, the seed fails to germinate, seedlings die, anthers abort etc.... (b) hybrid sterility - first generation hybrids are produced .... which flower, but they have aborted stamens, or more typically, infertile pollen. (c) hybrid breakdown - occurs in second or later-generation progeny. In some interspecific crosses in Vernonia, the first generation hybrids are completely fertile, but the 2nd generation progeny exhibit continuous variation from full fertility to complete inviability. Speciation - the evolutionary divergence and differentiation of a formerly homogeneous population system into two or more separate species. Speciation requires genetic variation ... which is common among most populations, plus reproductive isolation. ** - Most evolutionary scientists have stressed a need for spatial distance as a prerequisite for speciation .... i.e., allopatric speciation. IN plants, this may not be necessary .... and sympatric speciation occurs from time to time .... mainly via polyploidy but not always. ** - Two modes of Evolutionary Change: (a) Phyletic Evolution - a change from one state to another through time in which a species might evolve into something different from its ancestor, and (b) Divergent Evolution - a single population system that differentiates into two distinct evolutionary lines. ** - As an ecotype (a group that is separated from the parent population) becomes specialized and adapts to its environment, isolating mechanisms build up, reducing gene exchange. As divergence increases, the elements of the populations become less and less able to exchange genes. Divergence leads to reproductive isolation ... and this isolation promotes eventual speciation. ** - The question is whether sympatric speciation can occur (speciation from populations that overlap). The answer is yes in plants .... especially in those species that are self-fertilizing, because this tends to restrict gene flow (and in those species that produce polyploids). ** - In some plants (Clarkia for example) local populations acquire a set of distinctive features through a complex variety of evolutionary mechanisms and then the adaptive novelties spread over certain areas and replace the less adaptive genotypes. Darwinian Gradualism Versus Punctuated Equilibrium ** Darwin believe that organisms were in a constant struggle for survival throughout the earth s history. IN each generation, the fittest produce the most offspring ... and thus were evolutionarily the most successful. Basically, this was seen (and still is by many) as a constant gradual process resulting eventually in speciation. ** - Another view to evolution and speciation was put forth by Stephen Jay Gould and Niles Eldridge in 1977 in which they claim that the fossil evidence frequently suggests that for many taxa organic evolution is not a gradual unfolding process but a process of homeostatic equilibria, punctuated on occasion by rapid and episodic events of speciation via flooding, temperature changes, meteorite impact, etc... followed by long periods of stasis or non-change. ** - In the big scheme of things, the fossil record seems to support Punctuated Equilibrium Theory showing that indeed there were long periods of stasis punctuated by shorter periods of rapid change. The association of rapid speciation of plants with such events as mountain uplifts and climate changes .... is compatible with this theory. ** - Hybridization ..... the role of hybridization in plants is controversial. Helianthus and Quercus species readily form hybrids, which contradicts the biological species concept. Thus, species must be more loosely defined and arbitrary in plants than in animals. Plant hybrids are more readily seen than animal hybrids for three reasons: (a) ethological isolation is more effective in animals, (b) zoologists may have overlooked hybrid animals since they are more difficult to find, and (c) animals have more complex developmental sequences and structure which makes successful hybridization less likely. Hybrid plants are likely to have little affect in stable environments. However, they may be at an advantage in disturbed and changing environments. Polyploidy - the multiplication of the chromosome set a distinct process affecting the evolution of flowering plants. In a diploid species, n is the basic haploid set of chromosomes, but in a polyploid species, n is a multiple of x. Some species have diploid numbers of 18, 36, 54 and 90 (as in some composites such as Chrysanthemum in which diploid = 18 and haploid = 9). (Additional Information on Polyploidy) - Cells (and their owners) are polyploid if they contain more than two haploid (n) sets of chromosomes; that is, their chromosome number is some multiple of n greater than the 2n content of diploid cells. For example, triploid (3n) and tetraploid cell (4n) cells are polyploid. Polyploidy is very common in plants, especially in angiosperms. From 30% to 70% of today's angiosperms are thought to be polyploid. Species of coffee plant with 22, 44, 66, and 88 chromosomes are known. This suggests that the ancestral condition was a plant with a haploid (n) number of 11 and a diploid (2n) number of 22, from which evolved the different polyploid descendants. ** - In fact, the chromosome content of most plants groups suggests that the basic angiosperm genome consists of the genes on 7-11 chromosomes. Domestic wheat, with its 42 chromosomes, is probably hexaploid (6n, where n (the ancestral haploid number) was 7. Some other examples: Plant Probable ancestral haploid number domestic oat Chromosome number Ploidy Level 7 42 6n peanut 10 40 4n sugar cane 10 80 8n banana 11 22, 33 2n, 3n white potato 12 48 4n tobacco 12 48 4n cotton 13 52 4n apple 17 34, 51 2n, 3n Polyploid plants not only have larger cells but the plants themselves are often larger. This has led to the deliberate creation of polyploid varieties of such plants as watermelons, marigolds, and snapdragons. ** - Polyploidy has occurred often in the evolution of plants. ** - Polyploidy and Speciation When a newly-arisen tetraploid (4n) plant tries to breed with its ancestral species (a backcross), triploid offspring are formed. These are sterile because they cannot form gametes with a balanced assortment of chromosomes. However, the tetraploid plants can breed with each other. So in one generation, a new species has been formed. Polyploidy even allows the formation of new species derived from different ancestors. In 1928, the Russian plant geneticist Karpechenko produced a new species by crossing a cabbage with a radish. Although belonging to different genera (Brassica and Raphanus respectively), both parents have a diploid number of 18. Fusion of their respective gametes (n=9) produced mostly infertile hybrids. However, a few fertile plants were formed, probably by the spontaneous doubling of the chromosome number in somatic cells that went on to form gametes (by meiosis). Thus these contained 18 chromosomes - a complete set of both cabbage (n=9) and radish (n=9) chromosomes. Fusion of these gametes produced vigorous, fully-fertile, polyploid plants with 36 chromosomes. (Unfortunately, they had the roots of the cabbage and the leaves of the radish.) These plants could breed with each other but not with either the cabbage or radish ancestors, so Karpechenko had produced a new species. The process also occurs in nature. Three species in the mustard family appear to have arisen by hybridization and polyploidy from three other ancestral species: B. oleracea (cabbage, broccoli, etc.) hybridized with B. nigra (black mustard) -> B. carinata (Abyssinian mustard). B. oleracea x B. campestris (turnips) -> B. napus (rutabaga) B. nigra x B. campestris -> B. juncea (leaf mustard) Modern wheat and perhaps some of the other plants listed in the table above have probably evolved in a similar way. Aneuploidy refers to instances where the chromosome number is not an exact multiple of the basic number - Clarkia (Onagraceae) - haploid (n) = 5 in some, 6 in some, 7 in some, 8 in some and 9 in some species. 40% of plants and a higher percentage of ferns are polyploids. Polyploidy is related to growth habit. Annuals tend to have the lowest frequency of polyploidy and herbaceous perennials have the highest. Finally, Apomixis, the replacement of sexual reproduction with asexual reproduction .... occurs in some plant species. It is often associated with polyploidy. Many apomictic plant species are adapted to particular microhabitats. ** - Vegetative propagation by buds, bulbs, rootstocks, etc... is widespread among many perennials. This permits the multiplication of a well-adapted genotype that might disappear in the course of sexual reproduction.