Question

1. 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, and these contained 18 chromosomes — a complete set of both cabbage (n=9) and radish (n=9) chromosomes. Fusion of these plants’ gametes produced vigorous, fully fertile, polyploid plants with 36 chromosomes. Unfortunately, they had the roots of the cabbage and the leaves of the radish, and these plants could breed with each other but not with either the cabbage or radish ancestors. Therefore, Karpechenko produced a new species. This is an example of what type of speciation?

2. Bird guides once listed the myrtle warbler and Audobon's warbler as distinct species that lived side by side in parts of their ranges. However, recent books show them as eastern and western forms of the same biological species, the yellow-rumped warbler. What evidence must have been produced to support the claim that these birds are the same species under the biological species concept?

Answer 1

1) The biological definition of species, which works for sexually reproducing organisms, is a group of actually or potentially interbreeding individuals. There are exceptions to this rule. Many species are similar enough that hybrid offspring are possible and may often occur in nature, but for the majority of species this rule generally holds. In fact, the presence in nature of hybrids between similar species suggests that they may have descended from a single interbreeding species, and the speciation process may not yet be completed.
Polyploidy is a condition in which a cell or organism has an extra set, or sets, of chromosomes. Scientists have identified two main types of polyploidy that can lead to reproductive isolation of an individual in the polyploidy state. Reproductive isolation is the inability to interbreed. In some cases, a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition called autopolyploidy. The prefix “auto-” means “self,” so the term means multiple chromosomes from one’s own species. Polyploidy results from an error in meiosis in which all of the chromosomes move into one cell instead of separating. The other form of polyploidy occurs when individuals of two different species reproduce to form a viable offspring called an allopolyploid. The prefix “allo-” means “other” (recall from allopatric): therefore, an allopolyploid occurs when gametes from two different species combine.
Sympatric speciation occurs in such cases of allopolyploidy when two sets of chromosomes from two different species forms a polyploid of 36 chromosomes instead of 18 chromosomes, giving rise to a species that is reproductively compatible with only its new species and not with its ancestral population with 2n=18.In this way, sympatric speciation can occur quickly by forming offspring with 4n called a tetraploid. These individuals would immediately be able to reproduce only with those of this new kind and not those of the ancestral species.

2)Species are distinctly different kinds of organisms. Birds of one species are, under most circumstances, incapable of interbreeding with individuals of other species. Indeed, the "biological species concept" centers on this inability to successfully hybridize, and is what most biologists mean by "distinctly different." That concept works very well when two different kinds of birds live in the same area. For example, Townsend's and Yellow-rumped Warblers are clearly distinct kinds because their breeding ranges overlap, but they do not mate with one another. If they did, they might produce hybrid young, which in turn could "backcross" to the parental types, and (eventually) this process could cause the two kinds of warblers to lose their distinctness.
The Yellow-rumped Warbler has two distinct subspecies that used to be considered separate species: the "Myrtle" Warbler of the eastern U.S. and Canada's boreal forest, and "Audubon’s" Warbler of the mountainous West. The Audubon’s has a yellow throat; in the Myrtle subspecies the throat is white. Male "Audubon's" Warblers have more white in the wing than the "Myrtle" Warbler. Female Audubon's have less distinctly marked faces, lacking the dark ear patches of the "Myrtle" Warbler. Intermediate forms occur where the two subspecies' breeding ranges overlap, such as in the Canadian Rockies. The western populations of the Yellow-rumped Warbler (which have yellow throats) were previously considered a species, Audubon's Warbler, distinct from the eastern Myrtle Warblers (which have white throats), largely because of differences in appearance. Then it was discovered that the breeding ranges of Audubon's and Myrtle Warblers overlap broadly in a band from southeastern Alaska through central British Columbia to southern Alberta, and that the two "species" hybridize freely within this area. The forms intergrade, and taxonomists now consider them to be subspecies of a single species, the Yellow-rumped Warbler. Subspecies are simply populations or sets of populations within a species that are sufficiently distinct that taxonomists have found it convenient to formally name them, but not distinct enough to prevent hybridization where two populations come into contact.
Geographic variation -- birds showing different characteristics in different areas -- is inevitable among the populations of all species with extensive breeding distributions. It is largely the result of populations responding to different pressures of natural selection in different habitats. If populations of a single bird species become geographically isolated, those different selection pressures may, given enough time, cause the populations to differentiate sufficiently to prevent interbreeding if contact is reestablished. In nature, degrees of differentiation and of abilities to hybridize fall along a continuum, so one finds what is expected in an evolving avifauna -- some populations intermediate between subspecies and species, populations (members of superspecies) that have differentiated to the point where they will not hybridize but have not yet regained full contact, and populations so distinct that they can be recognized as full species whether or not they occur together.