2. The Problem of Paleospecies Paleontologists have to recognize species from their fossil remains. The problem of "What is a paleospecies?" led Niles Eldredge and Stephen Jay Gould to propose the theory of punctuated equilibria. The term "paleospecies" makes explicit the distinction between the classification of species from fossil remains and the process of recognizing species in modern populations. This problem involves geology, taphonomy, taxonomy, and -- though often ignored -- geography.
Mayr's Biological Species Concept uses the criterion of reproductive isolation to distinguish species in modern populations. Paleontologists who pursue taxonomic endeavors (which includes most of them) have to classify their finds generally based upon morphological features. The rareness of preservation of tissues containing DNA, or even of soft tissues, limits the range of diagnostic characters which may be utilized. The paleontologist has no access to such information. (Whether modern biologists really do have access to that information is a matter of some little debate in the literature.) The fossil record is incomplete. This incompleteness has many contributing factors. Geological processes may cause to confusion or error, as sedimentary deposition rates may vary, erosion may erase some strata, compression may turn possible fossils into unrecognizable junk, and various other means by which the local fossil record can be turned into the equivalent of a partially burned book, which is then unbound, pages perhaps shuffled, and from which a few pages are retrieved. Beyond geology, there remains taphonomy -- the study of how organisms come to be preserved as fossils. Here, there are further issues to be addressed. Hard parts of organisms fossilize preferentially. The conditions under which even those parts may become fossilized are fairly specialized. All this results in a heavily skewed distribution of even what parts of organisms become fossilized, and that affects which features of morphology are available for use in classification. The issue of geography enters into all this, as a consequence of the fact that living lineages occupy ecological niches, and those niches are bound to certain features of geography. Paleospecies, then, have to be recognized as species from morphology alone, where the available morphological characters are drawn from a skewed distribution, the pattern of fossilization is skewed, and the geographic correlates of fossilization are limited in extent. 3. Patterns of speciation from neontological study Eldredge and Gould's insight into paleontological processes was to derive their understanding of paleospecies from living biological species. In this manner, it can be made clear what PE means for the concept of paleospecies. First, modern species appear to have derived from cladogenesis, the splitting of a daughter species from an ancestral species rather than transformation of the ancestral species in toto. This is a multiplication of species, and without it, the diversity of the living systems that we see would be impossible. Second, the mode of speciation most often seen is also identified from modern populations. That mode is allopatric speciation of peripheral isolates, or peripatric speciation in Mayr's terminology. Peripatric speciation states that a population of an ancestral species in a geographically peripheral part of the ancestral range is modified over time until even when the ancestral and daughter populations come into contact, there is reproductive isolation. While saltational speciation by change in ploidy is observed to occur in modern populations, this form of speciation is also known to be rare (except in plants). The incipient speciation of clinal forms has recently become very controversial, and is also likely to be rare in any case. Sympatric speciation, the production of a daughter species within the geographic range of the parent species, likewise is held to be a very rare event seen primarily in insect and parasite lineages. Third, the frequency with which peripatric speciation occurs in modern lineages can be seen as "rare". This rarity is different from the rarity discussed in the last item. There, we compared how often certain modes of speciation were seen compared to other modes of speciation. Here, we want to know how common it is for a species to produce a daughter species by peripatric speciation. The answer is, "Not very common at all." This rarity means that a species may produce zero, one, or perhaps a few daughter species during its entire time span of existence. Fourth, the period of transition between parent species and daughter species is short compared to the period of time a species exists as a distinct form. When a small sub-population is isolated from the rest of the population of a species, the particular set of variations in the sub-population is much smaller than that in the remainder of the population. These variations, when in conjunction with suitable features of geographic locale, climate, and resources, can lead to rapid development of reproductive isolation from the ancestral population. The reduction in variation due to small sub-population size is known as the "founder effect". Fifth, significant adaptations developed or accentuated in the daughter species can lead to the rapid dispersal and establishment of a daughter species throughout the range of the ancestral species, or into new ranges. The ecological processes of dispersal and succession can occur very quickly compared to evolutionary processes of change. Sixth, the principles of gene flow, genetic homeostasis, and large population size inhibit widespread ancestral populations from much directional (adaptive) change. Any adaptive change found in the ancestral population is likely to be small and unrelated to evolutionary trends. --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] Visit IAGI Website: http://iagi.or.id IAGI-net Archive 1: http://www.mail-archive.com/iagi-net%40iagi.or.id/ IAGI-net Archive 2: http://groups.yahoo.com/group/iagi Komisi Sedimentologi (FOSI) : F. Hasan Sidi([EMAIL PROTECTED])-http://fosi.iagi.or.id Komisi SDM/Pendidikan : Edy Sunardi([EMAIL PROTECTED]) Komisi Karst : Hanang Samodra([EMAIL PROTECTED]) Komisi Sertifikasi : M. Suryowibowo([EMAIL PROTECTED]) Komisi OTODA : Ridwan Djamaluddin([EMAIL PROTECTED] atau [EMAIL PROTECTED]), Arif Zardi Dahlius([EMAIL PROTECTED]) Komisi Database Geologi : Aria A. Mulhadiono([EMAIL PROTECTED]) ---------------------------------------------------------------------
