Coyne talks about Homo floresiensis on page 207. They were the only population of earlier hominins not suddenly replaced by H. sapiens about 300,000 years ago. The two theories used to explain this sudden replacement are the "multiregional" theory and the "out of Africa" theory (see page 206). Under the "multiregional" theory, why didn't Homo floresiensis too evolve into H. sapiens? Under the "out of Africa" theory, what conditions could of allowed Homo floresiensis to reach the island of Flores in Indonesia, but then prevented H. sapiens from reaching the same island and replacing the Homo floresiensis?
Also, Coyne mentioned that Homo floresiensis may have preyed on Komodo dragons. How could a species that is so closely related to us as to be in the same genus, eat something that is so poisonous to us now?
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ReplyDeleteAccording to the multiregional theory, the H. erectus and H. neanderthalensis “evolved into H. sapiens independently in several areas”. According to this theory, then, the Homo floresiensis were populations of H. erectus in which natural selection did not act in the same direction to evolve into Homo sapiens. According to Coyne, the H. floresiensis represents “an evolutionary dead end”, one that evolved in isolation on Flores, one of the Lesser Sunda Islands in Indonesia. The multiregional theory stipulates that the evolution of H. floresiensis proceeded by allopatric speciation, that is, the development of species that “because they occupy areas separated by space or time, are reproductively isolated” (“Natural selections and speciation”). Conversely, sympatric speciation is the development of species which occupy the same habitat, yet cannot interbreed due to isolating mechanisms. “Isolating mechanisms” may also be geographical and the evolution of H. floresiensis exemplifies speciation contingent on spatial, or geographic, separation. When the populations were isolated, gene flow between the populations of H. erectus were interrupted. Since the colonization of Flores represented a new environment, the population of H. floresienses subsequently encountered substantially different selective pressures than did other “races” of H. erectus. In fact, the allopatric model of speciation states that “The reasons the physically isolated populations will genetically diverge are genetic drift and differences in natural selection between the two populations“(“Speciation”).
ReplyDeleteAccording to the “out of Africa” theory, H. sapiens originated on the African continent and replaced the existing populations of H. erectus by out-competing them for resources and habitat. H. sapiens likely occupied a similar “niche”, that is “all the interactions of a population with the other members of its community…a variety of abiotic factors also determine a species’ niche” (“Species”). Niche and Habitat selection thus resulted in competition between populations of H. sapiens and H. erectus and H. sapiens likely had selective advantages that eventually resulted in the species’ dominance. The isolated populations of H. erectus that colonized Flores and eventually evolved into H. floresiensis may have evaded competition by H. sapiens by niche modification.
cont. The spreading of individuals of a population from a regions of high population density is called dispersal and influences the “natural range expansion”, that is the geographic distribution of a species as determined by biotic and abiotic factors (Campbell & Reece, 1152).
ReplyDeleteBecause of the relative inaccessibility of the island of Flores, the dispersal of H. sapiens to Flores may have initially been hampered by geographic location; although this seems unlikely since floresiensis fossils date to 18000 years ago, at which point H. sapiens had already colonized Australia. It is possible that habitat selection dictated the isolation and evolution of H. floresiensis. Certain biotic or abiotic factors may have hindered the spread of H. sapiens as evidenced by the diet of H. floresiensis. According to Coyne, H. floresiensis had “adult height [which] was a scant of one meter…and they may have preyed on the Komodo dragons and dwarf elephants that populated the island”. This may have been an example of niche modification resulting from competition for food with inland populations of H. erectus (Shimada & Fujii). Thus, the unique selective pressures of Flores likely resulted in the development of a diet that would be poisonous to the modern-day human. H. erectus had spread all the way from Africa to Indonesia by 1.5 million years ago whereas the emergence of H. sapiens did not occur until 300 000 years ago. By then, it is possible that the unique environmental pressures of the isolated Flores would have selected for “traits” in the H. erectus populations that inhabited the Indonesian island which allowed for modifications in habitat selection and consequently non-competitive coexistence of H. sapiens and H. floresiensis.
Sources
Jerry Coyne. “Why Evolution is true”
“Natural Selection and Speciation”. http://biology.clc.uc.edu/Courses/bio303/natural%20selection.htm
“Speciation”. http://evoled.dbs.umt.edu/lessons/speciation.htm
niche. (2010). In Encyclopædia Britannica. Retrieved April 2, 2010, from Encyclopædia Britannica Online School Edition: http://www.school.eb.com/eb/article-9055711
Shimada, Masakazu; Fujii, Koichi. “Niche modification and stability of competitive systems. I. Niche modification process”. Researches on population ecology. volume 27. 1985.