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"Darwinists don't accept direction in evolution." -- Swenet
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[QUOTE]Originally posted by Swenet: [QB] [QUOTE] Originally posted by Swenet: Adaptations are tweaks [b]relative to the highly unpredictable environment, not to some ideal like "better design" or "complexity" or "intelligence".[/b][/QUOTE][IMG]https://snag.gy/V0sF9k.jpg[/IMG] [QUOTE][b]Long-term studies of evolution involving annual or more frequent sampling have many potential benefits.[/b] These include documentation and understanding of [b]slow and cryptic directional evolutionary change[/b], perhaps in association with gradual global warming, reversals in the direction of evolution, rare events with strong effects such as genetic bottlenecks caused by population crashes, phenomena recurring at long intervals, and processes with high interannual variability such as erratic and intermittent gene flow. These benefits are beginning to be realized (8–15), but [b]few studies have persisted long enough for us to be able to generalize about the temporal pattern and predictability of basic evolutionary processes in unconstrained natural populations.[/b] Here, we report the results of a [b]30-year study of evolution of size and shape traits in two populations of Darwin’s finches based on annual sampling and measurement.[/b] Distinctive features of the study are its length, continuity, entirely natural environmental setting, the availability of pedigree information to construct and interpret evolutionary change, and the macroevolutionary context of an adaptive radiation. [b]The study reveals the irregular occurrence, frequency, and consequences of two evolutionary processes that are more often inferred than directly studied: natural selection and introgressive hybridization.[/b] Natural selection and evolution. Populations of Geospiza fortis (medium ground finch) and G. scandens (cactus finch) have been studied on the Gala´pagos island of Daphne Major every year since 1973; adults that year were born (hatched) no later than 1972. Survival of marked and measured individuals has been recorded every year, and reproduction of most individuals has been recorded in most years (16). Six measured traits on adults whose growth has ceased have been reduced by principal components analyses to three interpretable synthetic traits: body size, beak size, and beak shape (17–20). The [b]null expectation is that, subject to sampling error, means of these traits have remained constant across the period of study. This expectation of no change is clearly not supported by the data (Fig. 1).[/b] Lack of independence of samples in successive years precludes year-by-year significance testing of the total samples. Nevertheless, [b]comparisons across years show nonoverlapping 95% confidence estimates of the means at different times.[/b] Mean body size and beak shape were [b]markedly different at the end of the period (2001) than at the beginning (1973) in both species (21).[/b] Between these two times [b]mean body and beak size of G. fortis initially decreased, then increased sharply, and decreased again more slowly (Fig. 1, A and B).[/b] Beak shape [b]abruptly became more pointed in the mid-1980s and remained so for the next 15 years (Fig. 1C). G. scandens, a larger species, displayed more gradual and uniform trends toward smaller size and blunter beaks (Fig. 1, D to F), thereby converging toward G. fortis in morphology.[/b] Apart from random sampling effects, annual changes in morphological means are caused by selective losses, as a result of mortality and emigration, and selective gains, as a result of breeding and immigration (22). [b]Previous work has demonstrated directional natural selection on beak and body size traits associated with survival, in G. fortis at three times and in G. scandens once, when a scarcity of rain caused a change in the composition of the seed supply that forms their dryseason diets (23–25). Evolutionary responses of G. fortis to the two strongest selection episodes occurred in the following generations (26), as expected from the high heritabilities of the morphological traits[/b] [h2 5 0.5 to 0.9 after corrections for misidentified paternity arising from extrapair copulations (18, 27)]. Figure 2 provides the long-term perspective of repeated natural selection in both species (28). There are four main features of the figure. First, body and beak size traits were subject to selection more often than was beak shape. Setting a at 0.01, to allow for the lack of complete independence of the traits (29), we find that body size was subject to selection about once every 3 years in both species (30), that is, once each generation of 4.5 years (G. fortis) or 5.5 years (G. scandens) (31) on average. Second, considering only the statistically significant selection differentials, [b]the species differed in the directions of net selection on size traits. G. fortis experienced selection in both directions with equal frequency (Fig. 2, A to C), whereas G. scandens experienced selection that repeatedly favored large body size and in no instance favored small beak size (Fig. 2, D and E).[/b] Third, unidirectional selection occurred in successive years, up to a maximum of 3 years in both species (Fig. 2, A, D, and E). Fourth, selection events in the two species were usually not synchronous, except in the late 1970s, when large size was selectively favored in both species during a drought (23). The demonstration here of natural selection occurring repeatedly in the same populations over a long time [b]complements the widespread detection of natural selection in many different species of plants and animals over much shorter times (32, 33, 34)[/b]. As in these broad surveys, and in three studies of birds lasting for 11 to 18 years (15, 35, 36), the magnitude of selection on the finch populations was usually less than 0.15 SD and rarely more than 0.50 SD (33, 34). Median values (0.03 to 0.06) are well within the normal range (0.00 to 0.30) of other studies (34). Evolution followed as a consequence of selection in both species because all traits are highly heritable (18, 20, 27). We compared the mean of a trait before selection with the mean of the same trait in the next generation by one-tailed t tests (P , 0.05) (26). Significant evolutionary events occurred in G. fortis eight times (body size, four; beak size, three; beak shape, one) and in G. scandens seven times (body size, two; beak size, five). [b]Evolution below the level of statistical detectability may have followed other instances of directional selection, may have been masked by annual variation in environmental effects on growth to final size (37), or may have been nullified by countervailing selection on correlated traits not included in the analyses (32).[/b] Magnitudes of evolution of the two independent beak traits (size and shape) are correlated with values predicted from the products of selection differentials and heritabilities (Fig. 3). Similar results were obtained in analyses of the direct effects of selection on the six measured traits of G. fortis at two times of intense selection, taking into account genetic correlations among them (26). Thus evolution, as an immediate response to selection, was predictable.[/QUOTE] http://science.sciencemag.org/content/296/5568/707.full [QUOTE][b]Conclusion. The long-term study of Darwin’s finch populations illustrates evolutionary unpredictability on a scale of decades.[/b] Mean body size and beak shape of both species at the end of the study [b]could not have been predicted at the beginning. Moreover, sampling at only the beginning and at the end would have missed beak size changes in G. fortis in the middle[/b]. The temporal pattern of change shows that [b]reversals in the direction of selection do not necessarily return a population to its earlier phenotypic state.[/b] Evolution of a population is [b]contingent upon environmental change, which may be highly irregular[/b], as well as on its demography and genetic architecture (33, 46). The study also illustrates how [b]the value of long-term studies increases with time. Not only is regular monitoring at short intervals desirable, but sampling for many years is to be recommended, especially for long-lived organisms like vertebrates and perennial plants. Yet evolutionary studies are rarely pursued in the field for as many as 10 years (33). If we had stopped sampling after 10 years, our conclusions would have been different because at that time the only difference from the starting point was in beak size of G. fortis. By persisting beyond then, we witnessed a natural-selection event that affected beak shape in G. fortis[/b], documented interbreeding and morphological effects of introgression on G. scandens, and gained a better quantitative estimate of the frequency of evolutionary events.[/QUOTE] http://science.sciencemag.org/content/296/5568/707.full [/QB][/QUOTE]
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