Dillon RT. 1984. Geographic distance, environmental difference, and divergence between isolated populations. Systematic Zoology 33: 69-82.
This author examines the relative contributions to population divergence of selection and gene flow (or the lack thereof) using 25 populations of freshwater snails occurring in extremely stable drainages in the south-eastern USA. The system used here has clear advantages for a study that attempts to disentangle these frequently-confounded variables.
Divergence between populations can be correlated by distance in two non-mutually-exclusive ways. A reduction in gene flow that may be associated with longer dispersal distances means that novel mutations take increasingly long times to reach further populations. Environmental differences tend to be spatially autocorrelated such that distant populations are likely to have more different environments and selection will therefore be different. However, if divergence/distance and divergence/environmental difference can be separated, then gene flow and selection can be examined independently.
The drainages of the southern Appalachians appear to have been highly stable since the Cretaceous. The snail Goniobasis proxima appears to be incapable of dispersal overland, though very rare cases of transport by birds or mammals may be responsible for establishing some populations. It is restricted to smaller streams of intermediate flow rates, many of which are distributed on both sides of the southern Appalachians and on the Piedmont (low plateau of small foothills) east of the mountains. Most of the populations examined in this study are completely isolated from each other, such that snails would have to either pass through the marine environment or over the (often very short) land barriers between populations. Development is direct, with egg masses attached to solid substrates producing crawling juveniles. This author notes that at any time, the majority of individuals are crawling upstream against the current, which apparently allows populations to stay approximately in the same place despite the occasional individual that must lose its grip and be swept downstream.
The analysis of population divergence here included comparisons among eight 25 x 25 symmetric matrices, constructed using a comprehensive range of variables including morphological features (shell height, aperture width, etc.), allozyme alleles for seven loci, and 15 environmental variables (11 components of water chemistry, plus temperature, flow rate, stream gradient, and parasite infections by trematodes) and a further independent assessment of environmental similarity derived from an examination of the diatom species diversity in the diets of each population.
Varying levels of genetic divergence were found throughout the study system, but most differences were relatively high compared to similar studies of other organisms. Allozyme alleles fell primarily into two categories: either they were present in all four study drainages, or they occurred in only a single population or small group of neighbouring populations. This suggests that all alleles arose either during a period when rapid spread across drainages was possible, or during a later period when dispersal was more difficult. The geological evidence strongly indicates extreme drainage stability, indicating that something about either the environment and / or the dispersal capabilities of G. proxima was different, perhaps during the Tertiary, than today.
No cline in morphology or allozymes was observed, which may be the result of a lack of gene flow preventing the spread of beneficial alleles. In other words, while nearby populations (overland) may experience very similar environments, adaptations in one population cannot spread to the other.
This author summarizes with a statement that both selection and gene flow restriction seem to be equally important in promoting morphological divergence in isolated populations. However, time since divergence may be the more important diversifying factor, and may underlie both selection and gene flow in this system. Furthermore, measures of divergence using allozymes indicate that time since isolation or gene flow may be more important than selection in structuring differences between populations.
Friday, June 6, 2008
Wednesday, June 4, 2008
Alvarez-Fuster et al. 1991
Alvarez-Fuster A, Juan C, Petitpierre E. 1991. Genome size in Tribolium flour-beetles: inter- and intraspecific variation. Genetical Research 58: 1-5.
These authors measured genome size in nine species of tenebrionid beetles in the tribe Ulomini; all but one of which are members of the genus Tribolium. Tribolium castaneum was and remains an exceptionally well-studied beetle, but genome size information was lacking, and few studies of insects had sought information about intraspecific variation in a comprehensive manner prior to this study.
Genome size was estimated by Feulgen densitometry measures of spermatids. Twenty spermatid nuclei from each of 10 individuals (five individuals of Alphitobius diaperinus) per species were measured, with spermatids of Dermestes maculatus used as a standard. For reasons not clear to me, detailed comparisons between D. maculatus and T. castaneum were carried out prior to studies of the other species. The procedure for isolation, fixation, and staining nuclei is similar to what we currently use but has some differences. Fixation was carried out for only 10 minutes in 10% formalin, rather than > 12 hours in MFA, hydrolysis lasted only 45 minutes (rather than 120) but used 5N-HCl as we use, and staining was for two hours (like what we do), but details of the chemical preparation of the stain are not given, though the cited Juan & Petitpierre (1989) may provide more information.
Statistical analysis included measures derived from Gold & Amemiya (1987) of within and between taxa comparisons. Nested ANOVA was also used to support an argument about the significance of intraspecific variation.
The discussion section includes an argument that large and uniform sampling as carried here provides a “solid base from which to analyze the pattern of distribution of genome sizes both inter- and intraspecifically.” Later discussion includes an odd argument that one species, T. brevicornis, with the largest genome size measured here, may be ancestral to the other species (including, presumably, the one species in another genus) and that genome evolution has proceeded by reduction and associated specialization of the type proposed by e.g. Hinegardner (1976).
Within each species, two to four significantly different groups of genome sizes were found, with the exception of one species (T. destructor), which did not show such structure. This, plus the nested ANOVA that indicated significant variation due to between-individuals-within-species comparisons, is taken as evidence in favour of (biologically) significant intraspecific variation in these beetles. The authors acknowledge that such intraspecific variation is surprising, but their arguments for evolutionary trends are not well constructed, so I do not know what to make of this intraspecific variation.
These authors measured genome size in nine species of tenebrionid beetles in the tribe Ulomini; all but one of which are members of the genus Tribolium. Tribolium castaneum was and remains an exceptionally well-studied beetle, but genome size information was lacking, and few studies of insects had sought information about intraspecific variation in a comprehensive manner prior to this study.
Genome size was estimated by Feulgen densitometry measures of spermatids. Twenty spermatid nuclei from each of 10 individuals (five individuals of Alphitobius diaperinus) per species were measured, with spermatids of Dermestes maculatus used as a standard. For reasons not clear to me, detailed comparisons between D. maculatus and T. castaneum were carried out prior to studies of the other species. The procedure for isolation, fixation, and staining nuclei is similar to what we currently use but has some differences. Fixation was carried out for only 10 minutes in 10% formalin, rather than > 12 hours in MFA, hydrolysis lasted only 45 minutes (rather than 120) but used 5N-HCl as we use, and staining was for two hours (like what we do), but details of the chemical preparation of the stain are not given, though the cited Juan & Petitpierre (1989) may provide more information.
Statistical analysis included measures derived from Gold & Amemiya (1987) of within and between taxa comparisons. Nested ANOVA was also used to support an argument about the significance of intraspecific variation.
The discussion section includes an argument that large and uniform sampling as carried here provides a “solid base from which to analyze the pattern of distribution of genome sizes both inter- and intraspecifically.” Later discussion includes an odd argument that one species, T. brevicornis, with the largest genome size measured here, may be ancestral to the other species (including, presumably, the one species in another genus) and that genome evolution has proceeded by reduction and associated specialization of the type proposed by e.g. Hinegardner (1976).
Within each species, two to four significantly different groups of genome sizes were found, with the exception of one species (T. destructor), which did not show such structure. This, plus the nested ANOVA that indicated significant variation due to between-individuals-within-species comparisons, is taken as evidence in favour of (biologically) significant intraspecific variation in these beetles. The authors acknowledge that such intraspecific variation is surprising, but their arguments for evolutionary trends are not well constructed, so I do not know what to make of this intraspecific variation.
Bell and Collins 2008
Bell G, Collins S. 2008. Adaptation, extinction and global change. Evolutionary Applications 1: 3-16.
These authors review and synthesize the current theory surrounding evolutionary responses to gradual environmental changes, especially increasing atmospheric carbon dioxide concentrations. In this context, these authors argue that the primary task of evolutionary biologists in the early 21st century is to predict adaptive evolution (or extinction) of populations and species in a gradually changing environment.
Populations are assumed to have high fitness, based on historical events. In a stable environment, beneficial alleles will accumulate, thus environmental changes are likely to be changes to the worse. Additionally, populations that do not adapt will suffer gradually declining mean fitness as pathogens and predators should themselves be selected for the most common target genotype.
Environmental variance increases with increasing time scales, that is, events occurring more distantly in time will be more different from each other. This applies to environmental conditions as much as anything else. The major implication of this in the context of this paper is that lineages surviving over long time periods will experience increasingly variable conditions, which also means generally deteriorating conditions if populations start well-adapted.
Many previous studies have found either strong selection, and / or high heritability. This apparent contradiction can be explained by high variability in the direction and magnitude of selection, suggesting that environmental variability is ubiquitous.
Populations can respond to changing conditions in four ways: phenotypic plasticity, dispersal and migration, adaptation, or extinction. These are arranged in order from short time scales to long.
The current pace of global environmental conditions change is probably higher than in most or all previous episodes. Historical levels of CO2 were about 280 ppm (parts per million); currently they are about 380 ppm, with some projections as high as 1000 ppm within the next 100 years.
These authors analysed the effects of the severity and frequency of environmental change (always change for the worse). They found that frequency is much more important to the probability of adaptation or “evolutionary rescue” than is severity. Frequent changes in conditions is likely to lead to extinction because selection does not have enough time to fix beneficial alleles, such that some fraction of the population is not well adapted to start with when the next change hits. In contrast, rare but severe changes lead to strong selection, and expose a long series of potentially beneficial alleles, increasing the mutation supply rate. The mutation supply rate is the critical parameter in determining adaptation or extinction, specifically the fraction of the supply of mutations that are beneficial and can rescue a declining population, i.e. raise a genotype’s rate of growth from negative to positive. Even rapid environmental change can be adapted to if the (rescue) mutation supply is high enough.
A survey of both simulations and studies of natural and laboratory populations revealed several cases of adaptation to frequent or continuous environmental change, and several cases of failure to adapt, even in species with very large population sizes. This indicates primarily that the rate of rescue mutations is unknown, despite being the single most important variable in predicting the evolutionary fates of populations under global climate change. However, these authors argue that a rescue mutation rate of approximately one per generation, regardless of population size, is probably close to the critical value, but qualify this estimate by stating they intend it as a first guess, and stimulus for more exact research in future.
The current driver of global environmental change is increasing CO2 concentrations in the atmosphere. Unlike most other agents of environmental change, CO2 is not in itself likely to represent a lethal stress to organisms, rather the effects will be either direct and positive (increased photosynthesis), or indirect via either temperature and climate changes or via biotic interactions such as competition.
Experiments and simulations by these and other authors have suggested that the short-term physiological responses of phytoplankton to increased CO2 will be opposite in direction to long-term evolutionary responses; this is at least partly because phytoplankton appear to be not limited by CO2 for growth. More specifically, the CO2-importing pump, which is tightly regulated by CO2 concentrations, will become less efficient, removing much of the productivity gain that would otherwise be predicted. Evidence that this will lead to community-level effects such as succession is currently equivocal, and based on short-term studies or idiosyncratic species compositions.
Our current understanding of evolution and ecology is based on populations subjected to a sudden shift from one stable environment to another, and usually a shift to increased stress or scarcity. In contrast, current global change is gradual and continuous, and is a case of nutrient enrichment.
These authors conclude with three main points. First, rescue mutations are critical to the phenomenon of evolutionary rescue and adaptation, but their rate is almost totally unknown. Second, the evolutionary response to increasing atmospheric CO2 concentrations is likely to be reduced efficiency of photosynthesis, rather than increased productivity. Third, these authors urge a general increase in studies of the evolutionary effects of global change, at rates of environmental change between the very fast rates typical of laboratory experimental evolution, and the very slow rates typical of most of the Earth’s history.
These authors review and synthesize the current theory surrounding evolutionary responses to gradual environmental changes, especially increasing atmospheric carbon dioxide concentrations. In this context, these authors argue that the primary task of evolutionary biologists in the early 21st century is to predict adaptive evolution (or extinction) of populations and species in a gradually changing environment.
Populations are assumed to have high fitness, based on historical events. In a stable environment, beneficial alleles will accumulate, thus environmental changes are likely to be changes to the worse. Additionally, populations that do not adapt will suffer gradually declining mean fitness as pathogens and predators should themselves be selected for the most common target genotype.
Environmental variance increases with increasing time scales, that is, events occurring more distantly in time will be more different from each other. This applies to environmental conditions as much as anything else. The major implication of this in the context of this paper is that lineages surviving over long time periods will experience increasingly variable conditions, which also means generally deteriorating conditions if populations start well-adapted.
Many previous studies have found either strong selection, and / or high heritability. This apparent contradiction can be explained by high variability in the direction and magnitude of selection, suggesting that environmental variability is ubiquitous.
Populations can respond to changing conditions in four ways: phenotypic plasticity, dispersal and migration, adaptation, or extinction. These are arranged in order from short time scales to long.
The current pace of global environmental conditions change is probably higher than in most or all previous episodes. Historical levels of CO2 were about 280 ppm (parts per million); currently they are about 380 ppm, with some projections as high as 1000 ppm within the next 100 years.
These authors analysed the effects of the severity and frequency of environmental change (always change for the worse). They found that frequency is much more important to the probability of adaptation or “evolutionary rescue” than is severity. Frequent changes in conditions is likely to lead to extinction because selection does not have enough time to fix beneficial alleles, such that some fraction of the population is not well adapted to start with when the next change hits. In contrast, rare but severe changes lead to strong selection, and expose a long series of potentially beneficial alleles, increasing the mutation supply rate. The mutation supply rate is the critical parameter in determining adaptation or extinction, specifically the fraction of the supply of mutations that are beneficial and can rescue a declining population, i.e. raise a genotype’s rate of growth from negative to positive. Even rapid environmental change can be adapted to if the (rescue) mutation supply is high enough.
A survey of both simulations and studies of natural and laboratory populations revealed several cases of adaptation to frequent or continuous environmental change, and several cases of failure to adapt, even in species with very large population sizes. This indicates primarily that the rate of rescue mutations is unknown, despite being the single most important variable in predicting the evolutionary fates of populations under global climate change. However, these authors argue that a rescue mutation rate of approximately one per generation, regardless of population size, is probably close to the critical value, but qualify this estimate by stating they intend it as a first guess, and stimulus for more exact research in future.
The current driver of global environmental change is increasing CO2 concentrations in the atmosphere. Unlike most other agents of environmental change, CO2 is not in itself likely to represent a lethal stress to organisms, rather the effects will be either direct and positive (increased photosynthesis), or indirect via either temperature and climate changes or via biotic interactions such as competition.
Experiments and simulations by these and other authors have suggested that the short-term physiological responses of phytoplankton to increased CO2 will be opposite in direction to long-term evolutionary responses; this is at least partly because phytoplankton appear to be not limited by CO2 for growth. More specifically, the CO2-importing pump, which is tightly regulated by CO2 concentrations, will become less efficient, removing much of the productivity gain that would otherwise be predicted. Evidence that this will lead to community-level effects such as succession is currently equivocal, and based on short-term studies or idiosyncratic species compositions.
Our current understanding of evolution and ecology is based on populations subjected to a sudden shift from one stable environment to another, and usually a shift to increased stress or scarcity. In contrast, current global change is gradual and continuous, and is a case of nutrient enrichment.
These authors conclude with three main points. First, rescue mutations are critical to the phenomenon of evolutionary rescue and adaptation, but their rate is almost totally unknown. Second, the evolutionary response to increasing atmospheric CO2 concentrations is likely to be reduced efficiency of photosynthesis, rather than increased productivity. Third, these authors urge a general increase in studies of the evolutionary effects of global change, at rates of environmental change between the very fast rates typical of laboratory experimental evolution, and the very slow rates typical of most of the Earth’s history.
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