After the effective size of a population, Ne, declines, some slightly deleterious amino acid replacements which were initially suppressed by purifying selection become effectively neutral and can reach fixation. Here we investigate this phenomenon for a set of all 13 mitochondrial protein-coding genes from 110 mammalian species. By using body mass as a proxy for N e, we show that large mammals (i.e., those with low Ne) as compared with small ones (in our sample these are, on average, 369.5 kg and 275 g, respectively) have a 43% higher rate of accumulation of nonsynonymous nucleotide substitutions relative to synonymous substitutions, and an 8 -40% higher rate of accumulation of radical amino acid substitutions relative to conservative substitutions, depending on the type of amino acid classification. These higher rates result in a 6% greater amino acid dissimilarity between modern species and their most recent reconstructed ancestors in large versus small mammals. Because nonsynonymous substitutions are likely to be more harmful than synonymous substitutions, and radical amino acid substitutions are likely to be more harmful than conservative ones, our results suggest that large mammals experience less efficient purifying selection than small mammals. Furthermore, because in the course of mammalian evolution body size tends to increase and, consequently, N e tends to decline, evolution of mammals toward large body size may involve accumulation of slightly deleterious mutations in mitochondrial protein-coding genes, which may contribute to decline or extinction of large mammals.body mass ͉ effective population size ͉ substitution rates ͉ purifying selection ͉ body size-dependent extinction N atural selection is more efficient in large populations. In a population of effective size N e , mutations with selection coefficient s such that ͉s͉ Ͻ 1/N e are effectively neutral, in the sense that their dynamics are affected mostly by random drift (1, 2). Effectively neutral nucleotide substitutions, both slightly deleterious and slightly beneficial, play a major role in evolution at the molecular level (3). Provided that the distribution of selection coefficient is independent of N e , the fraction of mutations which are effectively neutral must be higher in populations with smaller N e . Because selection is mostly purifying, in species with low N e a larger fraction of slightly deleterious mutations can reach fixation.Purifying selection affects nonsynonymous substitutions much stronger than synonymous substitutions (4-6). Indeed, when the ratio of the rate of nonsynonymous (amino acid changing) substitutions over the rate of synonymous (silent) substitutions, K a /K s , is Ͻ1, it is indicative of purifying selection on nonsynonymous substitutions and reflects its strength: the closer the K a /K s to 1, the weaker is the purifying selection (3). Another measure of the strength of purifying selection is the ratio of the rate of radical (presumably more harmful) over the rate of conservative (less harmful) amino acid sub...

Predation on

Research on the role of top–down (predation) and bottom–up (food) effects in food webs has led to the understanding that the variability of these effects in space and time is a fundamental feature of natural systems. Consequently, our measurement tools must allow us to evaluate the effects from a dynamical perspective. A population‐dynamics approach may be appropriate to the task. More specifically, because food and predators both affect birth rate, birth rate dynamics may be a key to understanding their impact on the population of interest. Based on the Edmondson–Paloheimo model for birth rate, we propose a new population metric to assess the relative strength of top–down vs bottom–up effects. The metric is the ratio of contributions of changes in proportion of adults and fecundity to change in birth rate. Proportion of adults reflects a top–down effect (predators are assumed to be size‐selective), fecundity reflects a bottom–up effect, and birth rate appears as a common currency with which to compare the former and the latter. Using microcosm experiments and computer simulations on the cladoceran Daphnia, we calibrate the metric and show that, in both types of tests, the ratio of contributions is typically 0.5–0.7 under a strong bottom–up effect and 2.0–2.2 under a strong top–down effect. This provides experimental evidence that the ratio of contributions may allow one to distinguish a strong top–down effect from a strong bottom–up effect.

Three natural Daphnia populations in which the previously established impact of invertebrate predators on the death rate (but not on the birth rate) were investigated. Two other cladoceran populations primarily affected by food were used for comparison. The effect of predators on the per capita birth rate of Daphnia was detected by decomposing changes in birth rate into components dependent on changes in egg development rate, mean clutch size, and proportion of adults and then comparing them. This method revealed short-term changes in the proportion of adults, which dominated changes in birth rate. The effect can be attributed to size-selective feeding of the invertebrate planktivores that normally prey on young daphnids, and it did not occur in food-dependent cladocerans. Invertebrate predation pressure tends to increase the per capita birth rate of Daphnia, which may provide a mechanism that stabilizes prey.

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