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J Merilä

Publications and source records attributed to J Merilä.

10 recordsLinked to original sources
PubMed2001

Carry-over effects of ultraviolet-B radiation on larval fitness in Rana temporaria.

A number of studies have failed to find evidence for negative effects of ultraviolet-B radiation (UVBR) on amphibian early-embryonic performance, leading to the conclusions, first, that the embryonic stages of many species are tolerant to UVBR, and second, that the increased amount of UVBR reaching the Earth's surface is not likely to have any direct negative effects on many amphibian populations. However, possible carry-over effects of exposure to UVBR in the embryonic stages to the larval stages have received less attention. We studied the effects of UVBR experienced during the embryonic stages (age less than 11 days) on the later performance (age 11-75 days) of common frog, Rana temporaria, larvae. In a factorial laboratory experiment, newly fertilized embryos were divided into three different UVBR treatments (no UVBR (control), 1.25 kJm(-2) (normal) and 1.58 kJm(-2) (26% enhanced)), after which the individual larvae were raised until metamorphosis in the absence of UVBR. No effects of UVBR on embryonic survival rates, frequency of developmental anomalies or hatchling size were found, corroborating the earlier results indicating that R. temporaria embryos are tolerant to UVBR. However, analyses of larval performance revealed that larvae exposed to enhanced levels of UVBR as embryos suffered from an increased frequency of developmental anomalies and metamorphosed later and at a smaller size than larvae that had been protected from UVBR as embryos. These results suggest, in contrast to the earlier studies, that UVBR has direct negative effects on R. temporaria embryos, but these effects are expressed mostly or only during the later life stages. To this end, our results support the contention that carry-over effects from one life stage to another may be an important source of phenotypic variation in fitness.

Animals↗
PubMed2001

Cryptic evolution in a wild bird population.

Microevolution is expected to be commonplace, yet there are few thoroughly documented cases of microevolution in wild populations. In contrast, it is often observed that apparently heritable traits under strong and consistent directional selection fail to show the expected evolutionary response. One explanation proposed for this paradox is that a genetic response to selection may be masked by opposing changes in the environment. We used data from a 20-year study of collared flycatchers (Ficedula albicollis) to explore selection on, and evolution of, a heritable trait: relative body weight at fledging ('condition'). Despite consistent positive directional selection, on both the phenotypic and the additive genetic component (breeding values, estimated from an animal model) of condition, the mean phenotypic value of this trait in the population has declined, rather than increased, over time. Here we show that, despite this decline, the mean breeding value for condition has increased over time. The mismatch between response to selection at the levels of genotype and phenotype can be explained by environmental deterioration, concealing underlying evolution. This form of cryptic evolution may be common in natural environments.

Animals↗
PubMed2001

Comparison of nitrate tolerance between different populations of the common frog, Rana temporaria.

Euthrophication-associated changes in the physical and biological environment of lakes and ponds are potentially a source of major stress for many aquatic organisms. In Scandinavia, the nitrate concentrations in lakes and ponds decrease towards north due to a naturally lower productivity of the habitats, but also due to lower supplementation of anthropogenic nitrogen. A chronic experiment using ecologically relevant concentrations of sodium nitrate (0-5000 micro gl(-1)) was used to test whether common frog (Rana temporaria L.) larvae from northern parts of Scandinavia are less well adapted to cope with high nitrate concentrations than those from the southern parts. Slight, but significant differences in nitrate tolerance, as measured in terms of growth rate and size at metamorphosis, between the two regions were found. High concentrations of nitrate reduced the growth rates and metamorphic size in north, but not in south. However, there was no clear-cut impact of high nitrate concentrations on developmental rate or on mortality until metamorphosis. The general lack of large effects of nitrate treatment on the response variables suggests that nitrates per se do not pose any significant threat to the development of R. temporaria tadpoles under a natural range of concentrations. This was confirmed in an acute test where results suggest that ammonia and nitrite, compounds seldom found in high concentrations in Fennoscandian lakes, are possibly responsible for the larger negative effects of "nitrate" observed in previous studies of amphibians.

Animals↗
PubMed2001

Explaining stasis: microevolutionary studies in natural populations.

Microevolution, defined as a change in the genetic constitution of a population over time, is considered to be of commonplace occurrence in nature. Its ubiquity can be inferred from the observation that quantitative genetic divergence among populations usually exceeds that to be expected due to genetic drift alone, and from numerous observations and experiments consistent with local adaptation. Experimental manipulations in natural populations have provided evidence that rapid evolutionary responses may occur in the wild. However, there are remarkably few cases where direct observations of natural populations have revealed microevolutionary changes occurring, despite the frequent demonstration of additive genetic variation and strong directional selection for particular traits. Those few cases where responses congruent with expectation have been demonstrated are restricted to changes over one generation. In this article we focus on possible explanations as to why heritable traits under apparently strong directional selection often fail to show the expected evolutionary response. To date, few of these explanations for apparent stasis have been amenable to empirical testing. We describe new methods, derived from procedures developed by animal breeding scientists, which can be used to address these explanations, and illustrate the approach with examples from long-term studies of collared flycatchers (Ficedula albicollis) and red deer (Cervus elaphus). Understanding why most intensively studied natural populations do not appear to be evolving is an important challenge for evolutionary biology.

Animals↗
PubMed2000

Lifetime Reproductive Success and Heritability in Nature.

The observation that traits closely related to fitness ("fitness traits") have lower heritabilities than traits more distantly associated with fitness has traditionally been framed in terms of Fisher's fundamental theorem of natural selection-fitness traits are expected to have low levels of additive genetic variance due to rapid fixation of alleles conferring highest fitness. Subsequent treatments have challenged this view by pointing out that high environmental and nonadditive genetic contributions to phenotypic variation may also explain the low heritability of fitness traits. Analysis of a large data set from the collared flycatcher Ficedula albicollis confirmed a previous finding that traits closely associated with fitness tend to have lower heritability. However, analysis of coefficients of additive genetic variation (CVA) revealed that traits closely associated with fitness had higher levels of additive genetic variation (VA) than traits more distantly associated with fitness. Hence, the negative relationship between a trait's association with fitness and its heritability was not due to lower levels of VA in fitness traits but was due to their higher residual variance. However, whether the high residual variance was mainly due to higher levels of environmental variance or due to higher levels of nonadditive genetic variance remains a challenge to be addressed by further studies. Our results are consistent with earlier suggestions that fitness-related traits may have more complex genetic architecture than traits more distantly associated with fitness.

Ficedula albicollis↗
PubMed1999

Heritable variation and evolution under favourable and unfavourable conditions.

Genetic variability in quantitative traits can change as a direct response to the environmental conditions in which they are expressed. Consequently, similar selection in different environments might not be equally effective in leading to adaptation. Several hypotheses, including recent ones that focus on the historical impact of selection on populations, predict that the expression of genetic variation will increase in unfavourable conditions. However, other hypotheses lead to the opposite prediction. Although a consensus is unlikely, recent Drosophila and bird studies suggest consistent trends for morphological traits under particular conditions.

Journal Article↗
PubMed1999

Genetic architecture of fitness and nonfitness traits: empirical patterns and development of ideas.

Comparative studies of the genetic architecture of different types of traits were initially prompted by the expectation that traits under strong directional selection (fitness traits) should have lower levels of genetic variability than those mainly under weak stabilizing selection (nonfitness traits). Hence, early comparative studies revealing lower heritabilities of fitness than nonfitness traits were first framed in terms of giving empirical support for this prediction, but subsequent treatments have effectively reversed this view. Fitness traits seem to have higher levels of additive genetic variance than nonfitness traits - an observation that has been explained in terms of the larger number loci influencing fitness as compared to nonfitness traits. This hypothesis about the larger functional architecture of fitness than nonfitness traits is supported by their higher mutational variability, which is hard to reconcile without evoking capture of mutational variability over many loci. The lower heritabilities of fitness than nonfitness traits, despite the higher additive genetic variance of the former, occur because of their higher residual variances. Recent comparative studies of dominance contributions for different types of traits, together with theoretical predictions and a large body of indirect evidence, suggest an important role of dominance variance in determining levels of residual variance for fitness-traits. The role of epistasis should not be discounted either, since a large number of loci increases the potential for epistatic interactions, and epistasis is strongly implicated in hybrid breakdown.

Animals↗
PubMed1998

Adaptive sex ratio variation in pre-industrial human (Homo sapiens) populations?

Sex allocation theory predicts that in a population with a biased operational sex ratio (OSR), parents will increase their fitness by adjusting the sex ratio of their progeny towards the rarer sex, until OSR has reached a level where the overproduction of either sex no longer increases a parent's probability of having grandchildren. Furthermore, in a monogamous mating system, a biased OSR is expected to lead to lowered mean fecundity among individuals of the more abundant sex. We studied the influence of OSR on the sex ratio of newborns and on the population birth rate using an extensive data set (n = 14,420 births) from pre-industrial (1775-1850) Finland. The overall effect of current OSR on sex ratio at birth was significant, and in the majority of the 21 parishes included in this study, more sons were produced when males were rarer than females. This suggests that humans adjusted the sex ratio of their offspring in response to the local OSR to maximize the reproductive success of their progeny. Birth rate and, presumably, also population growth rate increased when the sex ratio (males:females) among reproductive age classes approached equality. However, the strength of these patterns varied across the parishes, suggesting that factors other than OSR (e.g. socioeconomic or environmental factors may also have influenced the sex ratio at birth and the birth rate.

Adaptation, Physiological↗
PubMed1998

Genetic variation and causes of genotype-environment interaction in the body size of blue tit (Parus caeruleus).

In several studies of natural populations of birds, the heritability of body size estimated by parent-offspring regression has been lower when offspring have developed in poor feeding regimens than when they developed in good feeding regimens. This has led to the suggestion that adaptation under poor regimens may be constrained by lack of genetic variation. We examined the influence of environmental conditions on expression of genetic variation in body size of nestling blue tits (Parus caeruleus) by raising full sibs in artificially reduced and enlarged broods, corresponding to good and poor feeding regimens, respectively. Individuals grown in the poor regimen attained smaller body size than their sibs grown in the good regimen. However, there was among-family variation in response to the treatments--i.e., genotype-environment interactions (GEIs). Partitioning the GEI variance into contributions attributable to (1) differences in the among-family genetic variance between the treatments and (2) imperfect correlation of genotypic values across treatments identified the latter as the main cause of the GEI. Parent-offspring regressions were not significantly different when offspring were reared in the good environment (h2 = 0.75) vs. when they were reared in the poor environment (h2 = 0.63). Thus, there was little evidence that genetic variance in body size was lower under the poor conditions than under the good conditions. These results do not support the view that the genetic potential for adaptation to poor feeding conditions is less than that for adaptation to good conditions, but they do suggest that different genotypes may be favored under the different conditions.

Animals↗