Mats Björklund

HISTORICAL DEMOGRAPHY AND PRESENT DAY POPULATION STRUCTURE OF THE GREENFINCH, CARDUELIS CHLORIS-AN ANALYSIS OF MTDNA CONTROL-REGION SEQUENCES

Genetic variability within and among 10 geographically distinct populations of Greenfinches (Carduelis chloris) was assayed by directly sequencing a 637 BP part of the mtDNA control region from 194 individuals. Thirteen variable positions defined 18 haplotypes with a maximum sequence divergence of 0.8%. Haplotype (h = 0.28–0.77) and nucleotide (π = 0.058–0.17%) diversities within populations were low, and decreased with increasing latitude (h:rs = –0.81; π: rs = –0.89). The distribution of pairwise nucleotide differences fit better with expectations of a “sudden expansion” than of an “equilibrium” model, and the estimates of long term effective population sizes were considerably lower than current census estimates, especially in northern European samples. Selection is an unlikely cause of observed patterns because the distribution of variability conformed to expectations of neutral infinite alleles model and haplotype diversity across populations was positively correlated with heterozygosity (HE) in nuclear genes (rs = 0.74, P < 0.05). Hence, a recent bottleneck, followed by serial bottlenecking during the process of post‐Pleistocene recolonization of northern Europe, together with recent population expansion provide a plausible explanation for the low genetic diversity in the north. Genetic distances among populations showed a clear pattern of isolation‐by‐distance, and 14% of the haplotypic variation was among populations, the rest being distributed among individuals within populations. In accordance with allozyme and morphological data, a hierarchical analysis of nucleotide diversity recognized southern European populations as distinct from northern European ones. However, the magnitude of divergence in mtDNA, allozymes and morphology were highly dissimilar (morphology > mtDNA > allozymes).

Are 'comparative methods' always necessary?

Comparative analyses of traits across taxa taking explicit notice of phylogenetic relationships among the taxa involved, are today commonplace in modern evolutionary ecology. This approach grew from an increasing awareness among ecologists that evolution has a clear historical component such that closely related taxa are not solely a product of their current environment but have also inherited a certain proportion of their phenotypes from a common ancestor (see reviews by Brooks and McLennan 1991, Harvey and Pagel 1991, Maddison and Maddison 1992). Since evolution modifies what already exists this is to be expected. This fact posed a problem, for example, for ecologists who are interested in convergent evolution as a response to common selection pressures. If two species share the same adaptation, shared selection pressures need not be the cause of the similarity, but the fact that their common ancestor had already evolved this particular adaptation. Thus, from an evolutionary point of view, the two species do not represent two independent evolutionary events. This problem is opposite to the one faced in phylogenetic systematics, where the null hypothesis is that similarities are homologous rather than being a result of convergence. Characters that vary widely are generally avoided because of the risk of finding similarities that in fact are an effect of convergence, which makes the character potentially misleading in a phylogenetic analysis. For ecologists, however, convergence due to ecological reasons is often what is of interest, and thus similarity due to shared ancestry is the confounding factor in comparative studies. Consequently, there has been a considerable interest among ecologists and evolutionary biologists to try to find ways of sorting out to what extent the distribution of traits among taxa is due to convergence and what is due to shared ancestry (see reviews by Brooks and McLennan 1991, Harvey and Pagel 1991, Maddison and Maddison 1992). However, while it is increasingly appreciated that many character distributions may have a component of shared ancestry, this must not necessarily be the case. As is evident from systematic studies some characters may vary so much that they are of no use in systematics, and this conversely must mean that the very same characters can be used in comparative ecological studies without any precautions with regard to phylogeny. As no comparative methods are free from assumptions that may limit their usefulness or may have other properties that may render them less suitable (e.g. Bjdrklund 1994, Ricklefs and Starck 1996), use of raw data seems desirable as much as possible. I will argue here that the approaches commonly used in systematics can be of great value in analysing the phylogenetic component of character state distribution over a set of taxa, and can help to decide whether the data can be used as they stand, or whether we will have to use comparative methods. This will perhaps also help to settle the conflict among the widely different schools of thought with regard to comparative methods and their usefulness (see review by Ricklefs and Starck 1996). Several methods explicitly try to estimate the phylogenetic component of trait distributions by means of elaborate statistical procedures. In particular, these methods aim to either estimate the relative importance of shared ancestry vs convergence (e.g. Cheverud et al. 1985, Lynch 1991), or to find the taxonomic level of analysis where the influence of shared ancestry is minimal (e.g. Steams 1983, Derrickson and Ricklefs 1988, Miles and Dunham 1992). The method presented here is a means of testing if there is any influence of shared ancestry in the trait distribution that requires special concern in further analysis. The method has also the virtue of being very simple and relies on well-established methods used in phylogenetic systematics.

A phylogenetic interpretation of sexual dimorphism in body size and ornament in relation to mating system in birds

Sexual dimorphism in body size and ornament is commonly assumed to be a result of sexual selection. Therefore, a pattern among species is expected where the type of mating system correlates with the degree of sexual dimorphism. This was analysed using comparative data on birds using an autocorrelation model. Mating system did correlate with sexual dimorphism in ornament (tail), but was only weakly correlated with sexual size (tarsus) dimorphism. Polygynous species were significantly larger than monogamous species. After correcting for size, there was no difference in size dimorphism between polygynous and monogamous species. The reverse was true for tail dimorphism; polygynous species exhibited a larger degree of tail dimorphism, but there was no difference in absolute size of the tail. This pattern suggests that an evolutionary change in mating system is associated with a change in overall size, and in degree of dimorphism in secondary sexual traits.

Extra-pair copulations in the Pied Flycatcher (Ficedula hypoleuca)

SummaryIn an attempt to evaluate the importance of mate guarding in the Pied Flycatcher several removal experiments were performed during the breeding season 1982. Males were captured 1–3 days before the onset of egglaying, since this is a period when the females are likely to be receptive to fertilization. Females were followed continuously during the experiment and all incidents were recorded. A total of five experiments were conducted and as a control we used observations on other pairs conducted in a similar way during the same time of the breeding season. The number of other males visiting the territory in the experiment significantly increased compared with controls as did the frequency of extra-pair copulations. All extra-pair copulations were performed by breeding neighbour males.

The importance of evolutionary constraints in ecological time scales

SummaryThe importance of constraints, defined as factors that retard or prevent a population from reaching its immediate adaptive peak on an ecological time scale is analysed. This is done by means of simple quantitative genetic models, which if anything underestimate the importance of constraints. The results show that even in the simplest case the response to selection will not generally be in the same direction as the selection vector, i.e. the direction to the nearest optimum. Adding complexity identifies cases where selection may lead the population in suboptimal directions. It is concluded that information about univariate genetic variances is not sufficient to predict evolutionary responses and may even be misleading. However, genetic covariances are not always acting as constraints, but can under certain circumstances promote evolution towards the nearest optimum. This can be understood by a spectral decomposition of the genetic variance—covariance matrix, where it is shown that the eigenvector associated with the largest amount of variance will to various degrees determine the outcome of selection. A literature survey of the pattern of character covariation in morphological characters in natural populations shows a wide variety of correlation patterns, but quite often shows a high level of covariance between traits. This suggests that constraints to short-term evolution may be more common than generally appreciated.

Dynamics in the evolution of sexual traits: losses and gains, radiation and convergence in yellow wagtails (Motacilla flava)

We analyse patterns of genetic diversity and song complexity in the Palaearctic yellow wagtail (Motacilla flava), a highly polytypic species complex. Mitochondrial and nuclear DNA show that the complex is polyphyletic, despite parallel plumage variation in western and eastern clades. In the western clade there is genetic structure among southern subspecies, haplotype diversity decreases with latitude, and northern subspecies show evidence of bottlenecking and rapid expansions, as expected from isolation in glacial refugia followed by postglacial colonization. However, northern subspecies, which have more divergent male plumages, lack genetic structure and sing simpler songs. Loss of song complexity and evolution of plumage in founder populations are consistent with the Kaneshiro model, which posits that variation among species is a consequence of founder‐induced shifts in female preference leading to loss of ancestral male sexual traits. Our results suggest possible postglacial founder‐effect mechanisms for the morhological diversification of the yellow wagtail complex.

Reliability of noninvasive genetic census of otters compared to field censuses

Conservation and management actions are often highly dependent on accurate estimations of population sizes. However, these estimates are difficult to obtain for elusive and rare species. We compared two census methods for Eurasian otter: snow tracking and noninvasive genetic census based on the genotyping of faecal samples. With the noninvasive genetic census we detected the presence of almost twice as many otters as with snow tracking (23 and 10–15, respectively), and mark-recapture estimates based on the genetic census indicated that the real number of otters could be even higher. Our results indicate that snow tracking tends to underestimate the number of individuals and also that it is more susceptible to subjective assessment. We compared the strengths and weaknesses of the two methods.

EVOLUTION, PHYLOGENY, SEXUAL DIMORPHISM AND MATING SYSTEM IN THE GRACKLES (QUISCALUS SPP.: ICTERINAE)

According to theory, two consequences of sexual selection are sexual dimorphism in size and secondary sexual characteristics, due to either intra‐ or intersexual selection. In this paper I suggest three criteria for the test of an evolutionary hypothesis involving quantitative morphological characters. First, the postulated change must be shown to have occurred in evolutionary time. Second, this change must be positively correlated with a change in the proposed selective agent. Third, given two taxa with different degrees of sexual size dimorphism and different mating system, the possible influence of drift must be rejected. If the hypothesis is not rejected by these three criteria, then we still have no proof of causality, but we can at least be more confident about its plausibility.

If FST does not measure neutral genetic differentiation, then comparing it with QST is misleading. Or is it?

The comparison between neutral genetic differentiation (FST) and quantitative genetic differentiation (QST) is commonly used to test for signatures of selection in population divergence. However, there is an ongoing discussion about what FST actually measures, even resulting in some alternative metrics to express neutral genetic differentiation. If there is a problem with FST, this could have repercussions for its comparison with QST as well. We show that as the mutation rate of the neutral marker increases, FST decreases: a higher within‐population heterozygosity (He) yields a lower FST value. However, the same is true for QST: a higher mutation rate for the underlying QTL also results in a lower QST estimate. The effect of mutation rate is equivalent in QST and FST. Hence, the comparison between QST and FST remains valid, if one uses neutral markers whose mutation rates are not too high compared to those of quantitative traits. Usage of highly variable neutral markers such as hypervariable microsatellites can lead to serious biases and the incorrect inference that divergent selection has acted on populations. Much of the discussion on FST seems to stem from the misunderstanding that it measures the differentiation of populations, whereas it actually measures the fixation of alleles. In their capacity as measures of population differentiation, Hedrick’s G′ST and Jost’s D reach their maximum value of 1 when populations do not share alleles even when there remains variation within populations, which invalidates them for comparisons with QST.

Genetic structure of lions (Panthera leo L.) in the Selous Game Reserve: implications for the evolution of sociality

Abstract We use 14 microsatellites to examine the genetic structure of a lion (Panthera leo L.) population in southern Tanzania. Heterozygosity levels were high (0.75 ± 0.08). Relatedness estimates showed that prides often had close relatives in neighbouring prides, whereas few relatives were found in prides not sharing a border. The drop‐off in relatedness with distance was highly significant. Female pridemates exhibited a higher mean relatedness (0.26 ± 0.07) to one another than did pride males (0.11 ± 0.07). Mean relatedness among females was significantly higher in small prides than in large ones. Prides exhibited significant inbreeding avoidance (FIL: −0.11). Mating did not detectably differ from random across prides (FIT: −0.02 ns). In addition to being recognizable behavioural and demographic units, prides were statistically distinct genetic units (FLT: 0.07). Some neighbouring prides grouped together both geographically and genetically, forming ‘superprides’ in the population (FZT = 0.05). Thus, although individual prides were genetically distinct, there was an important genetic structure above the level of social groups.