No abstract

Dispersal is a ubiquitous trait in living organisms. Evolutionary theory postulates that the loss or death of propagules during dispersal episodes (cost of dispersal) should select against dispersal. The cost of dispersal is expected to be a strong selective force in fragmented habitats. We analyzed patchy populations of the weed Crepis sancta occupying small patches on sidewalks, around trees planted within the city of Montpellier (South of France), to investigate the recent evolutionary consequences of the cost of dispersal. C. sancta produces both dispersing and nondispersing seeds. First, we showed that, in urban patches, dispersing seeds have a 55% lower chance of settling in their patch compared with nondispersing seeds and, thus, fall on a concrete matrix unsuitable for germination. Second, we showed that the proportion of nondispersing seeds in urban patches measured in a common environment is significantly higher than in surrounding, unfragmented populations. Third, by using a quantitative genetic model, we estimated that the pattern is consistent with short-term evolution that occurs over Ϸ5-12 generations of selection, which is generated by a high cost of dispersal in urban populations. This study shows that a high cost of dispersal after recent fragmentation causes rapid evolution toward lower dispersal.fragmentation ͉ short-term evolution ͉ human-altered habitat D ispersal has evolved in almost all living organisms and is thus considered as a central life-history trait (1, 2). The evolution of dispersal is usually understood as the result of a cost-benefit process. On the one hand, three main factors selecting for dispersal have been identified (3): reduction of the competition among kin (4, 5); the temporal heterogeneity of the environment, such as local population extinction (6, 7); and last, the avoidance of inbreeding depression when mating occurs between related individuals (8). However, various costs of dispersal have been postulated in theoretical models. For instance, dispersal structures can be costly for organisms [e.g., fleshy fruits dispersed by animals (9)]. More generally, dispersing organisms may pay a high cost of dispersal because they may get lost during the displacement. This phenomenon is encapsulated under the term ''cost of dispersal.'' Various theoretical models have been studied, including various selective factors (3). These theoretical models conclude that increasing the cost of dispersal selects for lower dispersal. Although this cost may appear as obvious in natural populations, the strength of this selection pressure on dispersal traits is almost unknown in the wild because of the difficulties of measuring it in natural systems. When dispersal is passive (wind or water transport) and habitat choice is random, the probability of settling in a suitable site is positively dependent on the frequency of suitable sites in the landscape. Many empirical studies have reported a reduction in dispersal structures in organisms that live on islands, such as plants (10) or insects (ho...

Hermaphroditic individuals can produce both selfed and outcrossed progeny, termed mixed mating. General theory predicts that mixed-mating populations should evolve quickly toward high rates of selfing, driven by rapid purging of genetic load and loss of inbreeding depression (ID), but the substantial number of mixed-mating species observed in nature calls this prediction into question. Lower average ID reported for selfing than for outcrossing populations is consistent with purging and suggests that mixed-mating taxa in evolutionary transition will have intermediate ID. We compared the magnitude of ID from published estimates for highly selfing (r > 0.8), mixed-mating (0.2 ≤ r ≥ 0.8), and highly outcrossing (r < 0.2) plant populations across 58 species. We found that mixed-mating and outcrossing taxa have equally high average lifetime ID (δ = 0.58 and 0.54, respectively) and similar ID at each of four life-cycle stages. These results are not consistent with evolution toward selfing in most mixed-mating taxa. We suggest that prevention of purging by selective interference could explain stable mixed mating in many natural populations. We identify critical gaps in the empirical data on ID and outline key approaches to filling them.

656I. 657II. 658III. 660IV. 661V. 663VI. 663VII. 664VIII. 664 665 References 665 Summary Baker's law refers to the tendency for species that establish on islands by long‐distance dispersal to show an increased capacity for self‐fertilization because of the advantage of self‐compatibility when colonizing new habitat. Despite its intuitive appeal and broad empirical support, it has received substantial criticism over the years since it was proclaimed in the 1950s, not least because it seemed to be contradicted by the high frequency of dioecy on islands. Recent theoretical work has again questioned the generality and scope of Baker's law. Here, we attempt to discern where the idea is useful to apply and where it is not. We conclude that several of the perceived problems with Baker's law fall away when a narrower perspective is adopted on how it should be circumscribed. We emphasize that Baker's law should be read in terms of an enrichment of a capacity for uniparental reproduction in colonizing situations, rather than of high selfing rates. We suggest that Baker's law might be tested in four different contexts, which set the breadth of its scope: the colonization of oceanic islands, metapopulation dynamics with recurrent colonization, range expansions with recurrent colonization, and colonization through species invasions.