Rapid life‐history changes caused by size‐selective harvesting are often interpreted as a response to direct harvest selection against a large body size. However, similar trait changes may result from a harvest‐induced relaxation of natural selection for a large body size via density‐dependent selection. Here, we show evidence of such density‐dependent selection favouring large‐bodied individuals at high population densities, in replicated pond populations of medaka fish. Harvesting, in contrast, selected medaka directly against a large body size and, in parallel, decreased medaka population densities. Five years of harvesting were enough for harvested and unharvested medaka populations to inherit the classically predicted trait differences, whereby harvested medaka grew slower and matured earlier than unharvested medaka. We show that this life‐history divergence was not driven by direct harvest selection for a smaller body size in harvested populations, but by density‐dependent natural selection for a larger body size in unharvested populations.

Sex is a pervasive factor that underpins functional phenotypic variation across a range of traits. Although sex can usually be distinguished morphologically, in some species this is not possible. The development of genetic markers for sex identification is, thus, key if we are to incorporate sex into an understanding of ecological or evolutionary process. Here we develop genetic markers for the identification of sex within an iconic Australian lizard group, the Egernia group, which is notable for its complex social behaviour. We used restriction-site associated DNA sequencing to characterise sex-specific genetic sequences for a key member of the group, Liopholis whitii, and designed primers for four of these putative sex-specific sequences. These primers amplified across some, but not all, species of the group. Our results provided several important insights. They suggest conservatism of a XX/XY sex determination system within the group as well as sex-specific genomic regions that appear independent of the conserved genomic regions identified in other skink species. More broadly, the development of sex markers for the Egernia group opens up a range of potential research questions related to the role that sex plays in the mediation of social behaviour and, through this, the emergence and stability of social life.

Hatching synchrony is wide-spread amongst egg-laying species and is thought to enhance offspring survival, notably by diluting predation risks. Turtle and snake eggs were shown to achieve synchronous hatching by altering development rates (where less advanced eggs may accelerate development) or by hatching prematurely (where underdeveloped embryos hatch concurrently with full-term embryos). In Natricine snakes, smaller eggs tend to slow down metabolism throughout incubation in order to hatch synchronously with larger eggs. To explore the underlying mechanism of this phenomenon we experimentally manipulated six clutches, where half of the eggs were reduced in mass by removing 7.2% of yolk, and half were used as the control. The former experienced higher heart rates throughout the incubation period, hatched earlier and produced smaller hatchlings than the latter. This study supports the idea that developmental rates are related to egg mass in snake eggs and demonstrates that the relationship can be influenced by removing yolk after egg-laying. The shift in heart rates however occurred in the opposite direction to expected, with higher heart rates in yolk-removed eggs resulting in earlier hatching rather than lower heart rates resulting in synchronous hatching, warranting further research on the topic.

Harvesting has been demonstrated to cause rapid, yield-decreasing trait change towards slower somatic growth and earlier maturation in wild populations. These changes are largely considered to result from direct, density-independent harvest selection on traits. Here, we show that exact same trait changes may also indirectly result from a harvest-induced relaxation of density-dependent (K) natural selection for faster growth and delayed maturation. We exposed 12 pond populations of medaka fish (Oryzias latipes) to contrasted size-selective harvesting during 5 years, and show that harvesting effectively changed juvenile natural mortality from density-dependent to density-independent. We then laboratory-reared medaka progeny under contrasted food levels mimicking the environmental effects of a harvest-induced density gradient. Interaction between past harvest regime and present food environment on progeny traits revealed that harvest-induced trait changes in medaka resulted from selection in a low-food environment only, i.e., were driven by relaxed K-selection only, not by direct harvest selection. Feeding trials further demonstrated that trait changes were associated with reorganizations in rates of food acquisition, assimilation and allocation that were contingent upon the food environments. This is the first study to demonstrate that harvesting can induce undesirable distortions of natural selection that impair productivity traits. We conclude that sustaining harvesting yields over extended time scales requires a preservation of high population densities.Significance statementFisheries management often opposes a density-dependent approach which prioritize the preservation of high population densities, and an evolutionary approach which consider that alleviating change towards smaller body sizes is paramount to the sustainability of harvesting. The evolutionary approach consider harvest-induced body downsizing to be density-independent, i.e., to result only from direct harvest selection against large-bodied individuals. Here, we show instead that harvest-induced body downsizing may be density-dependent because, by decreasing population density, fishing relaxes natural, density-dependent selection for large-bodied individuals. Therefore, preserving population numbers and alleviating body downsizing in harvested populations are not independent lines of management, but are in fact two necessary and complementary routes to reaching the same management objectives.