Demographic and spatial determinants of hybridization rate

Abstract

Summary1. Hybridization is a key evolutionary process with major consequences for conservation and speciation. However, sexual barriers interact with the local context to determine hybridization rates in a way that is still poorly explored. For instance, in the context of an expanding or introduced plant population, where a few individuals are isolated in populations dominated by heterospecific individuals, what is the hybridization potential? 2. To obtain baseline predictions on hybridization rate between two species differing in abundance and to evaluate the effect of pollen limitation, we first used a mean-field model. We then explored a spatially explicit individual-based mating model relying on pollen dispersal kernels to predict hybridization rates in a mixed-species stand, under different scenarios for (i) the strength of sexual barriers; (ii) the spatial distribution of individuals within the site; (iii) the variation in individual fecundity; and (iv) the magnitude, shape and asymmetry of pollen dispersal kernels. 3. Pollen limitation was shown to have the potential to greatly increase hybridization rates. Similarly, fine-scale variation in species composition can result in elevated hybridization rates compared to mean-field predictions, especially with strong sexual barriers. However, species clustering in combination with reduced pollen dispersal has the opposite effect, protecting from hybridization. 4. Synthesis. Our simulation results show that variation of the pollen pool composition at the scale of individuals or stigmas due to spatial configurations or pollen limitation can substantially modify hybridization rates. We explain this by the disproportionate effect of some pollen environments on average hybridization rates. This suggests thoroughly evaluating individual behaviour in terms of hybridization, especially for rare or patchy species.

The oak syngameon: more than the sum of its parts

“…One of the more perplexing aspects of the syngameon is how species retain their cohesiveness and distinctiveness despite occasional hybridization. To solve this paradox, it is important to recall that hybridization is both frequency‐ and density‐dependent: hybrids are produced in large numbers only episodically under conditions of extreme demographic imbalance among species, a process called Hubb's effect or the rare‐species effect (reviewed and modeled in Klein et al ., ). Such conditions are highly unstable.…”

Diversification, adaptation, and community assembly of the American oaks (Quercus), a model clade for integrating ecology and evolution

“…Without reproductive isolation, coexistence of distinct species cannot be maintained because introgression can cause species to merge over time (Petit et al, 2004), eliminating their co-occurrence (Losos, 1990;Levin, 2006;Pollock et al, 2015), and potentially contributing to the observed patterns of overdispersion. In contrast, functional divergences between close relatives that allow them to occupy contrasting habitats likely limit gene flow between species (Klein et al, 2016), particularly if contrasting environments promote assortative mating, such as in sympatric live oaks in the southeastern USA, discussed in Section IV (Cavender-Bares & Pahlich, 2009). Empirical and theoretical evidence indicates that the likelihood of hybridization is impacted both by community structure and by demographic factors that influence pollen limitation (Klein et al, 2016).…”

Interspecies gene flow is a key evolutionary process for the adaptation of long-lived species to new environmental conditions

“…Theory predicts that, besides species intrinsic properties regarding pre-zygotic barriers and postzygotic isolation forces (see Vallejo-Marin & Hiscock, 2016, for a review), interspecies hybridization depends on environmental factors, more specifically the habitat and climate conditions, determining the relative abundance of species and the relative frequency of gametes involved in intra-and interspecific reproduction (Cannon & Lerdau, 2015;Klein et al,2017). In long-lived species predominantly consisting of large primarily outcrossing organisms with slow replacement rate (see above), under unfavourable environmental conditions, individual's survival is generally favoured over their reproductive investment (Bromham, 2011).…”