The speed of ecological speciation

Abstract

Adaptation can occur on ecological time-scales (contemporary evolution) and adaptive divergence can cause reproductive isolation (ecological speciation). From the intersection of these two premises follows the prediction that reproductive isolation can evolve on ecological time-scales. We explore this possibility in theory and in nature. Finding few relevant studies, we examine each in some detail. THEORY: Several models have demonstrated that ecological differences can drive the evolution of partial reproductive barriers in dozens to hundreds of generations. Barriers likely to evolve quickly include dispersal rate, habitat preference and selection against migrants/hybrids. PLANTS: Adjacent populations adapting to different fertilizer treatments or to mine tailings can develop reproductive barriers within at least 100 generations. These barriers include differences in flowering time and selection against migrants/hybrids. INVERTEBRATES: Populations on native and introduced host plants can manifest reproductive barriers in dozens to hundreds of generations. These barriers include local host preference and selection against migrants/hybrids. VERTEBRATES: Salmon adapting to divergent breeding environments can show restricted gene flow within at least 14 generations. Birds evolving different migratory routes can mate assortatively within at least 10-20 generations. Hybrid sculpins can become isolated from their ancestral species within at least 20-200 generations. Ecological speciation can commence within dozens of generations. How far it goes is an important question for future research.

Fig. 1

Anthoxanthum odoratum shows higher zinc tolerance (top panel) and earlier flowering (bottom panel) on the tailings of a zine mine than on the adjacent pasture (adapted from fig. 1 in Antonovics & Bradshaw 1970). These data are based on eight samples along a 100 m transect (x-axis) perpendicular to the transition between the mine and pasture. Flowering time represents stigma emergence in days after 9 June 1966.

Fig. 2

Weevils collected from exotic milfoil (EXO) show a significant preference to oviposit on exotic milfoil rather than native milfoil (NAT) in no-choice trails (top panel; adapted from fig. 1 in Sheldon & Jones 2001) and in choice trails (middle panel; adapted from fig. 2 in Sheldon & Jones 2001). Moreover, females collected from exotic milfoil produce significantly more larvae when mated with males collected from exotic milfoil than with males collected from native milfoil (bottom panel; adapted from fig. 3 in Sheldon & Jones 2001). Weevils collected from native milfoil show no differences in preference or larval production on native vs. exotic milfoil.

Fig. 3

Evidence for the contemporary evolution of reproductive isolation for introduced salmon populations adapting to river and beach breeding environments over the course of approximately 14 generations. The top panel shows that body depth (standardized to a common body length) is significantly greater for males hatched and breeding at the beach (beach residents; BR) than for males hatched and breeding at the river (river residents; RR). The middle panel shows that female body length (at a common age) is significantly less for beach residents than for river residents. For both traits, fish hatched in the river but breeding at the beach (beach immigrants, BI) are intermediate and not significant different from either river or beach residents. In both panels, boxes contain 50% of the data and bars contain the remainder; horizontal lines indicate medians, arrows indicate means, and the circle indicates an outlier. The bottom panel shows that gene flow is limited between beach residents and river residents. Specifically, F_ST based on six microsatellite loci is significant between beach residents and both river residents and beach immigrants but not between river residents and beach immigrants.