Rapid Formation of Reproductive Isolation between Two Populations of Side-Blotched Lizards, Uta stansburiana
Determining the factors that give rise to reproductive isolation is critical for understanding how species form. Observing reproductive isolation between closely related populations is especially interesting because it can show the factors initially involved in species formation. We studied two phenotypically different populations of Side-Blotched Lizards (Uta stansburiana), which diverged less than 22,500 years ago according to geologic evidence. The lava flow population has dark dorsal coloration for crypsis and is dimorphic for throat coloration used for signaling male mating type. The off-lava population has light dorsal coloration and is trimorphic for throat coloration. We tested whether there was reproductive isolation between these two populations in order to understand the factors that maintain the phenotypic differences between these populations. Our genetic crosses revealed evidence of reproductive incompatibilities because females from the dimorphic lava population produced fewer fertilized eggs and more unfertilized eggs when mated outside their population. In addition, male morphs varied in their reproductive compatibility because females from both populations produced fewer fertilized eggs and fewer clutches with orange-throated males from outside their population. The reproductive incompatibilities observed between the populations suggest that cryptic female choice of sperm may act as a post-mating, prezygotic barrier that contributes to the rapid formation of new species.
Clutch size of the lava and off-lava populations as measured in the lab and wild. Only within-population crosses were used for the lab clutch size.
Factors affecting the number of fertilized eggs laid during the first clutch. (A) Effects of population and type of cross. (B) Effects of male morph and type of cross. Within-population crosses = In. Between population crosses = Out. Least square means and standard errors are depicted and an asterisk highlights adjoining means that were significantly different.
Factors affecting the number of unfertilized eggs laid during the first clutch. (A) Effects of population and type of cross. Within-population crosses = In. Between population crosses = Out. (B) Trend in the interaction of morph and type of cross. (C) Effects of population and male morph. Orange and Not-Orange refer to the morph type of the male in the cross. Least square means and standard errors are depicted and an asterisk highlights adjoining means that were significantly different.
(A) Effects of male morph on the amount of time individual females took to lay a first clutch. The line with the asterisk above it connects the two means that are significantly different. (B) Effects of male morph on the number of clutches produced by a female. An asterisk highlights adjoining means that were significantly different. Least square means and standard errors are depicted for both graphs. Within-population crosses = In. Between population crosses = Out.
Mean luminosity of hatchlings from different types of population crosses. “L” = a lava parent and “O” = an off-lava parent. Low values of luminosity indicate dark lizards. Least square means and standard errors are depicted.
Contributor Notes
Associate Editor: B. Stuart.