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Abstract:
The migration of Europeans to the new world was accompanied by a rash of invasive flora that greatly disturbed the native ecological landscape. Colonizing plant populations frequently experienced strong founder effects, but they were nevertheless able to succeed in diverse habitats. I will discuss the demography and genetic basis for phenotypic variation in two such species, A. thaliana and C. bursa-pastoris. North American A. thaliana experienced an extreme founder effect with most individuals being derived from a single lineage. We have sequenced 76 genomes of extant individuals and 27 genomes of herbarium specimens from this lineage, together covering the time span from 1863 to 2006. Only three individuals show evidence of within-lineage recombination, greatly simplifying genetic analysis. We date the introduction to the mid-17th century, and we directly compare mutation rates throughout the genome. This comparison reveals that genetic drift predominates, but that purifying selection in this rapidly expanding population is nevertheless evident even over short historical times. Furthermore, a genome wide association study identifies mutations affecting traits likely to be under selection and important for adaptation in the wild. In contrast, genome-wide genetic analysis of 1000 C. bursa-pastoris collections suggests that adaptation to new environments was predominantly driven by standing variation. Climate stratified population structure is reiterated after colonization, and preliminary analysis suggests that the genetic basis of phenotypes relevant to climate adaptation can be mapped. Our analysis reveals alternative paradigms for success during recent invasions, and provides a glimpse into the genetic basis of adaptation on short timescales.
