In the plant "Arabidopsis arenosa" some of the alleles conferring adaptation to drought and phytotoxic levels of metal have been introgressed from "A. lyrata". Even in humans there is evidence for adaptive introgression of e.g. immunity alleles, skin pigmentation alleles and alleles conferring adaptation to high altitude environments from Neanderthal and Denisovans. If traits important for species recognition or reproductive isolation introgress into a population of another species, the introgressed population may become reproductively isolated against other populations of the same species. Examples of this include "Heliconius" butterflies, where selective introgression of wing pattern genes between diverged lineages occurs, and wing patterns contribute to reproductive isolation in some species pairs with low (e.g. between "H. t. florencia" and "H. t. linaresi") and intermediate levels (e.g. "H. c. galanthus"/"H. pachinus") of divergence. Many empirical case studies start with exploratory detection of putative hybrid taxa or individuals with genomic clustering approaches, such as those used in the software STRUCTURE, ADMIXTURE or fineSTRUCTURE. These methods infer a user-specified number of genetic groups from the data and assign each individual to one or a mix of these groups. They can be applied to closely related taxa without having to preassign individuals to taxa and may thus be particularly useful in the study of closely related taxa or species complexes. However, uneven sampling of the parental taxa or different amounts of drift in the included taxa may lead to erroneous conclusions about evidence for hybridization. If genomic data of multiple species is available, phylogenetic methods may be better suited to identify introgression. Introgressive hybridization leads to gene trees that are discordant from the species tree, whereby introgressed individuals are phylogenetically closer to the source of introgression than to their non-introgressed conspecifics. Such discordant gene trees can also arise by chance through incomplete lineage sorting, particularly if the species compared are still young. Therefore, discordant gene trees are only evidence of introgression if a gene tree produced by excess allele sharing between the hybridizing taxa is strongly overrepresented compared to alternative discordant gene trees. An entire suite of methods have been developed to detect such excess allele sharing between hybridizing taxa, including Patterson’s D statistics or ABBA-BABA tests or f-statistics.