Which scenario causes gene flow

Migration is one way that genes are transferred from one population to another.

Explanation:  Gene flow is caused by a significant change in the population of individuals due to the addition of other individuals whose alleles are slightly different from those of the existing individuals

Introgression refers to gene flow from another taxon. While low levels of gene flow between species are not uncommon, it is a serious issue when an introduced invasive species hybridizes with a native species and overwhelms its genetic constitution (genetic swamping). This is a major threat in canids, ducks, fish and plants. For example, exotic rainbow trout introduced into the habitats of native cutthroat trout in the Western United States led to hybrid swarms that eventually overwhelmed the local species. Introgression is readily detected by molecular genetic and genomic methods.

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