An invasive species is an organism or even an organism's seeds or eggs, that is not indigenous, or native, to a particular area.
While all species compete to survive, invasive species appear to have specific traits that allow them to outcompete the native species. Initially they often co exist with native species but gradually the superior competitive ability of an invasive species becomes apparent as it adapts to its new environment. This is possible because the new species is able to use resources that were previously unavailable to the native species.
Invasive species can cause great economic and environmental harm to the new area. They may alter the environment by releasing chemical compounds, modifying abiotic factors, or affecting the behaviour of herbivores, creating a positive or negative impact on other species.
Since there is a rapid adaptive evolution in such species, their resulting offsprings have higher fitness and are better suited for the environment.
Invasions mostly occur in ecosystems that were similar to the one in which the species previously evolved.