Genetic drift is defined as a shift in the allelic frequency of a population. This can be caused by natural selection which is the differential survival and reproduction of individuals across a population due to differences in phenotype. Natural selection favors particular traits in a population hence these individuals are able to reproduce while the individuals with disadvantageous traits will pass their genes to succeeding generations. This way the allelic frequency of succeeding generation will be diferent from that of the parent population.
Aquatic ecosystems can be classified as freshwater and marine ecosystems.
By knocking at the bend of the knee with a small "hammer". If the leg "spasms" then the superficial reflex is in working condition. Hope this helps :)
