Which of these is required for natural selection?
a.)all individuals of a population must be identical.
b.)there must be a small number of individuals in a population.
c.)there must be variation among the members of a population.
d.)a population of individuals must forced out of their natural habitat.

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.

the answer is cell division : )

estuaries are the partially enclosed coastal bodies of brackish water formed at the mouth of sea where river meets an ocean. it is a transition zone between river and sea water so estuaries are influenced to both marine environment such as tides, waves and to riverine such as flow of fresh water and sediments. the mixing of sea and fresh water provide high level of nutrients, making estuaries a high product habitats.

many estuaries suffers degradation from a variety of human activities like erosion from deforestation, overgrazing, overfishing, drainage and eutrophication.

so from the above statement answers will be

1. false as estuaries are formed due to both brackish and freshwater.

2.true, due to high productivity estuaries are breeding ground for many marine species.

3. direct and indirect activities of man that contribute to pollution and deposition of sediments.