According to the biological species concept, for speciation to occur, according to the biological species concept, for speciation to occur, the number of chromosomes in the gene pool must change large numbers of genes that affect numerous phenotypic traits must change at least one gene, affecting at least one phenotypic trait, must change changes to centromere location or chromosome size must occur within the gene pool

The answer is; at least one gene, affecting at least one phenotypic trait, must change

This change, due to mutation, may result in an extreme trait in a small proportion of the population . If this extreme trait proves to be favorable with the environment it will continue to increase in the population with every generation. Eventually, the trait will become common in most individuals and the population will have speciated.

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.