It is generally believed that there are four species of Coilia distributed in China, namely Coilia ectenes, Coilia ectenes, Coilia ectenes and Coilia ectenes, among which Coilia ectenes and Coilia ectenes have important economic value. (1) The internal cause of biological evolution is that mutation produces the raw materials for evolution, while the external cause is that the living environment changes to form geographical isolation, and natural selection leads to the directional change of gene frequency.

(2) Under natural conditions, the gene exchange between Coilia ectenes from Yangtze River and Coilia ectenes from Pearl River does not occur because of geographical isolation.

(3) Biodiversity refers to all biological (animal, plant, microorganism) species, their genetic variation and the complexity of ecosystem in a certain period of time and in a certain area. It includes three levels: genetic diversity, species diversity and ecosystem diversity, which are different variation types here, so it is genetic diversity.

(4) Because the gene frequency of A is 0.2, the gene frequency of A is 0.8, aa=A2=4%, Aa=2×A×a=32%, aa=a2=64%, if the frequency of A is gradually reduced by the selection of environment, the number of AA genotype individuals will gradually decrease.

(5) Environmental changes play a selective role in different variation types, and isolation is a necessary condition for species formation. There may be geographical isolation between different populations of the same species, and biological evolution is bound to be accompanied by changes in gene frequency.

Therefore, the answer is: (1) Genetic and mutation change the living environment to form geographical isolation.

(2) Both of them belong to different waters, and they both have the habit of short-distance migration in estuaries, so there is geographical isolation, which leads to the inability of gene exchange.

(3) heredity

(4)4%, 32% and 64% gradually decreased.

（5）A B C