They decrease because the zebra mussel were taking their resources. Leaving them nothing to eat.
Answer:
Phenotypic variation is the variation which appears in the phenotype or morphology of the organism. Natural selection is the mechanism of evolution in which the adapated organism to the environment is selected or highly favoured during the course of evolution.
A phenotype is the results of the genetic change which to be established in a population has to be passed on to the generation.
Since variation in phenotypic traits allows the organism to leave indifferent circumstances therefore phenotypic variation proves essential condition to natural selection.
Infertility is the incapacity to conceive an offspring while erectile dysfunction is the incapacity to get aroused so as to engage in sexual intercourse (mostly in men).
Erectile dysfunction could be caused by stress, cardiovascular diseases, and diabetes. Therefore treating it requires maintaining a healthy lifestyle of nutrition and exercise.
Infertility can be caused by disorders of reproductive organs such as endometriosis and occlusion in the tubes. This can be treated by assisted reproduction (IVF), corrective surgery and education.
