单项选择题

TEXT E
In the 1960s scientists begin to recognize that environmental contaminants could not only affect the health and survival of individual animals but also alter the prospects for their off-spring and thereby potentially change the genetic makeup of entire populations.
Researchers were first altered to problems in wildlife in the 40s after the populations of eagles, falcons, and the other fish-eating birds in Britain plummeted. In nest after nest the birds’ eggshells were so thin that they cracked under the weight of the adults during incubation. In the 1960s David Peakall and other wildlife toxicologists demonstrated that the accumulation of very high levels of such pesticides as DDT in the birds’ tissues had seriously impaired their productive capabilities. Some of these declines resulted in the complete disappearance of populations from large portions of their former range. In North America, for example, the eastern population of the peregrine falcon was virtually wiped out. More recently, the Golf Coast population of the brown pelican disappeared as a result of eggshell thinning thought to be caused by the organochlorine pesticides dieldrin and endrin.
Since then, researchers have provided additional evidence that environmental pollution can affect future generations. For example, exposure to high levels of PCBs has been shown to affect the learning and behavior of children. In the 1980s Snadra W. Jacobson and Joseph L. Jacobson of Wayne State University, Detroit, Michigan, studied a group of children whose mothers had eaten PCB—contained fish from Lake Michigan. The researchers found that the children’s prenatal exposure to these compounds resulted in neurological anomalies at birth and developmental delays in motor function during infancy. The Jacobson retested the children at age 11. In a 1996 report they noted that the children exhibited significantly poorer intellectual function, amounting to a 6.2 point deficit in the IQs of the most highly exposed subjects.
Contaminants also have been linked to a critical loss of genetic variability in populations of living organisms. One of the best studies of this phenomenon was published in 1994 by M. H Murdoch and P.D.N. Hebert of the University of Guelph, Ontario. The study measured the variations in the mitochondrial DNA (mtDNA) of populations of brown bullhead catfish in the Great Lakes, comparing bullheads from pristine reference areas with bullheads living in heavily contaminated with such pollutants as organochlorines and petrochemicals. The two researchers used one of the most powerful tools of modern molecular population genetics-molecular analysis of DNA. By revealing differences in the specific code, i.e., in the sequence of nucleotides, contained in the DNA of a particular gene, the technique can help identify and quantify genetic variety within and among populations. For their study, Murdoch and Hebert examinated variations in genes of the cellular mitochondria, which possess their own DNA (mtDNA) that is distinct from the DNA found in the cell nucleus. Because mitochondrial genes are not "shuffled" in the production of sperm and egg cells, as are nuclear genes, and because they are transmitted to offspring only by the mother, they are ideal for charting the relatedness and evolutionary history of spaces.
The researchers found that although the numbers of fish were abundant in both types of sites, the levels of genetic variability were always significantly higher in the pristine areas. The most likely explanation is that bullheads populations in polluted waters crashed after their initial contact with contaminants, but the remaining fish were able to repopulate because a few individuals possessed rare genes that allowed them to adapt and survive. Thus, even though the bullhead populations appeared to be thriving in contaminated areas, the genetic makeup of their populations had undergone a damaging simplification, a depletion of the storehouse of adaptations that animals can draw upon to surmount environmental challenges such as the introduction of a new disease of fluctuations in climate. Their genetic diversity potentially could be quickly increased by the influx of new genes from migrant fish, but most fish from other populations might survive in the polluted sites long enough to contribute to the gene pool.
The main purpose of the first 3 paragraphs of the passage is to ______.