Asexually reproducing animal populations are rare, but they typically consist of females who produce female offspring, so all offspring can produce more offspring. By contrast, half of the offspring of the sexual reproducers are male and cannot produce offspring. Thus, a clonal population of females can expand much more quickly than a population of sexual reproducers. Potamopyrgus antipodarum is a species of freshwater snail that has spread far beyond its native New Zealand. Some populations of these tiny snails reproduce sexually, and other populations reproduce asexually. Huge asexual populations are now disrupting ecosystems all over the world, particularly where the water is contaminated with phosphorus. Fertilizers and detergents contain phosphorus, so agricultural runoff and other forms of water pollution are common in these regions. Like most animals that cannot reproduce sexually, the asexual snails have at least three sets of chromosomes; like most sexual organisms, the sexual snails have two. Explain why sexually reproducing snail populations predominate in regions where the water is unpolluted.