Answer:
Declining crude birth rates.
Explanation:
Birth rate may be defined as the number of births takes place per year per thousand of the population. The birth rate and death rate may change the population dynamics.
The developing countries shows the expanding population graph as they have more number of people of the reproductive age. The advancement in the medical field results in the decline birth rate that causes the decline of the nature resources.
Thus, the answer is declining crude birth rates.
The number of phenotypes produced for a given trait depends on how many genes control the trait. Anyhow, The distribution of phenotypes for a typical polygenic trait can often be expressed as a bell-shaped curve.
Many traits are controlled by two or more genes and are, therefore, called polygenic traits<span>. Each gene of a polygenic trait often has two or more alleles. As a result, one polygenic trait can have many possible genotypes and phenotypes.</span>