Final answer:
Disease genes like obesity and heart disease are common due to complex genetic and environmental factors that interact, and modern societal conditions have altered the impact of natural selection, allowing individuals with such predispositions to reproduce and pass on their genes.
Step-by-step explanation:
The presence of disease genes like obesity and heart disease in most populations can mainly be attributed to complex genetic and environmental factors. These conditions often occur when numerous genes interact with lifestyle and environmental influences, rather than being caused by a single genetic mutation. For instance, the genes that contribute to obesity may increase one's tendency to gain weight, but this tendency will be realized only in an environment with abundant food and limited physical activity. Likewise, heart disease can be influenced by both genetic susceptibility and factors such as diet, exercise, and stress.
It's also important to note that the natural selection process, which favors the survival of more fit individuals who are more likely to reproduce, operates in a vastly different context in modern human societies compared to the ancestral environments. Today, many people with genetic predispositions to these diseases survive and reproduce, passing on these genes to the next generation. Additionally, mutations contributing to these diseases may not significantly reduce the individual's ability to reproduce before they are passed on to offspring.
In summary, while natural selection can be a potent force, the current prevalence of genes associated with obesity and heart disease is due to a multitude of factors, not solely because they are currently being strongly selected for in natural environments.