The idea that two humans could not repopulate the entire human race without dying out is based on genetic diversity concerns. When a population is very small, there is a higher risk of inbreeding, which can lead to the expression of harmful recessive traits and reduce overall genetic diversity. In the long term, this can result in decreased fitness and increased vulnerability to diseases, environmental changes, and other challenges.
The minimum viable population size required for long-term survival and genetic diversity is a topic of debate among biologists, and estimates vary depending on the species and environmental factors. For humans, the consensus is that a population of at least a few thousand individuals would be necessary to ensure a healthy and genetically diverse population over the long run.
Now, as for domesticated animals like Syrian hamsters and thoroughbred horses, their survival with a small initial stock can be attributed to human intervention and selective breeding practices.
Selective Breeding: Humans have carefully selected and bred these animals for specific traits over generations. In the case of thoroughbred horses, for example, they were selectively bred for speed and agility. This artificial selection allows for the preservation of certain desired traits within a smaller population.
Controlled Environment: Domestic animals typically live in controlled environments, such as farms or zoos, where their access to food, shelter, and healthcare is managed by humans. This controlled environment can help mitigate some of the challenges they would face in the wild with a smaller population.
Genetic Management: For many domestic animals, breeders and conservationists utilize genetic management techniques to monitor and maintain genetic diversity. They may introduce new genetic material periodically through controlled breeding programs or even artificial insemination.
Reduced Natural Selection Pressure: In the wild, small populations would be more vulnerable to predators, diseases, and other environmental factors that can reduce their chances of survival. However, domestic animals often face reduced natural selection pressures due to the controlled environment and human intervention.
While these factors help ensure the survival of domestic animals with small initial populations, it is important to note that these populations are often kept in controlled settings with human oversight. In the wild, a small population could be more at risk due to natural challenges. Moreover, domestic animals might still face certain genetic issues over time if not managed appropriately. The situation with humans is unique as we cannot be as selectively bred or managed in the same way as domestic animals. As a species, we must prioritize maintaining a healthy population size and genetic diversity to ensure our long-term survival and adaptability to changing environments.