7.12 Variations in Populations
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Genetic Diversity | 遗传多样性 | The total number of genetic characteristics in the genetic makeup of a species; crucial for adapting to environmental changes. |
| Resilience | 韧性/恢复力 | The ability of a population or ecosystem to recover quickly from perturbations or environmental stressors. |
| Deleterious Trait | 有害性状 | A genetic trait that reduces the fitness of an organism in a specific environment. |
| Inbreeding Depression | 近交衰退 | The reduced biological fitness in a given population as a result of breeding between related individuals, which concentrates deleterious alleles. |
1. Diversity Equals Resilience
As we explored with Hardy-Weinberg, populations have varying frequencies of different alleles. The overall level of genetic variation in a population profoundly affects its population dynamics and its ultimate survival.
Genetically diverse populations are more resilient to environmental perturbation. Why? Because the environment is constantly changing (new diseases, climate shifts, new predators). If a population has a vast library of different alleles, it is highly likely that at least some individuals will possess the specific phenotype needed to withstand the new environmental pressure, survive, and rebuild the population.
- Example: Antibiotic resistance in bacteria. Not all individuals in a diverse bacterial population are susceptible to a specific antibiotic. The few that have a random resistance allele survive the "outbreak" and repopulate.
2. The Danger of Low Diversity
Conversely, a population's ability to respond to changes in the environment is severely compromised if it lacks genetic diversity. Species and populations with little genetic diversity are at extreme risk of decline or extinction.
This often happens after a severe Bottleneck Effect (see 7.4). The remaining population is highly inbred, concentrating deleterious (harmful) recessive mutations.
- Animal Examples: Species like California condors, black-footed ferrets, and prairie chickens suffered massive habitat loss and hunting. Their populations plummeted to just a few dozen individuals. Even through conservation breeding, their lack of genetic diversity makes them highly vulnerable to a single disease outbreak wiping them out entirely.
- Agricultural Examples: Humans often artificially select for just one high-yielding strain of a crop (monoculture). Because these plants are virtually identical genetically, outbreaks of potato blight (which caused the Irish Potato Famine) or corn rust can obliterate millions of acres overnight because zero plants possess a resistance allele.
3. Context-Dependent Alleles
It is important to remember that natural selection is entirely dependent on the current environment.
Alleles that are highly adaptive in one environmental condition may become deleterious in another because of different selective pressures.
- Example: The allele for sickle-cell trait. In high altitudes or normal environments, it can be deleterious (causing anemia). However, in tropical environments where Malaria is endemic, possessing one copy of this allele provides resistance to the malaria parasite, making it highly adaptive. The environment dictates the value of the variation.