32.3
突变，基因流和遗传漂移
在不处于哈代 - 温伯格原理平衡的群体中，等位基因的频率随时间而变化。因此，与哈代 - 温伯格原理平衡的五个条件的任何偏差都可以改变给定群体的遗传变异。改变群体遗传变异性的条件包括突变，自然选择，非随机交配，基因流和遗传漂变（小群体大小）。
遗传变异机制
基因变异的原始来源是突变，即DNA核苷酸序列…
In addition to natural selection, allele frequencies in a population can change over time through mutation, gene flow, and genetic drift.
Random mutations can generate genetic variation in a population, such as in these beetles.
Harmful mutations are often reduced in frequency by natural selection, especially if they strongly lower survival or reproduction. Beneficial mutations can spread if they increase reproductive success.
Genes from outside a population can also increase genetic variation through the immigration of new individuals. When these new individuals breed with the resident population, they introduce new alleles into the gene pool.
When beetles from two populations regularly exchange individuals, they reproduce in each other’s populations. As a result, genetic variation flows in both directions. Their gene pools become more similar over time, though other forces can counteract this effect.
Finally, if a random event such as a storm reduces population size, allele frequencies may change dramatically because only a small number of individuals survive. This change is called genetic drift.
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Q1: How do mutations create genetic variation in populations?
Mutations are changes in DNA nucleotide sequences that generate new alleles and increase genetic variability. Random mutations can produce beneficial traits that spread if they enhance reproductive success, or harmful mutations that natural selection typically reduces. Most mutations cause no significant health effects, but those reducing survival chances are often eliminated before reproduction occurs.
Q2: What is genetic drift and how does it affect allele frequencies?
Genetic drift occurs when random events, such as storms, drastically reduce population size, causing allele frequencies to change dramatically. Only surviving individuals contribute to the new gene pool, and their genetic composition was selected randomly rather than by fitness advantage. This process can significantly alter allele frequencies independent of natural selection.
Q3: How does gene flow introduce new alleles into a population?
Gene flow occurs when individuals migrate between populations and breed with resident members, introducing new alleles into the gene pool. When beetles from two populations regularly exchange individuals, genetic variation flows in both directions, making their gene pools more similar over time. This transfer of alleles can counteract other evolutionary forces.
Q4: What is a genetic bottleneck and how does it reduce genetic variation?
A genetic bottleneck occurs when a major disturbance drastically reduces population size, randomly eliminating many alleles from the gene pool. Surviving individuals are selected by chance rather than genetic fitness, resulting in significantly diminished genetic diversity. This random loss of variation can severely limit a population's adaptive potential.
Q5: Why is genetic variation essential for population survival?
Genetic variation provides the raw material for natural selection and enables populations to adapt when environments change. Without diverse alleles conferring survival and reproductive advantages, populations cannot evolve and may vanish. Smaller populations face greater vulnerability to stochastic events because their gene pools contain fewer variants.
Q6: What is the founder effect and how does it alter gene frequencies?
The founder effect occurs when a small group establishes a new population, and by chance, previously rare alleles become relatively frequent in the new population. This random change in gene frequencies happens when populations fragment due to geographic barriers or urban development. The new population's genetic composition differs significantly from the original population.
Q7: How do mutations, gene flow, and genetic drift differ from natural selection?
Unlike natural selection, which favors beneficial alleles, mutations, gene flow, and genetic drift change allele frequencies through random or non-selective mechanisms. Mutations introduce new variation; gene flow transfers alleles between populations; and genetic drift causes random frequency changes in small populations. These processes operate independently of an organism's fitness or survival advantage.