Why is genetic drift considered random whereas natural selection is considered nonrandom?

In the context of evolution, allele frequencies refer to how common an allele is in a population. An allele is a version of a gene, and in a stable population, certain alleles are present at predictable rates. When genetic variation is high, a population can have multiple alleles at varying frequencies. Factors like genetic drift and natural selection influence these frequencies over time.

How It Relates to Genetic Drift and Natural Selection:
Genetic drift can dramatically fluctuate allele frequencies, especially in small populations, often independently of the alleles’ fitness. On the other hand, natural selection alters allele frequencies by favoring those that provide adaptive advantage, thus being more deliberate and consistent in changing allele frequencies towards beneficial traits.

Random sampling can cause significant changes in a population, especially in smaller ones. In a large population, the random loss or survival of individuals with certain alleles might not make a noticeable difference. However, in a small population, if certain individuals do not reproduce for any reason unrelated to their traits, the alleles they carry can become less frequent or disappear. This is the essence of genetic drift — the change in allele frequencies that occurs due to chance alone.

Examples of Random Events:

• Some organisms may have more offspring by chance, not necessarily due to higher fitness.
• A natural disaster could randomly eliminate individuals, regardless of their genetic advantages.

An adaptive trait is a characteristic that increases an organism’s chance of survival and reproduction in its specific environment. These traits are the focus of natural selection. For example, a moth with coloring that camouflages it from predators will likely survive longer and produce more offspring.

Survival of the Fittest: 'Fitness' in evolutionary terms does not just mean physical strength. It refers to the ability to survive and reproduce. Hence, adaptive traits are those that improve an organism's fitness relative to the current environment.

Success in reproduction is essentially the transmission of genes to the next generation. The more successful an organism is at reproducing, the greater its influence on the gene pool of the future population.

Natural Selection's Role: Through natural selection, organisms with adaptive traits often have higher reproductive success because their traits allow them to survive and produce more offspring. Over time, these traits become more common within the population as they are successfully passed down through generations.

The environment plays a crucial role in shaping populations through natural selection. Environmental factors include everything that can impact survival, such as predators, climate, availability of food, and competition.

How Does the Environment Influence Evolution? It selects for traits that are advantageous in a given environment. For instance, in a cold climate, animals with thicker fur might survive better and leave more descendants than those with thinner fur. Over time, the population's gene pool could shift towards thicker fur as the frequency of that trait increases due to higher reproduction of individuals possessing it.