Contrast evolution by natural selection with evolution by genetic drift.

Evolution by natural selection and evolution by genetic drift are two mechanisms that can lead to changes in the allele frequencies of a population over time, which is the essence of evolution. However, they operate under different principles and conditions.

**Evolution by Natural Selection:**

Natural selection is a process described by Charles Darwin in which individuals with traits that are advantageous for survival and reproduction in their environment are more likely to pass on their genes to the next generation. Over time, this can lead to the accumulation of beneficial traits within a population and can drive the adaptation of species to their environments.

Key characteristics of evolution by natural selection include:

1. Variation: There must be variation in traits within a population.
2. Heritability: The traits must be heritable, meaning they can be passed from parents to offspring.
3. Differential Survival and Reproduction: Individuals with certain traits are more likely to survive and reproduce than others.
4. Adaptation: Over time, the traits that confer a reproductive advantage will become more common in the population, leading to adaptation.

Natural selection is a directional process that tends to increase the frequency of alleles that contribute to reproductive success in a given environment.

**Evolution by Genetic Drift:**

Genetic drift, on the other hand, is a stochastic (random) process that affects the allele frequencies in a population. It is most pronounced in small populations where chance events can lead to large changes in allele frequencies from one generation to the next.

Key characteristics of evolution by genetic drift include:

1. Randomness: Genetic drift is random with respect to fitness; the changes in allele frequency are not driven by any systematic environmental pressures.
2. Population Size: The effects of genetic drift are more significant in smaller populations where random events can have a larger impact.
3. Founder Effect: When a new population is started by a small number of individuals (founders), the allele frequencies can be significantly different from the original population due to genetic drift.
4. Bottleneck Effect: A sudden reduction in population size due to an event (e.g., natural disaster) can lead to a reduction in genetic variation and a change in allele frequencies due to drift.

Genetic drift does not necessarily lead to adaptation or improved fitness. Instead, it can lead to the loss of genetic variation and potentially beneficial alleles.

In summary, while both natural selection and genetic drift can lead to evolution, natural selection is a non-random process that tends to produce adaptation to the environment, whereas genetic drift is a random process that can lead to changes in allele frequencies independent of their effect on fitness and adaptation.