Here's why:
* Evolution is about changes in allele frequencies. Alleles are different versions of genes, and their frequencies within a population determine the traits that are expressed.
* Individuals cannot evolve. An individual's genetic makeup is fixed at birth. While an individual might change over time due to environmental factors, these changes are not passed on genetically to offspring.
* Populations evolve through natural selection. Natural selection acts on the variation within a population, favoring traits that increase survival and reproduction. Over time, this leads to changes in the allele frequencies within the population.
Example: Imagine a population of beetles with varying colors, some brown and some green. If the environment favors brown beetles (e.g., providing better camouflage), then brown beetles are more likely to survive and reproduce. Over generations, the brown allele will become more common in the population, resulting in evolution.
Important Note: While the smallest unit of evolution is a population, individuals within a population can still experience mutations, which are the raw material for evolution. However, these mutations only become relevant to evolution when they are passed on to offspring and contribute to changes in the population's gene pool.
