Biological evolution describes the process by which heritable characteristics of biological populations change over successive generations. These characteristics, passed from parents to offspring, are expressions of genes. This fundamental process explains the diversity of life on Earth, from the smallest microbes to the largest mammals.
Evolution by Natural Selection
Natural selection is a process where organisms better suited to their environment tend to survive and reproduce more successfully, passing on their advantageous genes. This mechanism relies on several core principles, beginning with variation within a population, where individuals possess differing traits like size or color. These variations are heritable, meaning they can be passed from parents to offspring.
In any given environment, resources are limited, leading to overproduction of offspring and competition for survival. Individuals with traits that offer a better fit to their environment are more likely to survive and reproduce, a concept known as differential survival and reproduction. For instance, in bacteria, some individuals may have a genetic variation that provides resistance to an antibiotic. When exposed to the antibiotic, susceptible bacteria die, while the resistant ones survive and multiply, leading to a population dominated by resistant strains over time.
Similarly, in prey animals, camouflage provides a survival advantage. Individuals with coloration that blends better with their surroundings are less likely to be detected and eaten by predators. These camouflaged individuals are more likely to survive to reproductive age and pass on their beneficial genes, causing the trait to become more common in subsequent generations.
Evolution by Artificial Selection
Artificial selection is a process where humans intentionally choose organisms with desired traits to breed, thereby influencing the characteristics of future generations. Unlike natural selection, where environmental pressures drive changes, human intervention is the deliberate force behind artificial selection. This method has been practiced for thousands of years, leading to significant changes in domesticated plants and animals.
A prime example is the domestication of dogs, which began an estimated 20,000 to 40,000 years ago from wolf ancestors. Through generations of selecting and breeding wolves with desirable traits like tameness or specific hunting abilities, humans have created the vast array of dog breeds seen today, from small Chihuahuas to large German Shepherds.
Artificial selection has also profoundly impacted agriculture, transforming wild plants into the crops we consume. For instance, various vegetables like broccoli, cabbage, and kale were all developed from a single wild mustard plant through selective breeding. Farmers consistently chose and bred plants with larger leaves, specific flower buds, or other preferred characteristics, leading to the diverse forms of these common vegetables.
Evolution by Sexual Selection
Sexual selection is a specific type of natural selection that focuses on traits enhancing an individual’s ability to obtain a mate. These traits may not directly aid survival but offer a reproductive advantage, ensuring genes are passed to the next generation. The process often leads to distinct differences between males and females of a species, known as sexual dimorphism.
There are two main forms of sexual selection. Intersexual selection involves mate choice, where individuals of one sex, typically females, select mates based on certain desirable characteristics. A classic example is the elaborate tail feathers of male peacocks. Females prefer males with the brightest and most impressive plumage, leading to the prevalence of this trait.
In contrast, intrasexual selection involves competition among members of the same sex, usually males, for access to mates. This competition can manifest as physical battles or displays of dominance. For example, male deer grow large antlers and engage in fierce fights to establish dominance and secure breeding rights with females. The winners of these contests are more likely to reproduce, ensuring that traits associated with strength and combat ability become more common in the male population over time.