What Is Natural Selection?
Natural selection: The process by which individuals with traits better suited to their environment have higher reproductive success, passing these traits to the next generation , driving evolutionary change over time.
Natural selection is the central mechanism of evolution. It does not operate on individuals during their lifetimes; rather, it changes the genetic composition of populations across generations.
Natural selection and evolution are not the same thing. Natural selection is one mechanism that drives evolution , do not use the terms interchangeably in exams.
Before Darwin, several competing explanations for species change existed. Lamarck proposed two laws: the law of use and disuse (body parts strengthen with use and weaken with disuse) and the law of inheritance of acquired characteristics (traits modified during an organism's lifetime are passed to offspring , e.g., a giraffe stretching its neck and passing a longer neck to its young). However, special creation (the idea that species were individually and immutably created) was arguably the more dominant view in broader society at the time. Other frameworks, such as Cuvier's catastrophism, also competed for scientific acceptance.
Darwin's theory of natural selection replaced inheritance of acquired characteristics with a mechanism grounded in heritable variation and differential reproductive success.
Paradigm shift: A fundamental change in the dominant framework of scientific understanding. The replacement of Lamarckian inheritance by Darwinian natural selection is a classic example of how a more powerful, evidence-supported explanation can displace an existing one.
This shift exemplifies how science progresses: a new, more powerful explanation , supported by evidence , replaces an existing one.
The Five Key Principles of Natural Selection
Natural selection can be understood through five interconnected principles that form a logical causal chain:
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Overproduction of offspring , Most species produce far more offspring than the environment can support. This creates an inevitable excess of individuals.
Because far more individuals are born than can survive, and resources are limited…
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Variation among individuals , Individuals within a population differ in their traits (e.g., size, colour, speed). Much of this variation has a genetic basis and is therefore heritable.
Because individuals vary in their traits, and some variants cope better with limited resources…
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Struggle for survival , Not all individuals survive to reproduce. Food, water, shelter, and mates are limited; overproduced individuals must compete for them.
Because not all can survive, and those that do tend to have advantageous traits…
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Differential reproduction , Individuals with traits that provide an advantage are more likely to survive and reproduce, passing those traits to more offspring than less-advantaged individuals.
Because advantageous traits are heritable and passed to more offspring…
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Change in populations over time , Advantageous traits become more common across generations; the allele frequencies underlying those traits increase in the gene pool , the population evolves.
Green Sea Turtles: Female turtles lay hundreds of eggs per season, but only a tiny fraction of hatchlings survive to adulthood. Hatchlings with traits such as stronger swimming ability or better camouflage are more likely to escape predators and reach the ocean, and later reproduce , passing those advantageous traits on.
Think of natural selection like an ongoing audition where the environment is the judge. Every generation, individuals "audition" with their traits. Those whose traits best match the current environment "pass" and get to reproduce , the others do not. The audition criteria (environment) can change over time, so what counts as a winning trait can shift.