Question
What is natural selection? Explain Darwin’s theory of natural selection with an example.
Solution — Step by Step
Within any population, individuals are not identical — some beetles are green, some are brown, some run faster, some digest food better. This variation is the starting point. Without it, natural selection has nothing to act on.
Resources — food, mates, shelter — are always limited. More individuals are born than can possibly survive. This creates competition, both with other species and within the same species.
Here’s the key: “fittest” doesn’t mean strongest. It means best suited to the current environment. A brown beetle in a brown-soil habitat survives better than a green beetle — not because it’s stronger, but because it’s harder for predators to spot. It’s about fit between organism and environment.
The survivors reproduce. Because traits are heritable (passed from parent to offspring), the next generation has more individuals with the advantageous trait. The green beetles get eaten before reproducing; the brown ones pass on “brownness.”
Over thousands of generations, this small-per-generation shift adds up. The population changes. Given enough time and different environmental pressures acting on isolated groups, new species can emerge. This is the bridge from natural selection → evolution.
Why This Works
Natural selection isn’t a force that “wants” to improve species — it has no goal or direction. It’s a filter. Variation enters the population (through mutation, recombination), the environment filters out the less-fit variants, and what remains is whatever happened to work right now in this environment.
This is why the same selection pressure in a different environment produces a different result. The peppered moth (Biston betularia) is the classic NCERT example — before industrialisation, light-coloured moths survived better on lichen-covered trees. After soot darkened the trees, dark moths had the survival edge. The environment changed; the “fit” trait changed.
For NEET, always link natural selection to its three conditions: heritable variation exists → differential survival → differential reproduction. If any one of these is absent, natural selection cannot operate. Questions often remove one condition and ask “will natural selection occur?” — the answer is no.
Alternative Method — The Logical Argument Form
Darwin himself presented natural selection as a logical argument, not just an observation. This framing appears in CBSE short-answer questions asking you to “explain Darwin’s logic.”
Premise 1: All species produce more offspring than can survive (overproduction).
Premise 2: Resources are finite → struggle for existence.
Premise 3: Individuals in a population vary in heritable traits.
Conclusion: Individuals with favourable traits survive and reproduce more → those traits increase in frequency over generations.
Write it as a numbered argument in exams when the question says “explain Darwin’s reasoning” — it shows you understand the logical structure, not just the definition.
Common Mistake
“Survival of the fittest means the strongest individual survives.”
This is the most common misreading, and it costs marks. Fitness in biology means reproductive success in a given environment — the ability to survive long enough to pass genes. A sickle-cell trait carrier in a malaria-prone region is “fitter” than a normal individual there, even though they carry a disease allele. Always define fitness relative to the environment, not as an absolute measure of strength or size.
A second mistake: students write “organisms adapt to survive” — implying the organism consciously changes. Natural selection works on existing variation; it doesn’t create new traits on demand. The variation must already be present in the population.