Natural Selection
இயற்கைத் தேர்வு
Evolution is not a choice; it is a filter. Nature does not reward the strongest, but the most fit. If you think 'survival of the fittest' means physical strength, you are failing the NEET biology section already.
The idea
Darwinian Natural Selection dictates that individuals with heritable variations better suited to their environment survive and reproduce. These individuals leave more progeny, increasing the frequency of advantageous alleles in the population. Over generations, this differential reproductive success drives evolutionary change. It is a process of selection, not creation.
Why it matters
It acts as the primary mechanism of evolution, ensuring that traits beneficial for survival and reproduction are passed on to subsequent generations.
NEET lens
NEET focuses on the mechanism: variation leads to differential reproduction, not just survival. Do not confuse Lamarckism with Darwinism; Lamarck focused on 'use and disuse,' while Darwin focused on 'selection of existing variations.' Expect questions linking natural selection to industrial melanism or pesticide resistance. Memorize the phrase 'differential reproductive success'—it is the key to the answer.
Clinical significance
Antibiotic resistance in bacteria is the most immediate clinical application of natural selection. When antibiotics are used indiscriminately, they kill susceptible bacteria, leaving only the resistant strains to proliferate. This leads to the rapid evolution of 'superbugs' that render standard medical treatments ineffective.
Introduction
Natural selection is the process by which individuals with heritable variations better suited to their environment survive and leave more offspring. Darwin and Wallace proposed it as the main mechanism of evolution.
Core concept
Darwin's key ideas: populations show variation; organisms produce more offspring than can survive (struggle for existence); individuals with useful variations have greater fitness (reproductive fitness = more progeny); these characters are inherited, so over generations the population changes. Examples: industrial melanism in Biston betularia (peppered moth) in England — after industrialisation, tree trunks became dark with soot and lichens died, so dark (melanic) moths were less visible to predators and increased; resistance of insects to pesticides, weeds to herbicides, bacteria to antibiotics; man-made breeds of dogs (artificial selection). Types of natural selection: stabilising (more individuals acquire the mean character value), directional (more individuals acquire a value other than the mean) and disruptive (more individuals acquire values at both extremes).
Go deeper
Natural selection works on variation that already exists; it does not create variation. Artificial selection by humans (e.g. cows with higher milk output, dog breeds) works the same way but the 'selector' is the breeder.
From Class 10
Class 10 introduced natural selection with examples like beetles of different colours being eaten by crows. Here you add industrial melanism and the three types of selection.