Natural selection
Evolution explains the diversity of life. Charles Darwin and Alfred Russel Wallace proposed its central mechanism, natural selection, in 1858.
What evolution means
Biological evolution is change in the inherited traits of populations over generations. Individuals do not evolve; populations do, as the frequency of alleles shifts. Over long periods, these changes produce new species and the branching tree of life.
Darwin's observations
On the voyage of HMS Beagle, Darwin saw that species on the Galapagos Islands resembled those on the nearby South American mainland but differed from island to island. Finches, for example, had beaks suited to different foods. He also knew breeders could change animals dramatically by choosing which ones bred.
The logic of natural selection
Individuals in a population vary; much of that variation is inherited; more offspring are born than can survive; and individuals with traits better suited to the environment tend to survive and reproduce more. Over generations, those helpful traits become more common. Nothing in this requires a goal or plan.
Fitness and adaptation
In biology, fitness means reproductive success: how many offspring an individual leaves compared with others. An adaptation is an inherited trait that improves fitness in a particular environment, such as a cactus's water-storing stem or the camouflage of a moth. Adaptations fit the environment that shaped them, not every environment.
Peppered moths
In industrial England, soot darkened tree bark and dark peppered moths, harder for birds to spot, became more common. After clean air laws, bark lightened and pale moths increased again. The population's colour tracked which form survived predation better.
Antibiotic resistance
When antibiotics are used, sensitive bacteria die, but any with resistance mutations survive and multiply. Repeated or incomplete use favours resistant strains, which is why resistant infections are a serious medical problem. This is natural selection happening within years.
Artificial and sexual selection
In artificial selection, humans choose which individuals breed, producing dog breeds and crops like broccoli and cabbage from one wild mustard. Sexual selection favours traits that improve mating success, such as a peacock's tail, even when those traits are costly for survival.
Common misconceptions
Organisms do not evolve because they need to; variation arises randomly and selection sorts it. Traits acquired during life, like a giraffe stretching its neck, are not inherited. Evolution is not 'just a theory' in the everyday sense: in science, a theory is a well-tested explanation supported by large amounts of evidence.
Key takeaways
Evolution is inherited change in populations over generations. Natural selection needs variation, inheritance, overproduction, and differential survival and reproduction. Fitness means reproductive success. Peppered moths and antibiotic resistance show selection in action. Artificial and sexual selection follow the same logic.
Flashcard
Change in inherited traits of populations
Check your understanding
Why do bacteria become resistant to an antibiotic over time?