Natural Selection in Action
Natural Selection in Action
Natural selection is a key mechanism of evolution. It describes how certain traits become more or less common in a population depending on how they affect survival and reproduction.
But “natural selection in action” means actually observing this process happening in real time, in real organisms, often across just a few generations.
How natural selection works
Variation Individuals within a species are not identical. Some bacteria, for example, might have a slight mutation that changes how they grow.
Environmental pressure Something in the environment challenges survival — predators, diseases, climate, antibiotics, lack of food, etc.
Differential survival Individuals with helpful traits survive better than others.
Reproduction Survivors pass their traits to offspring.
Population changes Over many generations, the favorable trait becomes common or even widespread.
Why some species evolve faster
Species like bacteria, insects, and viruses reproduce extremely quickly. That means:
More generations in a short time
More opportunities for mutations
Natural selection happens rapidly
This is why doctors sometimes observe antibiotic resistance evolving within months, whereas changes in large animals (like wolves or elephants) take thousands of years.
What Are Bacteria?
Bacteria are tiny, single-celled living organisms. They are among the oldest life forms on Earth and exist nearly everywhere:
Deep oceans
Hot springs
Soil
Inside plants
Inside animals
On your skin
In the air you breathe
Characteristics of bacteria
Prokaryotic cells: They don’t have a nucleus.
Fast reproduction: They divide through binary fission, sometimes in as little as 20 minutes.
Huge diversity: There are thousands of species, each adapted to different environments.
Metabolic versatility: Some make their own food, some consume sugars, some break down waste, and some even eat chemicals like sulfur.
Roles of bacteria
Helpful bacteria:
Gut microbiome helps digest food and make vitamins.
Soil bacteria fix nitrogen for plants.
Some are used to make cheese, yogurt, and medicine.
Harmful bacteria:
Cause diseases such as pneumonia, tuberculosis, and strep throat.
Can spoil food.
May grow on wounds or cause infections.
What Are Antibiotics?
Antibiotics are chemical substances designed to target bacteria. They can be:
Naturally produced by fungi or bacteria (like penicillin)
Semi-synthetic (modified in labs)
Fully synthetic (engineered entirely by humans)
How antibiotics work
Different antibiotics attack bacteria in specific ways:
Destroy cell wallsBacteria rupture and die.(Example: Penicillin-type antibiotics)
Block protein productionBacteria can’t make essential proteins.(Example: Tetracycline)
Interfere with DNA replicationStops bacteria from multiplying.(Example: Ciprofloxacin)
Disrupt metabolismBacteria can’t produce energy.(Example: Sulfonamides)
Important note
Antibiotics do NOT work on viruses (like the flu or common cold), because viruses are completely different types of organisms.
Peppered Moths — A Classic Case of Natural Selection
Peppered moths (Biston betularia) show one of the most famous examples of evolution observed in real time.
Before industrial pollution
Most moths were light-colored, speckled with black.
They blended perfectly with lichen-covered tree bark.
Birds could not easily spot them.
During the Industrial Revolution (1800s)
Factories released soot into the air.
Soot darkened tree trunks and killed the lichens.
Light moths became visible and were eaten more.
Natural selection occurred
A rare dark-colored form (melanic form) suddenly had a survival advantage.
Dark moths blended into the soot-covered trees.
They survived more and reproduced more.
Within decades, dark moths became the majority in polluted areas.
After pollution control laws
Trees became lighter again.
Light moths regained their camouflage advantage.
Their population increased once more.
This is evolution happening rapidly in response to environmental change.
What Do Bacteria and Antibiotics Do to Each Other?
Their interaction is one of the best modern examples of natural selection.
Bacteria
Rapidly reproduce
Frequently mutate
Sometimes develop resistance genes that protect them from antibiotics
Antibiotics
Kill bacteria
Create strong selection pressure
Only bacteria with resistance survive treatment
How resistance evolves
A few bacteria happen to have mutations that make them resistant.
When antibiotics are used, they kill the non-resistant bacteria.
Resistant bacteria survive and multiply.
Over time, the population becomes mostly resistant.
How resistance spreads
Through reproduction (the resistant bacteria simply multiply)
Through gene transfer (bacteria can share DNA with each other, even different species)
Summary
Natural selection in action: Rapid evolution observed over time.
Bacteria: Single-celled organisms, incredibly diverse and often helpful, sometimes harmful.
Antibiotics: Medicines that kill or inhibit bacteria through different mechanisms.
Peppered moths: Classic example of natural selection responding to environmental changes.
Bacteria vs. antibiotics: Antibiotics create pressure that leads to the evolution of resistant bacteria.