Natural Selection Basics
What is the term for differences between individuals in a population?
Variation
What do we call an organism’s ability to survive and reproduce?
Fitness
If the environment stays the same, what happens to advantageous traits?
They become more common
Birds with longer beaks reach nectar better. What happens over many generations?
Longer beaks become more common
Natural selection is driven by differences in what?
Survival and reproduction
Natural selection acts on individuals, but what actually changes over time?
The population
If two traits give equal survival, what determines which trait increases?
Reproductive success
What happens to traits that lower fitness?
They decrease in frequency
Dark beetles are eaten more often on sand. Which trait increases?
Light beetles
Why can two organisms look different even in the same species?
Genetic variation
What is the term for something in the environment that makes one trait more helpful than another?
Selective pressure
Why does overproduction increase competition?
More offspring than resources → struggle to survive
What must be true about a trait for natural selection to change it?
It must be heritable
Bacteria exposed to antibiotics often develop what trait?
Resistance
What causes the population to shift toward one trait over another?
Differential reproductive success
What three conditions must be present for natural selection to occur?
Variation, overproduction, and competition (limited resources)
In a snowy biome, which fur color increases fitness?
Light/white fur
Give one reason why a population might NOT evolve even if variation exists.
No selective pressure / equal fitness
Wolves with thicker fur survive a freezing winter. What happens to fur thickness?
It increases in the population
Why can a harmful trait sometimes remain in a population?
Carriers survive; environment doesn’t eliminate it
A population has two traits: A and B. Trait A is favored in dry years. Trait B is favored in wet years.
The climate begins switching every 3 years between wet and dry.
Question:
Predict the long-term pattern of trait frequencies AND explain why neither trait goes extinct.
They oscillate; each becomes advantaged when the environment shifts.
A trait increases survival but lowers reproduction. Predict what happens to it over generations.
It decreases because reproduction drives evolution.
A population suddenly moves into a new environment where both traits become low fitness until a rare mutation appears.
Question:
Predict what happens to trait frequencies in the original population before and after the new mutation spreads.
Both original traits decline; the new, rare beneficial mutation increases.
In a fish population, a new predator arrives that hunts only during daylight.
One trait causes fish to be more active in daylight; another causes night activity.
Question:
Predict how the predator’s hunting pattern reshapes population behavior over generations.
Night-active trait increases; day-active trait decreases.
A drought causes seeds to become extremely hard.
Only birds with very deep beaks can crack them — but deep beaks reduce flight speed, lowering escape chance from predators.
Question:
Explain how these conflicting pressures shape the beak-depth distribution over generations.
The population shifts toward a balanced middle or fluctuates depending on which pressure is stronger.