Natural Selection
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Natural selection is the process by which organisms with traits better suited to their environment tend to survive and reproduce more successfully than those without such traits. Over time, this process leads to evolutionary change in populations.
Natural Selection
Natural selection is a mechanism of evolution in which individuals with heritable traits that increase their survival and reproduction in a specific environment pass those traits to offspring at higher rates, causing the traits to become more common in the population over generations.
Darwin's Four Postulates
Charles Darwin identified four key observations that explain how natural selection works:
- Variation: Individuals in a population differ in their traits
- Heritability: Some of this variation is passed from parents to offspring
- Overproduction: More offspring are produced than can survive
- Differential survival and reproduction: Individuals with certain traits survive and reproduce at higher rates
Key Concepts in Natural Selection
| Term | Definition | Example |
|---|---|---|
| Fitness | An organism's ability to survive and reproduce in its environment | A fast rabbit has higher fitness in a predator-rich environment |
| Adaptation | A trait that increases fitness in a particular environment | Thick fur is an adaptation to cold climates |
| Selective pressure | Environmental factors that favor certain traits | Predators, climate, food availability |
| Variation | Differences in traits among individuals | Different beak sizes in finches |
Types of Natural Selection
- Directional selection: Favors one extreme phenotype (e.g., larger size during food scarcity)
- Stabilizing selection: Favors intermediate phenotypes (e.g., medium birth weight in humans)
- Disruptive selection: Favors both extreme phenotypes (e.g., very large and very small seeds in a bird population)
What Natural Selection Does NOT Do
- Does NOT cause individuals to change during their lifetime
- Does NOT create perfect organisms
- Does NOT work toward a goal or direction
- Does NOT always lead to increased complexity
Survival of the Fittest
"Survival of the fittest" does not mean the strongest or most aggressive. "Fitness" in biology refers specifically to reproductive success - the ability to survive long enough to reproduce and pass genes to the next generation. A fit organism is one well-suited to its specific environment.
SAT/ACT Connection
Science passages may present data on population changes over time and ask you to identify which type of selection is occurring, explain why certain traits increase in frequency, or distinguish natural selection from other evolutionary mechanisms like genetic drift.
Examples
Example 1: Peppered Moths
Problem: Before the Industrial Revolution, light-colored peppered moths were common in England. After factories darkened tree bark with soot, dark moths became more common. Explain this using natural selection.
Step 1: Identify variation - moths existed in both light and dark colors.
Step 2: Identify selective pressure - bird predation, with visibility depending on background color.
Step 3: Before pollution: Light moths camouflaged on light bark; dark moths eaten more.
Step 4: After pollution: Dark moths camouflaged on dark bark; light moths eaten more.
Step 5: Dark moths had higher survival and reproduction (fitness).
Answer: Natural selection favored dark moths in polluted environments because they were better camouflaged from predators, leading to higher survival and reproduction rates. This is an example of directional selection.
Example 2: Antibiotic Resistance
Problem: A population of bacteria is exposed to an antibiotic. Most bacteria die, but a few survive and multiply. After several generations, most bacteria in the population are resistant. Explain this process.
Step 1: Variation existed - some bacteria had mutations conferring antibiotic resistance.
Step 2: The antibiotic created a strong selective pressure.
Step 3: Non-resistant bacteria died; resistant bacteria survived.
Step 4: Survivors reproduced, passing the resistance trait to offspring.
Step 5: Over generations, resistant bacteria became the majority.
Answer: This is natural selection in action. The antibiotic didn't cause the resistance - it selected for pre-existing resistant individuals. The resistance mutation existed before antibiotic exposure.
Example 3: Identifying Selection Type
Problem: In a population of lizards, both the smallest and largest individuals survive a drought because small ones can hide in cracks to find water and large ones can travel far to find it. Medium-sized lizards die at higher rates. What type of selection is this?
Step 1: Identify which phenotypes are favored - both extremes (small and large).
Step 2: The intermediate phenotype (medium size) is selected against.
Step 3: This pattern matches disruptive selection.
Answer: This is disruptive selection because both extreme phenotypes have higher fitness than the intermediate phenotype.
Example 4: Darwin's Finches
Problem: On one Galapagos island, finches with medium-sized beaks dominated the population. After a severe drought, only plants with very hard seeds or very soft seeds survived. How would you expect the finch population to change?
Step 1: Hard seeds require large, strong beaks to crack.
Step 2: Soft seeds can be eaten by birds with small, delicate beaks.
Step 3: Medium-beaked birds cannot efficiently eat either seed type.
Step 4: Selective pressure favors both large and small beak extremes.
Answer: The population would likely undergo disruptive selection, with an increase in both large-beaked and small-beaked finches, and a decrease in medium-beaked individuals.
Example 5: Giraffe Necks
Problem: Explain how natural selection could lead to the evolution of long necks in giraffes. Address a common misconception.
Step 1: Variation: Ancestral giraffe populations had individuals with varying neck lengths.
Step 2: Selective pressure: Competition for food, especially during dry seasons when leaves are scarce.
Step 3: Individuals with longer necks could reach more food and had higher fitness.
Step 4: Over many generations, alleles for longer necks became more common.
Common misconception: Giraffes did NOT stretch their necks and pass on stretched necks (Lamarckism). The variation in neck length already existed in the population.
Answer: Natural selection favored individuals with longer necks because they could access more food. These individuals survived and reproduced more, passing on alleles for longer necks. Over many generations, this led to the long-necked giraffes we see today.
Practice
Test your understanding of natural selection with these questions.
1. Which of Darwin's postulates states that organisms produce more offspring than can survive?
A) Variation B) Heritability C) Overproduction D) Differential survival
2. What is biological "fitness"?
A) Physical strength B) Reproductive success C) Running speed D) Intelligence
3. In a population of mice, brown mice survive better than white mice in a forest environment. Over time, the proportion of brown mice increases. This is an example of:
A) Stabilizing selection B) Directional selection C) Disruptive selection D) Artificial selection
4. For natural selection to occur, variation must be:
A) Acquired during an organism's lifetime B) Heritable C) Beneficial in all environments D) Caused by the environment
5. Human birth weight shows stabilizing selection because:
A) Very large babies are healthiest B) Very small babies are healthiest C) Medium-sized babies have the best survival rates D) All babies have equal survival rates
6. Antibiotic-resistant bacteria become common in hospitals because:
A) Antibiotics cause bacteria to mutate B) Bacteria learn to resist antibiotics C) Pre-existing resistant bacteria survive and reproduce D) Doctors create resistant bacteria
7. Which statement about natural selection is FALSE?
A) It acts on individuals B) It can lead to adaptation C) It changes individual organisms D) It requires heritable variation
8. An adaptation is:
A) Any trait an organism has B) A trait that increases fitness in a particular environment C) A trait acquired during an organism's lifetime D) Always beneficial in all environments
9. Natural selection works on:
A) Individuals B) Populations C) Species D) Ecosystems
10. A population of rabbits has individuals with short, medium, and long fur. After a period of climate change toward colder temperatures, what would directional selection predict?
A) More rabbits with medium fur B) More rabbits with short fur C) More rabbits with long fur D) Equal proportions of all fur lengths
Click to reveal answers
- C) Overproduction - This postulate recognizes that organisms produce more offspring than the environment can support.
- B) Reproductive success - In biology, fitness refers to an organism's ability to survive AND reproduce, passing genes to the next generation.
- B) Directional selection - One phenotype (brown) is favored, shifting the population in one direction.
- B) Heritable - Only heritable traits can be passed to offspring and change population genetics over time.
- C) Medium-sized babies have the best survival rates - Very small babies may be premature or underdeveloped; very large babies face birth complications.
- C) Pre-existing resistant bacteria survive and reproduce - Resistance mutations exist before antibiotic exposure; antibiotics select for these individuals.
- C) It changes individual organisms - Natural selection does NOT change individuals; it changes population frequencies over generations.
- B) A trait that increases fitness in a particular environment - Adaptations are context-dependent, beneficial in specific environments.
- A) Individuals - Selection acts on individual variation, but evolution occurs in populations over time.
- C) More rabbits with long fur - In colder temperatures, long fur provides better insulation, increasing survival and reproduction of long-furred individuals.
Check Your Understanding
1. Explain the difference between natural selection and evolution.
Reveal Answer
Natural selection is a mechanism or process that causes evolution. Evolution is the change in allele frequencies in a population over time - the result. Natural selection is one way evolution can occur, but evolution can also happen through genetic drift, gene flow, and mutation. Think of it this way: natural selection is like a driver; evolution is the journey that results from driving.
2. Why can't individuals evolve, only populations?
Reveal Answer
Evolution is defined as a change in allele frequencies in a population over generations. An individual organism's DNA does not change during its lifetime (except through mutations that usually don't spread). While selection acts on individuals (some survive, some don't), the effect of selection - changing the proportion of alleles in the gene pool - can only be observed across a population over multiple generations.
3. A pesticide is applied to a field, killing 99% of insects. The surviving 1% reproduce, and their offspring are resistant to the pesticide. Did the pesticide cause the resistance?
Reveal Answer
No, the pesticide did not cause the resistance. The resistant mutation already existed in the population before the pesticide was applied. What the pesticide did was create a selective pressure that killed non-resistant individuals and allowed resistant ones to survive and reproduce. The resistance allele was already present (perhaps rare), and selection made it common. This is a crucial distinction in understanding natural selection.
4. Explain why a trait that is advantageous in one environment might be disadvantageous in another.
Reveal Answer
Fitness is environment-specific. A trait that increases survival or reproduction in one context may decrease it in another. For example: thick fur helps mammals survive cold climates but could cause overheating in hot climates; dark coloration provides camouflage on dark backgrounds but makes organisms visible on light backgrounds; antibiotic resistance may have a metabolic cost that reduces fitness when antibiotics are absent. This is why natural selection doesn't create "perfect" organisms - the best traits depend on environmental conditions, which can change.
🚀 Next Steps
- Review any concepts that felt challenging
- Move on to the next lesson when ready
- Return to practice problems periodically for review