Unit 2 · Topic 2.6 Beta

Adaptations

Populations respond to environmental change through natural selection.

Practice 1: Concept ExplanationPractice 5: Data Analysis

Part 1 · Hook

Why this matters

On a small island in the Galápagos, a drought killed most of the plants that make small, soft seeds. The seed-eating finches that survived were mostly those with deeper beaks, strong enough to crack the large, hard seeds that remained. Their chicks, hatched the next year, had deeper beaks on average than the birds before the drought. No bird changed its own beak. The population changed, because of who survived to breed.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. What is a population bottleneck?

  1. A sharp drop in population size that leaves little genetic variety
  2. A slow, steady rise in population size
  3. A barrier that keeps two species apart
  4. A food shortage that affects only one season
Show the answer

A bottleneck cuts the population to a few survivors, who carry only part of its genes.

  • Correct: A sharp drop in population size that leaves little genetic variety:
  • A slow, steady rise in population size:
  • A barrier that keeps two species apart:
  • A food shortage that affects only one season:

2. How is a drought that happens every few years at uneven gaps classified?

  1. Episodic
  2. Periodic
  3. Random
  4. Constant
Show the answer

Recurring at irregular intervals is episodic.

  • Correct: Episodic:
  • Periodic:
  • Random:
  • Constant:

Part 4 · See it

See it first

Bar chart of beak depth in an invented seed-eating bird. Parents before a drought: 10% at 8 mm, 25% at 9 mm, 35% at 10 mm, 20% at 11 mm, 10% at 12 mm. Offspring after the drought: 4% at 8 mm, 15% at 9 mm, 32% at 10 mm, 30% at 11 mm, 19% at 12 mm. The distribution shifts toward deeper beaks.
Beak depth in an invented seed-eating bird before and after a drought: the offspring generation has more deep-beaked birds. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Individuals in a population differ in heritable traits, such as beak depthsome individuals are better suited than others to the current environment
  2. The environment changes, for example a drought leaves only hard seedsindividuals with suited traits (deep beaks) survive and reproduce more than others
  3. Survivors pass their heritable traits to their offspringthe suited trait becomes more common in the next generation
  4. This repeats over many generationsthe population becomes adapted to its environment; if change is too fast, it may die out instead

Part 6 · Key ideas

Key ideas

  • Natural selection: individuals with heritable traits that suit their environment survive and reproduce more, so those traits become more common in the population over generations.
  • An adaptation is a heritable trait that helps an organism survive and reproduce in its environment, such as a deep beak for cracking hard seeds or thick fur in a cold climate.
  • Natural selection acts on populations over generations. An individual does not adapt during its life; the population shifts because some individuals leave more offspring.
  • Populations need genetic variety to adapt. When the environment changes faster than a population can adapt, the population may decline or die out.

Part 7 · Misconception

A common mistake

The wrong idea: During the drought, each finch grew a deeper beak so it could crack hard seeds.

What actually happens: Beak depth is inherited and does not change because a bird needs it to. Finches that already had deeper beaks survived and bred more, so the next generation had deeper beaks on average.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Model or diagram

Beak depth before and after a drought

Bar chart of beak depth in an invented seed-eating bird. Parents before a drought: 10% at 8 mm, 25% at 9 mm, 35% at 10 mm, 20% at 11 mm, 10% at 12 mm. Offspring after the drought: 4% at 8 mm, 15% at 9 mm, 32% at 10 mm, 30% at 11 mm, 19% at 12 mm. The distribution shifts toward deeper beaks.
LevlPrep original diagram.

1. According to the figure, which beak depth became more common among the offspring than among the parents?

  1. 12 mm
  2. 8 mm
  3. 9 mm
  4. 10 mm
Show the answer

Compare the two bars at each beak depth: the deep-beak bars grew.

  • Correct: 12 mm: Right: 12 mm rose from 10% of parents to 19% of offspring.
  • 8 mm: 8 mm fell from 10% to 4%.
  • 9 mm: 9 mm fell from 25% to 15%.
  • 10 mm: 10 mm fell slightly, from 35% to 32%.

2. Which explanation accounts for the shift shown in the figure?

  1. Each bird’s beak grew deeper during the drought as it cracked hard seeds
  2. Deep-beaked birds cracked hard seeds, survived and bred more
  3. Birds with deep beaks flew in from another island after the drought
  4. The drought caused new beak depths that the parents did not have
Show the answer

Natural selection changes how common existing traits are; it does not make individuals change.

  • Each bird’s beak grew deeper during the drought as it cracked hard seeds: A beak’s inherited depth does not grow to meet a need; the population shifted through who bred.
  • Correct: Deep-beaked birds cracked hard seeds, survived and bred more: Right: survivors with deeper beaks passed the trait to their chicks.
  • Birds with deep beaks flew in from another island after the drought: Nothing in the figure shows birds arriving; the offspring came from the surviving parents.
  • The drought caused new beak depths that the parents did not have: The offspring have the same five depths as the parents, in different shares.

3. What percent of the offspring had beaks 11 mm deep or deeper?

Type a number, then its unit (%). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

Add the two bars: 30%+19%=49%30\% + 19\% = 49\% of offspring, compared with 20%+10%=30%20\% + 10\% = 30\% of parents.

  • Answer: 49 %

4. If the rains return and small, soft seeds become common again for many years, which change in the population is most likely?

  1. Each adult bird grows a smaller beak within one season
  2. The population keeps getting deeper beaks whatever seeds are present
  3. The share of birds with smaller beaks rises again over several generations
  4. Beak depth stops being passed on to offspring
Show the answer

Natural selection follows the environment: when conditions reverse, the favored trait can reverse too.

  • Each adult bird grows a smaller beak within one season: Individual birds do not change their inherited beak depth.
  • The population keeps getting deeper beaks whatever seeds are present: Selection depends on the environment, so it can reverse when the food changes.
  • Correct: The share of birds with smaller beaks rises again over several generations: Right: if small beaks handle small seeds better, selection now favors them.
  • Beak depth stops being passed on to offspring: Beak depth stays heritable whatever the weather.

Data

Moths on dark and light tree bark

Percent of dark-colored moths caught in a forest near a city, by year, as soot from coal smoke first darkened the bark and then was cleaned up (invented data for practice)
YearTree barkDark moths (% of catch)
1850light, covered in pale lichen2
1900darkened by soot70
1950darkened by soot90
1970clean-air laws begin; soot falls85
2000light, lichen returning20

5. Write the setup for the percent change in the share of dark moths from 1950 to 2000, then give the answer, rounded to the nearest whole percent.

Type the setup as on the exam, with brackets and units, then the answer.

Show the answer

Start at 90 (1950): 20−9090×100≈−78%\frac{20 - 90}{90} \times 100 \approx -78\%. The share of dark moths fell by about 78% as the bark grew light again.

  • Setup: 20−9090⋅100\frac{20 - 90}{90} \cdot 100; answer -78 %

6. Which statement best defines an adaptation?

  1. A change an animal makes in its body during its life to cope with a problem
  2. Any trait that an organism has, helpful or not
  3. An inherited trait that helps survival and reproduction
  4. A behavior learned from watching other species
Show the answer

Adaptations are heritable and helpful in the environment where the organism lives.

  • A change an animal makes in its body during its life to cope with a problem: Changes within one life are not passed on, so they are not adaptations in this sense.
  • Any trait that an organism has, helpful or not: An adaptation is a trait that helps in the organism’s environment.
  • Correct: An inherited trait that helps survival and reproduction: Right: it is inherited and it raises survival or reproduction.
  • A behavior learned from watching other species: Learned behaviors are not passed on in genes, so they are not adaptations in this sense.

Part 9 · Summary

Summary

Populations respond to environmental change through natural selection. Individuals differ in heritable traits; those whose traits suit the environment survive and reproduce more, so those traits become more common over generations, and the population becomes adapted. Individuals do not adapt within their lifetime, and populations with little genetic variety, or facing very fast change, may not adapt in time.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

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