Evolution & Biodiversity — Science

  1. Grade 7 — Fossils & the Fossil Record
  2. Grade 7 — Natural Selection & Adaptation
  3. Grade 7 — Evidence of Common Ancestry
  4. Grade 7 — Selective Breeding & Biodiversity
  5. Grade 10 — Natural Selection

    ↑ Deepens: natural selection re-examined with genetics underneath it.

  6. Grade 10 — Evidence for Evolution
  7. Grade 10 — Speciation & Mechanisms of Evolution
  8. Grade 10 — Classification & Biodiversity

Apply it in a new setting

Problem 1

A patient takes an antibiotic. At first it kills almost all of the bacteria, but a few survive and multiply, and within days the infection is dominated by bacteria the drug can no longer kill. Which statement best explains what happened?
  1. The antibiotic caused the surviving bacteria to change and become resistant so that they could survive.
  2. A few bacteria already carried resistance genes; the antibiotic killed the rest, leaving the resistant ones to reproduce.
  3. The bacteria sensed the danger from the drug and deliberately developed resistance.
  4. Each surviving bacterium grew tougher during its own lifetime and passed that toughness to its offspring.
Hints

Did resistance appear because of the antibiotic, or was it already present in a few bacteria beforehand?

Natural selection acts on variation that already exists in a population.

The antibiotic selects which bacteria survive; it does not create the resistance.

Answer and explanation

A few bacteria already carried resistance genes; the antibiotic killed the rest, leaving the resistant ones to reproduce.

Resistance came from mutations already present in a few bacteria before the drug was ever used.

The antibiotic removed the non-resistant bacteria, and the pre-existing resistant survivors reproduced.

This is selection of existing variation, not the drug creating resistance — the correct answer is B.

Problem 2

Before the 1800s, most peppered moths in England were light-colored and blended with pale, lichen-covered bark; dark moths were rare. As factory soot darkened the tree trunks, dark moths became far more common than light ones. Which explanation fits natural selection?
  1. The soot settled on the light moths and stained them dark, and their offspring were then born dark.
  2. The moths chose to become darker in order to match the newly polluted trees.
  3. On soot-darkened bark, dark moths were better hidden from birds, so more survived and reproduced, passing on the dark trait.
  4. Pollution simply made all of the moths reproduce less, which somehow increased the number of dark moths.
Hints

What changed in the environment, and how would that change which moths get eaten?

Camouflage against predators such as birds is what drives survival here.

Selection favors whichever color is better hidden on the bark of the moment.

Answer and explanation

On soot-darkened bark, dark moths were better hidden from birds, so more survived and reproduced, passing on the dark trait.

Darkened bark made dark moths harder for birds to spot, so they survived and reproduced more often.

Light moths now stood out and were eaten more, so over generations the dark trait spread.

Differential survival based on camouflage is natural selection — the correct answer is C.

Problem 3

A human arm, a whale flipper, a bat wing, and a cat's front leg all contain the same set of bones in the same basic arrangement, even though they are used for very different jobs (grasping, swimming, flying, walking). What does this shared pattern most strongly suggest?
  1. These animals inherited the same limb blueprint from a shared common ancestor.
  2. These animals all need to perform the same job, so they evolved identical bones.
  3. The similarity is a coincidence and carries no evolutionary meaning.
  4. Each species separately invented the exact same bone pattern to suit its own environment.
Hints

Notice the limbs share a bone pattern but do very different jobs.

Same underlying structure with different functions is called homology.

Ask what a shared blueprint implies about where these animals came from.

Answer and explanation

These animals inherited the same limb blueprint from a shared common ancestor.

The limbs share one bone plan but perform different functions — they are homologous structures.

A shared underlying structure is best explained by inheritance from a common ancestor.

This points to common ancestry — the correct answer is A.

Problem 4

Scientists compare the same stretch of DNA across four species. Species W shares 99% of this sequence with species X, 92% with species Y, and 70% with species Z. Based only on these data, which pair is most closely related?
  1. W and Z, because the largest difference means they must have diverged back toward each other.
  2. W and Y, because 92% is the ideal middle value for judging relatedness.
  3. Relatedness cannot be inferred from DNA comparisons at all.
  4. W and X, because they share the greatest percentage of their DNA.
Hints

More shared DNA generally means a more recent common ancestor.

Which pair has the highest percentage of DNA in common?

Higher similarity points to a closer relationship.

Answer and explanation

W and X, because they share the greatest percentage of their DNA.

The more DNA two species share, the more recently they split from a common ancestor.

W and X share 99%, the highest of any pair listed.

So W and X are most closely related — the correct answer is D.

Problem 5

Sharks (fish) and dolphins (mammals) both have sleek, torpedo-shaped bodies and fins, yet dolphins are far more closely related to cats than to sharks. What best explains why sharks and dolphins look so similar?
  1. They look alike because they share a recent common ancestor.
  2. Living in the same watery environment, both faced similar selection pressures that favored a streamlined shape — convergent evolution.
  3. Dolphins are really a kind of fish, which is why they resemble sharks.
  4. The resemblance proves that body shape has nothing to do with how an animal lives.
Hints

These two animals are only distantly related, so shared ancestry cannot be the reason.

Both must move efficiently through water — what body shape does that favor?

Similar environments can produce similar traits in unrelated groups; this is convergent evolution.

Answer and explanation

Living in the same watery environment, both faced similar selection pressures that favored a streamlined shape — convergent evolution.

Sharks and dolphins are distantly related, so a recent common ancestor does not explain the resemblance.

Both independently evolved streamlined bodies because water favors that shape for fast swimming.

Independent evolution of similar traits under similar pressures is convergent evolution — the correct answer is B.

Problem 6

A single squirrel species lives across a region. A deep canyon forms, splitting it into two groups that can no longer reach each other. Over many thousands of generations the two groups build up different traits, and eventually they can no longer interbreed even if reunited. This process is best described as:
  1. A single population simply growing larger over time.
  2. One species deliberately transforming itself into two in order to fill new roles.
  3. Speciation caused by geographic isolation and the gradual buildup of genetic differences.
  4. Two species that were always separate simply happening to move apart.
Hints

What role does the canyon play in separating the two groups?

Isolated populations evolve independently and can drift apart genetically.

When two groups can no longer interbreed, a new species has formed.

Answer and explanation

Speciation caused by geographic isolation and the gradual buildup of genetic differences.

The canyon geographically isolates the two groups and stops gene flow between them.

Evolving independently, they diverge until they can no longer interbreed — becoming separate species.

This is speciation by geographic isolation — the correct answer is C.

Problem 7

In a beetle population, green beetles tend to live longer than brown beetles, but over their lifetimes the brown beetles each leave far more surviving offspring. In evolutionary terms, which beetles have the higher fitness, and why?
  1. The brown beetles, because fitness is measured by reproductive success — the number of offspring that survive to reproduce.
  2. The green beetles, because fitness means simply living the longest.
  3. The green beetles, because fitness means being the strongest or largest.
  4. They have equal fitness, because in evolution every individual counts exactly the same.
Hints

In biology, 'fitness' has a precise meaning — it is not about strength or lifespan.

Fitness is about how many offspring survive to reproduce.

Which beetle passes more of its genes to the next generation?

Answer and explanation

The brown beetles, because fitness is measured by reproductive success — the number of offspring that survive to reproduce.

Evolutionary fitness measures reproductive success, not lifespan or strength.

The brown beetles leave more surviving offspring, so they pass on more of their genes.

So the brown beetles have higher fitness — the correct answer is A.

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