Evolution

Where the answer comes from: the passage says the population already "includes both light-brown and dark-brown individuals," that "shell color is inherited," and that birds "hunt by sight." After dark ash covered the ground, the graph shows dark beetles rising from 10% to 85% over 20 generations.

Step by step (the parts of natural selection):

1. Variation: both colors existed before the eruption.
2. Inheritance: color is passed from parents to offspring.
3. Selection: on dark ash, light beetles stand out, so birds eat more of them. Dark beetles are more likely to survive and reproduce.
4. Time: over many generations, the dark-color trait becomes more common in the population.

Answer: On the dark ash, dark beetles were harder for birds to see, so more of them survived and reproduced, passing dark color to their offspring.

Why the others fail: individual beetles cannot change their inherited shell color because they need to (A). The passage says dark beetles existed before the ash, so the ash did not create them (C). Choice D reverses the predator effect: if birds preferred dark beetles, the dark share would fall.

Tip: natural selection acts on variation that already exists. Populations change over generations; individuals do not evolve during their lifetimes.

Where the answer comes from: the table says modern whales have "No hind legs; small pelvic and leg bones inside the body, not attached to the spine." The fossils show that whale ancestors such as Pakicetus had four working legs.

Step by step:

1. A vestigial structure is a reduced body part that had a function in an organism's ancestors but has little or no function now.
2. The whale's hip and leg bones cannot support a leg, but they match the hip bones of four-legged mammals.
3. Over millions of years, as whale ancestors moved into the water, the hind legs shrank (Rodhocetus: short; Basilosaurus: tiny; modern whales: hidden inside the body).

Answer: vestigial structures, reduced leftovers of body parts that were useful to the whale's ancestors.

Check: the bones are not developing into anything new; the fossils show legs getting smaller over time, not larger.

Tip: vestigial structures are evidence of ancestry: an organism would have no reason to build a useless part unless it inherited the instructions from ancestors that used it.

Where the answer comes from: the passage says beak depth "is largely inherited" and that after the drought "most of the food left was large, hard seeds," which deep-beaked birds can crack. The table shows the average beak depth was 9.4 mm before the drought, 9.9 mm in the survivors and 9.8 mm in their offspring.

Step by step:

1. Variation: finches already differed in beak depth before the drought.
2. Selection: with mostly hard seeds left, deep-beaked birds could eat and survive; many shallow-beaked birds died. The survivors' average rose to 9.9 mm.
3. Inheritance: the survivors passed their deep beaks to their young, so the next generation averaged 9.8 mm.

Answer: Finches with deeper beaks were more likely to survive the drought by eating hard seeds, and they passed this trait to their offspring.

Check: the offspring average (9.8 mm) is close to the survivors' average (9.9 mm), which shows the trait was inherited.

Tip: a correct natural selection answer always has three parts: existing variation, unequal survival or reproduction, and inheritance.

Where the answer comes from: the passage says a trait "is shared by every organism whose branch splits off after (above) that point." On the cladogram, the order along the main line is: vertebral column, Lamprey branch, jaws, Shark branch, four limbs, Frog branch, amniotic egg, Lizard branch, hair, Mouse.

Step by step:

1. Find the shark's branch point. The next mark above it is four limbs.
2. Four limbs is shared by every animal above that mark: frog, lizard and mouse. The shark branched off before it, so the shark lacks it.
3. Check the others:
   • Jaws appears before the shark branch, so the shark has jaws too.
   • Amniotic egg appears after the frog branch, so the frog lacks it.
   • Vertebral column is shared by all five animals.

Answer: four limbs.

Tip: on a cladogram, read the traits like a timeline along the main line. Everything above a trait mark has that trait; everything that branched off below it does not.

Where the answer comes from: the passage says "organisms that share a more recent common ancestor (a node closer to the top) are more closely related." On the cladogram, the lizard's branch splits off at the highest node, and the line continues only to the mouse.

Step by step:

1. Find the node where the lizard's branch splits off. It is the top node.
2. The only other animal above that node is the mouse, so the lizard and the mouse share the most recent common ancestor.
3. The frog split off earlier, at a lower node. The frog's common ancestor with the lizard is older, and it is also the ancestor of the mouse.
4. Supporting evidence: the lizard and the mouse share the amniotic egg, a trait the frog lacks. (Mammals' amniotic membranes surround the embryo even though most mammals do not lay eggs.)

Answer: mouse.

Why "frog" is tempting: the frog's name is printed right next to the lizard's, and both are often thought of as "cold-blooded." But a cladogram groups organisms by shared ancestry and derived traits, not by position on the page or by how similar they look.

Tip: to compare relatedness, trace both branches down to where they meet. The higher the meeting point, the closer the relationship.

Where the answer comes from: the table lists the fossil groups from oldest to youngest: Eohippus (55 million years ago), Mesohippus (35), Merychippus (15), Pliohippus (10), Equus (4 to present).

Step by step:

1. Shoulder height: 0.4 → 0.6 → 1.0 → 1.2 → 1.6 m, so horses got taller.
2. Toes on each front foot: 4 → 3 → 3 → 1 → 1, so the number of toes decreased.
3. Teeth: low-crowned in the two oldest groups, then high-crowned from Merychippus on, so teeth changed from low-crowned to high-crowned, not the reverse.

Answer: Over time, horses became larger and the number of toes on each front foot decreased.

Why the others fail: B reverses the toe trend. C reverses the size trend. D reverses the tooth trend; the oldest groups had low-crowned teeth.

Tip: when reading a table of change over time, first confirm which direction is "older." Many wrong answers on the GED simply reverse a trend.

Where the answer comes from: the hypothesis says high-crowned teeth became common "because their ancestors began eating tough grasses as grasslands spread across North America." The table shows high-crowned teeth first appearing in Merychippus, about 15 million years ago.

Step by step:

1. To support a cause-and-effect claim, show that the cause (grasslands spreading) happened at the right time, just before or alongside the effect (high-crowned teeth).
2. Fossil grass pollen and plant remains showing that grasslands became widespread at about the time high-crowned teeth appear would directly link the two.
3. Check the others:
   • Modern horses preferring grass says nothing about when or why the teeth changed millions of years ago.
   • More Eohippus fossils with low-crowned teeth repeat what we already know about one early group.
   • Measuring modern horse heights is unrelated to teeth or diet.

Answer: Fossil pollen and plant remains showing that grasslands became widespread in North America around the time high-crowned teeth appear.

Tip: the best supporting evidence tests the specific link in the hypothesis. Evidence that is true but does not connect the cause to the effect is "true but irrelevant."

Where the answer comes from: the passage says "fewer differences suggest that two species share a more recent common ancestor." The table gives: chimpanzee 0, rhesus monkey 1, horse 12, chicken 13, tuna 21, baker's yeast 44.

Step by step:

1. Sort the differences from smallest to largest: 0, 1, 12, 13, 21, 44.
2. Match the species: chimpanzee, rhesus monkey, horse, chicken, tuna, yeast.
3. Smallest difference = most closely related, so this order runs from most to least closely related.

Answer: chimpanzee, rhesus monkey, horse, chicken, tuna, baker's yeast.

Why it makes sense: this order matches other lines of evidence. Chimpanzees and monkeys are primates like us; horses are mammals; chickens are vertebrates with amniotic eggs; tuna are vertebrates without them; yeast is a fungus. Independent evidence from anatomy, fossils and molecules pointing to the same branching pattern is strong support for common ancestry.

Tip: in "differences" tables, small numbers mean close relatives. In "similarity" tables (percent of DNA shared), large numbers mean close relatives. Check which kind of table you have.

Where the answer comes from: paragraph 1 says the limbs "share the same basic plan because they were inherited from a common ancestor" and calls them homologous structures.

Step by step:

1. Flying and swimming are very different jobs, so the shared bones cannot be explained by a shared job.
2. The best explanation is that bats and whales both descended from an early mammal whose front limb had this set of bones.
3. Over time, natural selection reshaped the same bones in different ways: long thin finger bones in the bat's wing, short flat bones in the whale's flipper.

Answer: Bats and whales inherited the same basic limb plan from a common ancestor.

Check: the passage contrasts this with analogous structures (bird and butterfly wings), which do the same job with different structures. Bat wing and whale flipper are the opposite: different jobs, same structure.

Tip: same bones + different jobs = homologous = common ancestry. Same job + different structure = analogous = evolved separately.

Where the answer comes from: paragraph 2 says "farmers saved seeds only from the plants that had the traits they wanted" and calls this artificial selection.

Step by step:

1. Artificial selection: people choose which individuals reproduce, based on traits people want (big leaves, big flower clusters).
2. Natural selection: no one chooses. The environment (food, predators, climate) determines which individuals survive and reproduce more.
3. What they share: both need inherited variation, and both make chosen traits more common over generations. The farmers did not create new traits; they picked from variation already present in the wild mustard.

Answer: In artificial selection, people choose which individuals reproduce; in natural selection, the environment determines which individuals survive and reproduce more.

Check: the passage's six vegetables all came from one species in only hundreds of years, which shows how strong selection can be when people pick the parents every generation.

Tip: "artificial" = human-made choice; "natural" = nature's "choice" through survival and reproduction.

Where the answer comes from: the passage says "each layer is younger than the layers below it," that the ash bed "lies between Layer 3 and Layer 4," and that the ash is "about 250 million years old."

Step by step:

1. Layer 3 lies below the ash bed, so it formed before the ash fell.
2. Therefore Layer 3, and the amphibian bones in it, are older than about 250 million years.
3. Check the others: Layer 3 sits above Layer 2, so the amphibians are younger than the jawed fish; Layer 4 (reptiles) is above the ash, so it is younger than Layer 3, not the same age.

Answer: The amphibian bones are more than about 250 million years old.

Check: the rule only works because the passage says the layers "have not been folded or overturned." In a folded cliff, the order could be upside down.

Tip: a dated layer (like volcanic ash) works as a bookmark: everything below it is older, everything above it is younger.

Where the answer comes from: the passage says "Even before 2000, a few bacteria in the population carried a gene variant that let them survive the antibiotic" and "Resistance is inherited." The graph shows resistance rising from 5% in 2000 to 38% in 2012.

Step by step:

1. Variation: a few resistant bacteria already existed.
2. Selection: each time the antibiotic was used, it killed most non-resistant bacteria, but the resistant ones survived.
3. Inheritance and reproduction: the survivors divided, and their offspring were resistant too.
4. Result: year after year, resistant bacteria made up a larger share of the population.

Answer: The antibiotic killed non-resistant bacteria, while resistant bacteria survived and passed resistance on as they reproduced.

Check: this is natural selection happening fast, because bacteria can produce many generations in a single day.

Tip: the antibiotic is the selecting force, not the cause of the resistance gene. The variation must already be there.

Where the answer comes from: the passage says "Many finches died during the drought" and the table shows that the survivors averaged 9.9 mm, compared with 9.4 mm before. Beak depth "is largely inherited."

Step by step:

1. The student describes individual birds changing their bodies because of a need. That is not how inherited traits change.
2. What actually happened: birds that already had deeper beaks were more likely to survive; many shallow-beaked birds died.
3. The survivors' average was higher because of who survived, not because any bird's beak grew. Their offspring inherited deep beaks.

Answer: Individual finches do not change their inherited traits because of need; the average changed because finches that already had deeper beaks survived and reproduced.

Check: even if a bird's beak did wear or grow during its life, such changes are not stored in its DNA, so they would not be passed to its offspring.

Tip: watch for the words needed to, in order to, or tried to in an explanation of evolution. They usually signal the need-based (Lamarckian) error.

Where the answer comes from: the graph gives these values: 2000: 5; 2004: 10; 2008: 22; 2012: 38; 2016: 46; 2020: 48.

Step by step: subtract for each 4-year period.

• 2000–2004: 10 - 5 = 5 points
• 2004–2008: 22 - 10 = 12 points
• 2008–2012: 38 - 22 = 16 points
• 2012–2016: 46 - 38 = 8 points
• 2016–2020: 48 - 46 = 2 points

Answer: 2008 to 2012.

Check: on the graph, the segment from 2008 to 2012 is the steepest part of the line, which matches the largest change.

Tip: "increased the most" asks about the change during a period, not the highest value. The highest value (48) is in 2020, but the line was almost flat by then.

Where the answer comes from: the passage says "fewer differences suggest a more recent common ancestor." The table shows humans differ from chimpanzees by 1.2% and from orangutans by 3.1%.

Step by step:

1. 1.2% (chimpanzee) is less than 3.1% (orangutan).
2. So humans share a more recent common ancestor with chimpanzees than with orangutans.
3. Check the others: the table does not say humans came from chimpanzees; gorillas (1.6%) differ more than chimps (1.2%); and the table compares each species only with humans, so it cannot tell us how orangutans and macaques compare with each other.

Answer: Humans share a more recent common ancestor with chimpanzees than with orangutans.

Check: the full order from most to least closely related to humans is chimpanzee (1.2) → gorilla (1.6) → orangutan (3.1) → macaque (7.0).

Tip: a table of differences from one species supports conclusions about that species only. Comparing two other species needs data for that pair.

Where the answer comes from: the passage states the hypothesis: "whales evolved from four-legged mammals that lived on land." The fossils show legs shrinking over time, and modern whales keep "small pelvic and leg bones inside the body."

Step by step:

1. A supporting finding should show a trace of four-legged ancestry in whales.
2. Hind-limb buds in whale embryos are exactly that: the instructions for building legs are still in whale DNA, switched on briefly and then turned off. (This has in fact been observed in dolphin embryos.)
3. The other choices are true or plausible facts about whales or sharks, but they say nothing about whale ancestry. A shark's streamlined shape is an analogous trait shared by unrelated swimmers.

Answer: Early in development, whale embryos grow small hind-limb buds that later disappear.

Check: this finding fits the fossil pattern too: the ancestors had legs, the descendants lost them, and the embryos still show a brief trace.

Tip: to support a hypothesis about ancestry, look for evidence that links the organism to its supposed ancestors (shared bones, embryos, DNA). A fact about the present-day lifestyle is usually irrelevant.

Where the answer comes from: the passage says the table shows differences "from the human sequence," and that "fewer differences suggest that two species share a more recent common ancestor."

Step by step:

1. Tuna, chickens and horses are all living today. A living species cannot be the ancestor of another living species; each has had the same amount of time to evolve since their lineages split.
2. The data measure similarity to humans. They suggest that humans share a more recent common ancestor with horses than with chickens, and with chickens than with tuna.
3. That supports a pattern of branching from common ancestors, like a cladogram, not a ladder in which one modern species turns into the next.

Answer: The conclusion is not supported. The data suggest how recently each species shares a common ancestor with humans; they do not show that one living species evolved into another.

Why the others fail: A accepts the faulty "ladder" reasoning. B is wrong because the data are relevant to relatedness; they just do not support this conclusion. D treats the 1-amino-acid gap between horse (12) and chicken (13) as proof of descent, but closeness to humans says nothing about whether horses came from chickens.

Tip: common ancestry means branching. Humans did not evolve from chimpanzees; both evolved from a shared ancestor. Watch for GED choices that describe evolution as a straight line between living species.

Where the answer comes from: the table lists, from bottom to top: Layer 1 trilobites and shelled sea animals; Layer 2 jawed fish; Layer 3 amphibians; Layer 4 reptiles; Layer 5 mammals. The passage says "each layer is younger than the layers below it."

Step by step:

1. Jawed fish first appear in Layer 2.
2. Amphibians appear in Layer 3, reptiles in Layer 4, mammals in Layer 5. All are higher, so all are younger.
3. So fish fossils are found in older layers than the first amphibian, reptile and mammal fossils.
4. Check the others: trilobites and mammals are in different layers (1 and 5), so they did not live at the same time here; the ash only dates the rock, it does not cause anything; and fossils in neighboring layers do not prove one group turned directly into the next.

Answer: Fossils of fish appear in older layers than the first fossils of amphibians, reptiles and mammals.

Check: this matches the wider fossil record, in which fish appear long before the first four-legged land animals.

Tip: the safest conclusion from a rock column is about order (what came before what). Claims about causes or direct ancestors need more evidence.

Where the answer comes from: the passage says "Each branch point (node) represents the most recent common ancestor of all the organisms on the branches above it."

Step by step:

1. Pine and sunflower: their paths meet at the highest node, where the pine splits off.
2. Pine and moss: their paths meet much lower, at the node where the moss splits off.
3. A higher node is a more recent common ancestor. Pine and sunflower share a more recent ancestor, so they are more closely related.
4. The pine and the sunflower also share more marked traits (embryo protection, vascular tissue, seeds) than the pine and the moss (only embryo protection).

Answer: The pine and the sunflower share a more recent common ancestor than the pine and the moss do.

Check: the pine does not have flowers, because the Flowers mark lies above the pine branch.

Tip: relatedness on a cladogram is about how recently two lineages split, not about which organism "came from" which.

Where the answer comes from: the passage shows that beak depth is inherited and that the average shifted toward deeper beaks (9.4 mm → 9.8 mm) when hard seeds were the main food. The question now gives a new environment in which small, soft seeds dominate and shallow beaks handle them faster.

Step by step:

1. The population still contains variation in beak depth.
2. In the new conditions, shallow-beaked finches get food more efficiently, so they are likely to survive and raise more young.
3. Their offspring inherit shallower beaks, so over several generations the average beak depth would decrease.

Answer: The average beak depth would likely decrease, because finches with shallower beaks would survive and reproduce more.

Check: this kind of back-and-forth has actually been recorded in island finches: average beak size rises in dry years and falls again in wet years.

Tip: natural selection has no goal or fixed direction. When the environment changes, the favored trait can change too.

Where the answer comes from: the table shows an order in time: Pakicetus (50 million years ago, four legs) → Ambulocetus (49) → Rodhocetus (47, short hind legs) → Basilosaurus (40, tiny hind legs) → modern whales (no hind legs).

Step by step:

1. If whales came from four-legged land mammals, fully aquatic legless whales should appear after their legged ancestors.
2. A fully aquatic whale with no hind legs in rocks 70 million years old would be 20 million years older than Pakicetus. That would put the "end result" before the supposed ancestors and break the predicted order.
3. The other choices fit the hypothesis: a 45-million-year-old fossil with medium-sized hind legs fills a gap in the series; land-mammal-like teeth and hair in whale calves both link whales to land mammals.

Answer: A fossil of a fully aquatic whale with no hind legs, found in rock about 70 million years old.

Check: scientists would first re-check the dating, because one finding that conflicts with many others needs strong confirmation, which is why the question says "if confirmed."

Tip: to weaken a hypothesis, look for evidence that the hypothesis says should not exist, such as a descendant appearing before its ancestors.

Where the answer comes from: the passage says "fewer differences suggest a more recent common ancestor." The table orders the species from fewest to most differences: chimpanzee 1.2, gorilla 1.6, orangutan 3.1, macaque 7.0.

Step by step:

1. The macaque (7.0%) shares the oldest common ancestor with humans, so its branch splits off first, at the bottom.
2. Next comes the orangutan (3.1%), then the gorilla (1.6%).
3. The chimpanzee (1.2%) splits off last, so it shares the highest node with humans and sits next to the human branch.

Answer: The macaque's branch splits off first (lowest), and the chimpanzee's branch splits off last, closest to the human branch.

Check: this matches what many other studies have found: chimpanzees and humans are each other's closest living relatives, with gorillas next and orangutans further away.

Tip: to turn a "differences" table into a cladogram, put the most different species at the bottom and the least different species next to the reference species at the top.

Where the answer comes from: the passage says that after "two groups split from a common ancestor, small changes build up in each group's DNA," so "fewer differences suggest a more recent common ancestor." The chimpanzee's 1.2% difference is the smallest in the table.

Step by step:

1. The visitor assumes humans came from chimpanzees. The data say something different: humans and chimpanzees share a common ancestor.
2. After the split, both lineages kept changing. Modern chimpanzees are as "evolved" as modern humans; they are cousins, not ancestors.
3. So there is no puzzle: the common ancestor's population split into two groups, and each group gave rise to a different living species.

Answer: Humans did not evolve from chimpanzees; humans and chimpanzees both descended from a common ancestor, and each line changed separately after they split.

Check: a 1.2% difference across billions of DNA bases still means millions of differences, so the two are clearly separate species.

Tip: living species are never the ancestors of other living species. They are branch tips that meet at shared ancestors further back.

Where the answer comes from: the passage describes physical differences (leg length) and shared ancestry ("descended from the same ancestral population"), and it ends: "The scientists want to know whether the two populations have now become separate species."

Step by step:

1. By the biological species concept, a species is a group whose members can interbreed and produce fertile offspring. Separate species are reproductively isolated.
2. So the strongest evidence is a direct test of reproduction: when lizards from the two islands are brought together, they produce no offspring, or only offspring that cannot reproduce.
3. Check the others:
   • A difference in leg length is already known. Populations of one species can look different (consider the many breeds of dogs, which are all one species).
   • A difference in diet is a behavioral difference that does not show the populations cannot interbreed.
   • Shared ancestry is expected for any two populations that split from one, whether or not they have become separate species. It does not answer the question.

Answer: When lizards from the two islands are placed together, they produce no offspring, or only offspring that cannot reproduce.

Tip: for "strongest evidence" questions, pick the choice that tests the definition directly. Looks, habits and shared history are weaker clues than reproductive isolation.

Where the answer comes from: the stem says the two populations breed six weeks apart, "so in nature the two populations almost never mate with each other," although they still can in a lab.

Step by step:

1. New species form when populations stop exchanging genes (reproductive isolation). The barrier can be geographic, but it can also be timing, behavior or habitat.
2. Here the barrier is time: different breeding seasons keep the two gene pools apart even in the same ponds.
3. With no gene flow, mutations, natural selection and chance can make the populations more and more different. Eventually they might no longer be able to interbreed at all; then they would be separate species.
4. Right now they can still produce healthy offspring, so they are not yet fully separate species.

Answer: Because they breed at different times, their genes rarely mix, so differences could build up until they become separate species.

Check: this is the same logic as island populations separated by water, except that the separation here is in time rather than space.

Tip: for speciation questions, ask "What stops the two groups from interbreeding?" Any barrier that stops gene flow, not only mountains or water, can start speciation.

Where the answer comes from: the graph shows increases of 16 points (2008–2012), 8 points (2012–2016) and 2 points (2016–2020). The passage says the program began in 2014 and "cut the use of the antibiotic roughly in half."

Step by step:

1. Resistance kept rising after 2014 (38 → 46 → 48), but much more slowly. So the data are consistent with the claim.
2. Less antibiotic use means weaker selection for resistant bacteria, so a slowdown makes biological sense.
3. But the graph alone cannot prove cause: the 2012–2016 interval includes two years before the program began, so the graph cannot show exactly when the slowdown started, and there is no comparison hospital without the program. Other factors (for example, better hand-washing) could also explain the change.

Answer: The data are consistent with the claim, because resistance rose more slowly after 2014, but they cannot prove the program caused the slowdown, because there is no comparison group without the program.

Check: "resistance kept rising" does not contradict the claim, which is about the rate of rise, not about resistance going down.

Tip: a change that follows an action in time is evidence that fits a causal claim, not proof of it. Proof needs a controlled comparison.

Where the answer comes from: the stem says river populations "have working eyes and silver-gray skin," cave populations "have no working eyes and almost no skin color," and the cave fish "descended from river fish."

Step by step:

1. The cave fish evolved from ancestors that had eyes and color, and they lost both features.
2. That is evolution (an inherited change in a population over generations) that made the fish simpler, not more complex.
3. One clear counter-example is enough to disprove a claim that uses the word always.
4. In total darkness, eyes give no advantage and cost energy to build, so fish without them may do just as well or better. Evolution favors what works in the environment, not what is more complex.

Answer: The cave fish contradict the claim, because these populations evolved by losing eyes and skin color, becoming less complex.

Check: the change is inherited (shown by the genetic studies), so it counts as evolution, not just a change in individual fish.

Tip: evolution is not a march toward complexity. Parasites, cave animals and flightless island birds have all lost structures that their ancestors had.