TOEFL iBT Reading Practice: Main Idea, Detail, and Inference

Read an Academic Passage items test more than surface-level recall: they ask you to identify a passage's main idea (its central claim), distinguish that claim from the supporting details and examples used to back it up, infer meaning you weren't told directly, and determine a word's meaning from how it's actually used in context rather than its most common dictionary definition. A common trap is treating one vivid supporting example as if it were the passage's whole point, when it's really just one piece of evidence for a broader claim.

Below is an original academic-style passage. Practice all four skills against it.

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Passage (original, academic style -- genetic evidence and the domestication of maize):

Maize, known outside North America as corn, presents one of the most thoroughly documented case studies in plant domestication. Genetic and archaeological evidence now converges on a single origin: a wild grass called teosinte, native to the Balsas River valley in what is now southern Mexico, domesticated roughly nine thousand years ago. This convergence was not always so clear. Through much of the twentieth century, teosinte's candidacy was widely doubted, largely because its physical appearance differs dramatically from modern maize -- teosinte produces a handful of small, hard kernels enclosed in a tough casing, while maize produces hundreds of soft, exposed kernels packed onto a single large cob. Some researchers instead proposed that maize descended from an unidentified, now-extinct wild ancestor, reasoning that the anatomical gap between teosinte and maize was too large to have been closed by early farmers selecting among naturally occurring variants.

Genetic research beginning in the 1930s, and expanded dramatically by DNA sequencing techniques from the 1990s onward, eventually settled the debate. Researchers identified a small number of genes -- particularly one now called teosinte branched1 -- where a handful of mutations produce outsized effects on plant architecture, including the number and arrangement of kernels. This finding mattered because it demonstrated that the dramatic visual difference between teosinte and maize did not require thousands of small genetic changes; a comparatively small number of mutations, each with a large effect, could plausibly have been selected by early farmers over a few hundred generations, well within the timeframe archaeological evidence suggests domestication took.

This case is now frequently cited in introductory genetics courses specifically because it illustrates how a small number of genes with disproportionately large effects can produce rapid, dramatic change under selection -- a pattern that runs counter to an older assumption that major anatomical shifts must always accumulate gradually through many genes of small effect.