According to the passage, measured rates of sediment accumulation decline as the measurement window lengthens principally because

sediment compacts under its own weight as it is progressively buried.

erosion proceeds at a roughly constant rate in most depositional environments.

a longer window necessarily contains more intervals of non-deposition, and longer ones.

strata of greater age are inherently more difficult to date with precision.

The author mentions varved lake deposits primarily in order to

identify a case in which completeness can be established independently, so the gap explanation can be tested.

illustrate an environment in which sediment accumulates unusually rapidly.

establish that annual layering is the only trustworthy means of dating strata.

concede that stasis occurs more frequently than paleontologists had believed.

The author's attitude toward the view that punctuated equilibrium is "a signature of the archive" is best described as

convinced that it accounts for the observed pattern.

dismissive of it as methodologically naive.

neutral, reporting the position without evaluating it.

receptive to its reasoning but unpersuaded that the evidence bears it out.

It can be inferred that the author would most likely agree with which of the following?

Claims about evolutionary rates are meaningless in the absence of an estimate of completeness.

A quantitative rate is trustworthy only insofar as the completeness of the sequence yielding it is known.

Stasis has now been established as a genuine biological phenomenon rather than an artifact of sampling.

The fossil record should be set aside as evidence about the tempo of evolution.

Which of the following, if true, would most strengthen the skeptical position described in the second paragraph?

Sadler's compilation drew on a wider range of depositional environments than any earlier study.

Morphological change proceeds more slowly in marine lineages than in terrestrial ones.

Lineages that appear static in ordinary sequences show rapid change wherever unusually complete sequences are available.

Varved lake deposits have been identified on every continent.

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The Incomplete Archive: Stratigraphy and the Tempo of Evolution

PRACTICE

100:00+4 −1MCQ Single Answer

Reading Passage

The fossil record has long served as the principal archive from which paleontologists reconstruct the tempo of evolutionary change. Yet the archive is not a continuous ledger; it is a heavily edited one. Sedimentary accumulation is episodic, punctuated by long intervals of erosion or of no deposition at all, and the resulting stratigraphic column preserves only a fraction of elapsed time. Peter Sadler's compilation of accumulation rates demonstrated a consequence that is easy to state and difficult to internalize: measured rates of sediment accumulation decline systematically as the span over which they are measured increases, in some settings by five orders of magnitude. Because a longer measurement window necessarily encompasses more hiatuses, and longer ones, the apparent rate is diluted by the gaps it silently contains. The implications for evolutionary inference are corrosive. If the completeness of the record varies with the interval examined, then so does the apparent rate of morphological change. A lineage sampled at closely spaced horizons may appear to fluctuate rapidly and without direction; the same lineage sampled across an entire formation may appear static. Neither impression is straightforwardly a fact about the organisms. Some researchers have accordingly argued that the celebrated pattern of punctuated equilibrium—long stasis interrupted by brief bursts of change—could be generated, at least in part, by a record whose gaps are distributed in precisely the manner Sadler described. On this reading, the pattern is a signature of the archive rather than of the evolution it purports to record. This skeptical position has proven easier to assert than to sustain. Stasis has been documented in sequences whose completeness can be constrained independently—varved lake deposits, for instance, in which annual layering permits time to be counted rather than inferred. Where such control exists, morphological stability frequently persists across intervals long enough that a gap-based explanation becomes strained. The defensible conclusion is narrower than either camp initially advanced: the incompleteness of the record does not manufacture the pattern of stasis, but it does render any *quantitative* claim about evolutionary rates hostage to an estimate of completeness that is rarely available. What paleontology has gained is not a resolution of the tempo debate but a clearer sense of which of its questions the evidence is equipped to answer.

The primary purpose of the passage is to