Tempo and Mode in Evolution Genetics and Paleontology 50 Years After Simpson (1995) / Chapter Skim
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The Role of Extinction in Evolution
The Role of Extinction in Evolution
Pages 109-124
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From page 109... ...
This is surprising in view of the special importance Darwin attached to extinction, and because the number of species extinctions in the history of life is almost the same as the number of originations; present-day biodiversity is the result of a trivial surplus of originations, cumulated over millions of years. For an evolutionary biologist to ignore extinction is probably as foolhardy as for a demographer to ignore mortality.
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From page 110... ...
(ii) Sudden disappearances of many species, now called mass extinctions, did not actually occur.
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From page 111... ...
To be sure, Chapter II includes occasional mention of specific extinctions, but not as significant drivers of evolution. For example, Simpson suggests that mammals with long generation times (equated with large body size)
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From page 112... ...
Mass extinctions are acknowledged, but he follows Darwin in arguing that gaps, saltations, and other abrupt changes in the fossil record should not be taken at face value. Probably there is always a considerable period of time corresponding with the gap in morphology, taxonomy, and phylogeny.
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From page 113... ...
Lack of fossilization inevitably shortens the apparent life span of species, and this may explain why durations of dinosaur species are far shorter than is typical of other, better preserved organisms. On the other hand, short-lived, localized species have a low probability of appearing in the fossil record at all.
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From page 114... ...
The term "mass extinction" is most commonly reserved for the so-called "Big Five" events: short intervals in which 7~95% of existing species were eliminated (Table 1~. The K-T TABLE 1 Comparison of species extinction levels for the Big Five mass extinctions Extinction Age, Myr Percent episode before present extinction Cretaceous (K-T)
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From page 115... ...
Mass extinctions occupy the right-hand tail of the distribution. The mean extinction rate, 25% extinction per 1 Myr, is the approximate reciprocal of the mean species duration (4 Myr)
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From page 116... ...
. The 100-Myr events include the Big Five mass extinctions.
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From page 117... ...
This suggests, among other things, that the big mass extinctions cannot be explained by Darwin's species interactions unless one is willing to postulate an incredible degree of connectedness in the biosphere. A striking effect of the typical mass extinction is its aftermath.
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From page 118... ...
The approach carries the tacit assumption that genealogical relatedness implies similarity of physiology, ecology, or other attributes that determine susceptibility to extinction. Taxonomic selectivity has been documented, but the effect is generally quite small and requires massive samples for confirmation.
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From page 119... ...
Although this is supported by anecdotal data for several extinction events, a recent study based on a global data base (3514 occurrences of 340 genera) found no recognizable geographic pattern in extinction of bivalve mollusks at the end of the Cretaceous, once reef-dwelling rudists were omitted (Raup and Jablonski, 1993~.
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From page 120... ...
Extinction is evidently selective at certain times and places but the effects tend to be subtle and require careful analysis of large data bases. Darwin's contention that all extinction is selective cannot be sustained, although this may reflect only our inability to recognize complex patterns in an imperfect fossil record.
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From page 121... ...
The recent Pleistocene glaciation produced very few complete extincnons of species. To be sure, extinction rates during the last deglaciation were high among large mammals and some bird groups but overall, global data (including marine organisms)
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From page 122... ...
SUGARY The extinction of species is not normally considered an important element of neodarwinian theory, in contrast to the opposite phenomenon, speciation. This is surprising in view of the special importance Darwin attached to extinction, and because the number of species extinctions in the history of life is almost the same as the number of originations; present-day biodiversity is the result of a trivial surplus of originations, cumulated over millions of years.
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From page 123... ...
(ii) The largest mass extinctions produce major restructuring of the biosphere wherein some successful groups are eliminated, allowing previously minor groups to expand and diversify.
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From page 124... ...
M (1991a) A kill curve for Phanerozoic marine species.
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