Evolution, 4th Edition

494 CHAPTER 19

of species diversification (D), or in the maximum number of species (K) the

regions can support at equilibrium (see Fig ure 18.19). Exactly the same possible

explanations could account for why some clades have more species than others

(FIGURE 19.3).

Studying diversity in the fossil record

Most paleontological studies of diversity employ counts of higher taxa, such

as families and genera, because they generally provide a more complete fos-

sil record than individual species do. Although paleobiologists have used sev-

eral expressions for rates of origination, extinction, and diversification, the most

useful are the numbers per taxon per unit of time.

Because the fossil record is a very incomplete sample of past life, paleobiologists

have developed correction factors to estimate accurately the number of species

alive at different points in the past [20, 57, 67]. For example, rare species are more

likely to be included in large samples, which include more individual organisms,

than in small samples. If we want to compare the species diversity in two samples

that differ in size, we must correct for this problem, perhaps by picking the same

number of specimens at random from all the samples.

In addition, the geological or stratigraphic stages into which each geologi-

cal period is divided vary in duration, and more recent geological times are

represented by greater volumes and areas of fossil-bearing rock. Therefore it

may be necessary to adjust the count of taxa by the amount of time and rock

volume represented. Because fossils constitute a small sample of the organisms

that actually lived at the time they were formed, a taxon is often recorded from

several separated time horizons, but not from those in between. This means the

fossil record of these species is incomplete. In turn, that suggests that the actual

origination of a taxon may have occurred before its earliest fossil record, and

its extinction after its latest record. It follows that if many taxa actually became

extinct in the same time interval, the last recorded occurrences of some are

likely to be earlier, so that their apparent times of extinction will be spread out

over time. Conversely, if many taxa actually originated at the same time, some

of them may appear to have originated at later times.

Since our count of living species is much more complete than our count of

past species, taxa that are still alive today appear to have longer durations and

lower extinction rates than they would if they had been recorded only as fossils.

That is, we can list a living taxon as present throughout the last 10 My, let’s say,

even if its only fossil occurrence was 10 Mya. Because the more recently a taxon

arose, the more likely it is to still be extant, diversity will seem to increase as

we approach the present, even if it didn’t actually increase. This artifact, or bias,

is called the pull of the Recent. (The Recent epoch, more commonly referred to

now as the Holocene [see Ta b l e 17.1] began 12,000 years ago.) The bias can be

reduced by counting only fossil occurrences of each living taxon and not listing

it for time intervals between its last fossil occurrence and the Holocene.

Because of unusually favorable preservation conditions at certain times or

other chance events, a taxon may be recorded from only a single geological

stage, even though it lived longer than that. Such “singletons” make up a higher

proportion of taxa as the completeness of sampling decreases and therefore bias

the sample; moreover, they can create a spurious correlation between rates of

origination and rates of extinction because they appear to originate and become

extinct in the same time interval. Diversity may be more accurately estimated by

ignoring such singletons and counting only those taxa that cross the border from

one stage to another.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_19.03.ai Date 12-09-2016

Species richness

Time

Clade 1

Clade 2

Time for

diversication

T 1 > T 2

(C)

Species richness

Diversication rate

(S 1 – E 1 ) > (S 2 – E 2 )

(B)

Species richness

Species carrying

capacity

K 1 > K 2

(A)

FIGURE 19.3 ust as for contrasts in spe-J

cies richness between different geograph-

ic regions (see Figure 18.19), two clades

can differ in species richness because of

differences in (A) carrying capacity, the

equilibrium number that can stably coexist;

(B) their rate of diversification (specia-

tion rate minus extinction rate); or (C) their

age, meaning the time they have had to

diversify.

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