Evolutionary rates in the elephants

The rates of evolution can be measured in many different ways. Among the elephants (including mammoths), the rate of change of teeth characters has been quantified in detail by Vincent Maglio. Over the first 2 My after their emergence in Africa, all three genera—Loxodonta, Mammuthus, and Elephas— did not differ much in their rates of molar evolution. While Loxodonta continued to be conservative in molar development through the late Pliocene and the Pleistocene, Mammuthus and Elephas then surged ahead, with the woolly mammoth representing the most advanced stage.

A major dental change that characterized the evolution of the elephants was reduction in the grinding component of mastication in favor of the shearing component. A simplified "shearing index," suggested by Vincent Maglio and based on lamellar frequency and molar width, gives one indication of the relative adaptation of dental characters among various species (table 1.2).

Changes in individual molar characters, such as the number of lamellar plates, enamel thickness, and crown height, among the three genera reflect not only the differences in evolutionary rates, but also the nature of adaptive responses to changing ecological conditions encountered (fig. 1.7). It is clear that, in all these characters, the rates of change were much greater in Mammu-thus and Elephas than in Loxodonta, especially from about 3 My ago.

The noted evolutionary biologist J. B. S. Haldane suggested the use of a unit, the darwin, to depict the rate of evolutionary change. A rate of evolution

Table 1.2

Shearing ability of an elephant's third molar calculated from the mean lamellar frequency (LF3 = upper third molar; LF3 = lower third molar) and lower molar width (W3) as (2LF3 X 2LF3 X W3)/1000.

Table 1.2

Shearing ability of an elephant's third molar calculated from the mean lamellar frequency (LF3 = upper third molar; LF3 = lower third molar) and lower molar width (W3) as (2LF3 X 2LF3 X W3)/1000.

Species

LF3

LF3

W3

Shearing Index

Stegotetrabelodon syrticus

3.0

3.1

119.2

4.43*

Stegotetrabelodon orbus

2.8

2.9

97.9

3.18

Primelephas gomphotheroides

3.1

3.3

94.5

3.87

Loxodonta adaurora

3.7

3.4

90.2

4.54

Loxodonta atlantica

4.2

4.6

89.1

6.89

Loxodonta africana

4.1

4.2

75.4

5.19

Elephas ekorensis

4.4

4.0

91.4

6.34

Elephas recki

5.1

5.0

90.6

9.24

Elephas iolensis

4.0

5.3

104.7

8.88

Elephas antiquus

5.6

5.7

75.1

9.59

Elephas planifrons

4.2

4.2

94.0

6.63

Elephas celebensis

7.9

6.2

42.5

8.33

Elephas hysudricus

5.1

5.4

91.7

10.10

Elephas maximus

6.6

7.2

77.0

14.90

Mammuthus subplanifrons

3.5

3.3

94.4

4.36

Mammuthus africanavus

4.1

3.8

91.9

5.37

Mammuthus meridionalis

4.6

4.6

97.2

8.76

Mammuthus armeniacus

6.3

6.5

87.6

14.35

Mammuthus primigenius

8.5

9.0

87.6

26.81

Source: From Maglio (1973). Reproduced with the permission of the American Philosophical

Society. "Corrected from Maglio.

Source: From Maglio (1973). Reproduced with the permission of the American Philosophical

Society. "Corrected from Maglio.

Figure 1.7

Evolutionary changes in plate number, enamel thickness, and hypsodonty index for the molar M of three lineages of elephants: Loxodonta (open circles), Elephas (open squares), and Mammuthus (solid circles). The absolute rate of change, measured in darwins, is also given for each lineage for each of these characters. (From Maglio 1973. Reproduced with the permission of the American Philosophical Society.)

Figure 1.7

Evolutionary changes in plate number, enamel thickness, and hypsodonty index for the molar M of three lineages of elephants: Loxodonta (open circles), Elephas (open squares), and Mammuthus (solid circles). The absolute rate of change, measured in darwins, is also given for each lineage for each of these characters. (From Maglio 1973. Reproduced with the permission of the American Philosophical Society.)

of 1 darwin indicates doubling or halving of a character in 1 My. The rates of change in molar characters averaged around 0.2 darwin in Mammuthus, a little less in Elephas, and under 0.1 darwin in Loxodonta. Considering that all three genera arose in Africa, it is interesting that the rates of evolutionary change are also positively related very broadly to the distance of their eventual dispersal.

Variations in rates of change of particular characters within a lineage have been linked to local selective pressures in some cases. The rapid change in Mammuthus during the middle Pleistocene, for instance, is believed to have been driven in part by the appearance in Europe of Elephas antiquus, a potential competitor. Northern Europe also experienced a major glaciation at this time. With E. antiquus occupying the southern forests, Mammuthus was pushed further north and thus may have adapted more rapidly to the frigid conditions. Adrian Lister now believes that the entry of E. antiquus might have hastened the extinction of M. meridionalis in its woodland refugia. The rapid evolutionary change in the African line of Elephas, culminating in E. iolensis, during the later Pleistocene also coincided with the expansion of a competitor, Loxodonta atlantica, in the same regions. The dwarfing of elephants in several Mediterranean islands proceeded at the remarkable rate of about 10 darwins.

After the origin of the elephant family about 6 My ago, the extinction rates were initially low. The number of species began to increase gradually, concurrent with an increase in average species duration. About 3 My ago, the duration of a species averaged a high of 1.8 My. From the middle Pleistocene, 1 My ago, the elephant family witnessed a rapid phase of evolution with a spectacular increase in the number of species. As the pattern seen toward the end of the Pleistocene was one of "active diversification," Vincent Maglio and others have argued that the equally phenomenal extinctions at the end of the Pleistocene strongly suggest a cause such as human intervention.

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