Page 138 - Introduction to Paleobiology and The Fossil Record

MACROEVOLUTION AND THE TREE OF LIFE 125

Height of hyaline area (μm) 5.0 R. bergonii

4.0
3.0

2.0
1.0

0.0 R. praebergonii
Hyaline
3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6
area
Time (Ma)

Gauss Matuyama
Magnetochron

Figure 5.5 Phyletic gradualism and speciation in the planktonic diatom Rhizosolenia. Today there
are two distinct species, R. bergonii and R. praebergonii, that do not interbreed and that differ in
the height of the hyaline area. When tracked back through the past 3.4 myr, the species can be
seen to have diverged through a span of up to 500,000 years, from 3.2 to 2.7 Ma. The plot

shows samples taken from deep-sea boreholes in the central Pacific, and each measurement of the
height of the hyaline area is based on a large sample of hundreds of individuals; the means and
95% error bars for each sample are shown. The rock succession is dated by reference to the
magnetostratigraphic scheme of normal (black) and reversed (white) polarity. (Courtesy of Ulf
Sorhannus.)

The valves of Rhizosolenia are conical in shape, terminating in an apical process that is rooted
in a structure known as the hyaline area. The valves are usually broken at their distal ends, but
Sorhannus and his colleagues were able to measure three distinct biometric variables: the length of
the apical process, the height of the hyaline area, and the width of the valve at an arbitrary 8 μm
from its apex. The fi rst two characters are related to the overall size of the valve; the third is a shape
parameter related to both size and the conical angle of the valve. These measurements were con-
ducted on 5000 specimens in a number of populations in eight different cores, spanning 2 million
years of evolution and about 60° of longitude.
Planktonic diatoms generally reproduce asexually, but like many predominantly asexual organ-
isms they occasionally produce sexual offspring, probably to counteract the buildup of deleterious
mutations (see p. 200). This sexual reproduction means that the large populations of Rhizosolenia
can be considered as biological species, and speciation must be effected by a permanent barrier to
reproduction.
The morphometric data provide convincing evidence that speciation occurred at or before about
3 Ma. Prior to this, there is only one discernible population, but afterwards, two morphologically
distinct populations occur, within which there is a range of intergrading variation, but between which
there is a morphological gap. The distinction is visible in all three measured parameters. The descen-
dant species (R. praebergonii) later invaded the Indian Ocean where it appears abruptly in the sedi-
ment record.
Read more about speciation and punctuated equilibrium at http://www.blackwellpublishing.
com/paleobiology/.