Evidence for punctuated equilibrium
Cheetham has studied in detail the evolution of a group of the sessile aquatic
invertebrates called Bryozoa (also called Polyzoa). He concentrated on a genus
called Metra Rabdotus; some members of the genus are alive in the seas today
and they also have an extensive fossil record.
The stratigraphic record is reasonably complete: the average time interval
between sample populations was 160 000 years, and the completeness was
0.63 for time units of 160 000 years. He measured up to 46 morphological
characters per specimen in a total of about 1000 specimens, from about 100
populations. He concluded that these bryozoans mainly evolved in the
punctuated equilibria model.
Most of the species did not change in form over long periods of several million
years, and most of the new species appeared suddenly without intermediate
transitional populations. If there were intermediate forms they lasted (on average)
less than 160 000 years.
An example of gradualism: trilobites
The extinct arthropod group of trilobites is classified by external morphological
features.Sheldon made a rigorous biometrical study of their evolution at a site in
Wales. He measured the number of pygidial ribs in 3458 specimens from eight
generic lineages, taken from seven stratigraphic sections. The total time period
spanned by the sections is about 3 million years.In all eight genera, the average
number of pygidial ribs increased through time, and in all eight the evolution was
gradual as the (following) figure shows. A population at any one time was
usually intermediate between the samples before and after it. The study looks
like a good illustration of gradual evolution.
Cheetham has studied in detail the evolution of a group of the sessile aquatic
invertebrates called Bryozoa (also called Polyzoa). He concentrated on a genus
called Metra Rabdotus; some members of the genus are alive in the seas today
and they also have an extensive fossil record.
The stratigraphic record is reasonably complete: the average time interval
between sample populations was 160 000 years, and the completeness was
0.63 for time units of 160 000 years. He measured up to 46 morphological
characters per specimen in a total of about 1000 specimens, from about 100
populations. He concluded that these bryozoans mainly evolved in the
punctuated equilibria model.
Most of the species did not change in form over long periods of several million
years, and most of the new species appeared suddenly without intermediate
transitional populations. If there were intermediate forms they lasted (on average)
less than 160 000 years.
An example of gradualism: trilobites
The extinct arthropod group of trilobites is classified by external morphological
features.Sheldon made a rigorous biometrical study of their evolution at a site in
Wales. He measured the number of pygidial ribs in 3458 specimens from eight
generic lineages, taken from seven stratigraphic sections. The total time period
spanned by the sections is about 3 million years.In all eight genera, the average
number of pygidial ribs increased through time, and in all eight the evolution was
gradual as the (following) figure shows. A population at any one time was
usually intermediate between the samples before and after it. The study looks
like a good illustration of gradual evolution.
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