Page 134 - Introduction to Paleobiology and The Fossil Record
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MACROEVOLUTION AND THE TREE OF LIFE 121
So what is a species? The commonest defi - simply because part of the former genetic
nition is the biological species concept that range of the intact species has now been
states, “a species consists of all individuals separated off.
that naturally breed together and that produce 2 Selection pressures would be different,
viable offspring”. So, all modern humans can perhaps only subtly, on either side of the
breed together and produce fertile (viable) barrier.
children, and they therefore all belong to one
species. Wolves and domestic dogs are also The separation can cause a divergence in gen-
highly variable in external appearance, and otype, the genetic composition of an individ-
yet they interbreed successfully and so they all ual, population or species, and phenotype, the
belong to the one species Canis lupus. Domes- external appearance.
tic dogs belong to the subspecies C. l. famil- The allopatric model of speciation may
iaris, the European wolf to C. l. lupus, and take two main forms. The process may be
there are many other subspecies of wolves symmetric (Fig. 5.2a), with the ancestral
from other parts of the world. In other cases, species being divided roughly down the middle
the amount of physical variation may seem of its geographic range, and the two daughter
much less; there are certain species of frogs species starting out with similar-sized popula-
and birds, for example, that look identical but tions. More dramatic effects may be seen
are differentiated by their songs and never when the split is asymmetric (Fig. 5.2b).
interbreed with a frog or bird with a different Here, a small population, perhaps isolated
song. on an island, evolves independently of the
Local populations may be to a great extent parent species, which may continue roughly
autonomous, isolated from other populations unchanged. The smaller population may show
of the same species, and with a subtly different unusual and rapid evolution because of what
gene pool, the overall array of genetic material Mayr called the founder effect, the fact that
in all the individuals within the population. its gene pool is a small sample of the overall
The cohesion of a species is maintained over gene pool, and that new environmental pres-
its natural range by processes of gene fl ow, sures and opportunities may occur.
the occasional wandering of individuals from
one area to another, which interbreed with
members of neighboring populations. These
processes can be thought of as occurring on Speciation and evolution in the fossil record
many different scales, ranging from the whole Biologists generally assumed that speciation
Earth for humans, to a tiny patch of forest for happens gradually, with new species branch-
some insect species. ing off from their ancestors slowly. Up to
If species can show considerable, or little, 1970 this view was accepted by most paleon-
physical variation, and they can be held tologists, but then everything changed.
together by gene fl ow, how do they split? The Eldredge and Gould (1972) proposed an
process of splitting of a population to form alternative to the gradual model of evolution,
two species is speciation, and there are many which they termed the punctuated equilib-
models. The most convincing is the allopatric rium model. They argued that the fossil record
(“other homeland”) or geographic model that does not show evolution occurring in species
was proposed in the 1940s by Ernst Mayr, lineages: in fact, they argued, most species
based on the establishment of geographic bar- lineages show stasis (“standing still”, i.e. no
riers. He suggested that populations could be change) over long spans of time. Change
split and gene flow prevented by a barrier, occurs at the time of speciation. Eldredge and
such as a new strip of water, a new mountain Gould contrasted the two evolutionary models
chain or even the building of a major road – in terms of the shape of a phylogeny:
anything that stops free genetic mixing among
populations. The separated populations would 1 In the phyletic gradualism model (Fig.
then diverge for two reasons: 5.3a), with sloping branches, most evolu-
tion takes place within species lineages,
1 Each population, or set of populations, and speciation events involve no special
would start out with a different gene pool, additional amount of evolution;
