Principios de Taxonomia

4.4 In Sticklebacks (Gasterosteus aculeatus), a Single Gene Controls Many Phenotypesj73
Against all earlier expectations, many genes and their proteins have an astonishing
resemblance even among distant groups of animals. At first glance, one cannot
distinguish whether a genome is from a mouse or human (Prud homme et al., 2006).
For ninety-nine percent of human genes, there is a mouse counterpart. In the
genome, fairly little has changed since approximately 100 million years ago. The
coding sequences from different animals have mostly remained conserved. It is not
the genes, which encode for structural proteins that cause animals to look so
different. Here, the phenotypic appearance is deceiving. How an animal is built
anatomically, which and how many body parts and extremities it develops, which size,
form and color the body parts have, seems to lie predominantly in the modified
expression of the structural genes and not in the genes themselves. Different traits
between two different species do not result from differences in particular genes, but
instead from the fact that the exact same genes are activated in a species-specific
manner according to their local and temporal patterns of gene activity.
Coding sequences only make up approximately one and a half percent of the
genome of animals. Everything else is DNA sequence with other functions or no
function at all. Of this one and a half percent, less than ten percent is actually body
structure genes. The majority of genes are required for the everyday tasks that
regulate basic cellular metabolism and have nothing to do with taxon differences.
These considerations make clear that it is misleading to attempt to find an analog
for a trait pattern differing between two species (phenotype) in the gene pattern
(genotype). The very hope that classifying taxa on the gene level rather than the phene
level would be more exact is called into question. In some cases, a taxonomy based
solely on the similarity of structural genes or on the origin or evolutionary pathway of
these genes would result in a markedly different classification than the currently used
classification of organisms.
Genetic differences between species are complex networks of only a minor
number of genes. It is very difficult to find those genes that are really responsible
for species differences. It is a na€ıve conception to consider species differences as
nothing else than differences in a number of DNA base exchanges of arbitrarily
chosen DNA sequences (see criticism on barcoding below).
4.4
In Sticklebacks (Gasterosteus aculeatus), a Single Gene Controls Many Phenotypes
The Three-spined Stickleback (Gasterosteus aculeatus) occurs in Europe, northern
Asia and North America. It lives in fresh and brackish water close to the shore.
Originally, the Stickleback was a sea dweller. Only after the last glaciation ten
thousand years ago has it populated freshwater. Since then, many phenotypes have
developed that have multiple times been described as different species, variations
and forms, in part or synonymously (Prud homme et al., 2006). The differences in
body structure among the different forms are often larger than the differences
between different genera in other groups of fish. Among the most striking variations
in Sticklebacks are the almost 30-fold differences in size and number of the