Principios de Taxonomia

2.14 The Dualism of the Species Concept: the Epistemic vs. the Operative Goalj41
defining daytime by the trait of brightness is wrong. The definition is wrong because
daytime is not what brightness specifies.
Correctly, daytime (as opposed to nighttime) is defined by the position of Earth
relative to the sun, not by its brightness. Daytime is the period of time at which the
surface of the Earth is turned towards the sun. Nighttime is the period of time at
which the surface of the Earth is turned away from the sun. In contrast, brightness is
only a symptom of daytime, and darkness is a symptom of nighttime. These
definitions are operational definitions. Bright light can be present even at nighttime.
However, it can never be daytime in one location if the sun is positioned on the other
side of the Earth. Summer and winter on Earth are defined by the inclination of the
Earth s axis, not by their traits of being warm or cold.
Exactly the same type of reasoning applies to the biological species. The goal of
identifying a given individual as a member of a given species should never be
confused with the goal of knowing that a given group of organisms is a species.
Nothing expresses the difference between diagnostics and ontology better than a
statement by the classical author of evolutionary theory, American paleontologist
George Gaylord Simpson (Simpson, 1961): The definition of monozygotic twins
... provides a homologous causal sequence. ... Two monozygotic twins are
not twins because they are similar but, quite the contrary, are similar because
they are twins.
This quote expresses in a direct manner that on one level, there concern is about
identifying twins. On another level, the quote addresses a markedly different issue,
which is that twins are twins because they originate from a single egg. Twins have
identical traits as a result of being twins, but this is not the ontological definition of
twins because even non-twins can have identical traits. The fact that there are many
field guides on the identification of species on the market, all of which focus on
species diagnostics, may mislead us into falsely believing that diagnostic differences
are ontological differences.
2.14
The Dualism of the Species Concept: the Epistemic vs. the Operative Goal
How can we finally solve the species problem, meaning the worldwide dissension
about what a species is? Different authors mean different things by the word
species (Mallet, 1995). For more than a hundred years, this has led to what is
called the species problem . Disagreements about what a species is have led to deep
dissent, so much so that it is referred to as a never-ending story.
Taxonomists place varying weights on trait differences between organisms,
descent or sexual cohesion between organisms. Why are there approximately
9000 bird species on Earth (del Hoyo, Elliott, and Sargatal, 1992)? It would be easy
and consistent to defend other estimates, resulting in as many as 27 000 species
(Cracraft, 1997). The number depends entirely on the weight given to particular
delimiting criteria. There are many diagnostically easily recognizable subgroups,
distinguishable races, genetically far-distanced groups or allopatrically isolated