Biology Reference
In-Depth Information
(traits shared by two or more taxa and their most recent common ancestor
but not by more distant ancestors) and consideration of a parental pattern
of ancestry and descent. In seeking the most parsimonious explanation for
depicting character changes, recourse to taxonomic “outgroups” is often
needed for investigation of the polarity of characters and their stepwise
changes. This, of course, can be problematic, especially if there is limited
taxonomic material for inspection and “key” linking groups are missing,
i.e. so-called gaps in the evolutionary record. Identification of an evolu-
tionary informative character or “diagnostic” marker will therefore in this
case suggest that A. suum and A. lumbricoides still should be designated
two different species names. The evolutionary species concept defines
a species as a single lineage which maintains its identity from other such
lineages (evolving separately as inferred from tree-building methods
often using probabilistic approaches such as maximum likelihood), and
has a unitary evolutionary role, tendencies and historical fate through
time. So here focus is on evolutionary independence due to unique
features that can be ascribed to the organisms.
The most commonly used concept for a sexually reproducing
organism, however, is the biological species concept proposed by Mayr:
“Species are groups of actually or potentially interbreeding populations,
which are reproductively isolated from other such groups.”
23
As some
organisms can mate but not produce fertile offspring, Bock advocated
a reformulation: “a species is a group of actually or potentially inter-
breeding populations of organisms which are genetically isolated in
nature from other such groups,”
24
emphasizing the lack of gene flow
between populations as central for evolution of species and definition
hereof. Barriers to gene flow, and therefore the evolution of species, can be
pre-zygotic and post-zygotic.
22
Pre-zygotic barriers can be physical or
temporal, i.e. if A. suum and A. lumbricoides are not found in the same host
at the same time, mating cannot take place. They could also be found in
the same host at the same time but be unable to recognize each other, or, if
they do, transfer of male gametes may not take place or eggs may not be
fertilized (gametic incompatibility). The nature of asynchronous mating
populations in Ascaris should also not be forgotten, especially given that
parasites' eggs can remain dormant but viable within the environment for
several years. Another restriction in gene flow can be post-zygotic. If A.
suum and A. lumbricoides can mate, fertilized eggs may die, the produced
off-spring (F
1
hybrid) may be sterile or have reduced viability (fitness) or
lastly the worms may have reduced viability or fertility in F
2
or backcross
generations. According to the biological species concept, cases of cross-
infection or identification of hybrids does therefore not imply that
Ascaris in humans and pigs are one species. In addition, where two
populations are attributed to the same species based on the phenetic or
phylogenetic concepts, which is often the case in “newly developed
