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Species and Speciation

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Evolutionary Species concept
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are their shared past evolutionary
history and their common future evolutionary fate—at least until this species itself
bifurcates to form new descendant species. Notice that this definition is inherently
phylogenetic: species is a group of populations that have a shared past and will
have a shared future on a phylogenetic tree.
phenetic species concept; concept takes advantage of this fact, drawing species boundaries around clusters
of phenotypically similar individuals or populations individuals or populations are highly clustered in phenotype space and use this pattern of clustering to assign species boundariesA similar process
can be applied at higher levels of taxonomic organization to delineate genera,
families, orders, and other taxonomic levels.
how to weigh the relative importance of the characters or traits used to delineate species boundaries.
Should all traits be viewed as equally important in classifying organisms, or should some traits be weighed more heavily because they are particularly important?
Early numerical taxonomists tended to assign equal weights to all characters they
measured, but this approach was quickly abandoned by some, in favor of weighing
certain characters more heavily than others
it is the pattern of gene flow, rather than the pattern of phenetic similarity, that determines species
boundaries diagnosing what constitutes a species, the biological
species approach looks directly to the evolutionary mechanism—gene flow—
responsible for the "shared evolutionary fate" that is fundamental to the concept of
species. As a result, the biological species concept is not based on attributes of the
individuals, but rather it delineates species by properties possessed by populations.
If individuals in one population are capable of mating with individuals in another
population, then individuals in both populations are part of the same species, and
they are said to share the same gene pool. If populations are reproductively isolated
from one another then the individuals in such populations are not considered to be part of the same
Another problem for the biological species
concept is the occasional hybridization events
between individuals in populations that are, for
all practical purposes, reproductively isolated.
If individuals in population 1 consistently mate with those in population 2, individuals in these populations are classified as part of the same
species. But what if matings between individuals in different populations are rare or nonexistent? The two populations can still be part of a common
species, because the biological species concept allows the populations to be potentially interbreeding.
What if the offspring produced by cross-population
matings are nonviable or infertile? In this case, we clearly have two species. But what if the offspring merely have reduced viability or reduced fertility?
How rare do cross-population matings have to be,
and how poorly must the hybrid offspring fare,
before we can say that the two populations are two separate species
Another major limitation of the biological species definition is that it is
restricted to sexual species. With its emphasis on the reproductive isolation of
populations, the biological species concept makes little sense as a species concept
for asexual organisms. As Ernst Mayr notes, "[i]n an asexually reproducing species
every individual and every clone is reproductively isolated. It would be absurd to
call each of them a separate species"

forms that do not meet
many plants with extensive hybridization
Like the phenetic
species concept, this approach looks to character differences in order to distinguish
among species, but it does so in a different way. The basic problem in distinguishing
species remains the same: How do we determine whether two groups are behaving as
evolutionary species that are able to maintain distinct identities so that they have their
own evolutionary histories? If two groups have been separated long enough to have
diverged and produced distinguishing characters, they must have been reproductively
isolated from one another and, as evidenced by these distinguishing characters, they
must have already experienced unique evolutionary histories.