Defines A Monophyletic


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defines a monophyletic
Biological

Species Concept
Over the last few decades the Biological Species Concept (BSC) has become
predominately the dominant species definition used. This concept defines a
species as a reproductive community. This though has had much refinement through
the years. The earliest precursor to the concept is in Du Rietz (1930), then
later Dobzhansky added to this definition in 1937.But even after this the
definition was highly restrictive. The definition of a species that is accepted
as the Biological species concept was founded by Ernst Mayr (1942); "..groups
of actually or potentially interbreeding natural populations which are
reproductively isolated from other such groups" However, this is a definition
on what happens in nature. Mayr later amended this definition to include an
ecological component; "..a reproductive community of populations
(reproductively isolated from others) that occupies a specific niche in nature

The BSC is greatly accepted amongst vertebrate zoologists & entomologists.

Two reasons account for this .Firstly these are the groups that the authors of
the BSC worked with. (Mayr is an ornithologist & Dobzhansky has worked
mainly with Drosophila). More importantly Sexual reproduction is the predominate
form of reproduction in these groups. It is not coincidental that the BSC is
less widely used amongst botanists. Terrestrial plants exhibit much more greater
diversity in their mode of reproduction than vertebrates and insects. There has
been many criticisms of the BSC in its theoretical validity and practical
utility. For example, the application of the BSC to a number of groups is
problematic because of interspecific hybridisation between clearly delimited
species.(Skelton). It cant be applied to species that reproduce asexually ( e.g

Bdelloid rotifers,eugelenoid flagellates ).Asexual forms of normally sexual
organisms are also known. Prokaryotes are also left out by the concept because
sexuality as defined in the eukaryotes is unknown. The Biological species
concept is also questionable in those land plants that primarily self-pollinate.(Cronquist

1988). Practically the BSC has its limitations in the most obvious form of
fossils.-It cant be applied to this evolutionary distinct group because they no
longer mate.( Do homo Erectus and homo sapiens represent the same or different
species?) It also has limitations when practically applied to delimit species.

The BSC suggests breeding experiments as the test of whether a n organism is a
distinct species. But this is a test rarely made, as the number of crosses
needed to delimit a species can be massive. So the time, effort and money needed
to carry out such tests is prohibitive. Not only this but the experiment carried
out are often inconclusive. In practice even strong believers of the BSC use
phenetic similarities and discontinuties for delimiting species. Although more
widely known ,several alternatives to the biological species concept exist. The

Phenetic (or Morphological / Recognition) Species Concept proposes an
alternative to the BSC (Cronquist) that has been called a "renewed
practical species definition". This defines species as; "... the
smallest groups that are consistently and persistently distinct and
distinguishable by ordinary means." Problems with this definition can be
seen ,once again depending on the background of the user. For example
"ordinary means" includes any techniques that are widely available,
cheap and relatively easy to apply. These means will differ among different
groups of organisms. For example, to a botanist working with angiosperms
ordinary means might mean a hand lens; to an entomologist working with beetles
it might mean a dissecting microscope; to a phycologist working with diatoms it
might mean a scanning electron microscope. What means are ordinary are
determined by what is needed to examine the organisms in question. So once again
we see that it is a Subjective view depending on how the biologist wants to read
the definition. It also has similar difficulties to the BSC in defining between
asexual species and existence of hybrids. There are several phylogenetic species
definitions. All of them suggest hat classifications should reflect the best
supported hypotheses of the phylogeny of the organisms. Baum (1992) describes
two types of phylogenetic species concepts, one of thes is that A species must
be monophyletic and share one or more derived character. There are two meanings
to monophyletic (Nelson 1989). The first defines a monophyletic group as all the
descendants of a common ancestor and the ancestor. The second defines a
monophyletic group as a group of organisms that are more closely related to each
other than to any other organisms. So really, the species concepts are only
theoretical and by no means no standard as to which species should be grouped.

However it can be argued that without a more stuructured approached proper
discussion can not occur due to conflicting ... more

defines a monophyletic

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Concept of Species

Over the last few decades the Biological Species Concept (BSC)
has become predominately the dominant species definition used.

This concept defines a species as a reproductive community.

This though has had much refinement through the years. The
earliest precursor to the concept is in Du Rietz (1930), then
later Dobzhansky added to this definition in 1937.But even after
this the definition was highly restrictive. The definition of a
species that is accepted as the Biological species concept was
founded by Ernst Mayr (1942);

"..groups of actually or potentially interbreeding natural
populations which are reproductively isolated from other such groups"

However, this is a definition on what happens in nature. Mayr
later amended this definition to include an ecological component;

"..a reproductive community of populations (reproductively
isolated from others) that occupies a specific niche in nature

The BSC is greatly accepted amongst vertebrate zoologists &
entomologists. Two reasons account for this .Firstly these are
the groups that the authors of the BSC worked with. (Mayr is an
ornithologist & Dobzhansky has worked mainly with Drosophila).

More importantly Sexual reproduction is the predominate form of
reproduction in these groups. It is not coincidental that the BSC
is less widely used amongst botanists. Terrestrial plants
exhibit much more greater diversity in their mode of reproduction
than vertebrates and insects.

There has been many criticisms of the BSC in its theoretical
validity and practical utility. For example, the application of
the BSC to a number of groups is problematic because of
interspecific hybridisation between clearly delimited species.(Skelton).

It cant be applied to species that reproduce asexually ( e.g

Bdelloid rotifers,eugelenoid flagellates ).Asexual forms of
normally sexual organisms are also known. Prokaryotes are also
left out by the concept because sexuality as defined in the
eukaryotes is unknown.

The Biological species concept is also questionable in those
land plants that primarily self-pollinate.(Cronquist 1988).

Practically the BSC has its limitations in the most obvious form
of fossils.-It cant be applied to this evolutionary distinct
group because they no longer mate.( Do homo Erectus and homo
sapiens represent the same or different species?)

It also has limitations when practically applied to delimit
species. The BSC suggests breeding experiments as the test of
whether a n organism is a distinct species. But this is a test
rarely made, as the number of crosses needed to delimit a species
can be massive. So the time, effort and money needed to carry out
such tests is prohibitive. Not only this but the experiment
carried out are often inconclusive.

In practice even strong believers of the BSC use phenetic
similarities and discontinuties for delimiting species.

Although more widely known ,several alternatives to the
biological species concept exist.

The Phenetic (or Morphological / Recognition) Species Concept
proposes an alternative to the BSC (Cronquist) that has been
called a renewed practical species definition. This defines species as;
... the smallest groups that are consistently and
persistently distinct and distinguishable by ordinary means.

Problems with this definition can be seen ,once again depending
on the background of the user. For example ordinary means
includes any techniques that are widely available, cheap and
relatively easy to apply. These means will differ among different
groups of organisms. For example, to a botanist working with
angiosperms ordinary means might mean a hand lens; to an
entomologist working with beetles it might mean a dissecting
microscope; to a phycologist working with diatoms it might mean a
scanning electron microscope. What means are ordinary are
determined by what is needed to examine the organisms in
question. So once again we see that it is a Subjective view
depending on how the biologist wants to read the definition. It
also has similar difficulties to the BSC in defining between
asexual species and existence of hybrids.

There are several phylogenetic species definitions. All of them
suggest hat classifications should reflect the best supported
hypotheses of the phylogeny of the organisms. Baum (1992)
describes two types of phylogenetic species concepts, one of thes
is that A species must be monophyletic and share one or more
derived character. There are two meanings to monophyletic (Nelson

1989). The first defines a monophyletic group as all the
descendants of a common ancestor and the ancestor. The second
defines a monophyletic group as a group of organisms that
are more closely related to each other than to any other organisms.

So really, the species concepts are only theoretical and by no
means no standard as to which species should be grouped. However
it can be argued that without a more stuructured approached
proper discussion can not occur due to conflicting species names.

And so, if there are quite large problems with all of the
species concepts, the question about ... more

defines a monophyletic

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