Download Principle of Classification

Principle of Classification
 Humans primarily emphasize traits that can be
seen with their eyes
 Biologists also classify organisms into different
categories mostly by judging degrees of
apparent similarity and difference (greater the
degree of physical similarity, the closer the
biological relationship)
 Researchers begin their classification by looking
for anatomical features that appear to have the
same function as those found on other species
(determining whether or not the similarities are
due to an independent evolutionary
development or to descent from a common
ancestor)
 Homologies – are anatomical features of
different organisms, that have a similar
appearance or function because they were
inherited from a common ancestor that
also had them
For example: the forelimb of a bear, the wing
of a bird, and your arm have the same
functional types of bones as did our
shared reptilian ancestor
 There can also be non homologous structural similarities
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between species
Homoplastic structures can be the result of parallelism,
convergence, or mere chance
Parallelism or parallel evolution is similar evolutionary
development in different species lines after divergence
from a common ancestor that did not have the
characteristic but did have an initial anatomical feature
that led to it
Convergence or convergent evolution is the
development of a similar anatomical feature in distinct
species lines after divergence from a common ancestor
that did not have the initial trait that led to it
Both parallelism and convergence are thought to be due
primarily to separate species lines experiencing the
same kinds of natural selection pressures over long
periods of time
 Analogies are anatomical features that
have the same form or function in different
species that have no known common
ancestor.
For example: the wings of a bird and a
butterfly are analogous structures because
they are superficially similar in shape and
function
Problems in Classifying Organisms
 Species are physically and genetically diverse
 In newly discovered organism determining the specific
characteristics that actually distinguish it from all other
types of organisms
Splitter approach – first defines new species based on
minor differences between organisms
Lumper approach – emphasizes major differences
 Comparisons of DNA sequences are now becoming
more commonly used as an aid in distinguishing species
 Morphological characteristics is the most commonly
used criteria for identifying species differences
 Linnean scheme for classification of living
things lumps organisms together based on
presumed homologies
more homologies two organisms share, the
closer they must be in terms of
evolutionary distance
 Hierarchical system of classification with
the highest category consisting of all living
things. The lowest category consists of a
single species