Principle of ClassificationPrinciple of Classification

Humans primarily emphasize traits that can be Humans primarily emphasize traits that can be seen with their eyesseen with their eyes

Biologists also classify organisms into different Biologists also classify organisms into different categories mostly by judging degrees of categories mostly by judging degrees of apparent similarity and difference (greater the apparent similarity and difference (greater the degree of physical similarity, the closer the degree of physical similarity, the closer the biological relationship)biological relationship)

Researchers begin their classification by looking Researchers begin their classification by looking for anatomical features that appear to have the for anatomical features that appear to have the same function as those found on other species same function as those found on other species (determining whether or not the similarities are (determining whether or not the similarities are due to an independent evolutionary due to an independent evolutionary development or to descent from a common development or to descent from a common ancestor)ancestor)

Homologies – are anatomical features of Homologies – are anatomical features of different organisms, that have a similar different organisms, that have a similar appearance or function because they were appearance or function because they were inherited from a common ancestor that inherited from a common ancestor that also had themalso had them

For example: the forelimb of a bear, the For example: the forelimb of a bear, the wing of a bird, and your arm have the wing of a bird, and your arm have the same functional types of bones as did our same functional types of bones as did our shared reptilian ancestorshared reptilian ancestor

There can also be non homologous structural similarities There can also be non homologous structural similarities between speciesbetween species

Homoplastic structures can be the result of parallelism, Homoplastic structures can be the result of parallelism, convergence, or mere chanceconvergence, or mere chance

Parallelism or parallel evolution is similar evolutionary Parallelism or parallel evolution is similar evolutionary development in different species lines after divergence development in different species lines after divergence from a common ancestor that did not have the from a common ancestor that did not have the characteristic but did have an initial anatomical feature characteristic but did have an initial anatomical feature that led to itthat led to it

Convergence or convergent evolution is the Convergence or convergent evolution is the development of a similar anatomical feature in distinct development of a similar anatomical feature in distinct species lines after divergence from a common ancestor species lines after divergence from a common ancestor that did not have the initial trait that led to itthat did not have the initial trait that led to it

Both parallelism and convergence are thought to be due Both parallelism and convergence are thought to be due primarily to separate species lines experiencing the primarily to separate species lines experiencing the same kinds of natural selection pressures over long same kinds of natural selection pressures over long periods of timeperiods of time

Analogies are anatomical features that Analogies are anatomical features that have the same form or function in different have the same form or function in different species that have no known common species that have no known common ancestor.ancestor.

For example: the wings of a bird and a For example: the wings of a bird and a butterfly are analogous structures because butterfly are analogous structures because they are superficially similar in shape and they are superficially similar in shape and functionfunction

Problems in Classifying OrganismsProblems in Classifying Organisms

Species are physically and genetically diverseSpecies are physically and genetically diverse In newly discovered organism determining the specific In newly discovered organism determining the specific

characteristics that actually distinguish it from all other characteristics that actually distinguish it from all other types of organisms types of organisms

Splitter approach – first defines new species based on Splitter approach – first defines new species based on minor differences between organismsminor differences between organisms

Lumper approach – emphasizes major differencesLumper approach – emphasizes major differences Comparisons of DNA sequences are now becoming Comparisons of DNA sequences are now becoming

more commonly used as an aid in distinguishing speciesmore commonly used as an aid in distinguishing species Morphological characteristics is the most commonly Morphological characteristics is the most commonly

used criteria for identifying species differencesused criteria for identifying species differences

Linnean scheme for classification of living Linnean scheme for classification of living things lumps organisms together based on things lumps organisms together based on presumed homologiespresumed homologies

more homologies two organisms share, the more homologies two organisms share, the closer they must be in terms of closer they must be in terms of evolutionary distanceevolutionary distance

Hierarchical system of classification with Hierarchical system of classification with the highest category consisting of all living the highest category consisting of all living things. The lowest category consists of a things. The lowest category consists of a single speciessingle species