1. Principle of Classification

2.  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)

3.  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

4.  There can also be non homologous structural similarities 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

5.  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

7. 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

8.  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 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