How Biologists Classify Organisms

Chapter 14 Section 2

Grade 10 BiologySpring 2011

What is a Species?Biological species: a group of natural

populations that are interbreeding or that could interbreed, and that are reproductively isolated from other such groups

What is a Species?Hybrids: sometimes individuals of

different species interbreed and produce offspring Species are closely related when they can

interbreed and produce fertile hybrids

Evaluating the Biological Species ConceptWorks well for most members of kingdom

AnimaliaSome species are able to form fertile

offspring with closely related speciesFails to describe species that reproduce

asexually Biologist recognize species by studying an

organism’s features

Evolutionary HistoryClassification based on similarities should

reflect an organism’s phylogeny Its evolutionary history

This can be misleading, not all characters are inherited from a common ancestor Wings of bird vs. wings of insect

Evolutionary HistoryConvergent evolution: similarities evolve

in organisms not closely related to one another, often because the organisms live in similar habitats

Analogous structures: similarities that arise through convergent evolution

Evolutionary HistoryDivergent evolution: similarities evolve

in organisms not closely related to one another

CladisticsCladistics: method of analysis that

reconstructs phylogenies by inferring relationships based on shared characters Can be used to hypothesize the sequence in

which different groups of organisms evolved

CladisticsAncestral character: with respect to two

different groups, a character is defined as an ancestral character if it evolved in a common ancestor of both groups Ex. Birds and mammals, backbone is an

ancestral character

CladisticsDerived character: evolved in an

ancestor of one group but not of the other Ex. Feathers evolved in an ancestor of birds

that was not also ancestral to mammals

CladisticsCladogram: branching diagram, shows the

evolutionary relationships among groups of organisms

Evolutionary SystematicsEvolutionary systematics: taxonomists give

varying degrees of importance to characters and thus produce a subjective analysis of evolutionary relationships

Phylogenetic tree: branching diagram based on evolutionary systematics