1. How Biologists Classify Organisms Section 14.2

2. What Is a Species? In 1942, the biologist Ernst Mayr of Harvard University proposed the biological species concept. Mayr defined a biological species as a group of natural populations that are interbreeding or that could interbreed, and that are reproductively isolated from other such groups. Sometimes individuals of different species interbreed and produce offspring called hybrids.

3. Evaluating the Biological Species Concept The biological species concept works well for most members of the kingdom Animalia, in which strong barriers to hybridization usually exist. In practice, modern biologists recognize species by studying an organism’s features.

4. Number of Species The number of species in the world is much greater than the number described. Only about 1.5 million species have been described to date. Scientists estimate that 5 million to 10 million more species may live in the tropics alone.

5. Evolutionary History Classification based on similarities should reflect an organism’s phylogeny - its evolutionary history. Through convergent evolution, similarities evolve in organisms not closely related to one another because the organisms live in similar habitats. Similarities that arise through convergent evolution are called analogous characters. Example – the fin of a shark & a dolphin

6. Phylogenic Diagram of Mammals

7. Cladistics Cladistics is a method of analysis that reconstructs phylogenies by inferring relationships based on shared characters. A character is defined as an ancestral character if it evolved in a common ancestor of both groups. A derived character evolved in an ancestor of one group but not of the other.

8. Cladistics, continued A branching diagram called a cladogram shows the evolutionary relationships among groups of organisms. Organisms that share derived characters, are grouped together on the cladogram. All the characters are given equal importance.

9. Evolutionary Systematics In evolutionary systematics, taxonomists give different degrees of importance to characters. In this type of analysis, evolutionary relationships are displayed in a branching diagram called a phylogenic tree.