Transcript How Biologists Classify Organisms

How Biologists Classify
Organisms
Section 14.2
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.
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.
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.
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
Phylogenic Diagram of
Mammals
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.
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.
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.