I.On the Road to Speciation

                        A.What is a Species?

                              1.The morphological species concept expresses the
                                following:
                                   a.Species, in its simplest interpretation, means "kind."
                                   b.Attempting to determine whether similar, yet
                                     different, animals are the same species by
                                     appearance (phenotype) is not reliable due to the
                                     subtle variations that are displayed.

                              2.The biological species concept relies on reproduction to
                                define relatedness of species.
                                   a.Ernst Mayer says, "Species are groups of
                                     interbreeding natural populations that are
                                     reproductively isolated from other such groups."
                                   b.As good as it is, this definition is troublesome for
                                     organisms that are non-sexually reproducing and
                                     those known only from fossils.

                        B.Genetic Change and Speciations

                              1.Speciation is the attainment of reproductive isolation, but
                                genetic changes between populations of the same species
                                can be countered by gene flow.

                              2.Genetic divergence is the process whereby local units of a
                                population become reproductively isolated from other
                                units and thus experience changes in gene frequencies
                                between them.

                   II.Reproductive Isolating Mechanisms

                        A.Reproductive isolating mechanisms are any heritable features of
                           body form, functioning, or behavior that prevent interbreeding
                           between genetically divergent populations.

                        B.Prezygotic Isolation (mechanisms take effect before or during
                           fertilization)

                              1.Temporal Isolation: different groups may not be
                                reproductively mature at the same season, or month, or
                                year (for example: periodical cicadas).

                              2.Behavioral Isolation: patterns of courtship may be
                                altered to the extent that sexual union is not achieved (for
                                example: albatross courtship rituals).

                              3.Mechanical Isolation: two populations are mechanically
                                isolated when differences in reproductive organs prevent
                                successful interbreeding (for example: floral arrangements
                                in sage plants discriminate between different bee
                                pollinators).

                              4.Ecological Isolation: potential mates may be in the same
                                general area by not in the same habitat where they are
                                likely to meet (for example: different species of manzanita
                                shrubs live at different altitudes and habitats).

                              5.Gametic Mortality: incompatibilities between egg and
                                sperm prevent fertilization (for example: signals to pollen
                                grains to begin growing toward the egg).

                        C.Postzygotic Isolation (mechanisms take effect after fertilization)

                              1.Sometimes fertilization does occur between different
                                species, but the hybrid embryo is weak and dies.

                              2.In some instances the hybrids are vigorous but sterile
                                (example: mule produced by a male donkey and a female
                                horse).

                   III.Speciation in Geographically Isolated Populations

                        A.Allopatric Speciation Defined

                              1.In this model some physical barrier arises and prevents
                                gene flow between populations.

                              2.Allopatric refers to the "different lands" the two species
                                occupy.

                              3.Reproductive isolating mechanisms evolve in the
                                genetically diverging populations and will result in
                                complete speciation when the two species can no longer
                                interbreed.

                   IV.Models for Other Speciation Routes

                        A.Sympatric Speciation

                              1.In this model, species may form within the home range of
                                an existing species, in the absence of a physical barrier.

                              2.Literally means "together with others in the homeland."

                              3.In two crater lakes of East Africa exist small fish called
                                cichlids.
                                   a.The species in each lake are alike in their
                                     mitochondrial DNA and unlike the species in
                                     neighboring lakes and streams.
                                   b.The lakes are small so the fish must live in
                                     sympatry.

                              4.Polyploidy is the inheritance of three or more of each type
                                of chromosome due to improper separation of
                                chromosomes during meiosis or mitosis; speciation is
                                instantaneous for plants that are polyploid.

                        B.Parapatric Speciation

                              1.Daughter species form from a small proportion of
                                individuals along a common border between two
                                populations.

                              2.Literally means "near another homeland."

                              3.Interbreeding individuals produce hybrid offspring in this
                                region called a hybrid zone.

                   V.Patterns of Speciation

                        A.Branching and Unbranched Evolution

                              1.Cladogenesis applies to populations that become isolated
                                from one another and subsequently diverge in different
                                directions.
 
                        B.Evolutionary Trees and Rates of Change

                              1.Evolutionary trees summarize information about the
                                continuity of relationship among species.

                              2.The gradual model of speciation is represented by tree
                                diagrams with branches at slight angles to each other to
                                show slow change over time.

                              3.The punctuation model of speciation is drawn with short,
                                horizontal branches that represent abrupt periods of
                                speciation followed by stable periods.

                        C.Adaptive Radiations

                              1.An adaptive radiation is a burst of microevolutionary
                                activity that results in the formation of new species in a
                                wide range of habitats.

                              2.The presence of adaptive zones presents new ways of life
                                by physical, evolutionary, or ecological access.