The Origin of Species Chapter 16 Botany 4

Outline I. Introduction II. What is a species? Anagenesis vs. cladogenesis The biological species concept Reproductive isolating mechanisms Alternative species concepts III. Modes of speciation Allopatric Sympatric the tempo of change

What is a species?

Anagenesis is change within a lineage; cladogenesis is the divergence of one lineage into two.

The Biological Species Concept The biological species concept defines a species as a population or a series of populations whose members have the potential to interbreed and which are reproductively isolated from other such populations.

Reproductive Isolation and Isolating Mechanisms

ex. fireflies Biologists distinguish between prezygotic and postzygotic isolating mechanisms.

ex: mule

Limitations of the BSC: • Not applicable to asexually reproducing organisms • Useless with respect to the fossil record

Alternative species concepts: • Ecological species concept - defines a species in terms of its ecological niche • Morphological species concept - emphasis is on unique structural features • Genealogical species concept - emphasis is on ancestor-descendent relationships • Pluralistic species concept - acknowledges that, where species concepts are concerned, one size may not fit all!

Modes of Speciation

There are two general modes of speciation: • Allopatric - speciation takes place in populations with geographically separate ranges • Sympatric - speciation takes place in geographically overlapping populations

Ammospermophilus harrisi S Ammospermophilus leucurus N Allopatric speciation of antelope squirrels in the Grand Canyon

In allopatric speciation, speciation a new species originates while geographically isolated from its ancestor. As the new species evolves by genetic drift and natural selection, reproductive isolation from the ancestral species may evolve as a by-product of genetic change.

Sympatric speciation - a mode in which a new species arises in the geographic midst of its progenitor species.

In plants, sympatric species may arise by polyploidy a condition that results in extra sets of chromosomes in the derivative species. An example from the European holly ferns ….

European holly ferns (Polystichum) P. setiferum P. lonchitis P. aculeatum

The origin of European Polystichum aculeatum P. aculeatum (4 X) P. setiferum (2 X) primary diploid hybrid P. lonchitis (2 X)

The origin of European Polystichum aculeatum P. setiferum n=41 primary diploid hybrid P. lonchitis

Sympatric speciation requires the emergence of some sort of reproductive barrier that isolates the gene pool of a population subset without geographic separation from the parent population.