The Origin of Species

Chapter 24Chapter 24

BCOR 012BCOR 012

Jan. 31 and Feb. 2, Jan. 31 and Feb. 2, 20112011

Outline for January 31, Feb. 2, 2011:

I. IntroductionII. What is a species?

The biological species conceptReproductive isolating mechanismsAlternative species concepts

III. Modes of speciationAllopatricSympatricthe tempo of change

What is a species?

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

Biologists distinguish between prezygotic and postzygotic isolating mechanisms.

ex. fireflies

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

• Phylogenetic species concept - emphasis is on ancestor-descendent relationships

• Pluralistic species concept - acknowledges that, where species concepts are concerned, one size may not fit all!

There are an awful lot of living things; how did

there get to be so many?

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

Modes of Speciation

There are two general modes of speciation:

• Allopatric (other homeland)- speciation takes place in populations with geographically separate ranges

• Sympatric (same homeland) - speciation takes place in geographically overlapping populations

S N

Ammospermophilus harrisi Ammospermophilus leucurus

Allopatric speciation of antelope squirrels in the Grand Canyon

In In allopatric speciationallopatric 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 ancestralspecies may evolve as a by-product of genetic change.

Sympatric speciation - a mode in which a new species arises in thegeographic midst of its progenitorspecies.

Copper-tolerant Mimulus guttatus

In plants, sympatric species may arise by polyploidya condition that results in extra sets of chromosomes

in the derivative species.

An example from the European holly ferns ….

P. aculeatum

European holly ferns (Polystichum)

P. lonchitis

P. setiferum

The origin of European Polystichum aculeatum

P. aculeatum

P. setiferum P. lonchitisprimary diploid

hybrid(2X)

(4X)

(2X)

The origin of European Polystichum aculeatum

P. aculeatum

P. setiferum P. lonchitisprimary diploid

hybridP. setiferum n=41

Sympatric speciation in animals -East African Cichlids(sick-lids)

Pundamilia pundamilia

a) Normal light

b) Monochromatic orange light

Non-random mating, in which females select mates having a preferred appearance, is the main reproductive barrier keeping these two species separate in nature. This is an example of sympatric speciationsympatric speciation in response to sexual selection.

Pundamilia nyererei

Sympatric speciationSympatric 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.

Hybrid Zones

The Tempo of Evolution

Darwin’s original idea was that

species diverge gradually.

More recently, the idea that there are rapid speciation events

interrupting periods of no change has become

popular.

Punctuated equilibria vs. gradual divergence

http://evolution.berkeley.edu/evosite/evo101/VIIA1bPunctuated.shtml

http://evolution.berkeley.edu/evosite/evo101/VIIA1bPunctuated.shtml

Punctuated equilibria vs. gradual divergence

Possibly include spur story

Pollinator shifts drive increasingly long nectar spurs in columbine flowers. Whittall and Hodges, 2007 ---- Nature 447: 706-710.

Outline for January 31, Feb. 2, 2011:

I. IntroductionII. What is a species?

The biological species conceptReproductive isolating mechanismsAlternative species concepts

III. Modes of speciationAllopatricSympatricthe tempo of change