Concepts of Biology (BIOL115) - Dr. S.G. Saupe (ssaupe@csbsju.edu); Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321

Speciation

I.  Some Definitions.

• Macroevolution - large scale evolutionary changes; formation of new groups above the species level.

• Microevolution - smaller scale changes; changes in gene frequency in a gene pool

II.  Speciation.  
    What is a species?  There are two general criteria: (1) individuals that look alike belong to the same species (morphological species concept); and (2) individuals that can interbreed and produce fertile offspring, and that are isolated from other individuals, comprise a species (biological species concept).  Although these generally work to delimit a species, there are some problems.  For example, Eastern and Western meadowlarks look identical but are separate species.  And, some species don't sexually reproduce like prokaryotes.  And, we can only guess about the bedroom behaviors of extinct organisms like dinosaurs (who knows what went on behind the closed doors of time?).

    Subspecies, variety, races - distinct groups within a species

III.  Reproductive Barriers - maintaining genetic integrity of species.  
    A key feature of a species is that it is reproductively isolated from other individuals. Any intrinsic factor that prevents two species from producing fertile hybrids is a reproductive barrier.  Reproductive isolation can be achieved through mechanisms that are pre-zygotic or post-zygotic. 

A.  Pre-zygotic Barriers.  
    These prevent formation of zygote, even though the ranges of the species may overlap.  These include:

1. Mechanical isolation -  results from differences in size and shape.  For example, the sex organs would be incompatible in terms of size (i.e., chihuauha vs Great Dane; orchids and pollinators);

2. Gametic isolation - the sperm and egg are incompatible;

3. Behavioral isolation - unique mating signals or courtship rituals;

4. Temporal isolation - separation in time such that reproduction occurs at different times of the day, or season or even years.  For example, consider the time of day when Evening primrose or Morning glory flowers. The Western spotted skunk mates late summer, whereas the eastern spotted skunk mates in late winter.

5. Ecological isolation - within an area, species occupy different habitats, thus they don't have opportunity to reproduce.

6. Geographic isolation -  live in different areas

B.  Post-zygotic Barriers.
    These prevent hybrid from from developing into adult, mating or producing fertile offspring.

1. Hybrid inviability - the hybrid doesn't survive. 

2. Hybrid infertility - the hybrid is sterile.  For example, a cross between horses and donkeys results in a mule which is sterile.

IV.  Types of Speciation. 

A.  Allopatric (allo = different; patria = fatherland).  
    Speciation due to geographic isolation; populations have separate geographic ranges due to some geographical boundary (i.e., isthmus of Panama, creeping glaciers, emerging mountain range).  Speciation occurs because isolated gene pools accumulate microevolutionary differences through genetic drift (i.e., founder/bottleneck effect) and natural selection. 

    Once separated, gene flow between individuals of the populations stops and drift and natural selection act on the smaller populations.  These can evolve at a rapid rate.

    Examples:  colonization of islands such as the Galapagos and Hawaii are great examples.  Desert pupfish is another. 

    Adaptive radiation - emergence of numerous species from a common ancestor introduced to diverse environments.

B.  Sympatric Speciation (sym = same)
    Reproductive isolation occurs without geographical isolation.  A barrier develops, usually genetic, that isolations species within a range.  Common in plants.  Doubling of chromosomes (polyploidy) will result in sympatric speciation. Due to non‑disjunctions.  Mutations can also result in sympatric speciation (i.e., a mutation may result in a change of animal behavior and hence create a reproductive barrier).