The Origin of Species.
Two patterns of speciation.
1.
Anagenesis—phyletic evolution, the accumulation of heritable changes in a population transforming the population into a new species.
2.
Cladogenesis—branching evolution, a new species arises from a population that buds from a parent species.
Cladogenesis is responsible for biological diversity.
Species—Latin for kind or appearance.
A species is defined as a population or groups of populations whose members have the potential to interbreed with each other in nature, producing viable, fertile offspring.
Any thing that helps prevent two species from reproducing hybrid species is a reproductive barrier. While a barrier may impede reproduction it will not necessarily prevent reproduction all together.
Biological barriers to reproduction do not include geographic isolation. Reproductive isolation typically prevents organisms from reproducing even though their ranges may overlap.
Prezygotic barriers—cause a hindrance in mating between species or fertilization of the ova.

Postzygotic barriers—if an egg is fertilized postzygotic barriers prevent the zygote from being viable or fertile.
Prezygotic barriers
Habitat isolation—species live in different habitats in the same area. Insects feeding on different plants.
Behavioral isolation—behavior unique to one species is not recognized by other species. Signals that attract mates may not be recognized by similar species.
Temporal isolation—breeding during different times of day, seasons, or years limits the ability to mate with different similar species. The range may overlap but due to mating times the species “can’t mix their genes”
Mechanical isolation—Anatomical incompatibility.
Gametic isolation—Gametes will not fuse to form a zygote.
Many are due to receptor molecules around the egg.
Postzygotic barriers
Reduced hybrid viability—Genetic incompatibility may abort the hybrid zygote during an embryonic stage.
Reduced hybrid fertility—if two species mate and the offspring is viable it may be completely or largely sterile.
Typically failure of meiosis to produce normal gametes

Hybrid Breakdown—First generation hybrids are viable and fertile but subsequent generations may not be viable or fertile.
Biological Species Concept has limitations.
1.
No way to check for inbreeding in extinct species.
2.
Most living species are distinguished by morphology.
3.
Asexual life forms are distinguished by morphology.
Alternative Concepts of Species
1.
Ecological species concept—defines species by ecological niche. Or its adaptations to its environment.
2.
Pluralistic species concept—combination of reproductive isolation and ecological niche.
3.
Morphological species concept—defines species by features typically physical. Predominant method for identifying organisms.
4.
Genealogical species concept—species as a group of organisms with a unique genetic history.
Identification of species rather then evolutionary descent, however larger ‘families’ can be grouped.
MODES OF SPECIATION
Two modes of speciation, allopatric and sympatric

Allopatric speciation—occurs with organisms that are geographically separated.
Sympatric speciation—occurs with organisms in populations that share the same geographic location.
Allopatric speciation
1.
Population becomes isolated from main population.
2.
Over a period of time the gene flow is reduced or eliminated and new species can evolve.
3.
If the two populations are brought back together and they begin mating successfully again then speciation did not occur. If they do not resume mating then speciation did occur.
Be sure to review 470 to 472 for examples of speciation.
Sympatric speciation
1.
Requires some sort of reproductive barrier in a few individuals within the main population
2.
This isolation can be caused by genetic mutation, change of habitat, food source, or other resource, which leads to a smaller gene pool in a larger gene pool.
In plants sympatric speciation can be caused by a mutation leading to polyploidy.
One type is Autopolyploidy this is caused most often by a mutation that changes the plant from diploid (2n) to tetraploid (4n).

Another more frequent method is allopolyploid, where two species combine gametes and the offspring typically is sterile. If the plant is more successful then its two separate parent species then it may reproduce asexually and eventually a mutation may occur that makes the plant’s offspring may be fertile.
Pages 473 to 475 for examples of sympatric speciation.
Gradualism vs. punctuated equilibrium
1.
Tempo is what is at question.
2.
Evolution takes thousands to 100,000’s of years.
3.
Gradualism—species are always changing slowly in small increments from each generation.
4.
Punctuated Equilibrium—large changes over a short period of time (1000’s of years) then maintain a relative amount stability for long periods of time
(Hundred thousand to Millions of years)
5.
Gradualists claim that stasis (genetic equilibrium) can never occur.
And Now Macroevolution
Macroevolution is the development of general characteristics of groups of populations.
Example—Feathers when did they first occur? Or increase in eye complexity from a patch of cells that acts as a photo receptor to a complex camera type eye.

Most current specialized structures are modified versions of older structures.
Exapations—structures that evolve for one use but are modified for another.