V) Review Mechanisms of Speciation
A) Three Basic Types
1) Allopatric Speciation evolution of reproductive isolating barriers occur due to an extrinsic or physically isolating barrier.
a) Vicariance geological or physical barrier causes isolation
b) Founder Event rare colonization of new area and highly limited connection to parental population
2) Parapatric evolution of reproductive isolating barriers between geographically localized subsections of a continuously distributed species.
3) Sympatric evolution of reproductive isolating barriers within a randomly mating population.
VI) Hybridization
A) Causes of hybridization
1) two genetically different populations (species?) come into contact.
2) reproductive isolating barriers have not formed or are incompletely formed (i.e., weak)
3) the two populations mix and mating between them occurs.
B) Outcome of hybridization
1) Reinforcement if fitness of hybrid is lower than both parental species, selection will work to strengthen reproductive isolating mechanisms.
a) Gamete wastage since individuals that are involved in interspecies mating will be producing less fit offspring, natural selection will favor the formation of positive assortative mating.
Species Blue Hybrid Red
Fitness 1 + s 1 1 + t
W1 W3 W2
s
and t are positive numbers
b) Character displacement when species overlap, selection can promote the divergence of characters in the area of sympatry.
c) prezygotic isolating mechanisms should precede post zygotic isolating mechanisms and good species formation.
C) Hybrid Zones since hybridization occurs in geographically localized areas (i.e., in sympatry), a region where hybrids are common can be formed (i.e., hybrid zone).
1) Primary Hybrid Zones
a) differential, geographically localized selection causes species to differentiate in situ (i.e., parapatric speciation).
b) hybrids form in area of contact
2) Secondary Hybrid Zone
a) continuously distributed population is divided by a barrier into isolated, geographically localized areas (i.e., refugia)
b) geographically isolated populations differentiate (i.e., allopatric speciation)
c) barrier is
removed and population sizes increase
d) differentiated
populations come back into contact and hybrids form in the area of contact 
D) Dynamics in Hybrid Zones
1) Interplay between the magnitude of dispersal and the intensity of selection determines width of hybrid zone and the fate of hybridization.
2) If hybrids are less fit everywhere relative to parental species then natural selection will reinforce reproductive isolating mechanisms and speciation will be completed
3) If hybrids are equally or more fit everywhere than parental species, then species will merge.
4) If hybrids are less fit in parental ranges and equally fit in hybrid zone, then a stable hybrid zone will form.
5) If hybrids are less fit in parental ranges but more fit than parental types in hybrid zone, then a new, hybrid specie may form.
6) Overall:
E) Neutral Genes, Hybridization and Hybrid Zones
1) Not all genes are involved in the fitness differences in the various geographic regions.
2) Meiotic recombination in hybrids can unlink genes under selection from neutral genes.
3) Genes under selection will show a clinal distribution of alleles
4) Neutral genes may be uniformly distributed throughout the range of the hybridizing species
5) Different genes responding to different selection pressures may show different clines
G) Primary versus Secondary Hybrid Zones
1)
Secondary Hybrid Zone clines neutral and selected gene clines generally are
correlated and located at the point of hybridization since the genetic
differentiation occurred in isolation (i.e., neutral genes drifted while
selected genes were evolving).
1) Primary Hybrid Zone clines only genes responding to the specific selection pressure that produced the species in the first place have clines located at the point of hybridization. Other selected genes will have clines appropriate for the selection pressure and neutral genes wont have clines.
