Speciation and Reproductive Isolation


We now want to begin to address the progression of evolutionary change over the history of life on earth. We will begin this by examining how new species may arise from previously existing species.

What is a species? Fundamental similarity, actually or potentially interbreeding, reproductively isolated.

What is speciation? The evolution of reproductive isolation among once-interbreeding populations.

How is reproductive isolation achieved? Isolating mechanisms

How does reproductive isolation evolve? Speciation mechanisms

Reproductive Isolating Mechanisms

Prezygotic reproductive isolating mechanisms

geographic isolation: potential mates are allopatric
(allopatric = occupying separate geographic ranges)

they do not occupy the same geographic range: narrow-mouth toads, leopard frogs
habitat isolation: potential mates are sympatric on a geographic scale, but occupy different habitats within the common geographic range: Peromyscus maniculatus and P. leucopus

temporal isolation: potential mates occur in the same geographic range and in the same habitat, but are reproductively active at different times of day or in different seasons of the year: Eastern Fox squirrel and Eastern Gray squirrel: Gray squirrel mates in june and july, Fox squirrel in may and june.

behavioral isolation: potential mates occur in the same geographic range and in the same habitat, and are reproductively active at the same time, but individuals of the two species do not recognize one another as potential mates. ie: frog calls, bird songs, visual and vocal displays of many types

mechanical isolation: potential mates occur in the same geographic range and in the same habitat, and are reproductively active at the same time, and are behaviorally compatible, but reproductive organs of the male do not fit into the reproductive tract of the female.

penises must fit into vaginas; many organisms have complex sets of ducts in the female reproductive tract. Bifurcate uterus of opossum; copulatory bursa, spermatheca of insects.

sperm must be transferred and deposited into the proper location within the female's tract

Copulation = insemination = fertilization

 gametic mortality: sperm are transferred but die before fertilizing any eggs
 

Postzygotic reproductive isolating mechanisms

Hybrid inviability: zygote is produced, but dies before developing into a potentially reproductive "organism"

zygote mortality
hybrid mortality
(development is a continuous process from fertilization to death)
Hybrid sterility: Hybrid is viable as far as survival is concerned but is unable to produce any gametes (mules)

Hybrid breakdown (hybrid reproductive disadvantage):

hybrids produce gametes (are fertile) but second generation hybrids are less well able to survive and reproduce than are individuals of either parental species
How will selection act? Selection acts to REINFORCE incipient reproductive isolation Hybrid without reproductive disadvantage:
species or no species? depends on organism we are talking about


Hybridization and introgression:

introgression is the introduction of genes from one species or gene pool into another species or gene pool. (Oak trees, Darwin's finches)

consequences:

increased genetic variability
increased phenotypic variability
increased opportunity for selection

hybridization: in some (plants, some vertebrates): instantaneous (allopolyploid) speciation (see later under speciation mechanisms)