The Origin of Species 2 December, 2005 Text Chapter 24

Historically, definitions of species have been based on appearance. Modern definitions focus on reproductive isolation. A species is a population or group of populations whose members have the potential to interbreed to produce viable, fertile offspring. Different species, then, cannot interbreed under natural conditions.  Breeding involves two individuals meeting at the same place and at the same time.  The behavior of mating individuals must generate sexual attraction.  The genitalia of the individuals must be mechanically compatible.  Gametes must recognize and fuse to produce a zygote.  The zygote must develop into a fertile adult.

Each of the steps in the production of viable fertile offspring can be a potential barrier to the interbreeding of different species. Habitat isolation and temporal isolation prevent individuals from different species from meeting. (examples: parasites on different host species, periodic cicadas) Behavioral isolation prevents individuals from attracting a mate from another species. (examples: bird song, courtship rituals, firefly flash codes)

Mechanical isolation occurs when the reproductive anatomy of different species is incompatible.

Gametic isolation prevents gametes form different species from fusing to form a zygote, even if they do happen to meet. (examples: aquatic and terrestrial spawning organisms)

Post zygotic reproductive isolation can arise when hybrid zygotes (arising from interspecific mating)  Fail to develop (example: Rana spp.).  Have reduced fertility (example: mules).  Produce a non-viable second generation (hybrid breakdown)

Summary and review of reproductive barriers.

The classification of species by reproductive isolation does not work for all organisms.  Extinct species are classified on the basis of morphology alone.  Reproductive isolation is not meaningful for species that reproduce completely asexually: prokaryotes, some plants animals, and fungi.  Speciation in progress is difficult to classify.

Investigating the genotype of individuals can answer questions about phylogeny (relatedness). Liquify mite Purify DNA PCR Mt COI gene

Species that do interbreed will (by definition) exchange DNA. Gene flow will keep even variable sequences similar within a species. Populations that do not exchange genetic information will have divergent sequence, especially for variable regions.

Sequence analysis allows detailed phylogenic relationships to be determined.