Speciation

Speciation is the origin of new species  A species is a population or group of populations whose members have the potential to interbreed with one another in nature and produces viable, fertile offspring This definition works only for sexually-reproducing individuals

Diversity Within A Species There is much diversity within a species  A businesswoman in Manhattan could produce offspring with a dairyman in Mongolia

Reproductive Barriers Reproductive barriers prevent organisms from two different species from producing viable, fertile offspring Prezygotic Barriers  Prevent mating between species  Do not allow the egg to be fertilized if members of different species attempt to mate Postzygotic Barriers  Prevent the zygote from developing into a viable, fertile adult if the egg does become fertilized

Prezygotic Barriers Habitat Isolation  Species that live in different habitats within the same area will rarely (if ever) encounter one another Behavioral Isolation  Special signals that attract mates are species-specific  Ie. courtship rituals

Prezygotic Barriers Temporal Isolation  Species that breed during different times of the day, different seasons, or different years cannot mix their gametes Mechanical Isolation  Anatomically incompatible species Gametic Isolation  Sperm of one species can’t survive in the female reproductive tract of another species  Ova may only recognize sperm of their own species

Postzygotic Barriers Reduced Hybrid Viability  Offspring dies during embryonic development Reduced Hybrid Fertility  Even if hybrids survive, they are usually sterile  Mule – horse & donkey Hybrid Breakdown  Even if the first generation is fertile and viable, offspring of the first generation is infertile

Modes of Speciation 1.Allopatric Speciation Speciation that occurs based on the geographic relationship of a new species to its ancestral species (Geographic barrier separates organisms). 2. Sympatric Speciation  Speciation that occurs when a subpopulation becomes reproductively isolated in the midst of its parent population (No geographic barrier)

Modes of Speciation

Allopatric Speciation Occurs as a result of geographical isolation  Mountain range, river, etc. Example: different species of closely- related squirrels on opposite rims of the Grand Canyon

Adaptive Radiation Adaptive radiation is the evolution of many diversely adapted species from a common ancestor  Darwin’s finches  Especially common on island chains (Galapagos, Hawaiian islands)

Sympatric Speciation Sympatric speciation occurs when new species arise within the range of the parent population Reproductive isolation evolves without geographical isolation

Sympatric Speciation Animals may become reproductively isolated within the geographical range of a parent population if genetic factors cause them to become fixed on resources not used by the parent population  Different food source, different breeding grounds, etc. This sets the stage for speciation to occur

The Tempo of Speciation There are 2 alternate views of how quickly speciation occurs:  Gradualism  Punctuated Equilibrium

Gradualism Species, descended from a common ancestor, diverge more and more in morphology as they acquire unique adaptations

Punctuated Equilibrium A new species changes most as it “buds” from a parent species and then changes little for the rest of its existence