{ Evolution & Speciation Mechanisms & Models

 Broad definition: species = one or more populations that share a trait derived from a common ancestor which sets them apart from other organisms  Biological Species Concept: a species includes only individuals that can and do produce fertile offspring and are isolated from others. Species defined

 1) one species becomes 2 different species  (the original species goes extinct)  2) one species turns into a new species.  (changes so much it would no longer be able to reproduce with an ancestor if one came forward in a time machine) Speciation: The process by which new species arise

 Adaptive Radiation = one species evolves into many similar species

 #1 Reproductive Isolation of some type  #2 Genetic divergence due to different selective pressures or genetic drift  #3…?.. Evolution of differences that prevent interbreeding even if reintroduced to original home land Speciation Requires

 1) Allopatric speciation = geographic isolation  different homeland, physical barriers prevent reproduction with other parts of a population  (prevent gene flow)  2)Sympatric speciation = reproductive isolation  together homeland, some part of population stops reproducing with another part of the population even though they are nearby  3) Parapatric Speciation = common border only  beside homeland Models of Speciation

 Hybrid organisms produced in border area by interbreeding between 2 species. No real gene flow because hybrids are either …  a) restricted to border land (die elsewhere)  Or  b) sterile and can’t pass on any genes Parapatric Hybrids

 Requires reproductive isolation to occur through differential natural selection or genetic drift.  Lake levels subside to form 2 separate lakes  Human developments separate populations  Canyons/mountains separate populations  Islands separated from mainland by water  (founder effect)  Continents separated by continental drift Allopatric

 Guppies in different pools of the same river.  Pools with predator = select for drab colors  Pools with no predator = select for bright color  Put the two together again they don’t recognize each other and won’t mate Example of Allopatric

 Requires some form of reproductive isolation  1)ecological isolation  2) behavioral isolation  3)temporal isolation  4) polyploid isolation (in plants)  May result from disruptive selection Sympatric

 1) Song birds change their mating song. (behavioral isolation & sexual selection)  2) Female fish select mates that are most similar in color to themselves (sexual selection)  3) One population of flowers blooms earlier than another (temporal isolation)  4) One group of fish feed at the top of the lake and the other group feed at the bottom so they seldom encounter each other and do not mate. (ecological isolation) Examples of Sympatric

 Plants can become 3n or 4n or more n rather than the standard 2n. Polyploid individuals can no longer interbreed with the 2n population = reproductive isolation = new species.  Allopolyploidy = hybridize with other species  Autopolyploidy = just keep two full sets of your own chromosomes Polyploidy & Sympatric speciation

allopolypoidy

 1) prezygotic = before zygote forms  2) Postzygotic = after zygote forms Mechanisms of Reproductive Isolation

 A) Mechanical Isolation….parts don’t fit  Chihuahua and Great Dane  Flowers with specific pollinator  B) Temporal Isolation… mate at dif times  17 vs 13 year cicada  nocturnal vs diurnal  C) Behavioral Isolation….courtship  D) Ecological Isolation…  E) Gamete Mortality Prezygotic Repro Isolation

 A) Embryo mortality  B) Death before maturity  C) Sterility  D) weak hybrid / low survival of hybrids Postzygotic …zygote forms and then….