Speciation Ch 24 p Modes of Speciation When gene flow is interrupted, new species arise
1) Allopatric Speciation: geographic separation of a popltn results in new species i.e.: through genetic drift 2) Sympatric Speciation: biological changes create new species in same area i.e.: chromosomal changes, nonrandom mating Conditions for Allopatric Speciation
1) Geological processes i.e.: formation of mountains, lakes, etc 2) Colonization of new area by individuals from another population If popltn is small & isolated, chances of allopatric speciation Ring Species (Allopatric)
Geographically isolated species may sometimes reconvene If they can not interbreed, speciation has occurred! Adaptive Radiation (Allopatric)
Evolution of many species from one common ancestor Individuals become isolated from parent popltn i.e.: by migration, wind dispersal, etc Develop different adaptations to new environment Evolution of Barriers Geographic isolation reproductive isolation
Reproductive barriers often occur by chance 1) Prezygotic i.e.: fruit fly eating habits 2) Postzygotic i.e.: sterile mule Sympatric Speciation New species arise from parent popltn due to reproductive barriers i.e.: polyploidy (plants), resource differences, sexual selection Polyploidy in Plants (Sympatric)
Autopolyploid: >2 sets chromosomes from single species i.e.: due to faulty meiosis Offspring usually infertile Causes reproductive isolation & interrupts gene flow Polyploidy in Plants (Sympatric)
Allopolyploid: extra chromosome sets from 2 different species i.e.: species interbreed & combine chromos. Offspring sterile, but may be hearty Asexual reproduction may lead to fertile polyploids Speciation!! Sympatric Speciation in Animals
Reproductive isolation is due to: 1) differences in resources required i.e.: food, host etc. 2) Nonrandom mating i.e.: sexual selection Punctuated Equilibrium Model
Species diverge in spurts of relatively rapid change (geologically speaking) Determined by fossil evidence in strata layers From Speciation Macroevolution
Microevolution: change over generations in popltns gene frequencies By genetic drift or natural selection Speciation: popltns genetic divergence results in reproductive isolation Macroevolution: cumulative change due to millions of speciations Evolutionary Novelties
Characteristics that define taxonomic groups i.e.: feathers, 4 legs, etc Evolve by: 1) Simpler structures transforming into complex ones i.e.: eyeballs from photoreceptors 2) Exaptations: structures that perform different functions in new environment i.e.: lightweight bird bones from land animals Heterochrony (Evo-Devo)
Evolutionary change in rate/timing of development 1) Allometric Growth: genes that control development determine specific adult form i.e.: rate, timing, spatial patterns 2) Paedomorphosis: sexually mature organisms retain ancestral juvenile structures i.e.: gills on mature salamander Homeotic Genes Determine the placement of basic features
i.e.: wings, legs Hox Genes: determine placements in embryos i.e.: whether leg or fin will develop Evolutionary Trends Fossil records indicate certain trends in evolutionary lineage i.e.: towards larger/smaller species Species Selection: long-living & greatly reproducing species determine evolutionary trends Greatest natural selection influences See Fig on p. 487