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Chapter 24 The Origins of the Species
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Biological Species Concept
Species: population whose members can interbreed and produce viable, fertile offspring (reproductively compatible) These happy face spiders look different, but since they can interbreed, they are considered the same species: Theridion grallator
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Speciation lineage-splitting event that produces two or more separate species Original species cannot produce viable offspring together or that they avoid mating with members of the other group
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speciation and extinction
Microevolution: changes within a single gene pool Macroevolution: evolutionary change above the species level Speciation explains similarities and differences among species. speciation video speciation and extinction
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New species are formed by being isolated from each other
New species are formed by being isolated from each other. Reproductive isolation barriers that prevent members of 2 species from producing viable, fertile hybrids
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Reproductive Barriers/Isolation
Limits gene flow between species and formation of hybrids
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Types of Reproductive Barriers
Prezygotic: impede mating or hinder fertilization if mating occurs
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Habitat Isolation Ex: garter snakes
Two species occupy different habitats rarely encounter each other Ex: garter snakes aquatic or terrestial
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Temporal Isolation Species that breed during different times of day, different seasons, or different years cannot mix gametes. eastern spotted skunk western spotted skunk mates in late winter mates in late summer
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Behavioral Isolation blue footed boobie
Unique behavioral patterns / rituals isolate species - identifies members of species - attract mates of same species courtship rituals, mating calls blue footed boobie
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Mechanical Isolation Mating is attempted, but morphological differences prevent its successful completion different direction spirals prevent genital allignment
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Gametic Isolation Sperm of one species not able to fertilize eggs of another species Sea urching egg and sperm can’t fuse due to different proteins on gamete surface
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Types of Reproductive Barriers
Postzygotic: prevent a hybrid zygote from developing into viable, fertile adult
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Reduced Hybrid Viability
Genes of different parent species may interact in ways that impair hybrid’s development or survival in its environment. Hybrids don’t complete development and are frail
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Reduced Hybrid Fertility
Robust hybrids are sterile - parent species chrom. # differ - meiosis does not form normal gametes Horse + donkey = mule mules are sterile
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Hybrid Breakdown Some first generation hybrids are viable and fertile, but when they mate with one another, offspring of next generation are feeble or sterile. Hybrid offspring of rice are vigorous and fertile Next generation are small and sterile Species have begun separation
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Haldane’s Rule In the early stage of speciation, if in a species hybrid only one sex is sterile more likely to be the heterogametic sex. (one with two different sex chromosomes) major form of post-zygotic reproductive isolation
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Other Definitions of Species
Morphological : by body shape, size, and other structural features Ecological : by niche/role in community Phylogenetic : smallest group of individuals that share common ancestry, branch on tree of life * holds true for sexual and asexual species*
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Two Main Modes of Speciation
Allopatric Speciation “other” “homeland” Geographically isolated populations Caused by geologic events or processes Mutations occur- evolution by natural selection & genetic drift Reproductive isolation Eg. Squirrels on N/S rims of Grand Canyon Sympatric Speciation “together” “homeland” Overlapping populations within home range (new species within original area-not isolated) Gene flow between subpopulations blocked by: polyploidy sexual selection habitat differentiation Eg. polyploidy in crops (oats, cotton, potatoes, wheat)
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Modes of Speciation
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Allopatric Speciation by Geographical Isolation
Population of wild fruit flies minding their business munching on their bananas Hurricane washes bananas and unhatched fruit flies out to sea. Bananas washes up on island off coast of mainland Fruit flies hatch and live on island Two population too far apart for gene flow between them
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Populations diverge due to different selective pressures on populations
Morphology Food preferences Courtship behaviors Rivers, mountains, continental drift, migration, etc.
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Allopatric speciation of antelope squirrels
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Sympatric Speciation by Reduction of Gene Flow
Populations not totally isolated Populations live at distant locations in geographic range Mating throughout range is not random Causes reduced gene flow
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Sympatric Speciation by Polyploidy
Polyploidy: extra sets of chromosomes (2, 3n, 4n) More common in plants Types Autoploid: 2+ sets of chrom. from a single species - produce fertile offspring Alloploid: 2+ sets of chrom. from different species - produce sterile hybrids- propagate asexually - can become fertile and reproduce with each other but not parent species - represent new biological species
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Sympatric speciation: autoploidy
Same species Error in meiosis
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Sympatric Speciation: alloploidy
chromosomes not homologous (different species) can’t undergo meiosis undergo asexual reproduction new species - diploid number equal to two parent species
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Sympatric speciation by Sexual Selection
Under normal light, two cichlid species are different in coloration, and females mate only with males of their own species. Under orange light, the two species appear identical, and females mate with males of either species, yielding fertile hybrids. Intersexual selection by females based on coloration is the reproductive barrier
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What happens when new or partially formed species come into contact with each other?
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Notice difference in allele frequencies
Hybrid Zone: region where different sp. mate and produce hybrid offspring - Incomplete reproductive barriers Notice difference in allele frequencies
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Change in Hybrid Zones Over Time
Result: 1. reinforcement: barriers strengthened (limit hybrid formation) 2. fusion: barriers weakened (formation of single sp.) 3. stability: barriers same (continuation of hybrids) 1. Three populations connected by gene flow barrier to gene flow established Separated population begins to diverge from other 2 populations Gene flow is re-established in a hybrid zone
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Weakened barriers: grizzly and polar bears hybrid
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Rate of Speciation/Evolution
Punctuated Equilibium Eldridge & Gould Long period of stasis punctuated by short bursts of rapid, significant change Gradualism Common ancestor Slow, constant change
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Remember Evolution begins with small changes in a species that become more and more pronounced eventually leading to macroevolution.
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