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The Origin of Species chapter 24 Campbell and Reece

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Speciation process by which one species splits into 2 or more species Speciation explains both the diversity of life and the unity of living things.

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Speciation : forms bridge between: MICROEVOLUTION Evolutionary change below species level Example: change in allele frequencies in population over generations MACROEVOLUTION Evolutionary change above the species level Examples: origin of new group of organisms impact of mass extinctions

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Biological Species Concept Species: a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring members of a species cannot produce viable, fertile offspring with other groups emphasizes the separateness of species due to reproductive barriers

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What holds the gene pool of a species together? Gene Flow: transfer of alleles between populations of same species exchange of alleles tends to hold populations together genetically

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Reproductive Isolation existence of biological barriers that keep members of 2 populations from interbreeding over long periods of time

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Reproductive Isolation block gene flow between the species & limit formation of hybrids

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Reproductive Isolation hybrids: offspring that result from the mating of individuals from 2 different species or from 2 true- breeding varieties of same species

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Reproductive Isolation single barrier may not prevent all gene flow combination of several barriers can effectively isolate a species gene pool 2 categories: 1. prezygotic barriers 2. postzygotic barriers

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Prezygotic Barriers block fertilization from happening by: 1. impeding members of different species from attempting to mate 2. preventing attempted mating from being completed successfully 3. hindering fertilization if mating was completed successfully

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Postzygotic Barriers reproductive barrier that prevents hybrid zygotes produced by 2 different species from developing into viable, fertile adults 1. lethal developmental errors 2. infertility in viable offspring

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Types of Prezygotic Reproductive Barriers 1. Habitat Isolation 2 species that occupy different habitats w/in same area may rarely interact example:

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Types of Prezygotic Reproductive Barriers 2. Temporal Isolation: species breed during different times of day, different seasons, or different years eastern & western spotted skunks: 1 breeds in late summer other in late winter

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Types of Prezygotic Reproductive Barriers 3. Behavioral Isolation: courtship rituals used to attract mates are effective barriers

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Types of Prezygotic Reproductive Barriers 4. Mechanical Isolation: morphological differences prevent successful completion even if attempted

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Types of Prezygotic Reproductive Barriers 5. Gametic Isolation: sperm of 1 species may not be able to fertilize egg of another: reproductive tract hostile to sperm sperm does not have enzymes to penetrate zona pellicida of another species

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Types of Postzygotic Reproductive Barriers 1. Reduced Hybrid Viability: hybrids development or survival is impaired

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Types of Postzygotic Reproductive Barriers 2. Reduced Hybrid Fertility: hybrids may develop and be healthy but they are not fertile

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Types of Postzygotic Reproductive Barriers 3. Hybrid Breakdown: Some 1 st generation hybrids are fertile but those offspring are feeble or sterile

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Species There is no single, universally applicable species concept that can define, explain, and identify all species. There are multiple ways to think about & define species.

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Biological Species Limitations unable to use these characteristics on fossils of extinct species only applies to organisms that reproduce sexually only applies where there is no gene flow

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Other Definitions of Species these dfns emphasize the unity w/in a species. 1. morphological species concept: characterizes a species by a structural feature applies to species that reproduce sexually or asexually how scientists distinguish most species disadvantage: subjective

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Morphological Species Concept Problems: domestic dogs may look very different but are still same species mouse lemurs look very similar but there are 18 species of them grey mouse lemur lesser mouse lemur

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Ecological Species Concept views species in terms of its niche the sum of how members of the species interact with the nonliving & living parts of their environment asexual or sexual species emphasizes role of disruptive NS as organisms adapt to different environmental conditions

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Phylogenetic Species Concept defines species as smallest group of individuals that share a common ancestor, forming one branch on the tree of life determining degree of differences is difficult

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Species There are >20 other ways to define species

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Speciation can take place with or w/out geographic separation Speciation can occur in 2 main ways: 1. Allopatric Speciation 2. Sympatric Speciation

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ALLOPATRIC SPECIATION other country gene flow is interrupted when population is divided into geographically isolated subpopulations

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Allopatric Speciation Process once geographic separation has occurred, the separated gene pools will each have their own mutations NS & genetic drift may alter allele frequencies in different ways in each subpopulation group

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Allopatric Speciation

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Evidence of Allopatric Speciation are many studies & examples supporting this type speciation indirect support: regions that are isolated or highly subdivided have more species than regions w/out those features

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Drosophila Experiment

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Sympatric Speciation same country occurs in populations in same geographic area less common than allopatric occurs if gene flow is reduced by factors like: polyploidy habitat differentiation sexual selection

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POLYPLOIDY means extra sets of chromosomes can occur in animals gray tree frog ( Hyla versicolor) around Great Lakes

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Polyploidy much more common in plants estimate: 80% of todays plants species have ancestors that formed by polyploid speciation 2 forms

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1. Autopolyploid individual has >2 chromosome sets all derived from a single species plant polyploidy

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Plant Polyploidy tetraploid plant can produce fertile tetraploid offspring by self- pollinating or mating with other tetraploids

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2. Allopolyploid 2 different species interbreed making a hybrid hybrid reproduces asexually over generations sterile hybrid fertile polyploid (called an allopolyploid)

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Allopolyploids can breed with each other but not with either of their parents so are a new species rare: 5 new plant species since 1850 documented Mimulus peregrinus

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Allopolyploids include many agricultural crops Triticum aestivum (bread wheat) has 6 sets chromosomes (2 pair from each of 3 parents), an allohexaploid 1 st polyploidy event probably occurred ~8,000 yrs ago as spontaneous hybrid

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Top 2 parents bottom: Triticum aestivum

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Allopolyploids plant geneticists create new polyploids making hybrids with desired characteristics use chemicals that induce meiotic & mitotic errors

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Habitat Differentiation Sympatric speciation can occur when genetic factors enable a subpopulation to exploit a habitat or resource used by the parent population

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Rhagoletis pomonella North American apple maggot fly original habitat was the native hawthorn tree

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Habitat Differentiation apples mature faster than hawthorn fruit, NS has favored flies with rapid development have an allele that benefits flies that feed off only 1 or the other not both (post-zygotic barrier to reproduction) flies feeding on apple trees now show temporal isolation from flies still eating hawthorn fruit (prezygotic restriction to gene flow)

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Sexual Selection can also drive sympatric speciation: cichlid fish Pundamilia pundamilia

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Cichlid Fish >600 species found in Lake Victoria originated in past 100 000 yrs hypothesis : subgroups of original population adapted to different food sources genetic divergence female preference for mates may also be a factor: 1 species breeding males have blue back another species has orange back

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Breeding Cichlids Colors

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Sexual Selection Study placed the 2 subspecies in same tank used monochromatic orange light so both appeared very similar females bred with either conclusion: mate choice by females is based on male coloration so its the main reproductive barrier (prezygotic behavior)

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Allopatric & Sympatric Speciation are the 2 main modes of speciation Allopatric geographic isolation NS genetic drift sexual selection Sympatric requires emergence of a reproductive barrier that isolates a subgroup less common polyploidy sexual selection

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Hybrid Zones A region in which members of different species meet & mate, producing at least some offspring of mixed ancestry. Hybrids are more likely to die as embryos & show a variety of morphological abnormalities

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Hybrids of yellow-bellied & fire-bellied toads Bombino bambino Bombino variegata Hybrids are more likely to die as embryos & show a variety of morphological abnormalities

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Hybrid Zones

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Hybrid Zones over Time barriers between parent species may increase (strengthening of reproductive barriers) eventually hybrids no longer formed

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Hybrid Zones over Time Or barrier weakens 2 species fuse

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Hybrid Zone over Time 3 rd possibility: status quo is stabilized and hybrids continue to be produced

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Speciation Time Course Patterns in the Fossil Record Punctuated Equilibrium: geologic periods of apparent stasis then sudden dramatic changes demonstrated by some species other species only show gradual changes coined Gradualism

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Punctuated Equilibrium

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Punctuated Equilibrium & Gradualism

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Speciation Rates

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Hybrid sunflower that diverged reproductively from parents to form new species over short time period F1 generation 5% fertility F5 generation >90% fertile

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Genetics of Speciation fundamental question for biologists: How many genes must change to form a new species?

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Single Gene Examples found in a few cases: Euhadra snail : single gene resulted in mechanical barrier to reproduction

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Small # of Genes 2 monkey flower species have different pollinators = strong barrier to cross-pollination Mimulus lewisii Mimulus cardinalis

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Large #s of Genes Hybrid of 2 species of fruit fly: Drosophila pseudoobscura results from gene interactions among at least 4 loci

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Large #s of Genes Western sunflower: hybrid species thrives in dry sand but 2 parents live in nearby moister environments

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