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Ch. 24 The Origin of Species

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Essential Question:

How does a species evolve?

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Two Types of Speciation:

1. microevolution­ adaptations to a single gene pool

2. macroevolution ­ evolutionary change above the species level, including the appearance of major evolutionary developments

ex. flight

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Two patterns of evolutionary development:

1. anagenesis ­ (phyletic evolution) = accumulation of changes that gradually transform a given species into a species with different characteristics

2. Cladogenesis ­ (branching evolution) = splitting the gene pool into two or more separate pools, each giving rise to one or more new species

­leads to diversity, increases # of species

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Species = a group whose members possess similar anatomical characteristics and have the ability to interbreed and produce viable offspring

Biological species concept­ Ernst Mayr ­1942

based on interfertility rather than physical similarity

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Barriers that affect the ability of two species from producing viable, fertile hybrids (reproductive isolation)

classification:according to reproductive isolation before or after fertilization

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1. Prezygotic barriers:impede mating between species or hinder fertilization of ova if members of different species attempt to mate

a. habitat isolation­ two species occupy different habitats within the same area, not isolated from each other, but rarely encounter each other

ex. 2 species of parasites living on different hosts 2 garter snakes, one lives in water, other on land

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b. temporal isolation­ species that breed during different times of the day, different seasons or yearsEx. eastern spotted skunk­ mates in late winter western spotted skunk ­ mates in late summer

eastern spotted skunk western spotted skunk

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c. behavioral isolation ­ courtship rituals that attract mates and other behaviors unique to a species

ex. 1. Blue­footed boobies mate after ritual "high­step" by male

2. song birds with different calls

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d. mechanical isolation ­ structural differences prevent mating

ex. ­flowers that attract different pollinators ­organisms where copulatory organs don't fit together

http://www.fws.gov/pollinators/Index.html

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e. gametic isolation­ sperm of one species not able to fertilize egg of another species; biochemical or ability to survive in reproductive tract

ex. sea urchins ­ release sperm and eggs into water, different species gametes do not fuse together (molecule

recognition)

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ex. of evolution of prezygotic barrier ­ Diane Dodd's exp. with fruit flies

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2. Postzygotic barriers­ after zygote ­ prevents hybrid from becoming a viable, fertile adult

a. reduced hybrid viability ­genes of different parent may interact and impair development

ex. salamanders that live in same area, some may hybridize, but hybrids do not develop or if they do are very sickly

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b. reduced hybrid fertility ­ hybrids that develop may be sterile, chromosomes of two parent species differ in # or structure ­meiosis may not form normal gametes

ex. hybrid offspring of donkey and horse ­ produce sterile mule

+ =

mule

http://www.equusite.com/cgi­bin/pictures/potdArchivesDisplay.cgi?picture_id=8191

http://www.freefoto.com/preview/01­12­14?ffid=01­12­14

http://petcaretips.net/mule­photo.html

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c. hybrid breakdown ­ first generation of hybrids are viable and fertile, but generations after are sterile or feeble

ex. different cotton species produce fertile offspring, next generation­ seeds die or grow to weak and defective

plant

http://encarta.msn.com/media_461562280/Cotton_Plant.html

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ex. of evolution of postzygotic barriersRobert Vickery's experiment

crossed monkey flowers (Mimulus glabratus) from different areas

­found the closer the monkey flowers populations were to each other the better they interbred, produced viable offspring

­if the populations were far apart, offspring not viable, some sterile

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other definitions of a species:1. morphological species concept­ species is characterized by body

shape, size and structural features

2. paleontological species concept ­ morphologically categorizes species based on fossil evidence

3. ecological species concept ­ views species in terms of niche

4. phylogenic species concept (genealogical)­ species is a set of organisms with unique history

compare physical characteristics or molecular sequences

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Two modes of speciation based on how gene flow among populations is interrupted:

1. Allopatric speciation­ gene flow is interrupted when a population is divided into geographically isolated subpopulationsex. ­river changing course forming lakes

river separating two areas­Galapagos finches

­depends on the ability of an organism to move­pollen may not have a problem with this­larger organisms

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to confirm allopatric speciation has made two different species unable to interbreed and produce viable offspring, remate them after and if they will not interbreed, then have allopatric speciation

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2. sympatric speciation ­ speciation takes place in geographically overlapping populations­due to chromosomal changes and nonrandom mating that reduces gene flow

a. Polyploidy= extra sets of chromosomes

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Autopolyploid ­individual that has more than two chromosome sets, all from a single species

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allopolyploid ­ changes a sterile hybrid into a fertile polyploid­are fertile with each other, but can't interbreed with parental species

polyploidy is common in plants ­ oats, cotton, potatoes, tobacco, wheatscientists can create polyploids in lab by using chemicals to induce meiotic and mitotic errors

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sympatric speciation in animalsex. if a subpopulation can use resources that can't be used by

parents Ex. North American apple maggot fly

­original habitat = hawthorn trees­200 yrs ago ­ some colonized apple trees­apple trees grow faster than hawthorn trees ­ species on

apple trees were selected for rapid development

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ex. cichlids in Lake Victoria, Africa­500 species ­ adapted to different food sources, nonrandom mating­ females select males based on appearance

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Hybrid Zones

­A region in which members of different species meet and mate, producing at least some offspring of mixed ancestry.

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Hybrid Zones over Time

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2. Fusion ­ Weakening of reproductive barriers ­ gene pools of two species become more alike to eventually fuse into a single species

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Possible outcomes of hybrid zones over time

1. Reinforcement ­ strengthening of reproductive barriers; hybrids gradually cease to be formed.

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3. Stability ­ hybrids continue to be produced

In general, hybrids are not that common due to environment constraints.

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fossil records indicate episodes of new species that appear suddenly and then disappear

Punctuated equilibrium ­ long periods of apparent stasis (no change) interrupted by relatively brief periods of sudden change

Speciation can occur rapidly or slowly

Once speciation begins it can be completed relatively rapidly

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Gradualism ­ organisms change gradually over time

"Speciation begins only after gene flow between populations is interrupted." Ex by a storm

"Once gene flow is interrupted, the populations must diverge genetically to an extent that they become reproductively isolated."

For speciation to occur:

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