Transcript AP Biology (An Introduction)

Chapter 24
Species & Speciation
Chapter 24 in a nutshell
 Microevolution v. Macroevolution
 Speciation
 Prezygotic
 Postzygotic
 Gradualism v. Punctuated equilibrium
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Species
 Morphological species concept
 Divisions of organisms into discrete units called species
 If two organisms look different enough, then they are
considered different species
 Biological species concept
 Population or group of populations whose members have
the potential to interbreed in nature and produce viable,
fertile offspring
 This population is unable to produce viable, fertile
offspring with members of another population
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Micro- vs. Macro- Evolutions
 Microevolution
 Change in the genetic makeup of a population over
generations
 Changes (adaptations) confined to a particular gene pool
or population
 Macroevolution
 Changes (evolution) above the species level
 Changes used to define higher taxa
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Speciation
 The process that produces a new species
 May be the result of microevolution OR
macroevolution
 How could microevolution lead to speciation?
 How could a new species arise from macroevolution?
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Reproductive Isolation
 Biological barriers that impede members of 2 species
from producing viable, fertile offspring
 There are 2 types of barriers that lead to reproductive
isolation
 Prezygotic
 Pre-mating
 Post-mating
 Postzygotic
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Prezygotic Barriers
 Pre-Mating
 Habitat isolation
 Temporal isolation
 Behavioral isolation
 Post-Mating
 Mechanical isolation
 Gametic isolation
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Prezygotic Barriers (Page 1)
 Pre-Mating
 Habitat isolation
 2 species occupy different habitats in the geographic area
 Tree-dwelling species, Bush-dwelling species
 Temporal isolation
 Temporal = time
 Species breed at different times
 Different time of day, different seasons, or different years
 Behavioral isolation
 Certain signals or types of behavior are integral to
courtship, and these signals/behaviors are unique to the
species
 Other species do not respond to these signals or behaviors
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Boobies!!
(blue-footed)
Different mating season –
late winter or late summer
Different habitats –
water or terrestrial
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Prezygotic Barriers (Post-Mating)
 Mechanical isolation
 Anatomical incompatibility
 Insect copulatory organs are elaborated for a particular
species
 Gametic isolation
 Even if gametes meet, they are unable to fuse
 Sperm may not be able to survive in the reproductive
tract of females from another species
 Purple + Red sea urchins cannot mate b/c gametes
cannot fuse
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Postzygotic Barriers
 Reduced hybrid vitality
 Reduced hybrid fertility
 Hybrid Breakdown
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Postzygotic Barriers
 Reduced Hybrid vitality
 Although zygote forms, genetic incompatibility causes
developmental cessation
 In some salamanders, hybrids form but do not complete
development
 Reduced Hybrid fertility
 Sterile hybrids
 Donkey + Horse = mule (Sterile)
 Hybrid breakdown
 Although first-generation hybrids survive & reproduce,
their offspring are feeble or sterile
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 Geographic Barrier
 No Geographic Barrier
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Allopatric Speciation
 Causes
 Geologic events or processes that fragment the
population
 Emergence of a mountain range
 Formation of a land bridge
 Evaporation of a large lake into multiple smaller lakes
 Leads to a significant alteration of a gene pool
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Sympatric Speciation
 Sympatric = sym (same) + patr (fatherland)
 Same country
 Geographically overlapping populations
 Major mechanism of sympatric speciation is Polyploidy
 Autopolyploidy
 Allopolyploidy
 Preferential Habitation
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Polyploidy
 Extra set(s) of chromosomes
 Autopolyploidy
 > 2n
 Typically, nondisjunction (Meiosis) in Plants
 All chromosomes from the same species
 Example
 Tetraploid (4n) + diploid (2n)
 Produce triploid (3n) offspring which are sterile
 But plant tetraploids can self fertilize or mate with other
tetraploids = viable, fertile polyploid offspring
 Tetraploids are reproductively isolated from the diploid
population
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Polyploidy (Page 2)
 Allopolyploidy
 Plants
 2 different species interbreed = hybrid
 Hybrids cannot breed with either original species
 But hybrids can successfully mate with each other
 Self-pollination successfully occurs as well
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Polyploidy (Page 3)
 Can happen in animals but not common
 In animals, sympatric speciation is usually due to part
of the population switching to a new habitat, food
source, or other resource
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Adaptive Radiation
 Many new species arise from a common ancestor given
new environmental opportunities or challenges
 Think Darwin’s finches & Galapagos islands
 As the finches moved from South America to the
Galapagos, different environments and food sources
were encountered
 Hawaiian Plants
 As the plants spread to Hawaiian islands, different
environments on different islands = evolution into
several distinct forms
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Hawaiian Plants that had a common
ancestor
 Arose from Adaptive Radiation
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Gradualism vs. Punctuated Equilibrium
 Gradualism – species descended from a common
ancestor and gradually diverge in morphology
(acquiring unique adaptations) until they are
reproductively isolated
 Punctuated Equilibrium – long periods of apparent
stasis punctuated by brief periods of sudden and rapid
change
 Fossil record confirms the punctuated equilibrium model
 Also eliminates the necessity of finding “missing links”
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