Chapter 24Chapter 24
The Origin of Species
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• Speciation = origin of new species
• Microevolution = adaptations that evolve within a population (confined to one gene pool)
• Macroevolution = evolutionary change making new species or new groups of organisms
In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth
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I. Reproductive Isolation
• Species = a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring
• Reproductive isolation = existence of biological factors (barriers) that impede two different species from producing viable, fertile offspring.
• Hybrids are the offspring of crosses between different species (horse + donkey = mule)
Fig. 24-2
(a) Similarity between different species
(b) Diversity within a species
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• Prezygotic barriers block fertilization from occurring
• Postzygotic barriers prevent hybrid from forming viable fertile adult
A. Reproductive Barriers
Reproductive Barriers Between Species
Prezygotic BarriersHabitat Isolation
Individualsof
differentspecies
Temporal Isolation
BehavioralIsolation
Matingattempt
MechanicalIsolation
Gametic Isolation
Fertilization
Reduced Hybrid Viability
Reduced Hybrid Fertility
Postzygotic Barriers
Hybrid Breakdown
Viable,fertile
offspring
(a)
(b)
(d)
(c) (e) (f) (g) (h) (i)
(j)
(l)
(k)
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• Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats
PreZygotic Reproductive Barriers
Habitat Isolation
Water-dwelling Thamnophis
Habitat Isolation
Terrestrial Thamnophis
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• Temporal Isolation: Species that breed at different times (day, seasons, years)
• Behavioral isolation: Courtship rituals and other behaviors
• Mechanical isolation: Morphological differences can prevent successful mating.
• Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species.
PreZygotic Reproductive Barriers
Behavioral Isolation Occurs Without Appropriate Mating Rituals
Courtship ritual of blue-footed boobies
Mechanical Isolation:Bradybaena with shells spiraling in opposite directions
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– Reduced hybrid viability -- weak offspring
– Reduced hybrid fertility -- sterile offspring
– Hybrid breakdown.
PostZygotic Reproductive Barriers
PostZygotic Reproductive Barrier
Mule: sterile hybrid offspring between horse and donkey
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Concept 24.2: Speciation can take place with or without geographic separation
• Speciation can occur in two ways:
– Allopatric speciation: geographic barrier separates populations.
– Sympatric speciation: no geographic barrier
Speciation
(a) Allopatric speciation (b) Sympatric speciation
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II. Allopatric Speciation
• Gene flow is interrupted
• Gene pool is reduced when a population is divided into geographically isolated subpopulations
• Separate populations may evolve independently through mutation, natural selection, and genetic drift.
• Reproductive isolation between populations generally increases as the distance between them increases.
• Barriers to reproduction are intrinsic; separation itself is not a biological barrier.
Allopatric Speciation
A. harrisi A. leucurus
Continental Drift Caused Allopatric Speciation
Mantellinae(Madagascar only):100 species
Rhacophorinae(India/SoutheastAsia): 310 species
Other Indian/Southeast Asianfrogs
Millions of years ago (mya)1 2 3
1 2 3
100 80 60 40 20 0
88 mya 65 mya 56 mya
India
Madagascar
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III. Sympatric Speciation
• Speciation takes place in geographically overlapping populations.
• Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division.
• Polyploidy is common in plants. Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids.
Sympatric Speciation via Polyploidy is Common in Plants
2n = 6 4n = 12
Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.
2n
Gametesproducedare diploid..
4n
Offspring withtetraploidkaryotypes maybe viable andfertile.
Sympatric Speciation - Polyploidy --> Allopolyploid
Species A2n = 6
Normalgameten = 3
Meioticerror
Species B2n = 4
Unreducedgametewith 4chromosomes
Hybridwith 7chromosomes
Unreducedgametewith 7chromosomes
Normalgameten = 3
Viable fertilehybrid(allopolyploid)2n = 10
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A. Habitat Differentiation and Sexual Selection
• Sympatric speciation can also result from the appearance of new ecological niches.
– New roles open for organisms to fill (new trees introduced into an area)
• Sexual selection can drive sympatric speciation.
– Ex. Selection for mates of different colors has contributed to the speciation in cichlid fish in Lake Victoria.
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OVERVIEW: Allopatric and Sympatric Speciation
• Allopatric speciation, geographic separation restricts gene flow between populations.
• Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations.
– Even if contact is restored between populations, interbreeding is prevented.
• Sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species.
– Can result from polyploidy, natural selection, or sexual selection.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
IV. Time Needed for Speciation
• Punctuated equilibrium = periods of apparent stasis (no change) punctuated by brief periods of rapid change.
• Gradualism = slow continuous change over time in a species’ existence
Patterns in Speciation
Punctuated Equilibriumpattern
Gradualism pattern
Change / Time
Speciation - Andersen 12 min
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
You should now be able to:
1. Define and discuss the limitations of the four species concepts.
2. Describe and provide examples of prezygotic and postzygotic reproductive barriers.
3. Distinguish between and provide examples of allopatric and sympatric speciation.
4. Explain how polyploidy can cause reproductive isolation.
5. Define the term hybrid zone and describe three outcomes for hybrid zones over time.