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Chapter 24
Origin of Species
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• Speciation• Evolutionary theory must explain how new
species originate and how populations evolve• Microevolution consists of changes in allele
frequency in a population over time• Macroevolution refers to broad patterns of
evolutionary change above the species level
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24.1 – reproduction isolation
• Biologists compare morphology, physiology, biochemistry, and DNA sequences when grouping organisms
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The Biological Species Concept
• The biological species concept states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations
• Gene flow between populations holds the phenotype of a population together
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Reproductive Isolation
• Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring
• Hybrids are the offspring of crosses between different species
• Reproductive isolation can be classified by whether factors act before or after fertilization
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• Prezygotic barriers block fertilization from occurring by:
– Impeding different species from attempting to mate
–Preventing the successful completion of mating
–Hindering fertilization if mating is successful
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• Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers
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• Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes
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• Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers
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• Mechanical isolation: Morphological differences can prevent successful mating
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Figure 24.3f
(f)
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• Gametic Isolation: Sperm of one species may not be able to fertilize eggs of another species
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• Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult:
–Reduced hybrid viability–Reduced hybrid fertility–Hybrid breakdown
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• Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development
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• Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile
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• Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile
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Other Definitions of Species
• The morphological species concept defines a species by structural features
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• The ecological species concept views a species in terms of its ecological niche
• The phylogenetic species concept defines a species as the smallest group of individuals on a phylogenetic tree- shared genetic history
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Concept 24.2: Speciation can take place with or without geographic separation
• Speciation can occur in two ways:–Allopatric speciation–Sympatric speciation
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Figure 24.5
(a) (b)Allopatric speciation.A population forms anew species whilegeographically isolatedfrom its parent population.
Sympatric speciation.A subset of a populationforms a new specieswithout geographicseparation.
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Allopatric (“Other Country”) Speciation
• In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations
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Figure 24.6
A. harrisii A. leucurus
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Evidence of Allopatric Speciation• 15 pairs of sibling species of snapping shrimp (Alpheus)
are separated by the Isthmus of Panama• These species originated 9 to 13 million years ago,
when the Isthmus of Panama formed and separated the Atlantic and Pacific waters
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Figure 24.8
A. formosus
Atlantic Ocean
A. nuttingi
Isthmus of Panama
Pacific Ocean
A. panamensis A. millsae
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• Regions with many geographic barriers typically have more species than do regions with fewer barriers
• Reproductive isolation between populations generally increases as the distance between them increases
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Sympatric (“Same Country”) Speciation
• In sympatric speciation, speciation takes place in geographically overlapping populations
PolyploidyHabitat differentiationSexual selection
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Polyploidy• Polyploidy is the presence of extra sets of
chromosomes due to accidents during cell division
• more common in plants than in animals autopolyploid is an individual with more than two
chromosome sets, derived from one speciesAllopolyploid – a hybrid that reproduces asexually
or may later become fertile and reproduce with other allopolypoids.
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Figure 24.11-4
Species A2n = 6
Species B2n = 4
Normalgameten = 3
Meiotic error;chromosome number notreduced from 2n to n
Unreduced gametewith 4 chromosomes
Hybrid with7 chromosomes
Unreduced gametewith 7 chromosomes
Normalgameten = 3
New species:viable fertile hybrid(allopolyploid) 2n = 10
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• Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids
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Habitat Differentiation
• appearance of new ecological niches• For example, the North American maggot fly
can live on native hawthorn trees as well as more recently introduced apple trees
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Sexual Selection
• Sexual selection can drive sympatric speciation• Sexual selection for mates of different colors
has likely contributed to speciation in cichlid fish in Lake Victoria
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Allopatric and Sympatric Speciation: A Review
• In allopatric speciation, geographic isolation 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
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• In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species
• Sympatric speciation can result from polyploidy, natural selection, or sexual selection
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Concept 24.3: Hybrid zones reveal factors that cause reproductive
isolation• A hybrid zone is a region in which
members of different species mate and produce hybrids
• Hybrids are the result of mating between species with incomplete reproductive barriers
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Figure 24.13
EUROPE
Yellow-belliedtoad, Bombinavariegata
Fire-belliedtoad range
Hybrid zone
Yellow-belliedtoad range
Fire-bellied toad, Bombina bombina
Freq
uenc
y of
B. v
arie
gata
-spe
cific
alle
le
Yellow-belliedtoad range
Hybridzone
Fire-belliedtoad range
Distance from hybrid zone center (km)40
0.99
0.9
0.5
0.1
0.01
30 20 10 0 10 20
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Figure 24.16
Pundamilia nyererei Pundamilia pundamilia
Pundamilia “turbid water,”hybrid offspring from a locationwith turbid water
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Patterns in the Fossil Record
• The fossil record includes examples of species that appear-persist- disappear
• punctuated equilibria to describe periods of apparent stasis punctuated by sudden change
• The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence
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Figure 24.17
(a) Punctuatedpattern
Time
(b) Gradualpattern
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• The interval between speciation events can range from 4,000 years (some cichlids) to 40 million years (some beetles), with an average of 6.5 million years
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From Speciation to Macroevolution
Macroevolution is the cumulative effect of many speciation and extinction events
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