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The Origin of Species Chapter 24
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Overview: That “Mystery of Mysteries”
In the Galápagos Islands Darwin discovered unique plants and animals
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Speciation The origin of new species, the focal point of evolutionary theory Evolutionary theory must explain how new species originate and how populations evolve Microevolution - changes in allele frequency in a population over time Macroevolution - broad patterns of evolutionary change above the species level Macroevolution
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Biological species concept emphasizes reproductive isolation
Species is a Latin word meaning “kind” or “appearance” Compare morphology, physiology, biochemistry, and DNA sequences
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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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(a) Similarity between different species
Figure 24.2 (a) Similarity between different species Figure 24.2 The biological species concept is based on the potential to interbreed rather than on physical similarity. (b) Diversity within a species
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Reproductive Isolation
The existence of biological factors or barriers that prevent 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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Reproductive Barriers
Prezygotic barriers Postzygotic barriers Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Isolation Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Individuals of different species MATING ATTEMPT VIABLE, FERTILE OFFSPRING FERTILIZATION (a) (c) (e) (f) (g) (h) (i) (l) (d) (j) (b) Figure 24.3 Exploring: Reproductive Barriers (k)
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Prezygotic barriers Block fertilization from occurring by:
Prevent species from attempting to mate Prevent successful completion of mating Stop fertilization if mating is successful
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Habitat isolation Two species encounter each other rarely or not at all. They occupy different habitats, even though not isolated by physical barriers © 2011 Pearson Educatio, Inc.
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Temporal isolation Species that breed at different times of the day, season, or years Cannot mix their gametes
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Behavioral isolation Courtship rituals and other behaviors unique to a species Blue Footed Boobie dance
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Mechanical isolation Morphological differences can prevent successful mating © 2011 Pearson Education, Inc.
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Gametic Isolation Sperm of one species may not be able to fertilize eggs of another species © 2011 Pearson Education, Inc.
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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 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 © 2011 Pearson Education, Inc.
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Hybrid breakdown Some first-generation hybrids are fertile, but when they mate with another species, offspring are feeble or sterile
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Limitations of the Biological Species Concept
The biological species concept cannot be applied to fossils or asexual organisms The biological species concept emphasizes absence of gene flow However, gene flow can occur between distinct species Grizzly bears + polar bears = “grolar bears”
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Other Species Concepts
Other species concepts emphasize the unity within a species rather than the separateness of different species Morphological species concept defines a species by structural features Ecological species concept views a species in terms of its ecological niche Phylogenetic species concept defines a species as the smallest group of individuals on a phylogenetic tree
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Speciation can take place with or without geographic separation
Figure 24.5 Two main modes of speciation. (a) Allopatric speciation. A population forms a new species while geographically isolated from its parent population. (b) Sympatric speciation. A subset of a population forms a new species without geographic separation.
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Allopatric Speciation
“Other Country” Gene flow is interrupted/reduced when a population is divided geographically The flightless cormorant of the Galápagos originated from a flying species on the mainland
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The Process of Allopatric Speciation
The definition of barrier depends on the ability of a population to disperse A canyon may create a barrier for small rodents, but not birds, coyotes, or pollen A. harrisii A. leucurus Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon.
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Reproductive isolation may arise as a result of genetic divergence
Separate populations may evolve independently through mutation, natural selection, and genetic drift Reproductive isolation may arise as a result of genetic divergence Mosquitofish in the Bahamas comprise several isolated populations in different ponds (a) Under high predation (b) Under low predation Figure 24.7 Reproductive isolation as a by-product of selection.
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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 originated 9 to 13 Mya, when the Isthmus formed, separating the Atlantic and Pacific
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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 For example, reproductive isolation increases between dusky salamanders that live further apart
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Degree of reproductive isolation
Figure 24.9 2.0 1.5 Degree of reproductive isolation 1.0 0.5 Figure 24.9 Reproductive isolation increases with distance in populations of dusky salamanders. Geographic distance (km)
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Barriers to reproduction are intrinsic; separation itself is not a biological barrier
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EXPERIMENT Initial population of fruit flies (Drosophila
Figure 24.10 EXPERIMENT Initial population of fruit flies (Drosophila pseudoobscura) Some flies raised on starch medium Some flies raised on maltose medium Mating experiments after 40 generations RESULTS Female Female Starch population 1 Starch population 2 Starch Maltose Figure Inquiry: Can divergence of allopatric populations lead to reproductive isolation? Starch population 1 Starch 22 9 18 15 Male Male population 2 Starch Maltose 8 20 12 15 Number of matings in experimental group Number of matings in control group
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Sympatric Speciation “Same Country”
In sympatric speciation, speciation takes place in geographically overlapping populations
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Polyploidy The presence of extra sets of chromosomes due to accidents during cell division More common in plants - oats, cotton, potatoes, tobacco, wheat Autopolyploid - an individual with more than two chromosome sets, derived from one species Allopolyploid – a species with multiple sets of chromosomes derived from different species
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Species A 2n = 6 Species B 2n = 4 Meiotic error; chromosome number not
reduced from 2n to n Normal gamete n = 3 Unreduced gamete with 4 chromosomes Figure One mechanism for allopolyploid speciation in plants.
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Species A 2n = 6 Species B 2n = 4 Meiotic error; chromosome number not
Figure Species A 2n = 6 Species B 2n = 4 Meiotic error; chromosome number not reduced from 2n to n Normal gamete n = 3 Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Figure One mechanism for allopolyploid speciation in plants.
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Species A 2n = 6 Species B 2n = 4 Meiotic error; chromosome number not
reduced from 2n to n Normal gamete n = 3 Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Figure One mechanism for allopolyploid speciation in plants. Normal gamete n = 3 Unreduced gamete with 7 chromosomes
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Species A 2n = 6 Species B 2n = 4 Meiotic error; chromosome number not
reduced from 2n to n Normal gamete n = 3 Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Figure One mechanism for allopolyploid speciation in plants. Normal gamete n = 3 Unreduced gamete with 7 chromosomes New species: viable fertile hybrid (allopolyploid) 2n = 10
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Habitat Differentiation
Sympatric speciation can also result from the appearance of new ecological niches 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 contributed to speciation in cichlid fish in Lake Victoria
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Monochromatic orange light
Figure 24.12 EXPERIMENT Monochromatic orange light Normal light P. pundamilia Figure Inquiry: Does sexual selection in cichlids result in reproductive isolation? P. nyererei
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Allopatric and Sympatric Speciation: A Review
In allopatric speciation - geographic isolation restricts gene flow between populations Reproductive isolation can then arise by natural selection, genetic drift, or sexual selection Even if contact is restored, interbreeding is prevented In sympatric speciation - a reproductive barrier isolates a subset of a population, without geographic separation Sympatric speciation can result from polyploidy, natural selection, or sexual selection
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Hybrid zones A region in which members of different species mate and produce hybrids Hybrids are the result of mating between species with incomplete reproductive barriers A hybrid zone can occur in a single band where adjacent species meet Two species of toad in the genus Bombina interbreed in a long and narrow hybrid zone
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B. variegata-specific allele
Figure 24.13 EUROPE Fire-bellied toad range Fire-bellied toad, Bombina bombina Hybrid zone Yellow-bellied toad range 0.99 Hybrid zone 0.9 Figure A narrow hybrid zone for Bombina toads in Europe. B. variegata-specific allele Frequency of 0.5 Yellow-bellied toad range Fire-bellied toad range Yellow-bellied toad, Bombina variegata 0.1 0.01 40 30 20 10 10 20 Distance from hybrid zone center (km)
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Hybrids often have reduced fitness compared with parent species
The distribution of hybrid zones can be more complex if parent species are found in patches within the same region
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Hybrid Zones over Time When closely related species meet in a hybrid zone, there are three possible outcomes: Reinforcement Fusion Stability
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Gene flow Population Barrier to gene flow
Figure Formation of a hybrid zone and possible outcomes for hybrids over time. Gene flow Population Barrier to gene flow
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Isolated population diverges Gene flow Population Barrier to gene flow
Figure Formation of a hybrid zone and possible outcomes for hybrids over time. Gene flow Population Barrier to gene flow
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Isolated population Hybrid diverges zone Gene flow Population Hybrid
Figure Formation of a hybrid zone and possible outcomes for hybrids over time. Gene flow Population Hybrid individual Barrier to gene flow
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Possible outcomes: Isolated population Hybrid diverges zone
Reinforcement OR Fusion OR Figure Formation of a hybrid zone and possible outcomes for hybrids over time. Gene flow Population Hybrid individual Barrier to gene flow Stability
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Reinforcement: Strengthening Reproductive Barriers
The reinforcement of barriers occurs when hybrids are less fit than the parent species Over time, the rate of hybridization decreases Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species In populations of European Flycatchers, males are more similar in allopatric populations than sympatric populations
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Females choosing between these males: Females choosing between
28 Sympatric pied male Allopatric pied male 24 Sympatric collared male Allopatric collared male 20 16 Number of females 12 8 Figure Reinforcement of barriers to reproduction in closely related species of European flycatchers. 4 (none) Own species Other species Own species Other species Female mate choice Female mate choice
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Fusion: Weakening Reproductive Barriers
If hybrids are as fit as parents, there can be substantial gene flow between species If gene flow is great enough, the parent species can fuse into a single species Researchers think that pollution in Lake Victoria has reduced the ability of female cichlids to distinguish males of different species This might be causing the fusion of many species
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Pundamilia pundamilia
Figure 24.16 Pundamilia nyererei Pundamilia pundamilia Figure Fusion: The breakdown of reproductive barriers. Pundamilia “turbid water,” hybrid offspring from a location with turbid water
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Stability: Continued Formation of Hybrid Individuals
Extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone
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Does speciation occur rapidly or slowly
Does speciation occur rapidly or slowly? Does it result from changes in few or many genes? Many questions remain concerning how long it takes for new species to form, or how many genes need to differ between species
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The Time Course of Speciation
Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data
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Patterns in the Fossil Record
The fossil record includes examples of species that appear suddenly, exist unchanged for some time, and then disappear Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibria to describe periods of stasis punctuated by sudden change The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence © 2011 Pearson Educati, Inc.
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(a) Punctuated pattern Time (b) Gradual pattern
Figure Two models for the tempo of speciation.
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Speciation Rates Patterns in the fossil record and evidence from lab studies suggest that speciation can be rapid Sunflower Helianthus anomalus originated from the hybridization of two other sunflower species © 2011 Pearson Education, Inc.
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chromosomes are shown)
EXPERIMENT H. annuus gamete H. petiolarus gamete F1 experimental hybrid (4 of the 2n = 34 chromosomes are shown) RESULTS H. anomalus Figure Inquiry: How does hybridization lead to speciation in sunflowers? Chromosome 1 Experimental hybrid H. anomalus Chromosome 2 Experimental hybrid
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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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Studying the Genetics of Speciation
A fundamental question of evolutionary biology persists: How many genes change when a new species forms? Depending on the species, speciation might require the change of only a single allele or many alleles
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Examples Japanese Euhadra snails, the direction of shell spiral affects mating and is controlled by a single gene In monkey flowers (Mimulus), two loci affect flower color, which influences pollinator preference Pollination that is dominated by either hummingbirds or bees can lead to reproductive isolation of the flower In other species, speciation can be influenced by larger numbers of genes and gene interactions © 2011 Pearson Education, Inc.
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M. cardinalis flower-color allele
Typical Mimulus lewisii (b) M. lewisii with an M. cardinalis flower-color allele Figure A locus that influences pollinator choice. (c) Typical Mimulus cardinalis (d) M. cardinalis with an M. lewisii flower-color allele
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From Speciation to Macroevolution
Macroevolution is the cumulative effect of many speciation and extinction events © 2011 Pearson Education, In
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