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The Origin of Species
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Overview: That “Mystery of Mysteries”
In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth Video: Galápagos Tortoise
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Fig. 24-1 Figure 24.1 How did this flightless bird come to live on the isolated Galápagos Islands?
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Animation: Macroevolution
Speciation, the origin of new species, is at the focal point of evolutionary theory Evolutionary theory must explain how new species originate and how populations evolve Microevolution consists of adaptations that evolve within a population, confined to one gene pool Macroevolution refers to evolutionary change above the species level Animation: Macroevolution
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Concept 24.1: The biological species concept emphasizes reproductive isolation
Species is a Latin word meaning “kind” or “appearance” 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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(a) Similarity between different species
Fig. 24-2a Figure 24.2 The biological species concept is based on the potential to interbreed rather than on physical similarity (a) Similarity between different species
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(b) Diversity within a species
Fig. 24-2b 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
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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Water-dwelling Thamnophis
Fig. 24-4c (a) Figure 24.4 Reproductive barriers Water-dwelling Thamnophis
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Terrestrial Thamnophis
Fig. 24-4d (b) Figure 24.4 Reproductive barriers Terrestrial Thamnophis
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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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(c) Eastern spotted skunk (Spilogale putorius) Fig. 24-4e
Figure 24.4 Reproductive barriers Eastern spotted skunk (Spilogale putorius)
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(d) Western spotted skunk (Spilogale gracilis) Fig. 24-4f
Figure 24.4 Reproductive barriers Western spotted skunk (Spilogale gracilis)
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Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers Video: Albatross Courtship Ritual Video: Giraffe Courtship Ritual Video: Blue-footed Boobies Courtship Ritual
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Courtship ritual of blue- footed boobies
Fig. 24-4g (e) Figure 24.4 Reproductive barriers Courtship ritual of blue- footed boobies
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Mechanical isolation: Morphological differences can prevent successful mating
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Bradybaena with shells spiraling in opposite directions
Fig. 24-4h (f) Figure 24.4 Reproductive barriers Bradybaena with shells spiraling in opposite directions
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Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species
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Fig. 24-4k (g) Figure 24.4 Reproductive barriers Sea urchins
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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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Fig. 24-4l (h) Figure 24.4 Reproductive barriers Ensatina hybrid
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Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile
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Fig. 24-4m (i) Figure 24.4 Reproductive barriers Donkey
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Fig. 24-4n ( j) Figure 24.4 Reproductive barriers Horse
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Fig. 24-4o (k) Figure 24.4 Reproductive barriers Mule (sterile hybrid)
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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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Hybrid cultivated rice plants with stunted offspring (center)
Fig. 24-4p (l) Figure 24.4 Reproductive barriers Hybrid cultivated rice plants with stunted offspring (center)
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Figure 24.4 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 24.4 Reproductive barriers (k)
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Limitations of the Biological Species Concept
The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)
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Other Definitions of Species
Other species concepts emphasize the unity within a species rather than the separateness of different species The morphological species concept defines a species by structural features It applies to sexual and asexual species but relies on subjective criteria
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The ecological species concept views a species in terms of its ecological niche
It applies to sexual and asexual species and emphasizes the role of disruptive selection The phylogenetic species concept: defines a species as the smallest group of individuals on a phylogenetic tree It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species
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Speciation can occur in two ways:
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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(a) Allopatric speciation (b) Sympatric speciation
Fig. 24-5 Fig 24.5 Two main modes of speciation (a) Allopatric speciation (b) Sympatric speciation
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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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The Process of Allopatric Speciation
The definition of barrier depends on the ability of a population to disperse Separate populations may evolve independently through mutation, natural selection, and genetic drift
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A. harrisi A. leucurus Fig. 24-6
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon
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Evidence of Allopatric Speciation
Regions with many geographic barriers typically have more species than do regions with fewer barriers
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Millions of years ago (mya)
Fig. 24-7 Mantellinae (Madagascar only): 100 species Rhacophorinae (India/Southeast Asia): 310 species Other Indian/ Southeast Asian frogs 100 80 60 40 20 1 2 3 Figure 24.7 Allopatric speciation in frogs Millions of years ago (mya) 1 2 3 India Madagascar 88 mya 65 mya 56 mya
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Barriers to reproduction are intrinsic; separation itself is not a biological barrier
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EXPERIMENT RESULTS Initial population Some flies raised on
Fig. 24-9 EXPERIMENT Initial population Some flies raised on starch medium Some flies raised on maltose medium Mating experiments after 40 generations RESULTS Female Female Starch Starch Starch Maltose population 1 population 2 Figure 24.9 Can divergence of allopatric populations lead to reproductive isolation? Starch population 1 Starch 22 9 18 15 Male Male Maltose 8 20 12 15 Starch population 2 Mating frequencies in experimental group Mating frequencies in control group
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Sympatric (“Same Country”) Speciation
In sympatric speciation, speciation takes place in geographically overlapping populations
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Polyploidy Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division
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2n = 6 4n = 12 Failure of cell division after chromosome
Fig 2n = 6 4n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue. Fig Sympatric speciation by autopolyploidy in plants
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2n = 6 4n = 12 2n Failure of cell division after chromosome
Fig 2n = 6 4n = 12 2n Failure of cell division after chromosome duplication gives rise to tetraploid tissue. Gametes produced are diploid.. Fig Sympatric speciation by autopolyploidy in plants
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2n = 6 4n = 12 2n 4n Failure of cell division after chromosome
Fig 2n = 6 4n = 12 2n 4n Failure of cell division after chromosome duplication gives rise to tetraploid tissue. Gametes produced are diploid.. Offspring with tetraploid karyotypes may be viable and fertile. Fig Sympatric speciation by autopolyploidy in plants
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Polyploidy is much more common in plants than in animals
Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids
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Habitat Differentiation
Sympatric speciation can also result from the 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 the speciation in cichlid fish in Lake Victoria
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Monochromatic orange light
Fig EXPERIMENT Monochromatic orange light Normal light P. pundamilia Figure 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 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 provide opportunities to study factors that cause reproductive isolation
A hybrid zone is a region in which members of different species mate and produce hybrids
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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
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Collared flycatcher
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Sympatric male pied flycatcher Allopatric male pied flycatcher
Fig a Figure Reinforcement of barriers to reproduction in closely related species of European flycatchers Sympatric male pied flycatcher Allopatric male pied flycatcher
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28 Pied flycatchers 24 Collared flycatchers 20 16 Number of females 12
Fig b 28 Pied flycatchers 24 Collared flycatchers 20 16 Number of females 12 8 Figure Reinforcement of barriers to reproduction in closely related species of European flycatchers 4 (none) Females mating with males from: Own species Other species Own species Other species Sympatric males Allopatric males
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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
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Pundamilia pundamilia
Fig Pundamilia nyererei Pundamilia pundamilia Figure The breakdown of reproductive barriers Pundamilia “turbid water,” hybrid offspring from a location with turbid water
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Concept 24.4: Speciation can occur rapidly or slowly and can 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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Frequency of individuals
Fig a Original population Frequency of individuals Phenotypes (fur color) Figure Modes of selection Original population Evolved population (a) Directional selection
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Frequency of individuals
Fig b Original population Frequency of individuals Phenotypes (fur color) Figure Modes of selection Evolved population (b) Disruptive selection
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Frequency of individuals
Fig c Original population Frequency of individuals Phenotypes (fur color) Figure Modes of selection Evolved population (c) Stabilizing selection
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Patterns in the Fossil Record
The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium 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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(a) Punctuated pattern
Fig (a) Punctuated pattern Time (b) Gradual pattern Figure Two models for the tempo of speciation
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Speciation Rates The punctuated pattern in the fossil record and evidence from lab studies suggests that speciation can be rapid The interval between speciation events can range from 4,000 years (some cichlids) to 40,000,000 years (some beetles), with an average of 6,500,000 years
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Studying the Genetics of Speciation
The explosion of genomics is enabling researchers to identify specific genes involved in some cases of speciation Depending on the species in question, speciation might require the change of only a single allele or many alleles
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Fig Figure Single-gene speciation
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(a) Typical Mimulus lewisii
Fig (a) 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
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Allopatric speciation Sympatric speciation
Fig. 24-UN1 Original population Fig. 24-UN1 Allopatric speciation Sympatric speciation
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Ancestral species: AA BB DD Triticum monococcum (2n = 14) Wild
Fig. 24-UN2 Ancestral species: AA BB DD Triticum monococcum (2n = 14) Wild Triticum (2n = 14) Wild T. tauschii (2n = 14) Product: Fig. 24-UN2 AA BB DD T. aestivum (bread wheat) (2n = 42)
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You should now be able to:
Define and discuss the limitations of the four species concepts Describe and provide examples of prezygotic and postzygotic reproductive barriers Distinguish between and provide examples of allopatric and sympatric speciation Explain how polyploidy can cause reproductive isolation Define the term hybrid zone and describe three outcomes for hybrid zones over time
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