Darwins Speciation
mystery of mysteries Involves the splitting of evolutionary lineages Need not involve adaptation
Outline
Anagenesis and c ladogenesis Species concepts Reproductive isolating mechanisms
Prezygotic Postzygotic
The geography of speciation
Allopatric Sympatric Parapatric
Examples
Ring species Reinforcement Adaptive radiations
Two basic patterns of evolutionary change can be distinguished
Anagenesis Cladogenesis
Figure 24.2 (a) A nagenesis
(b) C ladogenesis
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What is a species?
Are species real entities or a product of the human desire to pigeonhole or classify? How do we define them?
Species Concepts
The biological species concept, Ernst Mayr
Defines a species as a population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but are unable to produce viable fertile offspring with members of other populations
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(a) Similarity between different species. The eastern meadowlark (Sturnella magna, left) and the western meadowlark (Sturnella neglecta, right) have similar body shapes and colorations. Nevertheless, they are distinct biological species because their songs and other behaviors are different enough to prevent interbreeding should they meet in the wild. (b) Diversity within a species. As diverse as we may be in appearance, all humans belong to a single biological species (Homo sapiens), defined by our capacity to interbreed.
Figure 24.3 A, B
Biological species concept
Focuses the study of speciation on the study of the evolution of reproductive isolation Reproductive isolation
Is the existence of biological factors that impede members of two species from producing viable, fertile hybrids Is a combination of various reproductive barriers that may operate either before or after fertilization
Limitations of the Biological Species Concept
Some species hybridize frequently where their ranges overlap, but maintain their differences elsewhere. Are these two or one species? Biologists o ften disagree as to whether such situations represent two species or one species with two ecotypes
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Example: Mimulus aurantiacus (bush monkeyflower) in San Diego County: One species or two?
More limitations of the Biological Species Concept
The biological species concept cannot be applied to
Asexual organisms Fossils Organisms about which little is known regarding their reproduction
Other Definitions of Species
The morphological species concept
Characterizes a species in terms of its body shape, size, and other structural features
The paleontological species concept
Focuses on morphologically discrete species known only from the fossil record
The ecological species concept
Views a species in terms of its ecological niche
The phylogenetic species concept
Defines a species as a set of organisms with a unique genetic history
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Reproductive Isolation
Allows lineages to evolve separately (cladogenesis) Can be due to pre- or postzygotic barriers (or both).
Prezygotic barriers
Impede mating between species or hinder the fertilization of ova if members of different species attempt to mate
Postzygotic barriers
Often prevent the hybrid zygote from developing into a viable, fertile adult
Premating i solation mechanisms
Habitat isolation Temporal isolation Behavioral isolation
Post-mating pre-zygotic
Mechanical isolation
Prezygotic and postzygotic barriers
Individuals of different species
Mating attempt
HABITAT ISOLATION (b)
TEMPORAL ISOLATION
BEHAVIORAL ISOLATION
MECHANICAL ISOLATION (g)
(d) (e) (a) (f) (c)
Figure 24.4
blue footed booby dance
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Post-mating pre-zygotic,
Gametic isolation
Post-zygotic isolation mechanisms
Reduce hybrid viability Reduce hybrid fertility Hybrid breakdown
Viable fertile offspring
Fertilization
GAMETIC ISOLATION
REDUCED HYBRID VIABILITY
REDUCED HYBRID FERTILITY HYBRID BREAKDOWN (k)
(j) (m) (l) (h) (i)
The evolution of reproductive isolation
What causes it? Can it be a consequence of adaptation? Can reproductive isolation be directly selected for? Several examples
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Prezygotic isolation
Need not evolve as a mechanism to avoid hybridization Instead, usually evolves as a consequence of adaptation(s) to improve survival or mating success in a particular population or species.
Prezygotic Isolation
Could evolve to prevent matings that produce unfit hybrids This would be called reinforcement The best evidence for this would be greater premating isolation between sympatric than allopatric populations of the two species
Postzygotic isolation
Develops over time with genetic isolation of populations Due to divergence at one or (usually) more loci such that combinations of alleles from the two species result in low fitness Divergence could be due to selection or drift
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The geography of speciation
Speciation can take place with or without geographic separation Speciation can occur in three ways
Allopatric speciation Sympatric speciation Parapatric speciation
Figure 24.5 A, B
(a) Allopatric s peciation. A (b) Sympatric speciation. A small population forms a new population becomes a new species species while geographically without geographic separation. isolated from its parent population.
Allopatric (Other Country) Speciation
In allopatric speciation
Gene flow is interrupted or reduced when a population is divided into two or more geographically isolated subpopulations
Once geographic separation has occurred
One or both populations may undergo evolutionary change during the period of separation Even if ranges subsequently overlap, hybrids may not form (pre-zygotic isolation) or have reduced or zero fitness (post-zygotic isolation).
A. h arrisi
A. l eucurus
Figure 24.6
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Dobzhansky-Muller incompatibilities
Arise in allopatry and result in reproductive isolation. May arise with or without the aid of selection. Therefore, reproductive isolation and adaptation may be decoupled.
Dobzhansky-Muller incompatibilities
Two allopatric populations of a species are originally fixed for the same alleles at two loci (both populations A1A1B1B1 ). Through selection drift or one population becomes fixed for a new allele at the B locus (A1 A1B2 B2). In that population, a new allele at the A locus arises and is fixed (A2 A2B2 B2). When p opulations hybridize and recombine, the A2 allele will be mixed with the B1 allele in some offspring. T he A2B1 combination may be deleterious because A2 has never had to function with the B1 allele. Such reductions in hybrid fitness are called D-M incompatibilities.
Sympatric (Same Country) Speciation
In sympatric speciation
Divergence and speciation take place without geographic isolation This is difficult because genes causing divergence must be linked to or the same as those causing reproductive isolation (assortative mating) Some p otential e xamples:
Evolution of plant host races of the same insect species Genes that cause host preference may lead to assortative m ating
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Polyploidy
Polyploidy
Is the presence of extra sets of chromosomes in cells due to accidents during cell division Results in reproductive barriers between different ploidy levels. Has caused the sympatric speciation of some plant species
An autopolyploid
Is an individual that has more than two chromosome sets, all derived from a single species
Failure of cell division in a cell of a growing diploid plant after chromosome duplication gives rise to a tetraploid branch or other tissue.
Gametes produced by flowers on this branch will be diploid.
Offspring with tetraploid karyotypes m ay be viable and fertilea new biological species.
2n 2n = 6 4 n = 12 4n
Figure 24.8
An allopolyploid
Is a species with multiple sets of Unreduced gamete Unreduced gamete with 4 chromosomes with 7 chromosomes Hybrid with chromosomes derived from different 7 chromosomes Meiotic error; species chromosome Species A
2n = 4 number not reduced from 2n to n Viable fertile hybrid (allopolyploid)
2n = 10 Normal gamete n=3 Species B 2n = 6 Normal gamete n=3
Figure 24.9
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Parapatric speciation
Can occur in abutting populations, particularly if there is a steep ecological gradient and if divergence causes assortative mating
Heavy metal tolerant populations tend to bloom at a different time than adjacent non-tolerant populations, promoting reproductive isolation
Mimulus aurantiacus Parapatric distribution of flower color races Flower color controlled by one or two genes Hummingbirds prefer red flowers 95% of the time Hawkmoths prefer yellow flowers >99% of the time Flower color genes could cause assortative mating.
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Allopatric, Sympatric and Parapatric Speciation
In allopatric speciation
A new species forms while geographically isolated from its parent population
In sympatric speciation
The emergence of a reproductive barrier isolates a subset of a population without geographic separation
In parapatric speciation
Populations that abut one another diverge, usually in response to some strong ecological gradient.
Greenish Warbler (Phyloscopus trochiloides) An example of a split ring species
Color variation
Species range
Where forms overlap, they hybridize, except in Siberia (blue and red dashes)
Hypothesis: In the past, the species was restricted to the Himalayas, then spread north around both sides of the Tibetan plateau
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Genetic distance
Songs around the ring
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Greenish Warblers: a split ring
Gradual divergence between adjacent forms Evolution of more complex song as the bird moved into more structurally complex habitats (more trees, less visual communication) Song complexity increased in different ways around different sides of the ring Where forms overlap in Siberia they do not recognize each other as the same species
The E nsatina eschscholtzii complex: another split ring
Costus (Spiral gingers)
Tropical understory plants In the neotropics, some species are bee pollinated, others bird pollinated Species that share pollinators and may transfer pollen interspecifically Hybrids are commonly produced in greenhouses, but rarely seen in nature.
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Both species hummingbird pollinated. Pollen transfer is limited and asymmetrical.
Pollen deposition index Dye deposition index
0.3 0.25 0.2 0.15 0.1 0.05 0
(N=94)
Intraspecific Interspecific
(N=84)
5 array sites 39 array-days +1SE
C. pulverulentus
Maternal Plant
C. scaber
Reinforcement predicts that prezygotic isolation should be stronger between sympatric than allopatric p opulations.
Chiapas, Mexico
C. pulverulentus only sympatry
BCI, Panama
C. scaber only
La Selva, Costa Rica
Hypothesis:
Seed set is only lower for sympatric populations.
C. scaber from La Selva C. scaber from BCI
*Dashed line represents seed set in intraspecic crosses.
Reduced seed set is due to failure of interspecific pollen to germinate on stigmas of the other species, when they grow s ympatrically
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Costus (spiral gingers)
Hybridize freely when two allopatric forms are crossed Show increased prezygotic isolation when species grow sympatrically and share pollinators Provide evidence that prezygotic isolation evolved by reinforcement
Adaptive Radiation
Adaptive radiation
Is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities
Figure 24.11
The Hawaiian archipelago
Is one of the worlds great showcases of adaptive radiation
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Dubautia laxa
1.3 million years MOLOKA'I KAUA'I MAUI 5.1 million O'AHU LANAI years 3.7 million years
Argyroxiphium sandwicense HAWAI'I 0.4 million years
Dubautia waialealae
Figure 24.12
Dubautia scabra
Dubautia linearis
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Studying the Genetics of Speciation
The explosion of genomics
Is enabling researchers to identify specific genes involved in s speciation.
Loci responsible for adaptation Gamete recognition loci Loci responsible for D obzhansky-Muller i nteractions.
Summary
Cladogenesis provides independent evolutionary lineages Reproductive isolation arises for many reasons. Perhaps only rarely as a result of direct selection against hybridization Species can arise allopatrically or sympatrically. Sympatric speciation appears to require the simultaneous evolution of trait divergence and assortative mating.
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