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SPECIATION
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How do new species arise?
Note that mutation, recombination, and selection change the average characteristics of a population
This provides for “evolution” as adaptation
But it does not by itself explain “speciation,” the productionof new species and increase in the number of species
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What are species?
Taxonomic definition: the members of different species haveconsistent differences in a significant number of traits
same species different speciesTaxonomic definition clustered traits divergence of traits
Genetic definition: the members of different species arereproductively isolated
same species different speciesGenetic definition interbreeding reproductive isolation
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Genetic speciation implies taxonomic speciation
For interbreeding populations, any new allele (trait), generally small, will spread throughout the population; clusters of new alleles will tend to disperse randomly; differences will be gradual
For non-interbreeding-- “reproductively isolated” --populations, alleles will not be shared; differences in new alleles and clusters of new alleles will accumulate; populations will diverge; the more they diverge, themore isolated they become
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How are populations reproductively isolated?
Prezygotic mechanisms prevent fertilization
Different habitatSexual maturation at different seasonsDifferent mating behaviorDifferent sexual organsGamete incompatibility (no sperm-egg fusion)
Postzygotic mechanisms limit hybrid survival or reproduction
Hybrids inviableHybrids sterile (no gonads)Hybrids sterile (abnormal meiosis, incomplete gametes)F2 breakdown
Every natural species distinguished from every other species byone or more of these factors
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But how do populations BECOME reproductively isolated?
Geographical isolation (allopatric speciation)
Mutation, recombination,selection in differing environments stabilizedifferent traits, someproducing reproductiveIsolation
Once isolated, populationsdiverge further, givingdifferent genera, etc.
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Examples of speciation through reproductive isolation
Fruit flies in the Hawaiian islands
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Examples of speciation through reproductive isolation
Darwin’s finches
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Examples of speciation through reproductive isolation
Evolution of horses(from fossil evidence)
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Sympatric speciation
Polyploidy: change in chromosome number givesImmediate reproductive isolation
Diploid
ChromosomeDuplication,MitosisWithoutCytokinesis
Tetraploid
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Examples of polyploidy as speciation:
Tobacco
Nicotiana otophora x Nicotiana sylvestris 2n = 12 2n = 12
Nicotiana tabacum 2n = 24
(occurred naturally, but verified experimentally)
Rado-cabbage
radish (Raphanus sativus) x cabbage (Brassica oleracea)2n = 18 2n = 18
“radocabbage”2n = 36 (radish head, cabbage root)
(experimental, but shows possibility of intergeneric speciation)
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The tempo of evolution: is evolution gradual?
Fossil evidence shows :Species morphology is stable over long periods Species appear and disappear abruptly (in evolutionary time)
“Punctuated equilibrium” may describe life history better than “evolution”
Morphological trait Morphological trait
time
Gradualchange
Punctuatedequilibrium
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Flower compatibility may affect the tempo of evolution:
Self compatibility -> genetic constancy -> high speciation, high extinction -> low diversificationSelf incompatibility -> genetic variability -> low speciation, lower extinction -> high diversification
Self-incompatibleSelf-compatible
Science 22 Oct 2010, p. 493
Speciation rate Diversification rate
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Summary
Evolution of a population does not itself imply the formation of species
Speciation occurs through reproductive isolation
Reproductive isolation can occur through geographical isolation
Tetraploidy is one method of sympatric isolation
The appearance of new species, observed through study of fossils,occurs in a “punctuated equilibrium” pattern
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Other questions:
Are there any cases of gradual phenotypic change (instead of punctuated equilibrium)?
Are there alternatives to selection that can explain the pattern of changes in fossil samples? (Raup: the changes are not significantly different from random; also consider genetic drift)
What is the best way to sample fossils to estimate the patternof increase in species over geologic time? (Right: number of marine genera estimated from fossils in three different ways)