SpeciesDiversity is a consequence of cladogenesis, the branching or multiplication of lineages, each of which the evolved by anagenesis along it own path. Each branching in the phylogenetic tree marks a speciation event: the origin of two species from one.

Cladogenesis vs. Anagenesis

What are Species?Species are particular kinds or types of organisms.The word species in Latin means kind.There are many definitions of species.

Table 17.1And: Morphological Species Concept

Phylogenetic Species Concept - PSC"An irreducible (basal) cluster of organisms diagnosably different from other such clusters, and within which there is a parental pattern of ancestry and descent.

The smallest possible groups whose members are descended from a common ancestor and who all possess defining or derived characters that distinguish them from other such groups. Zimmer and EmlenPhylogenetic species concept emphasizes the phylogenetic history of organisms: common ancestry.

Phylogenetic Species Concept - PSCA species is the smallest lineage that can be united by synapomorphic characters.Synapomorphies are characters shared by two or more taxa that are derived from a common ancestor. The members of the species should share characteristics that other groups lack; these characteristics are diagnostic.This definition makes no reference to reproductive boundaries.

Biological Species Concept - BSCAs defined by Ernest Mayr (1942): Species are groups of actually or potentially interbreeding populations, which are reproductively isolated from other such groups. Populations of similar organisms that interbreed in the wild and produce viable and fertile offspring.

Biological Species Concept - BSCBased on Buffon's ideas.Reproductive isolation (no genetic exchange) is a key element of this definition.

Based on sexual reproduction.Reproduction without human interference.Morphological similarities and differences do no not suffice to define species.

Biological Species Concept - BSC

Biological Species Concept - BSCVariation within populations: characteristics vary among the members of a single population of interbreeding individuals.

Biological Species Concept - BSCGeographic variation: populations of a species differ; there exists a spectrum from slight to great difference, e.g. human populations.

Geographic VariationAdmiral ButterflyNortheastern US and CanadaSoutheastern USPacific Coast http://www.anoleannals.org/

Biological Species Concept - BSCSibling species: these are reproductively isolated populations that are difficult or impossible to distinguish by morphological features, but which are often recognized by differences in ecology, behavior, chromosomes and other characters.

Domain and Application of BSCAll concepts have limitationsDifficulties defining species with BSC:Testing allopatric populations.

Evaluating differences in fertility of offspring.

Fossils cannot be tested.

Domain and Application of BSCDifficulties defining species with BSC:Asexual reproducing organisms are genetically isolated because they cannot reproduce sexually.

Self-pollinated plants.

Plasmid-mediated horizontal gene exchange between different species.

Domain and Application of BSCDOMAINThe domain of the BSC is restricted to sexual, outcrossing organisms, and to short intervals of time.E. g. Fossils cannot be tested; asexual reproducing organisms are genetically isolated because they cannot reproduce sexually; self-pollinated plants, etc.

Domain and Application of BSCA second meaning of species is a taxonomic category, just like genus or family.

Borderline CasesInterbreeding versus reproductive isolation is not an either/or, all or none situation. Narrow hybrid zones exist where genetically distinct populations meet and interbreed to a limited extent, but in which there exist partial barriers to gene exchange.

Borderline CasesThe hybridizing entities are often recognized as species but may be called semispecies.A collection of semispecies is a superspecies.

Borderline CasesThe biological species concept is sometimes difficult to apply in botany.

There are many types of vegetative propagation and self-fertilization in plants.

Borderline CasesGeographic variation in status occurs when genetically different populations appear to be conspecific in certain geographic regions, but to be different species elsewhere.

Divergence processes in the ring species Ensatina eschscholtzii. a) Ecomorphotypes in Ensatina defined by color pattern. b) Nei's (1972) genetic distances based in 26 allozyme loci, showing increased differentiation towards the terminus of the ring (red) and lower genetic divergence between populations around the ring (black). c) Floristic provinces of California; different colors refer to distinct plant associations. Pereira et al. BMC Evolutionary Biology 2011 11:194

Borderline Cases - Ring SpeciesA connected series of neighboring populations, each of which can interbreed with close related populations, but for which there exist at least two "end" populations in the series, too distantly related to interbreed. There is a potential gene flow between each "linked" population. The non-breeding, though genetically connected, "end" populations may coexist in the same region, closing the "ring".

Ring species provide important evidence of evolution in that they illustrate what happens over time as populations genetically diverge, and are special because they represent in living populations what normally happens over time between long deceased ancestor populations and living populations, in which the intermediates have become extinct

Ring species "are only showing us in the spatial dimension something that must always happen in the time dimension. Richard Dawkins

Practical DifficultiesE. g. Testing allopatric populations; evaluating differences in fertility of offspring.The greatest practical limitation of the BSC lies in determining whether or not geographically segregated (allopatric) populations belong to the same species.

Geographic isolation is an extrinsic reproductive barrier.

Practical DifficultiesPopulations with intrinsic barriers to gene exchange can undergo independent evolutionary change, even if they should become sympatric. Range extension or colonization could well bring presently isolated populations in contact, so the evolutionary future of the populations depends on whether or not they have evolved reproductive isolation.Allopatric populations have been classified as species if their differences in phenotype or in DNA sequence are as great as those usually displayed by sympatric species in the same group.

When Species Concepts ConflictAllopatric populations that can be distinguished by fixed characters are species according to the PSC, but if the diagnostic differences are slight, advocates of the BSC may recognize the populations as geographic variants of a single species. In some cases, a local population of widespread species evolves reproductive isolation from other populations, which remain reproductively compatible with one another. Under the BSC, two species would be recognized; under the PSC, the various distinguishable populations of the paraphyletic group might be named as distinct species.

Figure 17.3 The phylogeny of some species and populations in the moth genus Greya

Barriers to Gene FlowGene flow between biological species is largely or entirely prevented by biological differences that have often been called isolating mechanisms.Other terms are isolating barriers or barriers to gene flow. Under BSC, speciation consists of the evolution of biological barriers to gene flow.

Barriers to Gene FlowBarriers to gene flow between two populations or species can be classified in various ways:

Premating vs. postmating

Prezygotic vs. postzygotic

Intrinsic vs. extrinsic

Premating Prezygotic BarriersA. Ecological isolation: potential mates do not meet.Geographic isolation: Do not live in same place, physically separated in space.

Temporal isolation: reproductive period occurs at different time of the year (or of the day).

Habitat isolation: live in the same locality but in different habitats, e.g. primarily aquatic while the other mostly terrestrial.

Immigrant inviability: Immigrants between populations do not survive long enough to interbreed.

Geographic Isolation by Formation of a Geographic Barrier (Vicariance)

Premating Prezygotic BarriersB. Potential mates meet but do not mate.Behavioral isolation: differences in courtship or life style.

Pollinator isolation: different pollinators respond to different colors, scents or forms of flowers.

Figure 17.4 Oscillograms of the songs of three morphologically indistinguishable species of green lacewings

Figure 17.6 Pollinator isolation in monkeyflowers

Postmating Prezygotic BarriersMating between species occurs but the fertilization of ova does not occur.Mechanical isolation: copulation occurs, but no transfer of male gametes takes place because of failure of mechanical fit of reproductive structures. Copulatory isolation: failure of fertilization because of behavior during copulation or because genitalia fail to stimulate properly.

Gametic isolation:

Postmating Prezygotic BarriersMating between species occurs but the fertilization of ova does not occur.Mechanical isolation:Copulatory isolation:Gametic isolation: gamete recognition is based on the presence of specific molecules on the coats around the egg, which adhere only to complementary molecules in the sperm.

Postzygotic BarriersPostzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult.Extrinsic: hybrid fitness depends on context.Ecological inviability: hybrids do not have ecological niche in which they are competitively equal to parent species.

Behavioral sterility: hybrids are less successful than parent species in obtaining mates.

Postzygotic BarriersIntrinsic: hybrid fitness is low because of problems that are relatively independent of environmental context.Reduced hybrid viability: hybrid zygote dies in the early stages of development or fails to reach sexual maturity.

Reduced hybrid fertility: hybrid does not produce functional gametes.

Hybrid breakdown: offspring of hybrids fail to produce functional gametes or do not reach sexual maturity.

Possible Causes of Hybrid InfertilityReduced fertility of hybrids can be caused by:

Structural differences between the chromosomes that cause segregation of some aneuploid gametes during meiosis: unbalanced number of chromosomes.

Different number of chromosomes in each of the parent species. Hybrid gametes cant undergo normal meiosis.Differences between genes from the two parents interact disharmoniously.

Possible Causes of Hybrid InfertilityHaldanes Rule (Haldane, 1922): Hybrid sterility and inviability is often reduced to the heterogametic sex: male in mammals and most insects; female in birds and butterflies. When in the offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous (heterogametic) sex.

Hybrid BreakdownHybrid breakdown occurs in the F2 generation and backcross offspring, between species and among different geographic populations of the same species The common interpretation of this phenomenon is that the F1 generation produced various combinations of alleles that were disharmonious. Alleles at different loci within the same population have presumably been selected to form harmonious combinations. They are coadapted and the population is said to have a coadapted gene pool.

F1 hybrid is not sterile but offspring are feeble and/or sterile.

Evolution3e-Table-17-01-0.jpg *Evolution3e-Fig-17-03-0.jpg *Evolution3e-Fig-17-04-0.jpg *Evolution3e-Fig-17-06-0.jpg *