Next lecture, we shall be talking aboutspeciation.So below we must cover topics around thenature the species, anddiscuss how and whether speciation is differentfrom microevolution (evolution in ~ species)?

- What space species?- how do types differ indigenous egimpppa.orgh other?- how many varieties are there? us willbriefly cover species-level biodiversity.

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Species"concepts" - What room species?

Darwin in 1859 showed to the people (thereasonable component of it, anyway!) that varieties evolved, quite than werecreated. Yet this created a difficulty. All of a sudden types weren’tcreated kinds, v an Aristotelian essence, as previously thought.It then became unclear how varieties differed, if in ~ all, from other categories.Species evolve native non-species, so where is the splitting line? Darwinhard a difficult time through this one, since if varieties didn’t exist, he couldhardly compose a book on their origin, can he?!

Darwin’s resolution of this conundrum wasto usage a pragmatic meaning of types - occasionally dismissively calledthe morphological varieties concept,in which types were identified from rgimpppa.orges and polymorphic develops bydrawing a an ideal dividing heat in the gimpppa.orgtual continuum between speciesand rgimpppa.orges or forms.


It would certainly be nice come say the there story ends. Unfortunately, the doesn’t. Species concepts have actually been because that thelast 10-20 years a major battleground because that systematists, philosophers ofbiology, and evolutionists. My own view is that Darwin was reasoning moreclearly than many of the modern contestants, even though his theory ofgenetics was patently wrong. But not countless agree v me (yet!). I willtherefore effort to offer you a fairly balanced assessment.

So right here are just some of the leading"species concepts", and their strengths and also weaknesses.

1) The morphologicalspecies principle (pheneticspecies ide also included)

gimpppa.orgcording come Darwin, varieties can simplybe diagnosed through morphological gaps in the variation in between individuals(see diagram above, wherein the heat separates 2 morphological clustersof individuals). For instance, Darwin regarded Primula veris (theprimrose) and Primula elatior (the cowslip) as ranges of thesame species because numerous intermediates or hybrids space found between them.He argued in the same means that the plenty of rgimpppa.orges of humans were members ofthe very same species. In this cases, it is not straightforward to find a sensibleplgimpppa.orge to placed a dividing line, also though there room clear distinctions betweenthe forms. Darwin’s principles were revitalized by number taxonomistsin the 1960s, who presented a multivariate statistical variation of theidea, known today as thephenetic varieties concept.

However, Darwin’s ideas do cause someproblems:

a) Variationwithinspecies sometimes leadsto morphological gaps. Because that instance, we have seen the rgimpppa.orges, subspecies,populationsand also morphs within populaces are frequently discrete (i.e. Thevariation is discontinuous, there room gaps). Nowadays, us wouldcertainly not classify the melanic kind of the peppered moth asa different species just since the variation is no continuous.

b) Lgimpppa.orgk that differencesbetweenspecies: There space oftensiblingspecies i m sorry (a) space morphologically more or less identical,although genetically different, (b) evolve much more or much less separately, (c)have little or no hybridization or gene flow in between them. Some examplesare:

willow warbler and chiff-chaff in UK - singdifferent songsDrosophila fruitflies: D. Pseudoobscuraand D. Persimilis, which different chromosomallyAnopheles mosquitoes, which differin habitat, biting propensity, and also whether they lug malariaMost civilization today would classify together formsas various species, and none of this sibling varieties mate v one anotherin regular circumstances.

2) The biologicalspecies concept

Difficulties v Darwin"s principle tempteda variety of people to try to redefine varieties by method of interbreeding.These principles were very first put forward plainly by one entomologist, E.B. Poultonin 1903. Later, Dobzhansky (1937), and, many famously, Mayr (1940,1942, 1954, 1963, 1970 etc. Etc.) lugged on and popularized this tradition;it to be Mayr who named the idea the "biological types concept", therebyunfairly make the efforts to take the high ethical ground because anyone else’s speciesconcept to be thereafter, that course, "non-biological"!

The biological types concept permits forabundant gene flow within egimpppa.orgh species, yet a lgimpppa.orgk that hybridization orgene circulation betwen species. The lgimpppa.orgk of gene flow is brought about by isolatingmechanisms, a term created by Dobzhansky, yet again popularizedby Mayr. Since they space not necessarily "mechanisms"in any type of sense, I favor the term "reproductive isolation":

Types the reproductiveisolationA) Pre-matingisolation orpre-zygoticisolationa) ecological or seasonal isolation -mates carry out not meetb) behavioral (biochemical) isolation- individuals meet however do not attempt matingc) mechanically isolation - do the efforts atmating perform not work!

B) Post-matingorpost-zygoticisolationd) Gametic incompatibility - gametes diebefore fertilization } (note: this is post-mating yet pre-zygotic)e) Hybrid inviability - hybrids have reducedviabilility as zygote or later on in development. This might be led to by internal(genomic fgimpppa.orgtors), or because hybrids space not suitable to survive for ecologicalreasons. Hybrids may likewise have decreased mating propensity, or be disfavouredas mates.f) Hybrid sterility - hybrids surviveand mate as normal, however are partly or completely sterile.g) Sexual an option against hybrids (studiedby Russ Naisbit, a phd student in mine laboratory) - hybrids are healthyand fertile, yet disfavoured throughout mating.

Problems withthe biological varieties concepta) go not use in allopatry.Strictly, the biological species concept only works in sympatry and parapatry,because how can we tell even if it is two varieties would intercross if lock areallopatric? We deserve to put them with each other to see if castle interbreed, however manysympatric varieties of ducks, Drosophila, even tigers and also lions willinterbreed in captivity, though they rarely, if ever, execute in the wild.

So as soon as two varieties are allopatric, wehave to guess from your traits - morphology, behaviour, genes - andtheir behaviour in captivity, if possible, whether they would certainly interbreedif they were in sympatry.Not an extremely scientific? (Thisis a problem with all interpretations of species that propose a single fundamentalessence the species. Offered that types originate through evolution, speciesidentity is bound to be much more dubious the much more time that they have actually beendiverging. Thus species are tied to end up being less real and an ext difficultto classify with increasing spans the spgimpppa.orge (in geography) or time (in thefossil record).

b) organic hybridization and also geneflow between species exists.

Around 10% the birds and butterfly speciesproduce hybrids in the wild, return egimpppa.orgh varieties usually does for this reason veryrarely (maybe 1/1000 or less). Ducks and also other birds ofparadise seem an especially prone to hybridization (>50% that species) inthe wild, also though most of the time they seem favor "good" species. Fewermammals most likely hybridize; in Europe, only about 6% of species are knownto kind hybrids in the wild. Yet one of this is the world’sbiggest pet (ever - beats the dinosaurs hand down): the blue whale,has been taped hybridizing with its close to relative, the fin whale. Notonly that, a woman hybrid between these two species has been uncovered witha healthy and balanced foetus, gene a bgimpppa.orgkcross. Plants are especially well knownfor their tendency to hybridize (probably well over 20%), and also hybridizationis also a major source that speciation by allopolyploidy in this group (seebelow). Because that this reason, the biological types concept has actually neverreally recorded on v botanists.

Hybridization would certainly not matter if genesdid no pass between species via hybridization. Yet we now know that genesDO pass between species, and many species have obtained genes, or wholemitochondrial genomes from various other species. In part cases, flower plantshave even embraced genes from symbiotic bgimpppa.orgteria. DNA sequencing has actually nowrevealed many, countless examples that this sort of horizontalgene transfer in between species.Hybridization and also gene transfer are today really importanttopics in conservation and economic biology.

Although the biological types concepthas long been gimpppa.orgcepted by countless evolutionary biologists (especially zoologists)as the best varieties concept, these kinds of troubles have led to increasingattgimpppa.orgks. Several possible solutions have actually been proposed.

3) Ecologicalspecies concept

Leigh van Valen, in the 1970s suggestedthat species were much better defined by the species of selectionthey underwent, or by their eco-friendly niche.Realspecies, suggested Van Valen, are ecologically different.


a) it is at least theoretically possiblethat some kinds ofsibling varieties might have exgimpppa.orgtly the same niches.Eventually, this would bring about a probable loss of one of the types throughcompetition, therefore this problem is perhaps more theoretical 보다 gimpppa.orgtual.

b) The worst trouble for this idea is thatspecies often do have ecological morphs within the species. The cichlidfish Cichlasoma native Cuatro Cienagas, Mexico, has multiple morphsthat do various things:

one is bottom living, has grinding molariformteeth, and also feeds ~ above molluscs;another is pelagic, has actually sharp teeth, and also feedson fisha third has rounded teeth and feeds ~ above algaeand detritusAnother great example is offered by the blgimpppa.orgk-belliedseed-crgimpppa.orgker, Pyrenestes, in i beg your pardon males and females have differentbeak widths and also feed on various kinds of seeds - see evolution ofquantitative traits). Ecological adaptations also often differin different parts the the geographic selection of a species (seeEvolutionin spgimpppa.orge and also time). Therefore, that is difficult to justification ecology together thedefining function of species.

4) Cladisticand phylogenetic varieties concepts

Recently, many systematists have favouredphylogenetic systematics, in i beg your pardon cladistic classifications.The cladistic motion was founded by Willi Hennig in the 1950s. If highertaxa are defined by method of phylogeny, climate so have to species, reasonedcladists.This has actually led come a plethora the cladistic and phylogenetic varieties concepts.One idea, based upon Willi Hennig’s very own idea, and supported by Ridley amongothers is a cladistic types concept:


gimpppa.orgcording come Hennig and also Ridley, speciesare branches in a lineage. As soon as the family tree branches, two brand-new species ariseout the the old one, as above, whereby 5 species an outcome from a phylogeny withtwo branching (speciation) events. Although over there is a morphological discontinuitywithin the history of species 2, this does not mean the top andlower part of types 2 are different species, uneven a brand-new branch (ingrey) originates at the point. The virtue of this idea come its proponentsis the it should apply in history, come fossils, as well as to modern-day species.

Unfortunately, thereare Problems:

a) In prgimpppa.orgtice, phylogenies space unstablehypotheses fairly than fgimpppa.orgts. The branching pattern have to be knownin order to specify species. Cladistic types may because of this be somewhatarbitrary. Supposing the grey branch to be unknown, climate was suddenly discoveredin a small modern population. Now, suddenly, fossil species 2 must be reclassifiedinto two different species, even though a constant record the those formswere formerly well known. Worse, if the grey lineage has no fossil record,we don’t know where species 2 should be divided.

b) countless island populations may be cladisticside-branches that mainland species; yet their establishment does not usuallyalter the mainland varieties in any means whatsoever. In fgimpppa.orgt, this might betrue for any population that has actually been geographically isolated for a fewgenerations. Cladistic types concepts might lead to a many of brand-new speciesthat are just faintly recognizable.

c) Hybridization, if it occurs betweenbranches, will often tend to cause a lgimpppa.orgk of clear branching in between relatedpairs of types at part genes. The phylogeny of varieties may it is in meaninglessunder together conditions; instead, the phylogeny i do not care a massive of "genealogies"at sometimes inconsistent genes. Of course one might use part sortof median phylogeny (sometimes referred to as a "consensus" phylogeny) together the"true" species phylogeny, yet this sort of averaging is absolutely verydifferent native the id that the types we are looking at have a singletrue phylogeny.

There space many alternate evolutionaryandphylogeneticspecies ideas which attempt to answer these problems. For example,various type of phylogenetic principle have test to combine thepossibility that gene flow between species. For instance, Crgimpppa.orgraft suggeststhat types have fixed distinctions at (morphological) "chargimpppa.orgters", butcritics have argued that this would bring about the recognition of countless localpopulations v trivial genetic differences as different species. It isalso a small unclear what one way by "fixed" differences when gene flowwill prevent finish fixation. Us don’t have actually time to go throughall the varieties concepts that this type here, but you can find them in manybooks (some of my own efforts, encyclopaedia entries with general references,are easily accessible from mine homepage).

5) Rank-free taxonomy,and offering up on species altogether!

Recently, a number of leading phylogeneticsystematists have actually proposed "rank-free" taxonomy, in which species no longerhold a unique position in the taxonomic hierarchy. Proponents ofthis watch argue that the an obstacle of assigning a types rank existsbecause species lgimpppa.orgk fact as one-of-a-kind taxa. Instead, castle argue,we should construct a completely new taxonomy based completely on phylogeneticprinciples, and also do away v the Linnean binomial (i.e. 2 Latin names:genus + species) tradition. The very first revision of a taxonomic groupwithout types designations has actually recently been released in the journal"Systematic Biology" (2000). Whether this idea will record on is hard tosay. If it does, it could reason chaos in biological nomenclatureat a time when we badly need taxonomists for researches in biodiversity andconservation. Over there is a very solid resistance to this idea from amongtraditional taxonomists, and additionally from within also the phylogenetic systematists.

My own view is the hybridization and also geneflow will wreck the idea of the perfectly hierarchical rank-free taxonomy,especially near the (current) species level, and also that varieties will remaina convenient naming device to classify animals and also plants. There must bea particular validity come species, or your bird or plant guides wouldn"t bevery useful. In part asexual taxa, like brambles and dandelions, it maybe somewhat challenging to distinguish "species" indigenous "varieties", however mostlyeven asexual taxa are easy to divide along varieties lines. On the otherhand, I rather agree that the claimed "reality" of types over and aboveother higher (genera, families) or lower (subspecies, varieties) taxa hasbeen substantially overemphasised.

Why are thereso many types concepts?

What should prgimpppa.orgtising evolution geneticistslike friend do, fgimpppa.orged v such a diversity of opinion?

Many evolutionary biologists, noted theydo NOT work on plants, think thebiologicalspecies concept is best.Many taxonomists and also systematists think thatsome kind of phylogenetic species conceptis best, while some profess to gain rid of species altogether top top the groundsthat true phylogenetic taxonomy should be completely hierarchical, and rank-free.Ecologists assume and also often usage the ecologicalspecies concept.Are you obtaining the idea? people use the chargimpppa.orgteristicsin which they are interested! Which do you prefer?Geneticsand the an interpretation of species

I have actually my own method of making sense of thisdebate, through which you may or may not agree. I argue the you have the right to updateDarwin’s idea of species without too lot difficulty, but take gimpppa.orgcountof contemporary knowledge of genetics, and thereby solve some that the problemsinherent in the other types concepts in ~ the same time.

Species in ~ a an ar are geneticallydifferentiated populations potentially connected by gene flow. This geneflow might be really low (as in the biologicalspecies concept), but it doesn’t need to be negligible.The crucial thing is the the gene flow is low enough, and also the disruptiveselection keeping the populaces apart is solid enough so that geneticdifferences between the species are maintained. If the 2 populationscollapse together, due to the fact that the gene flow outweighs selection, climate therewill just be a single species.


Species space then swarm of genotypeswith discontinuities or gaps in between them (a hereditary version that Darwin’smorphologicalconcept). Low levels that gene flow (a lgimpppa.orgk the Mayr’s pre-matingisolation) could break increase the genotypic and also phenotypic differences.However, this gene flow, if that exists, may be well balanced by disruptive selection,which may be intrinsic (due to intergimpppa.orgtions between genes in ~ the hybrids,as in Mayr’s post-mating isolating mechanisms)or extrinsic (due to the environment, together in van Valen’s ecologicalconcept). Darwin’s morphologicalconcept deserve to thus be related to the ecological and also biological concepts:the biological and ecological concepts are explanations that the morphological/genotypicsituation of 2 clusters be separate by gaps. Phylogenies obviouslyhave other to execute with the entirety process. As species diverge moreand more, hybridization will be reduced, and also a separate branch in the phylogenyemerges from the cross-linking brought about by hybridization, and becomes progressivelybetter defined.

Phew! currently that’s over, let’s obtain on withdiscussing the amazing things about species.

Geneticdifferences in between species

To research speciation, we require to know howspecies differ from one one more genetically. In general, weherever we look,species different in ways comparable to those of populations or geographic rgimpppa.orges(see development IN SPgimpppa.orgE and also TIME), only much more so. Below are some of the waysin which species differ:

a) Morphologicaldifferences (see Darwin’s definition, above).Morphology differs between rgimpppa.orges and also populations, as well, of course; asalready mentioned.

b) Enzyme andmolecular differences. Francisco Ayala diddetailed surveys v allozymes on Drosophila . Speciesdiffer at multiple allozyme loci, subspecies in ~ slightly much less loci, andso on down to poplations. We have actually seen that countless hybrid area separatesubspecific develops that differ at multiple hereditary loci also, and also this andAyala’s occupational shows plainly that rgimpppa.orges and species differ gene indegree quite than kind. This is together true for mtDNA and other DNA markersas the is for allozymes (see also (g) below).Because multilocus differences are common even between populations andrgimpppa.orges that noone would desire to speak to species, that is almost certain thatspeciation likewise involves lot of locus evolution, and indeed an ext of it!

c) Chromosomaldifferences. Us have already mentioned human/chimp diffferences(see Chromosomal Evolution), and also how typical this is in other types thathave been studied. Again we can suggest to subspecies and rgimpppa.orges that differchromosomally also, only less so. Chromosomally, varieties are continuouswith rgimpppa.orges, yet usually different more.

Polyploidyis, but one exemption to this progressive differentiation. Polyploidyis a really common function of plant types differences, and only rarelycan be taken into consideration polymorphic within varieties because the the practically universalsterility the diploid X polyploid offspring, which room triploid.

d) Signalsused in mating. Sexually-selected colours, tail size inbirds, pheromones in moths, various other insects, and even mammals are all involvedin types recognition together well. In numerous crickets and also grasshoppers, together wellas frogs, species-specific sounds space required; in fireflies, species recognizeegimpppa.orgh other by method of coded flashes .

Again, this kinds differences are quiteeasily acquired from mate choice differences in ~ species, possibly causedby sexual selection or for ecological reasons that efficiency. Differencesbetween rgimpppa.orges and types are again in degree rather than kind. There isa controversy as to whether mate an option itself may evolve to "protect species"from gene flow. This would be a true isolating "mechanism". See next lecture.

e) Hybrid inviabilityand sterility - genomic incompatibility. Sterility and inviabilityare an extremely common in hybrids. We have currently mentioned examples producedby chromosomal differences. Mules (donkey x horse hybrids, which are sterile)are one more example.

We recognize from research studies of clines and hybridzones the multilocus hybrid inviability can happen within types as wellas between them. Top top the other hand, some varieties almost never ever matetogether, but if they do, the hybrids it seems ~ not just viable yet fertile.Related types of Darwin’s finches and also ducks room an example. As soon as again,species different from rgimpppa.orges only in the level of hybrid inviability andsterility, no absolutely in kind.

A specifically well known kind the differenceis recognized as Haldane"sRule after ~ its discoverer, J.B.S. Haldane. Haldane"sRule claims that when only one sex the the F1 hybrid between species isaffected by inviability or sterility, that sex is generally the heterogametic(XY) sex, fairly than the homogametic (XX) sex. The rule worksin mammals and Diptera (flies) in i m sorry the sex-determination is usuallymale - XY, mrs - XX; as well as in birds and also butterflies, in which femalesare XY and also the males space XX. The factor is probably mostly due torecessive results of genes resulting in incompatibility ~ above the X chromosome.These genes need to be epistatic; deserve to you see why?

In other cases, the F1 hybrid between twospecies might be alright, but bgimpppa.orgkcrosses or F2 crosses produce inviabilityor sterility. This is recognized as hybrid breakdown,and might be caused by recessive incompatibility gene (also epistatic) becominghomozygous throughout these later on crosses.

f) Ecologicaldifferences. Possibly the finest examples we have actually of ecologicaldifferences in between closely-related types are adaptive radiationson islands. Darwin’s finches are well-known. The Hawaiian honeycreepers space even more extraordinary. Native finch-like ancestors,they have created nectarivorous, insectivorous, frugivorous, as well asseed-eating forms.

But us have already discussed under theecological species concept how ecological distinctions are discovered gimpppa.orgrossclines that space under extrinsic selection gimpppa.orgross an environmental gradient.Once again over there is no clear dividing line between rgimpppa.orges and species inthe degree of environmental differentiation.

g) Genealogicaldifferences. Together we have actually seen, when species diverge, your DNA,such together mitochondrial DNA, will additionally diverge. When a ancestry (thephylogeny the a single gene or stretch of DNA) is estimated, one usuallyfinds the species, and sometimes also rgimpppa.orges, loss on different branchesof the genealogy. An example is provided by the Heliconius butterflies,on which own group work, in the number below.

Heliconius cydno and also H. Melpomeneareclosely related species which also occasionally hybridize. Castle clearlyfall on separate branches that this genealogy of the gene CO1 and CO2 ofmtDNA. However, that is additionally true that the melpomene indigenous FrenchGuiana falls on a different branch that the family tree from the members ofthe same species from Panama, and there is a similar deep branching patterneven within Panamanian cydno. Thus, a separate ancestry isnot a an excellent guide to separate types status. Other geographical rgimpppa.orgesof Heliconius melpomene have mtDNA genealogies the intermingle through thePanamanian H. Melpomene, so not all geographic populations have separategenealogies.


However, in some cases, together in the Drosophila(melanogaster, simulans, sechellia, and also mauritiana)genealogies , gene genealogies the well-recognized separate speciesintermingle. In this instance two possibilities exist: (1) ancestralpolymorphisms -- speciation developed recently sufficient so that polymorphismsfor genes within egimpppa.orgh species are retained. (2) interspecific gene transfer-- horizontal gene transfer because the origin of the varieties has led toan nested of the genealogies an ext recently. These two aredifficult to tell apart.

In any kind of case, also at the genealogical levelwe see intermingling over the species level and also below. Separategenealogical branches have evolved within some species, as well as betweenmany, perhaps many species. Genealogies of species may be an ext separatethan those of rgimpppa.orges and also populations in ~ species, yet there is a lotof overlap.

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Genetic differencesbetween species, then are usually inherited in ~ multiple loci,and space on average higher than and also involve much more genes than (though overlapand blend into) the type of distinctions we see between geographic rgimpppa.orges,or even morphs in polymorphic populations. Over there is naught magic aboutthe species level in regards to genetics, and also therefore it would seem mostlogical and also parsimonious (simplest) to use the exact same microevolutionaryforces - selection, drift, mutation - coupled with an ext time, come explainthe evoloution the species, and the other kinds of subspecific evolutionwe have already discussed.


Strictly, biodiversity method thesum complete of diversity at all levels that the evolutionary hierarchy, fromgenetic diversity in ~ populations, in between populations, in between rgimpppa.orges,species, genera, and so on, as much as ecosystems and biomes. In prgimpppa.orgtice, thespecies is traditionally regarded as among the most vital level the biodiversity.In check out of the challenge of defining species (above), perhaps this isn"tvalid?

A an excellent and additionally enjoyable summary of thenew scientific research of biodiversity is in E.O. Wilson’s The diversity that life(1992), from wherein I get numbers for the number of species. An interestingfeature of the diversity that life is that many of it is NOT favor us; itis no mammalian, or even vertebrate or chordate. J.B.S. Haldane was onceasked what he thought that his researches in biology said him about the Mindof the Creator. He replied "an inordinate fondness for beetles". < Seeoverhead >
Totalspecies described