Chapter 17 Origin of Species

Chapter 17 Origin of Species
Figure CO: Mimulus Flower © LubaShi/ShutterStock, Inc.

Overview A few of Darwin’s predecessors had broken with western civilization’s and the Christian religion’s tradition of seeing species as unchanging and having originated from separate creations For Darwin and Wallace, speciation was the process by which new forms arose from ancestors, and natural selection was the main means by which speciation occurred

Overview Darwin and his contemporaries amassed evidence that adaptive radiations occurred when a species moved to a new environment, or the current environment changed, so that new niches and their resources could be utilized Darwin also demonstrated that evolutionary change could be driven by sexual selection, even when sexually selected phenotypic traits seemed maladaptive to general survival

Overview Proponents of the Modern Synthesis established that:
the Hardy-Weinberg Principle and its underlying assumptions provided the model for identifying causes of evolutionary changes in natural populations natural selection had three modes: stabilizing, directional and disruptive reproductive modes and sexual behaviors contribute to changes in genotype and phenotype some evolutionary changes in natural populations occur in the absence of natural or sexual selection

Adaptation and Differentiation
Adaptation to a changing environment, in the simplest case, will be driven by directional selection, and if enough change in genotypes and phenotypes occurs, an ancestral species will be transformed into a new descendant species Adaptive radiation signifies the rapid evolution of one or a few forms into many different species occupying a variety of habitats within a new geographical area

What Interests Us About Speciation?
Speciation provides evidence that evolution occurs Speciation provides important insights into the mechanisms of evolution Patterns of speciation provide insight into the distribution patterns of extinct and living organisms Speciation explains patterns in the ecology and reproductive biology of organisms

What Interests Us About Speciation?
What are the causes of speciation? What are the rates of speciation and do they differ among different taxa?

Speciation The evolutionary formation of new species in space or time, usually by the division of a single species into two or more genetically distinct ones Species share the same gene pool, or the sum of all genetic codes possessed by individual members of that species Speciation both marks and crosses the boundary between microevolution and macroevolution

The Species Problem Although most biologists accept the species as a real biological entity, just as they do the individual organism, it has not been easy to define the species or to indentify the boundaries between species often we still lack adequate data Anti-evolutionists seize on these difficulties to claim evolution does not occur They miss the point; the nature of reality is difficult to pin down, even for scientists! Physicists, chemists and geologists would agree

Species Definitions Morphological / Typological Species: a set of organisms sharing structural similarities between members and discontinuities in structure between different species Mayr’s Biological Species: groups of interbreeding natural populations that are reproductively isolated from other such groups Ecological Species: a set of organisms adapted to a particular set of resources, called a niche, in the environment Genetic species: A set of organisms exhibiting similarity of DNA

Species Definitions Agamospecies: a set of organisms in which sexual reproduction does not occur, represented typically as a collection of clones Chronospecies / Paleospecies*: a set of extinct organisms which changes in morphology, genetics, and/or ecology over time on an evolutionary scale such that the originating species and the species it becomes could not be classified as the same species had they existed at the same point in time [*Note: experts establish fine distinctions between chronospecies and paleospecies] Phylogenetic (Cladistic) / Evolutionary Species: a set of organisms that shares a common ancestor and maintains its integrity with respect to other lineages through both time and space Ring Species: a set of generally hybridizing species with a geographic distribution that forms a ring and overlaps without hybridization at the ends

An Important Reminder Regardless of species definitions, a species, to be a biological entity, must exist in an ecological niche Can a population of organisms which no longer has a niche still be a biological entity? Only Siberian tigers still exist within their range

Problems Defining Species Through Time
Morphospecies: Viewed today, at one moment in time, species A, C, and E are clearly distinct species, demarcated by current natural discontinuities between them

Problems Defining Species Through Time
Paleospecies (chronospecies): Viewed historically, through time, discovered fossil intermediates (B and D) fill in the missing gaps above, giving us a more or less continuous series with no obvious morphological discontinuities between them [still an over-simplification]

The Process of Species Formation
In the beginning, there is a single population with a common shared gene pool A discontinuity develops among some demes Changes in allele frequencies develop at various loci in the gene pools (and, usually, changes in phenotypes) of the demes Separate evolution of demes continues until one or more have diverged to the point that each deme now meets one of the definitions of a species concept

Species Can Change Without Speciation Being Initiated
Local adaptations may occur among the populations and demes of a species Even when a species consists of many subpopulations, gene flow between subpopulations may slow or even inhibit local specializations

Species Can Change Without Speciation Being Initiated
Gene flow promotes continuance and stability of the species

Speciation Branching speciation and adaptive radiations can be initiated when genetic exchange within or among populations is impeded Directional selection produces anagenesis Disruptive selection produces cladogenesis

Speciation Four different processes have been defined to describe different modes of speciation (cladogenesis) in sexually reproducing species, most of which satisfy the definition of Mayr’s Biological Species Concept These same four processes may also explain speciation in many asexual organisms, but isolating mechanisms will be different The processes differ in how the evolving demes are distributed geographically

Spatial Aspects of Speciation
Allopatric speciation – a physical barrier divides a continuous population Peripatric speciation – a small founding population enters a new or isolated niche Parapatric speciation – a new niche found adjacent to the original niche Sympatric speciation - speciation occurs without physical separation inside a continuous population

Spatial Aspects of Speciation
As you can see in this diagram, peripatric and parapatric are very similar Peripatric does involve crossing some sort of barrier to find favorable habitat

Speciation Initiated by Geographical Isolation
three steps geographical isolation local adaptation reproductive isolation three forms of geographical isolation sometimes called a porous or permeable barrier Figure 01C: Parapatric Figure 01A: Allopatric Figure 01B: Peripatric

Allopatric Speciation
Allopatric speciation: when an ancestral species population is divided because of a natural physical barrier or because intervening geographical populations become extinct This produces vicariance distributions

Since smaller habitats generally are somewhat different from one another, natural selection will alter the demes on the opposite sides of the barrier over time to improve their adaptations to the different habitats

Mechanisms that prevent interbreeding may be: ecological: seasonal breeding, migration, etc. habitat preference differing abiotic factors, etc. behavioral activity times food acquisition, etc. physiological reproductive biology fertilization embryonic development [More on this later in the chapter.]

Four steps lead to speciation: mountains and deserts are classic examples of new barriers A single species is an interbreeding reproductive community A barrier develops, or a dispersal event occurs, dividing the species Separated into different habitats, the divided populations diverge through the accumulation of gene and trait differences The separate populations become so different that, if and when the barrier disappears and they overlap again, interbreeding does not occur

The populations of Tamarin monkeys (family Callitrichidae) are separated on the sides of the Amazon River Where the river tributary is wide and individuals on opposite banks do not interbreed, the populations are diverging toward separate species Where the river tributary is narrow, the individuals still interbreed

Vicariance events provide complete geographic isolation for two robber fly genera, Tillobroma and Hypenetes

American dogwood, Cornus florida Chinese dogwood, Cornus kousa We mentioned a similar vicariant distribution for Catawba trees in CH 16

American alligator, Alligator mississippiensis Chinese alligator, Alligator sinensis

American hellbender, Cryptobranchus alleganiensis A. japonicus Andrias davidianus Chinese giant salamander , Andrias davidianus and Japanese giant salamander Andrias japonicus

Incomplete or Peripheral Isolation
Ernst Mayr and Theodosius Dobzhansky, speaking for the Modern Synthesis, emphasized that most speciation was allopatric, i.e., geographical isolation required Two modifications or variants have been proposed: Peripatric speciation – a small population enters a new or isolated niche [originally proposed by Mayr, and related to the founder effect and genetic drift altering the isolate’s gene pool] Parapatric speciation – a new niche found adjacent to the original niche

Peripatric speciation: when a population is divided because of the budding off of a small completely isolated founder colony from a larger population so that gene flow is minimal Parapatric speciation: when a population at the periphery of a species adapts to a different environment but remains contiguous with its parent so that gene flow is possible between them

Peripatric vs. Parapatric speciation
Peripatric speciation is caused by being at the edge of the range and almost isolated geographically ↔ geographic isolation leads to genetic isolation Parapatric speciation is by becoming genetically isolated which leads the population to become geographically isolated ↔ genetic isolation leads to geographic isolation Both are less common and more difficult to demonstrate since small niche and habitat differences rarely have fossil records

Peripatric and Parapatric Speciation
Speciation triggered by partial isolation (peripatric and parapatric) are now argued as being as or more important in explaining speciation events than classic allopatric speciation Eldridge and Gould’s Theory of Punctuated Equilibria is just one example of advocating for their importance dispersal or vicariance Systematists and paleontologists need more data to resolve that technical debate

Peripatric Speciation
sweepstakes dispersal events provide complete geographic isolation and reduce migration to near zero

Northern Chestnut-tailed Antbird Myrmeciza castanea dispersal Potential peripatric speciation triggered by partial isolation in a Bolivian natural forest island isolated 3000 years from the larger continuous forest habitat during a dry period Divergence in song and certain alleles frequencies between the two populations (reproductive isolation) suggest that incipient speciation is under way (Southern Chestnut-tailed Antbird, Myrmeciza hemimelaena and Northern Chestnut-tailed Antbird, Myrmeciza castanea)

Two male hammer-headed picture-wing Drosophila battle for mating territory. Figure B01A: “Picture-winged" group of Drosophila Figure B01B: “Picture-winged" group of Drosophila Figure B01C: “Picture-winged" group of Drosophila Modified from Edwards K.A, Doescher L.T., Kaneshiro K.Y., Yamamoto D., (2007) A Database of Wing Diversity in the Hawaiian Drosophila. PLoS ONE 2(5): e487. doi: /journal.pone Courtesy of Kevin Edwards , Illinois State University. Figure B02: “Picture-winged" group of Drosophila Adapted from Carson, H.L. Drosophila Inversion Polymorpism. CRC Press, 1992.

Parapatric Speciation
dispersal Potential parapatric speciation in sweet vernal grass/buffalo grass, Anthoxanthum odoratum, triggered by adaptation to heavy metal contaminated soils in many locations globally Divergence in flowering times (reproductive isolation) between the two populations suggests that incipient speciation is under way

Parapatric Speciation
Adjacent populations evolve into distinct species while maintaining contact along a common border BULLOCK’S ORIOLE HYBRID ZONE BALTIMORE ORIOLE

Ring Species—Salamanders
The ensatina salamander (Ensatina eschscholtzii) occurs from Canada to Southern California with interbreeding between adjacent populations through this range The Central Valley—a dry, hot lowland area—is divided into a coastal arm and inland arm However, where these two arms of the species meet again in Southern California, interbreeding does not occur Ring species are often considered examples of parapatric speciation

Ring Species - Herring Gulls
As glaciers retreated, herring gulls (Larus argentatus) were released out of a north Pacific refugia spreading one way across North America and into western Europe; and spreading in the other direction across Alaska into Siberia From Siberia, as the herring gull now extended its range further across Asia, it tended to differentiate, producing a subspecies (or species by some ornithologists) such as the vega gull (Larus vegae) and farther west the lesser blackbacked gull (Larus fuscus) Eventually its current circumpolar distribution became established (dashed lines) Adjacent subspecies interbreed (solid arrows), but where the ends of the circular range of the herring gull meet and overlap in Europe, there is very little interbreeding (dotted lines). (Simplified originally from Mayr, 1963) Ring species are often considered examples of parapatric speciation

Speciation Without Geographical Isolation
An important debate in evolutionary biology has been whether speciation can be initiated sympatricly by mechanisms that reduce gene flow within a population in the absence of initiating geographical isolation Can a deme experiencing disruptive selection accumulate gene pool change without the allele frequency changes being swamped by migration into the deme from neighboring demes?

Speciation Without Geographical Isolation
Recall that the other modes of speciation require three steps: geographical isolation local adaptation reproductive isolation Is it possible to skip the first step and have speciation without any geographical isolation within the continuous ancestral population? The biometricians thought so . . .

Sympatric Speciation Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region there is no geographic constraint to interbreeding the term was invented by the British entomologist Sir Edward Bagnall Poulton ( ) in 1904

Sympatric Speciation The first model was put forward by John Maynard Smith in 1966 Maynard Smith suggested that homozygotes (AA and aa) might, under particular environmental conditions, have a greater fitness than heterozygotes (Aa) for a certain trait Because of disruptive selection, therefore, homozygotes would be favored over heterozygotes, eventually leading to speciation (1920 – 2004)

Sympatric Speciation Ernst Mayr completely rejected sympatry
Debate continues on how important and widespread sympatric evolution may be But well documented empirical evidence for sympatric evolution exists, and sophisticated theories incorporating multilocus genetics have been developed Australian biologist Michael J.D. White ( ) wrote one of the first books to document the evidence: Modes of Speciation (1978)

Sympatric Speciation Sympatric divergence could also result from sexual selection Reproductive isolation may develop through changes in behavior, microhabitat, seasonality of breeding, or chromosomal mutation or ploidy events Sympatric speciation is common in plants; less common in animals It is often difficult to confirm sympatric origin

Sympatric Speciation 4N 4N The composites, salsify plants, from eastern Washington include a tetraploid hybrid derived from two diploid species The many polyploid hybrids species such as these are the best examples of sympatric speciation, including a few animal polyploid species

Sympatric Speciation Crater lakes and oceanic islands provide optimal locations for studying sympatric speciation because differentiation between sister taxa found at these locations is likely to have occurred in situ Clockwise from top left, Amphilophus citrenellus, A. zaliosus [cichlid fish], Howea forsteriana, H. belmoreana [palms], Lord Howe Island, S. Pacific, Craterlake Apoyo, Nicaraugua

Sympatric Speciation Sympatric African Indigobirds are host specific nest parasites Their hosts rear their young but their young do not destroy the host’s young, as cuckoos often do

Sympatric Speciation Polymorphic mimicry in Heliconius cydno alithea in western Ecuador, where the white form (middle left) mimics the white species Heliconius sapho (top left) and the yellow form (middle right) mimics the yellow species Heliconius eleuchia (bottom right). Heliconius cydno alithea Sympatric Neotropical Heliconius butterfly species are Mullerian mimics Their common toxicity is cyanide derived from cyanoglucosides in various Passiflora , passion flower vines, host plants eaten by the larvae The two H. cydno alithea color morphs exhibit assortative mating

Cichlid Fish, Family Cichlidae
To the right are cladograms for the cichlids of Africa (blue), the Neotropics (green), Madagascar (pink) and East Asia (yellow) to the level of genus The upper cladogram uses the fragmenting of Gondwana as its basis The lower cladogram uses the fossil record of the cichlids

East African Crater Lake Cichlid Fish
East African Crater Lake Cichlid Fish, members of the globally distributed tropical family Cichlidae, provide some of the better studied cases of sympatric speciation in nature Figure 02A: A cichlid Malawi (Maylandia estherae) © Wawritto/Dreamstime.com Figure 02B: A red terror (Cichlasoma festae) cichlid © Savone/Dreamstime.com Figure 02C: A blue cichlid (Cyphotilapia frontosa) © Showkontor/Dreamstime.com

Figure 02D: A blue cichlid (Pseudotrophues demasoni) Figure B03: Head of a cichlid fish © Moori/Dreamstime.com Figure 02E: A red cichlid (Hemichromis sp. Bangui) © Schoor/Dreamstime.com Fish with pharyngeal jaws can adapt their oral jaws to prey capture or food acquisition while the pharyngeal jaws can process the ingested food matter Figure 02F: A purple or kribensis cichlid (Pelvicachromis pulcher) © Ecophoto/Dreamstime.com

lower pharyngeal jaws and teeth of cichlid fish Figure B04A: Lower pharyngeal jaws and teeth of cichlid fish Figure B04E: East African cichlid Figure B04B: East African cichlid Figure B04F: East African cichlid Figure B04C: East African cichlid B and D show pharyngeal teeth adapted to cutting or chopping while the rest are adapted to grinding Figure B04D: East African cichlid Photos a, b and e, courtesy of Dr. Axel Meyer, University of Konstanz and photos c, d and f, courtesy of courtesy of Dr. Ann Huysseune, Ghent University

DNA and other characters confirm monophyly Figure 03B: Cichlid species - cladogram Adapted from Kocher, T. D., J. A. Conroy, K. R. McKaye, and J. R. Stauffer, Mol. Phylog. Evol., 2 (1993): Above are two clades from two east African lakes, Lakes Tanganyika and Malawi (Nyassa) There are many pairs of ecological equivalents present We presume they arose from a few founders and then speciated without any geographical isolation Their speciation was likely driven by ecological specialization, niche partitioning and by assortative mating Figure 03A: Cichlid species from Lakes Tanganyika and Malawi (Nyassa)

Speciation Types: Summary
A population with common gene pool Discontinuity develops among subpopulations Different selection pressures applied in different niches drive evolutionary change Reproductive isolation develops as the new species evolve peripatric

Recall that non-sympatric speciation requires three steps: geographical isolation local adaptation reproductive isolation Notice that sympatric speciation may end up producing the same species distribution patterns as allopatric speciation This is one reason it has been difficult to demonstrate sympatric speciation Figure 04a-e: Two modes of speciation Adapted from Strickberger, M. W. Genetics, Third edition. Macmillan, 1985.

All four modes of geographical speciation assume that local adaptation and reproductive isolation will occur Individual cases may show local adaptation to precede reproductive isolation; others may show reproductive isolation to precede local adaptation (assortative mating, changes in ploidy, etc.); and still others may show local adaptation and reproductive isolation to occur simultaneously Another accomplishment of the Modern Synthesis was to characterize the mechanisms of reproductive isolation

Reproductive Isolating Mechanisms (RIMs)
Different types of mechanisms that can prevent reproduction between individuals of different species RIMs are also referred to as prezygotic or premating, versus fertilization or mating, versus postzygotic or postmating mechanisms prezygotic or premating mechanisms geographic, ecological, behavioral, and temporal isolation (mating mechanisms) still prezygotic mechanical mechanisms and gamete incompatibility postzygotic or postmating mechanisms zygotic mortality, hybrid inviability, hybrid sterility, and hybrid breakdown

Geographical (Reproductive) Isolation
Iguana iguana Conolophus subcristatus Amblyrhynchus cristatus The two Galapagos iguana genera are, themselves, ecologically isolated from each other

Ecological (Reproductive) Isolation
Water or cotton-mouth moccasin is semi-aquatic, feeds on aquatic vertebrates, and is aggressive Copperhead is terrestrial, feeds on terrestrial vertebrates, and is less aggressive Agkistrodon piscivorus Agkistrodon contortrix

Behavioral (Reproductive) Isolation
Anolis garmani Anolis trinitatis Members of the genus Anolis on Jamaica chose different perches and use different patterns of head bobbing to attract female anoles They also have separate ecological niches Anolis opalinus

Temporal (Reproductive) Isolation
Members of the genus Magicicada, exist in temporaly separated populations, three species of 17 year cicadas, and four species of 13 year cicadas There is also some geographical isolation within the 17 year cicadas in the northeastern US and the 13 year cicadas in the southeastern US 13 year cicada 17 year cicada

Mechanical (Reproductive) Isolation
Members of the genus Parafontaria, Japanese millipedes, differ in body size and in the size and shape of their reproductive gonopodia

Mechanisms Facilitating Reproductive Isolation
Prezygotic mechanisms may be somewhat more advantageous to the species when they develop, because they prevent individuals from mating with members of the other species – this saves energy and prevents the waste of gametes Postzygotic mechanisms may be less advantageous from that perspective, but, on the other hand, may allow some alleles to pass from one species to the other, a form of horizontal gene transfer often called introgression

Introgression A 2010 study by Svante Pääbo and his colleagues confirms that genetic “introgression” occurred in the human lineage, and that between 1% and 4% of the DNA (SNPs) of Europeans, Asians, and Papua New Guineans is attributable to hybridization between “modern” humans and Neanderthals.

Mechanisms Facilitating Reproductive Isolation
On Isla Daphne in the Galapagos, hybrids formed between the medium ground finch and the cactus finch The hybrids were intermediate in phenotype for bill size and inferior competitors in normal years After a very rainy season, however, their numbers increased because their intermediate sized bill was best adapted to the plants that flourished in the wet Because female hybrids tended to mate with non-hybrid males with distinctive species specific mating songs, genes passed from one species to the other This horizontal gene transfer is probably beneficial to both species, even though its occurrence is accidental

Prezygotic mechanisms: Factors which prevent individuals from mating Geographical isolation already discussed Temporal isolation; Ecological isolation; Behavioral isolation; Mechanical isolation ― Gametic incompatibility: Sperm transfer takes place, but the egg is not fertilized Postzygotic isolating mechanisms: Genomic incompatibility, hybrid inviability or sterility Zygotic mortality: The egg is fertilized, but the zygote does not develop Hybrid inviability: Hybrid embryo forms, but is not viable Hybrid sterility: Hybrid is viable, but the resulting adult is sterile Hybrid breakdown: First generation (F1) hybrids are viable and fertile, but further hybrid generations (F2 and backcrosses) are inviable or sterile

Zygotic Mortality In sheep and goat crosses fertilisation takes place but the hybrid embryos die in the early developmental stages (Dobzhansky, Ayala, Stebbins and Valentine, Evolution. W. H. Freeman)

Hybrid Inviability Drosophila simulans Drosophila melanogaster Drosophila melanogaster and D. simulans have incompatible nuclear pore protein alleles

Hybrid Inviability The four groups of leopard frogs (Rana sp.) resemble one another closely in their external appearance But early tests of interbreeding produced defective embryos (hybrid inviability) in some combinations, leading biologists to suspect that these might be different subspecies or even different species Research on males’ mating calls indicates that the various groups differ substantially, and that such prezygotic behavioral isolation separates and reproductively isolates members of each group, producing four species: (1) Rana pipiens; (2) Rana blairi; (3) Rana utricularia; (4) Rana berlandieri

Hybrid Sterility mule hinny liger tigon
These hybrids have reduced, if not absent, fertility, though they are often otherwise healthy

Hybrid Breakdown In the parasitoid wasp genus Nasonia, F2 hybrid males of Nasonia vitripennis and Nasonia giraulti experience an increased larval mortality rate relative to the parental species. Previous studies indicated that this increase of mortality is a consequence of incompatibilities between multiple nuclear loci and cytoplasmic factors of the parental species, but could only explain ∼40% of the mortality rate in hybrids with N. giraulti cytoplasm. Hybrid larvae that carry the incompatible allele on chromosome 5 halt growth early in their development and ∼98% die before they reach adulthood. Nasonia giraulti

Over time, as a pair of sibling species diverge, reproductive isolation may increase by the development of multiple forms of both premating and postmating isolating mechanisms, as illustrated here with two hypothetical species of salamander

normally, the barriers separating species are not caused by a single isolating mechanism seasonal habitat behavioral mating isolating mechanisms have been the most thoroughly studied in Drosophila and in a a variety of plants

Sexual Isolation in Sympatric and Allopatric Populations
Here is Drosophila data which demonstrates that reproductive isolating mechanisms are more important when species arise in sympatry The X axis, genetic distance for species pairs (sister species) indicates that allopatric (geographically isolated) pairs may have almost no sexual isolation when their genetic distance is low, but sympatric pairs have much higher degrees of sexual isolation Figure 05: Sexual isolation for pairs of allopatric and sympatric Drosophila Adapted from Coyne, J. A., N. H. Barton and M. Turelli, Evolution 51 (1997):

Genetic and ecological isolation may be occurring at the same time, or before or after reproductive isolating mechanisms form Not all RIMs are required for any particular speciation event The sequence in which RIMs develops is also unique to each species

Patterns of Speciation
Regardless of species definitions, a species, to be a biological entity, must exist in an ecological niche Sometimes, the abiotic factors important in a species’ niche vary in a regular fashion across the range of the species If so, we can map those abiotic factors and then, sometimes, find patterns within the species itself, tracking the patterns in the abiotic factors of the niche

Biotic Clines A biotic cline, in reference to population biology, is a gradual change of phenotype (trait, character or feature) and underlying gene pool allele frequencies in a species over a geographical area, often as a result of environmental heterogeneity This meaning of "cline" was introduced by Sir Julian Huxley Many speciation events seem to track or parallel these environmental discontinuities

Clinal Variation In the leopard frog (Rana pipiens), tadpoles exhibit a range of temperature tolerances, generally enduring colder temperatures in higher (northern) latitudes and warm temperatures at lower (southern) latitudes

Reproductive Success In a study by J. Moore in of the leopard frog (Rana pipiens), eggs from females in the north were fertilized with sperm from males progressively farther to the south The degree of embryo or tadpole abnormalities was scored, from A (normal young) through progressively more abnormalities to F (high death rate) This study and others prompted biologists to divide leopard frogs into several different species egg mass

Biological Clines The flowering time of a plant may tend to be later at higher altitudes (an altitudinal cline) In species in which the gene flow between adjacent populations is high, the cline is typically smooth, whereas in populations with restricted gene flow the cline usually occurs as a series of relatively abrupt changes from one group to the next

Biological Clines We can also establish clines for polymorphic phenotypes in a single species

Speciation and Hybridization Zones
Hybrid populations becoming independent species is more common in plants Helianthus annuus Helianthus anomalus, the hybrid offspring formed less than 60 generations ago © Cousin Avi/ShutterStock, Inc. Helianthus petiolaris Courtesy of Clarence A. Rechenthin and USDA NRCS Texas State Office © Colin D. Young/ShutterStock, Inc. Figure 06: Western United States species show rapid evolution of a hybrid

Frequency and Impact of Hybridization in Nature
Hybridization is not particularly common in nature and difficult for scientists to identify Statistically, most hybrids will be inferior, especially in animals Nonetheless, episodic hybridization may provide for introgressions and this horizontal gene transfer increases the genetic variation on which selection can later act In some circumstances, hybrids are superior, persist in nature, and may give rise to new species, especially in plants

Hybridization in Agriculture
Most agricultural crops were artificially selected from more than one species which were hybridized as a part of the selection process Figure 07B: Corn © ailenn/ShutterStock, Inc. Figure 07A: Teosinte Photo by John Doebley

A Phylogeny of Mimulus cardinalis, Mimulus lewisii and Kin
Monkey-flowers Mimulus cardinalis Mimulus bicolor Mimulus lewisii

Monkey flowers, Mimulus sp.
Mimulus cardinalis and Mimulus lewisii are sister species (share the most recent common ancestor) Both species live in the California mountains They may be successfully cross-fertilized in the lab They do not hybridize in nature because they rely on different pollinators Bee pollination is the ancestral state Hummingbird pollination evolved twice within the larger clade; that is an example of parallelism

Mimulus cardinalis and M. lewisii
Mimulus lewisii M. lewisii is pollinated by bees which do not see red well but do need a landing platform M. cardinalis is pollinated by hummingbirds which prefer red flowers and do not need a landing platform Figure 08: Monkey flowers Photos courtesy Toby Bradshaw (University of Washington) and Douglas Schemske (Michigan State University)

The Mimulus Experiment
Investigators tested individual phenotypic traits against the preferences of pollinators Hummingbirds prefer nectar-rich flowers with much purple pigment Bees prefer large flowers with minimum yellow pigment This suggests that disruptive (sympatric?) selection shaped flower phenotypes which led to speciation

Speciation for Sexual Species
If species reproduce asexually, reproductive isolation is inherent in their formation; offspring form asexual clones, but may acquire new alleles through horizontal gene transfer If species reproduce sexually, the degree to which species may hybridize varies greatly The ability to hybridize does not necessarily contradict the reality of species distinction Some sister species never have the opportunity to reproduce across populations or form hybrids in nature

Genes and Speciation General statements about the evolution of new species: Adaptive and Sexual traits first undergo disruptive selection for differences between populations during speciation Adaptive and Sexual traits later undergo stabilizing selection for uniformity within sister species after attaining speciation Hybridization is limited by Reproductive Isolating mechanisms

Darwin’s Particular Genius
Over his career, Darwin wrote about 3 kinds of selection: (1) Natural selection - makes a species better adapted to its environment - increases survival (2) Sexual selection - makes one sex more appealing to the other - increases reproduction (3) Artificial selection - humans choose the desired traits and amplify them through selective breeding of domestic organisms In the 150 years since, scientists have added group and kin selection and species selection as additional modes of speciation

Darwin’s Particular Genius
Darwin defined 3 kinds of selection. However, you will hear biochemists use the term Directed Evolution: “A laboratory process used on isolated molecules or microbes to cause mutations and identify subsequent adaptations to novel environments.” A typical directed evolution experiment involves three steps: Diversification: The gene encoding the protein of interest is mutated and/or recombined at random to create a large library of gene variants. Techniques commonly used in this step are error-prone PCR and DNA shuffling. Selection: The library is tested for the presence of mutants (variants) possessing the desired property using a screen or selection. Screens enable the researcher to identify and isolate high-performing mutants by hand, while selections automatically eliminate all nonfunctional mutants. Amplification: The variants identified in the selection or screen are replicated manyfold, enabling researchers to sequence their DNA in order to understand what mutations have occurred. Together, these three steps are termed a "round" of directed evolution. Most experiments will perform more than one round. In these experiments, the "winners" of the previous round are diversified in the next round to create a new library. At the end of the experiment, all evolved protein or RNA mutants are characterized using biochemical methods. Biology Students: This is nothing but Darwin’s Artificial Selection using molecular tools. The biochemists are just showing their ignorance of the history of science.

Summary of Geographic Modes of Speciation
Allopatric speciation – a physical barrier divides a continuous population Peripatric speciation – a small founding population enters a new or isolated niche Parapatric speciation – a new niche found adjacent to the original niche Sympatric speciation - speciation occurs without physical separation inside a continuous population

Summary of Reproductive Isolating Mechanisms (RIMs)
Prezygotic mechanisms: Factors which prevent individuals from mating Temporal isolation; Ecological isolation; Behavioral isolation; Mechanical isolation Gametic incompatibility: Sperm transfer takes place, but the egg is not fertilized Postzygotic isolating mechanisms: Genomic incompatibility, hybrid inviability or sterility Zygotic mortality: The egg is fertilized, but the zygote does not develop Hybrid inviability: Hybrid embryo forms, but is not viable Hybrid sterility: Hybrid is viable, but the resulting adult is sterile Hybrid breakdown: First generation (F1) hybrids are viable and fertile, but further hybrid generations (F2 and backcrosses) are inviable or sterile

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Chapter 17 Origin of Species

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