Chapter 16 – 17 Short VersionChapter 16 – 17 Short Version

Species and Similarity:Species and Similarity:On Being the SameOn Being the Same

Yet DifferentYet DifferentFigure CO: Booby © rebvt/ShutterStock, Inc.

The Species Problem• In the first three decades of the 20th

century, species were envisioned as arising from saltations or macro-mutations, not from the gradual accumulation of small hereditary changes by natural selection

• One important voice for natural selection during its period of eclipse was Sir Edward Bagnall Poulton, (1856 - 1943), a British evolutionary biologist who was a life-long advocate of natural selection

• Ernst Mayr gave credit to Poulton as an early originator of the Biological Species concept

Sir Edward Bagnall Poulton

The Species Problem• The many biologists whose work generated

“The Modern Synthesis” conceived of species in terms of populations, demes and gene pools

• Ernst Mayr, revered as the greatest evolutionary biologist of the 20th Century, proposed the Biological Species ConceptBiological Species Concept in 1942, and revised it from time to time over his six decade profession career

• His last version, from 2001 states: "Species are groups of interbreeding natural populations that are reproductively isolated from other such groups."

Ernst Mayr

Limitations of the Biological Species Concept

• Reproductive isolation cannot be determined in fossil species

• Reproductive isolation cannot be determined in asexual species

• Populations separated geographically for long periods, often exhibiting vacariant distributions, may be shown not to have reproductive isolation in the laboratory, and yet have been distinct for thousands, even millions of years

Limitations of the Biological Species Concept

Catalpa ovata (left) from China and Catalpa bignoides (right) from the eastern US produce viable fertile offspring when crossed in the arboretum, but have been isolated on different continents for millions of years and are best considered as separate species; another species is found on Caribbean islands

“Catawba trees”

The Species Problem Again• Genetic species

– A set of organisms exhibiting similarity of DNA

• Agamospecies – A species in which sexual reproduction does not occur,

represented typically as a collection of clones – The absence of sexual reproduction means that the

biological species concept cannot be applied, and instead taxonomists must rely on identifying certain diagnostic traits to distinguish between closely related asexual lineages

– Consequently, the boundaries of agamospecies are often hard to define

– Taraxacum: the genus of dandelions, an agamic complex; it includes 26 (2n) species, [all of which can be crossed], and ~ 200 “microspecies” are recognized!

The Recognition Species Concept

• According to Patterson (1993), species have a specific mate recognition system (SMRS)

• Species can be defined as a set of organisms with a common method of recognizing mates

• Advantages:– Specific mate recognition systems (SMRS) are

easier to observe than interbreeding in nature– The Recognition Species Concept may more

accurately represent what happens when a new species originates

The Species Problem• Ecological Species: Populations that are adapted to

certain ecological niches, and because of their adaptations, will form discrete morphological clusters

• Advantages: Acknowledges the role the environment plays in controlling morphological development

• Disadvantages: – Cryptic species can be missed– Ecological niches tend to be assumed and are difficult to

define and describe completely– Many taxa exploit overlapping resources, or can suddenly

switch if a resource becomes scarce– This is not a very robust species concept

Leigh Van Valen(1935 – 2010)

Ring Species• Ring species are species with a geographic

distribution that forms a ring and overlaps at the ends.

• Although song sparrow (Passarella melodia) at the northern and southern extremes of the geographic range do not interbreed directly, the unity of the species is maintained by extensive gene flow between neighboring populations all along the Pacific coast

Shown on the map are representatives of 3 of the 34 subspecific populations of the song sparrow. Lines indicate the approximate breeding ranges of the illustrated subspecies, while those of 14 other distinct populations are shown by medium shading.

Chronospecies• A chronospecies is a species 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

• This implies directional selection• But does not rule out disruptive selection

mor

e le

gs

Chronospecies• A single lineage a evolves

over time into a morphologically distinct form b (left)

• If the connection is recognized, both forms will be assigned to a single evolutionary species

• Especially if the fossil record of the lineage is incomplete (right), forms a & b may be regarded and named as separate biological species

organism a undergoes a "pseudoextinction" when it

evolves into organism band

the fossil data can be interpreted as an example of punctuated equilibrium

• Evolutionary Species– George Gaylord Simpson: “An evolutionary

species is a lineage (an ancestor-descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies”

– One of the central “problems” of The Species Problem is that there are many options for species classification because there are many options for the process of speciation itself

The Species Problem

The Species Problem• Phylogenetic (Cladistic) Species

– A group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space

– At some point in the progress of such a group,

members may diverge from one another: when such a divergence becomes sufficiently clear, the two populations are regarded as separate species

– The smallest diagnosable monophyletic group sharing a set of unique synapomorphies

Willi Hennig1913 - 1976

Phylogenetic (Cladistic) / Evolutionary Species

• To be called a species under the PSC definition, populations must have been evolutionarily independent for a long enough time for diagnostic shared derived traits (synapomorphies) to emerge

• Actually, time for evolutionary independence is a necessity for most species definition concepts

A LIST OF 26 SPECIES CONCEPTS

Grades Versus Clades

• It has always been useful to classify higher taxa based on common traits, though this classification often implies a ladder of life

• When groups are united by common characteristics inherited from their common ancestor, the taxon is a grade

• Unfortunately, despite their descriptive value, identifying groups as grades can cloud their evolutionary relationships

• When similarity means shared ancestry: Homology

• When similarity means parallel evolution: Parallelism

• When similarity means similar solutions in independent lineages:

Convergence

Similarity: Knowing When CharactersAre the Same or Different

Homology and Analogy• Darwin’s contemporary, the

comparative anatomist and paleontologist, Richard Owen (1804-1892), coined the terms homology and analogy (and Dinosauria)

• To Owen, homologies indicated that organisms were created following a common (Divine/Vitalist) plan or archetype

Homology and Analogy

• Homology: Any similarity between phenotypic characters that is due to their shared ancestry. Homologous structures may retain the function they served in the common ancestor or they may evolve to fulfill different functions.

• Analogy: Any phenotypic characters that perform the same or similar function by a similar mechanism but evolved separately.

Analogies

• Analogy: Any phenotypic characters present in different taxa which perform the same or a similar function by a similar mechanism but which evolved separately – The structural carbohydrate chitin in fungal cell wall and in

arthropod exoskeletons

Analogies

• Thylacosmilus is a marsupial sabre-tooth cat

• Smilodon is a placental saber-tooth cat

Traits their

commonancestor

lacked

Analogy: Oxygen-Binding Proteins• Hemoglobin combines iron heme groups and globin proteins, red pigment found in

vertebrate erythrocytes• Myoglobin combines iron heme groups and globin proteins, red pigment found in

vertebrate muscle tissues • Hemocyanin combines copper prosthetic groups and globin proteins, blue pigment

found in arthropod and mollusc blood plasma• Hemerythrin combines iron-containing non-heme globin proteins, pink/violet vs

colorless pigment found in some marine invertebrate blood plasma• Chlorocruorin combines iron heme groups and globin proteins, red vs green

pigment found in many annelids• Vanabins (aka vanadium chromagens) combine the rare metal vanadium in

prosthetic groups and globin proteins, colorless molecules found in sea squirts blood plasma

• Erythrocruorin is a giant free-floating blood protein containing many dozens — possibly hundreds — of iron- and heme-bearing protein subunits bound together into a single protein complex with a molecular mass greater than 3.5 million daltons Found in many annelids, including earthworms

• PinnaglobinPinnaglobin is a brown manganese-based porphyrin protein only found in the mollusc Pinna squamosa

• Leghemoglobin combines iron heme groups and globin proteins, colorless compound found in leguminous plants, such as alfalfa or soybeans

Homology• A Brief History

– Richard Owen (1804–1892): regarded organs as the same (homologs), though they served different functions

– After Darwin, scientists realized that homologies are found in organisms with a shared evolutionary history, in particular, those with a shared common ancestor

Developmental Homology

• Developmental Homology: A related concept meaning that structures arose from the same tissue in embryonic development

• the ovaries and clitoris of female humans and the testes and penis of male humans are homologous in this sense

Classic Structural Homologies

Determining that

Phenotypic Traits are

Homologous Allows

Scientists to Build

PhylogeniesFigure 07: Skeletal structures of the forelimbs of

representative terrestrial vertebrates

Classic Structural Homologies

The vertebrate forelimbs have the same basic bone design: one long bone attached to two other long bones attached to a forefoot with many carpals attached to five or

fewer digits. The vertebrate hind limb offers similar evidence of homology

Classic Structural Homologies

• The number of digits was variable in the earliest fossil amphibians but pentadactyly became the standard for all later vertebrates, though there have been many reductions in number and a much fewer increases in certain finned marine reptiles

early fossil amphibians

6 8 6 5

Digit Increases From the Standard Vertebrate Pentadactyl Limb

mosasaur standard; plesiosaur not

ichthyosaur, like the plesiosaur, exhibits hyperphalangy

Note that a small change (mutation) in a homeotic gene might produce an initial dramatic skeletal change quite easily

Homologies

• Homologous structures may be adapted for different roles in which case their structure may vary considerably

• Consider mammalian incisors and canines

A Deep Homology: The Genetic Code

This is one of the most powerful of allhomologies, as it links all living organisms

on Earth to a common ancestor!a few microbes have

a few differences

Protein Homologies

• The gene that codes for the protein hormone insulin is homologous in humans and other mammals; the similar DNA sequences descended from a common ancestral gene sequence

• Human insulin (right) and pig insulin (left), showing their one amino acid difference

Protein Homologies

• Fig. 1. Phylogeny of the insulin superfamily. A typical phylogenetic tree of the animal kingdom is drawn and highlighted balloons indicate the animal classes/phyla in which insulin/insulin-related peptides were unambiguously identified in 1922, 1975 and 1998.

Physiological Homologies• Phylogeny showing the distribution of

menstruation in placental mammals and the inferred states of ancestral lineages

• Menstruating species/lineages are colored in pinkpink, non-menstruating species/lineages in black

• Species in which the character state is not known are not colored, and lineages of equivocal state are represented with black lines

• Monodelphis (short-tailed opossums) represents the outgroup

• Note that there is strong evidence for three independent originations of menstruation among placental mammals

Many primates, a few bats, and the elephant shrew

Analogy / Homology /Homoplasy• homology is a common element in structure

between parts of different organisms due to evolutionary differentiation from a corresponding part in a common ancestor

• analogy is a trait or an organ that appears similar in two distantly related organisms

– Biological analogies are often the result of convergent evolution

• homoplasy is when the organs of two different species have similar characteristics, functions and features as a result of similarity of environment rather than common heredity

– something of a special case of analogy, or– a more modern term for analogy– and an example of convergent evolution

Homoplasy• Homoplasy is the logical opposite of homology• Homoplasy is the structural similarity between

two traits in two species without phyletic continuity

• Even though the traits are similar, the common ancestor of species A and B did not present the trait – The last common ancestor may not be so far away in

time or phylogenetic history

• There are three different types of homoplasy: convergence, parallelism, and reversal

Homoplasy• Convergence is the evolution of

similar traits in response to similar adaptive pressures, but not built upon similar genes or developmental processes

• An example of convergence is found in the electroreception of mormyrids (African weak electric fish) and gymnotoids (South American electric eels): while the organs responsible for this perceptual capacity are similar, they are not derived from the same organ in their last common ancestor

Convergent Electric Organs in Fish• A fish is considered to be an electric

fish if it can generate electric fields• Some electric fish live in the ocean and

some live in freshwater rivers of South America and Africa

• The figure shows the location of the electric organ in each fish, and a sample of the waveform of the electric organ discharge

• Most electric fish use electroreception to swim in murky waters; some use electric shock as a defensive weapon

If a fish can generate electric fields it is said to be electrogenic. If a fish has the ability to detect electric fields, it is said to be electroreceptive. Most fish that are electrogenic are also electroreceptive.

Ecological EquivalentsWithout Convergence

Madagascar Aye-Aye (lemur)

PileatedWoodpecker ►AndWoodpeckerFinch ▼

Mimicry/Analogy/Convergence

• Pollination in the Bee orchid (Ophrys apifera) and Spider orchid (O. fuciflora) is enhanced by reproductive mimicry and the pheremones that attract specific male bumblebees

Homoplasy: Parallelism

• Parallelism occurs in closely related taxa, and is defined as the independent development of a descendant character trait that is not present on a common ancestor

• The evolution of similar features in closely related lineages that do not share a last common ancestor who exhibited the trait

Homoplasy• Parallelism occurs when two taxa

develop the same character after evolutionary divergence; since the trait is absent in a common ancestor, but present in both descendant species, it is probable that the developmental genetic program that produces the structures in the different taxa is the same, which means the developmental genetic program was inherited from the common ancestor

• Thus, there is homology between the developmental and genetic materials, but not on the final structure

• (Also ecological equivalents)

An extinct South American Litoptern

camel

litopterncamel

• Convergent evolution takes place when distantly related organisms give rise to species that resemble one another because they adapt in comparable ways

Convergent vs. Parallel Evolution?• Parallel evolution involves the

independent origin of similar features in more closely related organisms

ParallelEvolution

Recknagel, H., Kusche, H., Elmer, K.R. and Meyer, A., Two new endemic species in the Midas cichlid species complex from Nicaraguan crater lakes: Amphilophus tolteca and Amphilophus viridis

(Perciformes, Cichlidae), aqua, International Journal of Ichthyology, vol. 19 no. 4 - 25 October 2013.

Cichlid species from large lakes migrated to smaller lakes and in both the smaller lakes, a more elongated form evolved independently

Homoplasy: Reversals• hair and heterodont teeth

pelycosaur

brown bear

orcacynodont

The Panda’s Thumb?

• The panda has five digits like most mammals; however, opposing these is another digit, a “thumb, “ which is actually not a thumb at all but an enlarged wrist bone

2nd elongatedcarpal

analogy

Human (Intelligent) Design

• Machine functions vary and machine designs vary accordingly, using unrelated parts created from scratch

• Unlike biological organisms, human designed and built machines show no correspondence of parts from planes (flight), to mining machines (burrowing), or to boats (swimming)

From Homology to Phylogeny

• Homologous traits can be grouped into series• Homologous traits can include adaptive traits,

maladaptive traits, vestigial features and atavisms• All homologous traits can be potentially useful in

deriving phylogenetic trees• However, care must be taken to avoid using

analogous traits by mistake• Finally, homologous traits are excellent evidence

that evolution has, in fact, occurred!

Chapter 16

End

Chapter 17

Origin of Species

Figure CO: Mimulus Flower © LubaShi/ShutterStock, Inc.

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?

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 400-500 Siberian tigers still exist within their range

Problems Defining Species Through Time

a) 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

b) 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]

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

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 Initiatedby Geographical Isolation

• three steps– geographical isolation– local adaptation– reproductive isolation

• three forms of geographical isolation

Figure 01A: Allopatric Figure 01B: Peripatric

Figure 01C: Parapatric

sometimes called a porous or permeable barrier

Allopatric Speciation• 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.]

Allopatric Speciation

1. A single species is an interbreeding reproductive community

2. A barrier develops, or a dispersal event occurs, dividing the species

3. Separated into different habitats, the divided populations diverge through the accumulation of gene and trait differences

4. The separate populations become so different that, if and when the barrier disappears and they overlap again, interbreeding does not occur

mountains and deserts are classic examples of new barriersFour steps lead to speciation:

Allopatric Speciation

• 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

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

Incomplete or Peripheral Isolation

• 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

Figure B01A: “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:10.1371/journal.pone.0000487. Courtesy of Kevin Edwards , Illinois State University.

Peripatric Speciation

Figure B01B: “Picture-winged" group of Drosophila

Figure B01C: “Picture-winged" group of Drosophila

Figure B02: “Picture-winged" group of Drosophila

Adapted from Carson, H.L. Drosophila Inversion Polymorpism. CRC Press, 1992.

Two male hammer-headed picture-wing Drosophila battle for mating territory.

Parapatric Speciation

• 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

dispersal

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• Recall that the other modes of speciation require three

steps:1. geographical isolation2. local adaptation3. 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 (1856-1943) 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 (1910-1983) 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

• 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

• 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

• 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

Figure 04a-e: Two modes of speciation

Adapted from Strickberger, M. W. Genetics, Third edition. Macmillan, 1985.

Speciation Types: Summary

Recall that non-sympatric speciation requires three steps:

geographical isolationlocal adaptationreproductive 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

Speciation Types: Summary

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

• The two Galapagos iguana genera are, themselves, ecologically isolated from each other

Amblyrhynchus cristatusConolophus subcristatus

Iguana iguana

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 contortrix

Agkistrodon piscivorus

Behavioral (Reproductive) Isolation

• 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 garmani

Anolis opalinus

Anolis trinitatis

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

Reproductive Isolating Mechanisms (RIMs)

• 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 fertilization takes place but the hybrid embryos die in the early developmental stages (Dobzhansky, Ayala, Stebbins and Valentine, 1977. Evolution. W. H. Freeman)

Hybrid Inviability

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

Drosophila melanogaster

Drosophila simulans

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

liger

tigon

hinny

mule

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

Reproductive Isolating Mechanisms (RIMs)

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 1949 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

eggmass

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 GeniusOver 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 selection and kin selection and species selection as additional kinds of speciation

Darwin’s Particular GeniusDarwin 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.

3 Modes of Natural Selection

• Stabilizing

• Directional

• Disruptive

Figure 02: Selection and their effects on the mean (dashed lines) and variation of a normally distributed quantitative character

Summary ofGeographic 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 ReproductiveIsolating 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

Chapter 17End