Biology II

Chapter 22 NOTES

Evolutionary theory must explain:

• How adaptations evolve in populations.

• The origin of new species, which results in biological diversity.

The fossil record provides evidence for two patterns of speciation: anagenesis and cladogenesis.

Anagenesis (phyletic, evolution) = The transformation of an unbranched lineage of organisms, sometimes to a state different enough from the ancestral population to justify renaming it as a new species.

Cladogenesis (branching evolution) = The budding of one or more new species from a parent species that continues to exist; is more important than anagenesis in life's history, because it is more common and can promote biological diversity.

1. The biological species concept emphasizes reproductive isolation

Species exist in nature as discrete units, usually distinguishable from other species.

· Trained taxonomists often give scientific names to flora and fauna, which are already recognized by native populations. For example:

=> In 1927, Ernst Mayr traveled to the remote Arafak mountains of New Guinea to study its diverse animal fauna.

=> Using morphological differences, he identified 138 species of birds ‑ 137 of which the Papuan natives had already named.

· Although species are discrete in nature, it is difficult to devise a formal definition of a species.

Linnaeus (founder of modern taxonomy) described species in terms of their physical form (morphology). This morphospecies concept is still the most common method used for describing species.

Morphospecies = Species defined by their anatomical features.

· It is difficult to apply to some situations.

=>Sometimes, it is difficult to determine if a set of organisms represents multiple species or a single species with extensive phenotypic variation.

=>Two populations morphologically almost indistinguishable may be different species based on other criteria.

· Can be useful in the field, since its basis is observable and measurable anatomy.

· Does not address the discontinuity that exists between species from the standpoint of evolutionary theory.

In 1942, Ernst Mayr proposed the biological species concept as an alternative to the morphospecies.

Biological species = Groups of interbreeding natural populations that are reproductively isolated from other such groups.

· Is circumscribed by reproductive barriers that preserve its integrity by preventing genetic mixing with other species.

· Is the largest unit of population in which gene flow is possible.

· Is defined by reproductive isolation from other species in natural environments (hybrids may be possible between two species in the laboratory).

The biological species concept cannot be applied to:

· Organisms that are completely asexual in their reproduction. Some protists and fungi, some commercial plants (bananas), and many bacteria are exclusively asexual.

ð Asexual reproduction effectively produces a series of clones which, genetically speaking, represent a single organism.

ð Asexual organisms can be assigned to species only by grouping clones with the same morphology and biochemical characteristics.

· Extinct organisms represented only by fossils. These must be classified by the morphospecies concept.

· Population, which are geographically segregated. Can they potentially interbreed in nature even if they are similar enough to be placed into the same species based on morphological characteristics?

In some cases, unambiguous determination of species is not possible, even though the populations are sexual, contemporaneous and contiguous.

· Four phenotypically distinct populations of the deer mouse (Peromyscus maniculatus) found in the Rocky Mountains are geographically isolated and referred to as subspecies.

· These populations overlap at certain locations and some interbreeding occurs in these areas of cohabitation, which indicates they are the same species by the biological species criteria.

· Two subspecies (P. m. artemisiae and P. m. nebrascensis) are an exception, since they do not interbreed in the area of cohabitation. However, their gene pools are not completely isolated since they freely interbreed with other neighboring populations.

· This circuitous route could only produce a very limited gene flow, but the route is open and possible between the populations of P. m. artemisiae and P. m. nebrascensis through the other populations.

· If this route was closed by extinction or geographic isolation of the neighboring populations, then P. m. artemisiae and P. m. nebrascensis could be named separate species without reservation.

More examples are being discovered where there is a blurry distinction between populations with limited gene flow and full biological species with segregated gene pools.

· If two populations cannot interbreed when in contact, they are clearly distinct species.

· When there is gene flow (even very limited) between two populations that are in contact, it is difficult to apply the biological species concept.

· This is equivalent to finding two populations at different stages in their evolutionary descent from a common ancestor, which is to be expected if new species arise by gradual divergence of populations.

Other species concepts have been developed in an effort to accommodate the dynamic, quantitative aspects of speciation; however, the species problem may never be completely resolved as it is unlikely that a single definition of species will apply to all cases.

II. Reproductive barriers separate species

Reproductive barrier = Any factor that impedes two species from producing fertile hybrids, thus contributing to reproductive isolation.

· Most species are genetically sequestered from other species by more than one type of reproductive barrier.

· Only intrinsic biological barriers to reproduction will be considered here. Geographic segregation (even though it prevents interbreeding) will not be considered.

· Reproductive barriers prevent interbreeding between closely related species. The various reproductive barriers, which isolate the gene pools of species, are classified as either prezygotic or postzygotic depending on whether they function before or after the formation of zygotes.

· Prezygotic barriers impede mating between species or hinder fertilization of the ova should members of different species attempt to mate.

· In the event fertilization does occur, postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult.

A. Prezygotic Barriers

1. Habitat Isolation

Two species living in different habitats within the same area may encounter each other rarely if at all, even though they are not technically geographically isolated.

· For example, two species of garter snakes (Thamnophis) occur in the same areas but for intrinsic reasons, one species lives mainly in water and the other is mainly terrestrial.

· Since these two species live primarily in separate habitats, they seldom come into contact, as they are ecologically isolated.

2. Temporal Isolation

Two species that breed at different times of the day, seasons, or years cannot mix their gametes.

· For example, brown trout and rainbow trout cohabit the same streams, but brown trout breed in the fall and rainbow trout breed in the spring.

· Since they breed at different times of the year, their gametes have no opportunity to contact each other and reproductive isolation is maintained.

3. Behavioral Isolation

Species‑specific signals and elaborate behavior to attract mates are important reproductive barriers among closely related species. For example,

· Male fireflies of different species signal to females of the same species by blinking their lights in a characteristic pattern; females discriminate among the different signals and respond only to flashes of their own species by flashing back and attracting the males.

Many animals recognize mates by sensing pheromones (distinctive chemical signals).

· Female Gypsy moths attract males by emitting a volatile compound to which the olfactory organs of male Gypsy moths are specifically tuned: when a male detects this pheromone, it follows the scent to the female.

· Males of other moth species do not recognize this chemical as a sexual attractant.

Other factors may also act as behavioral isolating mechanisms:

· Eastern and western meadowlarks are almost identical in morphology and habitat, and their ranges overlap in the central United States.

· They retain their species integrity partly because of the difference in their songs, which enables them to recognize potential mates as members of their own kind.

4. Mechanical Isolation

Anatomical incompatibility may prevent sperm transfer when closely related species attempt to mate.

· For example, male dragonflies use a pair of special appendages to clasp females during copulation; when a male tries to mount a female of a different species, he is unsuccessful because his clasping appendages do not fit the female's form well enough to grip securely.

· In plants that are pollinated by insects or other animals, the floral anatomy is often adapted to a specific pollinator that transfers pollen only among plants of the same species.

5. Gametic Isolation

Gametes of different species that meet rarely fuse to form a zygote.

· For animals that use internal fertilization, the sperm of one species may not be able to survive the internal environment of the female reproductive tract of a different species.

· Cross‑specific fertilization is also uncommon for animals that utilize external fertilization due to a lack of gamete recognition.

Gamete recognition may be based on the presence of specific molecules on the coats around the egg, which adhere only to complementary molecules on sperm cells of the same species.

· Similar mechanisms of molecular recognition enables a flower to discriminate between pollen of the same species and pollen of different species.

B. Postzygotic Barriers

When prezygotic barriers are crossed and a hybrid zygote forms, one of several postzygotic barriers may prevent development of a viable, fertile hybrid.

1. Reduced Hybrid Viability

Genetic incompatibility between the two species may abort development of the hybrid at some embryonic stage.

· For example, several species of frogs in the genus Rana live in the same regions and habitats.

· They occasionally hybridize but the hybrids generally do not complete development, and those that do are frail and soon die.

2. Reduced Hybrid Fertility

If two species mate and produce hybrid offspring that are viable, reproductive isolation is intact if the hybrids are sterile because genes cannot flow from one species' gene pool to the other.

· One cause of this barrier is that if chromosomes of the two parent species differ in number or structure, meiosis cannot produce normal gametes in the hybrid.

· The most familiar case is the mule, which is produced by crossing a donkey and a horse; very rarely are mules able to backbreed with either parent species.

3. Hybrid Breakdown

When some species cross‑mate, the first generation hybrids are viable and fertile, but when these hybrids mate with one another or with either parent species, offspring of the next generation are feeble or sterile.

· For example, different cotton species can produce fertile hybrids, breakdown occurs in the next generation when progeny of the hybrids die in their seeds or grow into weak defective plants.

C. Introgression

Introgression = The transplantation of alleles between species.

· Introgression occurs when alleles occasionally seep through all reproductive barriers and pass between the gene pools of closely related species when fertile hybrids mate successfully with one of the parent species.

For example, corn (Zea mays) contains some alleles traceable to the closely related wild grass teosinte (Zea mexicana).

· Introgressions occur when the two species hybridize and a small number of the hybrids manage to cross with corn plants.

· The transplant of alleles increases the genetic variation that can be exploited by breeders attempting to produce new corn varieties by artificial selection.

· Occasional hybridization does not erase the boundary between corn and teosinte as only a very rare introgression occurs, the isolation of the two gene pools is not seriously breached and the two species remain distinct.

III. Geographical isolation can lead to the origin of species: allopatric speciation

Reproductive barriers form boundaries around species, and the evolution of these barriers is the key biological event in the origin of new species.

· An essential episode in the origin of a species occurs when the gene pool of a population is separated from other populations of the parent species.

· This genetically isolated splinter group can then follow its own evolutionary course as changes in allele frequencies caused by selection, genetic drift, and mutations occur undiluted by gene flow from other populations.

Speciation episodes can be classified into two modes based on the geographical relationship of a new species to its ancestral species: allopatric speciation and sympatric speciation.

Allopatric speciation = Speciation that occurs when the initial block to gene flow is a geographical barrier that physically isolates the population.

· Geological processes can fragment a population into two or more allopatric populations (having separate ranges).

o Such occurrences include emergence of mountain ranges, movement of glaciers, formation of land bridges, subsidence of large lakes.

o Also small populations may become geographically isolated when individuals from the parent population travel to a new location.

· The extent of development of a geographical barrier necessary to isolate two populations depends on the ability of the organisms to disperse due to the mobility of animals or the dispersibility of spores, pollen and seeds of plants.

o For example, the Grand Canyon is an impassable barrier to small rodents but is easily crossed by birds.

An example of how geographic isolation can result in allopatric speciation is the pupfish.

· 50,000 years ago, during an ice age, the Death Valley region of California and Nevada had a rainy climate and a system of interconnecting lakes and rivers.

· 10,000 years ago a drying trend began and by 4,000 years ago, the region had become a desert.

· Presently, isolated springs in deep clefts between rocky walls are the only remnants of the lake and river networks. Living in many of these isolated springs are small pupfishes (Cyprinodon spp.).

· Each inhabited spring contains its own species of pupfish, which is adapted to that pool and found nowhere else in the world.

· The endemic pupfish species probably descended from a single ancestral species whose range was fragmented when the region became arid, thus isolating several small populations that diverged in their evolution as they adapted to their spring's environment.

A. Conditions Favoring Allopatric Speciation:

When populations become allopatric, speciation can potentially occur as the isolated gene pools accumulate differences by microevolution that may cause the populations to diverge in phenotype.

· A small isolated population is more likely to change substantially enough to become a new species than is a large isolated population.

· The geographic isolation (peripheral isolate) of a small population usually occurs at the fringe of the parent population's range.

· As long as the gene pools are isolated from the parental population, peripheral isolates are good candidates for speciation for three reasons:

1. The gene pool of the peripheral isolate probably differs from that of the parent population initially, since fringe inhabiters usually represent the extremes of any genotypic and phenotypic clines in an original sympatric population. With a small peripheral isolate, there will be a founder effect with chance resulting in a gene pool that is not representative of the gene pool of the parental population.

2. Genetic drift will continue to cause chance changes in the gene pool of the small peripheral isolate until a large population is formed. New mutations or combinations of alleles that are neutral in adaptive value may become fixed in the population by chance alone, causing phenotypic divergence from the parent population.

3. Evolution caused by selection is likely to take a different direction in the peripheral isolate than in the parental population. Since the peripheral isolate inhabits a frontier with a somewhat different environment, it will probably be exposed to different selection pressures than those encountered by the parental population.

· Due to the severity of a fringe environment, most peripheral isolates do not survive long enough to speciate.

Although most peripheral isolates become extinct, evolutionary biologists agree that a small population can accumulate enough genetic change to become a new species in only hundreds to thousands of generations.

B. Adaptive Radiation on Island Chains

Allopatric speciation occurs on island chains where new populations, which stray or are passively dispersed from their ancestral populations, evolve in isolation.

Adaptive radiation = The evolution of many diversely adapted species from a common ancestor.

Examples of adaptive radiation are the endemic species of the Galapagos Islands, which descended from small populations, which floated, flew, or were blown from South America to the islands. Darwin's finches can be used to illustrate a model for such adaptive radiation on island chains.

4A single dispersal event may have seeded

one island with a peripheral isolate of the

ancestral finch, which diverged as it

underwent allopatric speciation.

4A few individuals of this new species may

have reached neighboring islands, forming

new peripheral isolates which also

speciated.

4After diverging on the island it invaded, a new species could re‑colonize the island from which its founding population emigrated and coexist with the ancestral species or form still another species.

4Multiple invasions of islands could eventually lead to coexistence of several species on each island since the islands are distant enough from each other to permit geographic isolation, but near enough for occasional dispersal.

Similar evolutionary events have occurred on the Hawaiian Archipelago. These volcanic islands are 3500 km from the nearest continent.

· Hawaii is the youngest (<one million years old), largest island and has active volcanoes.

· The islands grow progressively older in a northwesterly direction away from Hawaii.

· As each island was formed and cooled, flora and fauna carried by ocean and wind currents from other islands and continents became established.

· The physical diversity of each island provided many environmental opportunities for evolutionary divergence by natural selection.

· Multiple invasions and allopatric speciations have permitted such a degree of adaptive radiations that there are thousands of endemic species on the archipelago, which are found nowhere else on Earth.

In contrast to the Hawaiian Archipelago, islands such as the Florida Keys are close enough to a mainland to allow free movement from the island to the mainland.

· Such islands are not characterized by endemic species since there is no long‑term isolation of founding populations.

· Intrinsic reproductive barriers that block gene flow do not develop due to a steady influx of immigrants from the mainland parental populations.

IV. A new species can originate in the geographical midst of the parent species: sympatric speciation

Sympatric speciation = Formation of new species within the range of parent populations.

· Reproductive isolation evolves without geographical isolation.

· Can occur quickly (in one generation) if a genetic change results in a reproductive barrier between the mutants and the parent population.

Many plant species have originated from improper cell division that results in extra sets of chromosomes ‑ a mutant condition called polyploidy.

Depending on the origin of the extra set of chromosomes, polyploids are classified in two forms: autopolyploids and allopolyploids.

Autopolyploid = An organism that has more than two chromosome sets, all derived from a single species. For example,

· Nondisjunction in the germ cell line (in either mitosis or meiosis) results in diploid gametes.

· Self‑fertilization would double the chromosome number to the tetraploid state.

· Tetraploids can self‑pollinate or mate with other tetraploids.

· The mutants cannot interbreed with diploids of the parent population because hybrids would be triploid (3n) and sterile due to impaired meiosis from unpaired chromosomes.

· An instantaneous special genetic event would thus produce a postzygotic barrier, which isolates the gene pool of the mutant in just one generation.

· Sympatric speciation by autopolyploidy was first discovered by Hugo De Vries in the early 20th century while working with Oenothera, the evening primrose.

Allopolyploid = A polyploid hybrid resulting from contributions by two different species.

· More common than autopolyploidy.

· Potential evolution of an allopolyploid begins when two different species interbreed and a hybrid is produced.

· Such interspecific hybrids are usually sterile, because the haploid set of chromosomes from one species cannot pair during meiosis with the haploid set of chromosomes from the second species. These sterile hybrids may actually be more vigorous than the parent species and propagate asexually.

At least two mechanisms can transform sterile alloployploid hybrids into fertile polyploids:

1. During the history of the hybrid clone, mitotic nondisjunction in the reproductive tissue may double the chromosome number.

· The hybrid clone will then be able to produce gametes since each chromosome will have a homologue to synapse with during meiosis.

· Gametes from this fertile tetraploid could unite and produce a new species of interbreeding individuals, reproductively isolated from both parent species.

2. Meiotic nondisjunction in one species produces an unreduced (diploid) gamete.

· This abnormal gamete fuses with a normal haploid gamete of a second species and produces a triploid hybrid.

· The triploid hybrid will be sterile, but may propagate asexually.

· During the history of this sterile triploid clone, meiotic nondisjunction again produces an unreduced gamete (triploid).

· Combination of this triploid gamete with a normal haploid gamete from the second parent species would result in a fertile hybrid with homologous pairs of chromosomes.

· This allopolyploid would have a chromosome number equal to the sum of the chromosome numbers of the two ancestral species (as in 1 above).

Speciation of polyploids (especially allopolyploids) has been very important in plant evolution.

· Some allopolyploids are very vigorous because they contain the best qualities of both parent species.

· The accidents required to produce these new plant species (interspecific hybridization coupled with nondisjunction) have occurred often enough that between 25% and 50% of all plant species are polyploids.

Some of these species have originated and spread in relatively recent times and many others are of importance to humans.

· Spartina angelica is a species of salt‑marsh grass, which evolved as an allopolyploid in the 1870s.

o It is derived from a European species (Spartina maritima) and an American species (Spartina alternaflora).

o In addition to being morphologically distinct and reproductively isolated from its parent species, S. angelica has a chromosome number (2n = 122) indicative of its mechanism of speciation (S. maritima, 2n = , S. alternaflora, 2n = 62).

· Triticum aestivum, bread wheat, is a 42-chromosome allopolyploid that is believed to have originated about 8000 years ago as a hybrid of a 28 chromosome cultivated wheat and a 14 chromosome wild grass.

· Other important polyploid species include oats, cotton, potatoes, and tobacco.

· Plant geneticists are presently inducing these genetic accidents to produce new polyploids, which will combine high yield and disease resistance.

Sympatric speciation may also occur in animal evolution through different mechanisms.

· A group of animals may become isolated within the range of a parent population if genetic factors cause them to become fixed on resources not used by the parent population as a whole. For example,

o A particular species of wasp pollinates each species of figs. The wasps mate and lay their eggs in the figs.

o A genetic change causing wasps to select a different fig species would segregate mating individuals of the new phenotype from the parental population.

o Divergence could then occur after such an isolation.

o The great diversity of cichlid fishes in Lake Victoria may have arisen from isolation due to exploitation of differend food sources and other resources in the lake.

· Sympatric speciation could also result from a balanced polymorphism combined with assortative mating. For example,

o If birds in a population that is dimorphic for beak size began to selectively mate with birds of the same morph, speciation could occur over time.

While both allopatric speciation and sympatric speciation have important roles in plant evolution, allopatric speciation is far more common in animals.

V. Population genetics can account for speciation

Classifying modes of speciation as allopatric or sympatric emphasizes biogeographical factors but does not emphasize the actual genetic mechanisms. An alternative method, which takes genetic mechanism into account, groups speciation into two categories: speciation by adaptive divergence and speciation by shifts in adaptive peaks.

A. Speciation by Adaptive Divergence

Two populations, which adapt to different environments, accumulate differences in the frequencies of alleles and genotypes.

· During this gradual adaptive divergence of the two gene pools, reproductive barriers may evolve between the two populations.

· Evolution of reproductive barriers would differentiate these populations into two species.

A key point in evolution by divergence is that reproductive barriers can arise without being favored directly by natural selection.

· Divergence of two populations is due to their adaptation to separate environments, with reproductive isolation being a secondary development.

· Postzygotic barriers may be pleiotropic effects of interspecific differences in those genes that control development. For example,

o Hybrids may be inviable if both sets of genes for rRNA synthesis are not active (e.g. hybrids between D. melanogaster and D. simulans).

· Gradual genetic divergence of two populations may also result in the evolution of prezygotic barriers. For instance,

o An ecological barrier to inbreeding may secondarily result from the adaptation of an insect population to a new host plant different from the original population's host.

In some isolated populations, reproductive isolation has evolved more directly from sexual selection. For example,

· In Drosophila heteroneura, the male's wide head enhances reproductive success with females of the same species while reducing the probability that a male D. heteroneura will mate with females of other species.

· Sexual selection, in this case, probably evolved as an adaptation for enhanced reproductive success. A secondary consequence is that it prevents interbreeding with other Drosophila species.

· Since reproductive barriers usually evolve when populations are allopatric, they do not function directly to isolate the gene pools of populations.

o For this reason, the emphasis on reproductive isolating mechanisms is one criticism of the "biological species concept".

· An alternative to the biological species concept is the recognition concept of species.

The recognition concept of species assumes that the reproductive adaptations of a species consists of a set of characteristics that maximize successful mating with members of the same population.

· Characteristics include the molecular, morphological, and behavioral characteristics that permit individuals to recognize a mate of the same species.

· Reproductive isolation from other populations would thus be a side effect.

· Focuses on characteristics, which are actually subject to natural selection in, isolated populations that are in the process of speciation.

B. Speciation by Shifts in Adaptive Peaks

The concept of the "adaptive landscape" was proposed by Sewell Wright in the 1930's.

· The landscape includes many adaptive peaks separated by valleys.

· Each adaptive peak represents an equilibrium state in which allelic frequencies optimize the population's success in that environment.

· The landscape could include many adaptive peaks, even under stable environmental conditions, but natural selection would tend to maintain the population at a single peak.

For a population to reach an alternate peak by a change in its gene pool, it must pass through a valley where genetic combinations are of low average fitness.

· Thus, if a slight change in allele frequencies at one or more loci pushes a population from its adaptive peak, natural selection will tend to push that population back to its original adaptive peak.

Environmental changes redefine the landscape, making new adaptive peaks possible.

· For a population to survive the new conditions, it must reach a new adaptive peak through microevolution of its gene pool.

Population geneticists use the term peak shift for speciation caused by non‑adaptive changes in the genetic system.

· Peak shifts may be caused by founder effect or by bottlenecks.

· Genetic drift can remove a small population from its original adaptive peak and, if the gene pool is sufficiently destabilized, new adaptive peaks may be reached.

· Such a population that survives will be pushed to a new adaptive peaks by natural selection in succeeding generations.

· Adaptive evolution thus plays a major role in peak shift, but genetic drift is the mechanism that makes the shift possible.

· Peak shifts can also occur in bottlenecked populations, even if there is no environmental change, due to the many adaptive peaks possible under the same environmental conditions.

A small, genetically destabilized population that is dislodged from its adaptive peak may undergo a generation‑to‑generation climb to a new adaptive peak due to natural selection.

· With founder effect, the splinter population is affected not only by genetic drift moving the gene pool randomly over the adaptive landscape, but also by new environmental conditions which form a new set of adaptive peaks.

· The combination of genetic drift followed by natural selection under new environmental conditions may be responsible for the rapid radiation of island species (e.g. Hawaiian Drosophila).

C. Hybrid Zones and the Cohesion Concept of Species

Three possible outcomes are possible when two closely related populations that have been allopatric for some time come back into contact:

· The two populations may interbreed freely.

o The gene pools would become incorporated into a single pool indicating that speciation had not occurred during their time of geographical isolation.

· The two populations may not interbreed due to reproductive barriers.

o The gene pools would remain separate due to the evolutionary divergence, which occurred during the time of geographical isolation. Speciation has taken place.

· A hybrid zone may be established.

Hybrid zone = A region where two related populations that diverged after becoming geographically isolated make secondary contact and interbreed where their geographical ranges overlap. For example,

· The red‑shafted flicker of western North America and the yellow‑shafted flicker central North America are two phenotypically distinct populations of woodpecker that interbreed in a hybrid zone stretching from southern Alaska to the Texas panhandle.

· The two populations came into renewed contact a few centuries ago after being separated during the ice ages.

· The hybrid zone is relatively stable and not expanding.

o The introgression of alleles between the populations has not penetrated beyond the hybrid zone, although the two populations have been interbreeding for at least two hundred years.

o The genotypic and pheontypic frequencies that distinguish the two population form steep clines into the hybrid zone.

· Away from the hybrid zone, the two populations remain distinct.

Should two populations which form a hybrid zone be considered subspecies or separate species?

· Some researchers, who support species status for such populations, recognize that the presence of stable hybrid zones creates a problem for the biological species concept.

o If the taxonomic identity of two species is maintained, even though they hybridize, there must be cohesive forces other than reproductive isolation that maintain the species and prevent their merging into a single species.

· These researchers favor an alternative known as the cohesion concept of species.

The cohesion concept of species holds that the cohesion may involve a distinctive, integrated set of adaptations that has been refined during the evolutionary history of a population.

· Phenotypic variation would be restricted by stabilizing selection to a range narrow enough to define the species as separate from other species.

· In the adaptive landscape view:

o The red‑shafted and yellow‑shafted flickers are clustered around different adaptive peaks.

o Specific combinations of alleles and specific linkages between gene loci on chromosomes may form a genetic basis for the cohesion of phenotypes.

o The clinal change of genetic structure and phenotype noted in the hybrid zone may be correlated with transitions in environmental factors that help shape the two distinct populations.

D. How Much Genetic Change Is Required For Speciation?

No generalizations can be made about genetic distance between closely related species. Reproductive isolation may result from changes in many loci or only in a few.

· Two species of Hawaiian Drosophila (D. silvestris and D. heteroneura) differ at only one locus, which determines head shape, an important factor in mate recognition.

o The phenotypic effect of different alleles at this locus is multiplied by epistasis involving at least ten other loci.

o Thus, no more than one mutation was necessary to differentiate the two species.

· Changes in one gene in a coadapted gene complex can substantially impact the development of an organism.

V1. The Theory of Punctuated Equilibrium has stimulated research on the tempo of speciation

Traditional evolutionary trees diagram the descent of species from ancestral forms as branches that gradually diverge with each new species evolving continuously over long spans of time.

· The theory behind such a tree is the extrapolation of microevolutionary processes (allele frequency changes in the gene pool) to the divergence of species.

· Big changes thus occur due to the accumulation of many small changes.

Paleontologists rarely find gradual transitions of fossil forms but often observe species appearing as new forms suddenly in the rock layers.

· These species persist virtually unchanged and then disappear as suddenly as they appeared.

· Even Darwin, who believed species from a common ancestral stock evolve differences gradually, was perplexed by the lack of transitional forms in the fossil record.

Advocates of punctuated equilibrium have redrawn the evolutionary tree to represent fossil evidence for evolution occurring in spurts of relatively rapid change instead of gradual divergence.

· This theory was proposed by Niles Eldredge and Stephen Jay Gould in 1972.

· It depicts species undergoing most of their morphological modification as they first separate from the parent species then showing little change as they produce additional species.

· In this theory gradual change is replaced with long periods of stasis punctuated with episodes of speciation.

· The origin of new polyploid plants through genome changes is one mechanism of sudden speciation.

· Allopatric speciation of a splinter population separated from its parent population by be rapid. Geographical barriers may also

o For a population facing new environmental conditions, genetic drift and natural selection can cause significant change in only a few hundred or thousand generations.

A few thousand generations is considered rapid in reference to the geologic time scale.

· The fossil record indicates that successful species survive for a few million years on average.

· If a species survives for five million years and most of its morphological changes occur in the first 50,000 years, then the speciation episode occurred in just 1% of the species' lifetime.

· With this time scale, a species will appear suddenly in rocks of a certain age, linger relatively unchanged for millions of years, then become extinct.

· While forming, the species may have gradually accumulated modifications, but with reference to its overall history, its formation was sudden.

· An evolutionary spurt preceding a longer period of morphological stasis would explain why paleontologists find so few transitions in the fossils record of a species.

Because "sudden" can refer to thousands of years on the geological time scale, differing opinions of punctuationalists and gradualists about the rate of speciation may be more a function of time perspective than conceptual difference. There is clear disagreement, however, over how much a species changes after its origin.

· In a species adapted to an environment that stays the same, natural selection would counter changes in the gene pool.

o Once selection during speciation produces new complexes of coadapted genes, mutations are likely to impose disharmony on the genome and disrupt the development of the organism.

· Stabilizing selection would thus hold a population at one adaptive peak to produce long periods of stasis

Some gradualists feel that stasis is an illusion since many species may continue to change, after they have diverged from the parent population, in ways undetectable in fossils.

· Changes in internal anatomy, physiology and behavior would go unnoticed by paleontologists as fossils only show external anatomy and skeletons.

· Population geneticists also point out that many microevolutionary changes occur at the molecular level without affecting morphology.

Some biologists even contest the long periods of morphological stasis in the history of species. Peter Sheldon has analyzed approximately 15,000 trilobite fossils which represent an unusually complete record.

· Paleontologists have arranged the fossils into several evolutionary lineages and classified the youngest and oldest as different species based on morphological differences.

· Based on his extensive study, Sheldon concludes that specific species boundaries cannot be drawn among these trilobite fossils.

o He found a gradual change in the morphological characteristics used to name the new species as he examined successive rock layers.

· Sheldon suggests that giving fossil species names can lead to an artificial punctuated interpretation of a gradually changing fossil series.

Alan Cheetham's research on fossilized bryozoans supports punctuated equilibrium.

· His studies have found long periods of morphological stasis in the bryozoans, which are punctuated by the relatively rapid appearance of new species.

It is obvious that additional extensive studies of fossil morphology where specific lineages are preserved should be carried out to assess the relative importance of gradual and punctuated tempos in the origin of new species.