classification organisms are classified into a hierarchical classification that groups closely...

ClassificationClassification

Organisms are classified into a Organisms are classified into a hierarchical classification that groups hierarchical classification that groups closely related organisms and closely related organisms and progressively includes more and more progressively includes more and more organisms.organisms.

SpeciesSpecies

The species is the basic biological unit The species is the basic biological unit around which classifications are based.around which classifications are based.

However, what constitutes a species can However, what constitutes a species can be difficult to define and there are multiple be difficult to define and there are multiple definitions of species in use today. definitions of species in use today.

What is a species?What is a species?

The species is a basic biological unit and The species is a basic biological unit and humans seem to intuitively recognize humans seem to intuitively recognize species.species.

However, why do species exist?However, why do species exist?

Why don’t we see a smooth continuous Why don’t we see a smooth continuous blending of one species into another?blending of one species into another?

Why do we see discrete species?Why do we see discrete species?

Because intermediate forms between Because intermediate forms between closely related organisms are usually closely related organisms are usually selected against. selected against.

If they were not selected against, then the If they were not selected against, then the two forms would merge into one as their two forms would merge into one as their gene pools mixed. gene pools mixed.


Organisms are very well adapted to their Organisms are very well adapted to their environments having evolved over millions environments having evolved over millions of years. of years.

Each organism has specialized Each organism has specialized characteristics such as camouflage, characteristics such as camouflage, feeding structures, behavior, and genitalia feeding structures, behavior, and genitalia that equip it to survive well in its that equip it to survive well in its environment. environment.


An offspring that results from a cross between members An offspring that results from a cross between members of two different species or between members of different of two different species or between members of different populations that have been evolving in isolation from populations that have been evolving in isolation from each other, will probably have traits each other, will probably have traits intermediateintermediate between its parents. between its parents.

As a result, it likely will be less well adapted to its As a result, it likely will be less well adapted to its environment than either parental form and be selected environment than either parental form and be selected against.against.

Thus, we see distinctively different species.Thus, we see distinctively different species.

What is a species?What is a species?

John Ray (1627-1705) gave first general John Ray (1627-1705) gave first general definition of a species.definition of a species.

A species consists of all individuals that A species consists of all individuals that can breed together and produce fertile can breed together and produce fertile offspring.offspring.

A female donkey mated to a male horseproduces what?

A mule (which is sterile)Hence, donkeys and horses are separate species.

Biological Species ConceptBiological Species Concept

Ray’s idea was updated into the Ray’s idea was updated into the Biological Species Biological Species Concept. Concept. Two definitions of the BSC are given below:Two definitions of the BSC are given below:

““Species are groups of actually or potentially Species are groups of actually or potentially interbreeding natural populations, which are interbreeding natural populations, which are reproductively isolated from other such groups.” Ernst reproductively isolated from other such groups.” Ernst Mayr.Mayr.

““A species is a reproductive community of populations A species is a reproductive community of populations (reproductively isolated from others) that occupies a (reproductively isolated from others) that occupies a specific niche in nature.” Ernst Mayr.specific niche in nature.” Ernst Mayr.

Biological Species ConceptBiological Species Concept

The biological species concept emphasizes that The biological species concept emphasizes that a species is an interbreeding population of a species is an interbreeding population of individuals sharing common descent and that individuals sharing common descent and that members of that community because they share members of that community because they share a niche constitute an ecological entity in nature.a niche constitute an ecological entity in nature.

Members of a species we expect to be similar to Members of a species we expect to be similar to each other but different from other organisms,each other but different from other organisms,

Criticisms of the Biological Species Criticisms of the Biological Species ConceptConcept

The BSC has been criticized for several The BSC has been criticized for several reasons:reasons:

1. It applies only to sexually reproducing 1. It applies only to sexually reproducing species.species.

2. Distinguishing between species on the basis 2. Distinguishing between species on the basis of reproductive separation is problematic of reproductive separation is problematic because it can be difficult to determine how because it can be difficult to determine how much reproductive separation is needed to much reproductive separation is needed to distinguish between species.distinguish between species.

3. The definition refers only to current 3. The definition refers only to current populations and ignores the species status of populations and ignores the species status of ancestral populations.ancestral populations.

Evolutionary Species ConceptEvolutionary Species Concept

George Gaylord Simpson proposed the George Gaylord Simpson proposed the Evolutionary Species ConceptEvolutionary Species Concept in the in the 1940’s to add an evolutionary time 1940’s to add an evolutionary time dimension to the Biological Species dimension to the Biological Species Concept.Concept.


Evolutionary species concept “A single Evolutionary species concept “A single lineage of ancestor-descendant lineage of ancestor-descendant populations that maintains its identity from populations that maintains its identity from other such lineages and that has its own other such lineages and that has its own evolutionary tendencies and historical evolutionary tendencies and historical fate.”fate.”


Definition applies to both sexually and Definition applies to both sexually and asexually reproducing species and asexually reproducing species and emphasizes common descent. As long as emphasizes common descent. As long as diagnostic features are maintained a diagnostic features are maintained a lineage will be recognized as a single lineage will be recognized as a single species.species.

Phylogenetic species conceptPhylogenetic species concept

A third species concept is the phylogenetic A third species concept is the phylogenetic species concept.species concept.

““an irreducible (basal) grouping of an irreducible (basal) grouping of organisms diagnosably distinct from other organisms diagnosably distinct from other such groupings and within which there is a such groupings and within which there is a parental pattern of ancestry and descent.”parental pattern of ancestry and descent.”


The phylogenetic species concept also The phylogenetic species concept also emphasizes common descent and covers emphasizes common descent and covers both sexually and asexually reproducing both sexually and asexually reproducing organisms. organisms.

Under the PSC any population that has Under the PSC any population that has become separated and has undergone become separated and has undergone character evolution will be recognized as a character evolution will be recognized as a species.species.


Criterion of irrreducibility requires that no more Criterion of irrreducibility requires that no more than one diagnosibly distinct population can be than one diagnosibly distinct population can be included in a single species. Thus, the included in a single species. Thus, the emphasis is placed on monophyly: lineages that emphasis is placed on monophyly: lineages that contain all the descendents of a single common contain all the descendents of a single common ancestor.ancestor.

Main difference in practice between ESC and Main difference in practice between ESC and PSC is that PSC recognizes as species the PSC is that PSC recognizes as species the smallest groupings of organisms that have smallest groupings of organisms that have undergone independent evolutionary change.undergone independent evolutionary change.


The ESC would group into one species a series The ESC would group into one species a series of geographically disjunct populations that show of geographically disjunct populations that show some genetic divergence, but the PSC would some genetic divergence, but the PSC would treat them as discrete species.treat them as discrete species.

Thus, subspecies under the ESC would be Thus, subspecies under the ESC would be species under the PSC and in general more species under the PSC and in general more species would be recognized under the PSC species would be recognized under the PSC than either the BSC or ESC.than either the BSC or ESC.

Typological Species conceptTypological Species concept

For historical interest this is the pre-Darwinian idea For historical interest this is the pre-Darwinian idea that species are defined by fixed and unchanging that species are defined by fixed and unchanging features and do not change over time (i.e., evolve).features and do not change over time (i.e., evolve).

Biologists discarded the idea after Darwin’s theory Biologists discarded the idea after Darwin’s theory of evolution by natural selection became of evolution by natural selection became established.established.

Creationist’s still cling to the typological species Creationist’s still cling to the typological species concept and you’ll often see “types” referred to in concept and you’ll often see “types” referred to in creationist writings.creationist writings.

Applications of species conceptsApplications of species concepts

Diversification in marine copepods.Diversification in marine copepods.

Copepods are small abundant Copepods are small abundant crustaceans. Numerous populations of crustaceans. Numerous populations of EurytemoraEurytemora affinisaffinis have been described have been described from estuaries in the northern hemisphere from estuaries in the northern hemisphere and traditionally grouped into one species and traditionally grouped into one species on the basis of similarity of appearance.on the basis of similarity of appearance.

Diversification in marine copepodsDiversification in marine copepods

A study by Lee (2000) in which she compared A study by Lee (2000) in which she compared gene sequences of populations and also carried gene sequences of populations and also carried out breeding trials showed that at least 8 out breeding trials showed that at least 8 phylogenetic species exist, which are phylogenetic species exist, which are reproductively isolated.reproductively isolated.

Clearly, assuming species identity on the basis Clearly, assuming species identity on the basis of morphology alone will underestimate species of morphology alone will underestimate species diversity.diversity.

How many species of African How many species of African elephants are there?elephants are there?

Traditionally one species of elephant Traditionally one species of elephant Loxodonta Loxodonta africanaafricana has been recognized in Africa (a has been recognized in Africa (a second species second species ElephasElephas maximusmaximus occurs in occurs in Asia).Asia).

However, recent morphological studies have However, recent morphological studies have pointed out that forest dwelling elephants in pointed out that forest dwelling elephants in West Africa appear to differ from elephants West Africa appear to differ from elephants found in Savannah habitats elsewhere on the found in Savannah habitats elsewhere on the continent.continent.

How many species of African How many species of African elephants are there?elephants are there?

A comparison of DNA from 21 populations A comparison of DNA from 21 populations suggests that two phylogenetic species exist and suggests that two phylogenetic species exist and it has been suggested by Roca et al. (2001) that it has been suggested by Roca et al. (2001) that forest elephants be named forest elephants be named Loxodonta cyclotisLoxodonta cyclotis..

Whether the two populations are capable of Whether the two populations are capable of interbreeding is unclear, but the clear genetic interbreeding is unclear, but the clear genetic differences between populations suggest that differences between populations suggest that conservation biologists should be attempting to conservation biologists should be attempting to conserve members of both populations.conserve members of both populations.

How species formHow species form

Classically, speciation has been viewed as Classically, speciation has been viewed as a three stage process:a three stage process: Isolation of populations.Isolation of populations. Divergence in traits of separated populations Divergence in traits of separated populations

(e.g. mating system or habitat use).(e.g. mating system or habitat use). Reproductive isolation of populations that Reproductive isolation of populations that

maintains isolation when populations come maintains isolation when populations come into contact again (secondary contact).into contact again (secondary contact).

How species formHow species form

Recent research shows that steps one and Recent research shows that steps one and two may take place simultaneously in the two may take place simultaneously in the same place and often the third step does same place and often the third step does not occur.not occur.

Genetic Isolation: physical Genetic Isolation: physical isolationisolation

Physical separation reduces or stops gene flow Physical separation reduces or stops gene flow between populations and as a result there may be a between populations and as a result there may be a balance between gene flow and natural selection balance between gene flow and natural selection (recall the Lake Erie water snake example from (recall the Lake Erie water snake example from chapter 6).chapter 6).

On the islands selection favors elimination of alleles On the islands selection favors elimination of alleles for banding, but migration constantly introduces for banding, but migration constantly introduces them. If the islands were to be completely separated them. If the islands were to be completely separated so no snakes migrated natural selection would result so no snakes migrated natural selection would result in the island populations becoming different from the in the island populations becoming different from the mainland ones.mainland ones.

Allopatric speciationAllopatric speciation

This is the essence of Ernst Mayr’s This is the essence of Ernst Mayr’s allopatric model of speciation.allopatric model of speciation.

A physical barrier isolates a population or A physical barrier isolates a population or populations from the rest of the species populations from the rest of the species and selection favors genetic divergence of and selection favors genetic divergence of that population.that population.

Allopatric speciationAllopatric speciation

Separation of populations can occur by Separation of populations can occur by two major means:two major means: Dispersal of some individuals across a barrier.Dispersal of some individuals across a barrier. Development of a new barrier that separates Development of a new barrier that separates

populations [populations [VicarianceVicariance] (the vicariance event ] (the vicariance event could be e.g. change in flow of a river, lava could be e.g. change in flow of a river, lava flow, development of a mountain range, flow, development of a mountain range, habitat destruction)habitat destruction)

Geographic isolation through Geographic isolation through dispersaldispersal

We have already encountered example sof We have already encountered example sof speciation after individuals crossed a barrier.speciation after individuals crossed a barrier.

The ancestors of Darwin’s finches colonized the The ancestors of Darwin’s finches colonized the Galapagos Islands after dispersing from South Galapagos Islands after dispersing from South America and speciated into the current range of America and speciated into the current range of species. species.

Similarly, the Hawaiian Islands were colonized by Similarly, the Hawaiian Islands were colonized by ancestral ancestral DrosophilaDrosophila fruit flies that appear to have fruit flies that appear to have speciated to produce more than 500 endemic speciated to produce more than 500 endemic species of Drosophila on the islands.species of Drosophila on the islands.

Evidence for founder hypothesis of Evidence for founder hypothesis of

speciation in Hawaiian Islandsspeciation in Hawaiian Islands The main hypothesis for how the Hawaiian The main hypothesis for how the Hawaiian

Islands became populated with a diverse variety Islands became populated with a diverse variety of endemic species most of which occur on only of endemic species most of which occur on only a single island is the founder hypothesis.a single island is the founder hypothesis.

According to the founder hypothesis new According to the founder hypothesis new species are formed when a small population of species are formed when a small population of individuals disperses to a new island and after individuals disperses to a new island and after being separated diverges from the ancestral being separated diverges from the ancestral form.form.

Evidence for founder hypothesis of Evidence for founder hypothesis of speciation in Hawaiian Islandsspeciation in Hawaiian Islands

The Hawaiian Islands were formed by a The Hawaiian Islands were formed by a stationary geological “hot spot” over which the stationary geological “hot spot” over which the continental plate drifts northwest.continental plate drifts northwest.

Periodically, the hot spot produces magma Periodically, the hot spot produces magma flows, which form islands that are then carried flows, which form islands that are then carried away on the plate and ultimately erode away. away on the plate and ultimately erode away. Thus the newest islands are close to the hot Thus the newest islands are close to the hot spot and the oldest further northwest.spot and the oldest further northwest.


Based on the geological information te Based on the geological information te founder hypothesis makes two predictions founder hypothesis makes two predictions about the pattern of speciation that should about the pattern of speciation that should be observed.be observed. Closely related species should be found on Closely related species should be found on

adjacent islands andadjacent islands and Some speciation sequences should match the Some speciation sequences should match the

sequence in which islands formed.sequence in which islands formed.


A study of mitochondrial DNA of four A study of mitochondrial DNA of four species of closely related Drosophila by species of closely related Drosophila by DeSalle and Giddings (1986) found the DeSalle and Giddings (1986) found the predicted patterns. predicted patterns.

The most recent species occur on the The most recent species occur on the youngest islands and several of the youngest islands and several of the branching events match the order of island branching events match the order of island formation.formation.

Geographic isolation through Geographic isolation through vicariance eventsvicariance events

There are many ways in which a species There are many ways in which a species distribution may be split into two by a physical distribution may be split into two by a physical event. Some such as mountain formation are event. Some such as mountain formation are slow, others such as a lava flow are rapid.slow, others such as a lava flow are rapid.

The Isthmus of Panama closed about 3 million The Isthmus of Panama closed about 3 million years ago separating marine populations on years ago separating marine populations on either side. Did these populations speciate?either side. Did these populations speciate?


A DNA sequence study by Knowlton et al. 1993 A DNA sequence study by Knowlton et al. 1993 of snapping shrimp populations from both sides of snapping shrimp populations from both sides of the isthmus suggests they did.of the isthmus suggests they did.

Seven pairs of morphologically closely related Seven pairs of morphologically closely related species pairs occur, one of each pair on each species pairs occur, one of each pair on each side of the isthmus and the DNA sequence side of the isthmus and the DNA sequence results confirm that these are each others results confirm that these are each others closest relatives, which is consistent with the closest relatives, which is consistent with the vicariance hypothesis.vicariance hypothesis.

Phylogenetic tree of numbered species ofsnapping shrimp. P and C refer to Pacific and Caribbean species respectively.


Mating experiments with the snapping Mating experiments with the snapping shrimp fund that males and females with shrimp fund that males and females with the greatest genetic divergence were least the greatest genetic divergence were least interested in each other and almost none interested in each other and almost none the pairs produced clutches that yielded the pairs produced clutches that yielded fertile young.fertile young.

Polyploidization as a mechanism of Polyploidization as a mechanism of speciationspeciation

Polyploidy (production of multiple sets of Polyploidy (production of multiple sets of chromosomes) appears to have played a chromosomes) appears to have played a major role in the speciation patterns of major role in the speciation patterns of plants.plants.

An estimated 70% of flowering plants An estimated 70% of flowering plants appear to have had polyploid events in appear to have had polyploid events in their evolutionary history as have 95% of their evolutionary history as have 95% of fern species.fern species.

Mechansims of divergenceMechansims of divergence

Dispersal, vicariance and polyploidization Dispersal, vicariance and polyploidization create opportunity for speciation to take create opportunity for speciation to take place.place.

For speciation to occur populations must For speciation to occur populations must diverge genetically from each other.diverge genetically from each other.

Genetic driftGenetic drift

Genetic drift is a sampling phenomenon in Genetic drift is a sampling phenomenon in which only some alleles occur in a which only some alleles occur in a population as a result of its small size population as a result of its small size because of founder effect and because of founder effect and bottlenecking.bottlenecking.

If a population remains small for a period If a population remains small for a period of time many alleles may be lost from the of time many alleles may be lost from the gene pool. gene pool.

Genetic driftGenetic drift

The length of time the population is bottlenecked The length of time the population is bottlenecked has a strong influence on how great allele has a strong influence on how great allele frequency changes will be. Theoretical studies frequency changes will be. Theoretical studies show that if populations remain very small for show that if populations remain very small for only a short time then only rare alleles are likely only a short time then only rare alleles are likely to be lost and little effect on speciation is likely.to be lost and little effect on speciation is likely.

Thus, scientists are increasingly focusing on Thus, scientists are increasingly focusing on natural selection as a more important force natural selection as a more important force driving speciation than drift.driving speciation than drift.

Natural selection and speciation in Natural selection and speciation in apple and hawthorn maggot fliesapple and hawthorn maggot flies

The apple maggot fly (The apple maggot fly (Rhagolestis Rhagolestis pomonellapomonella) is a major pest of apples that ) is a major pest of apples that occurs throughout the northeastern U.S. It occurs throughout the northeastern U.S. It also parasitizes hawthorn trees a close also parasitizes hawthorn trees a close relative of apples.relative of apples.

Maggot flies recognize trees on the basis Maggot flies recognize trees on the basis of visual, tactile and olfactory cues and of visual, tactile and olfactory cues and mate on or near the fruit.mate on or near the fruit.


Eggs are laid on fruits and larvae develop Eggs are laid on fruits and larvae develop in them. When the fruit falls the larvae in them. When the fruit falls the larvae burrow into the ground and pupate burrow into the ground and pupate emerging as adults the next year.emerging as adults the next year.

Apple trees are a novel food source for Apple trees are a novel food source for these native flies, which exploited apples these native flies, which exploited apples after they were introduced about 300 after they were introduced about 300 years ago.years ago.


The question is does the new food source The question is does the new food source represent an island and are the represent an island and are the populations that breed on apples populations that breed on apples genetically distinct form those that breed genetically distinct form those that breed on hawthorn trees?on hawthorn trees?

Do apple and hawthorn populations Do apple and hawthorn populations interbreed or not and are they diverging?interbreed or not and are they diverging?


Hawthorn and apple trees are often in very Hawthorn and apple trees are often in very close proximity so it would seem hard for close proximity so it would seem hard for the populations to diverge.the populations to diverge.

However, a protein electrophoresis study However, a protein electrophoresis study by Feder et al. (1988,1990) showed that by Feder et al. (1988,1990) showed that the populations are genetically distinct.the populations are genetically distinct.


Each population shows a strong preference for Each population shows a strong preference for its own fruit type, which because mating takes its own fruit type, which because mating takes place on fruit results in strong nonrandom place on fruit results in strong nonrandom mating. mating.

There is gene flow between populations There is gene flow between populations because about 6% of matings are cross-because about 6% of matings are cross-population matings, but despite this gene flow population matings, but despite this gene flow natural selection appears to driving the natural selection appears to driving the populations apart.populations apart.


Natural selection favors divergence because Natural selection favors divergence because hawthorn fruits ripen 3-4 weeks after apples. As hawthorn fruits ripen 3-4 weeks after apples. As a result hawthorn fly larvae experience cool a result hawthorn fly larvae experience cool temperatures before pupating whereas apple fly temperatures before pupating whereas apple fly larvae experience warmer temperatures.larvae experience warmer temperatures.

Hawthorn flies and apple flies thus depend on Hawthorn flies and apple flies thus depend on different temperature signals to time their different temperature signals to time their pupation and emergence the next spring and pupation and emergence the next spring and have different developmental timetables.have different developmental timetables.


Experimental tests show that these Experimental tests show that these developmental schedules have a genetic basis developmental schedules have a genetic basis and individuals need the correct alleles to and individuals need the correct alleles to develop under each temperature regime.develop under each temperature regime.

Individuals that are the result of crosses Individuals that are the result of crosses between apple and hawthorn flies are thus between apple and hawthorn flies are thus selected against and the populations have selected against and the populations have diverged and continue to do so.diverged and continue to do so.

Secondary contactSecondary contact

Theodosius Dobzhansky (1937) the famous Theodosius Dobzhansky (1937) the famous geneticist reasoned that for populations that had geneticist reasoned that for populations that had diverged and come back into contact hybrid diverged and come back into contact hybrid offspring between them would have reduced offspring between them would have reduced fitness.fitness.

As a result there should be strong selection As a result there should be strong selection favoring assortative mating (individuals mating favoring assortative mating (individuals mating within their own population) and as a result a within their own population) and as a result a variety of isolating mechanisms should evolve to variety of isolating mechanisms should evolve to reduce the likelihood of interbreeding.reduce the likelihood of interbreeding.

Isolating mechanismsIsolating mechanisms

Isolating mechanisms fall into two Isolating mechanisms fall into two categories: categories: prezygoticprezygotic (those that reduce chances of (those that reduce chances of

mating and fertilization taking place) and mating and fertilization taking place) and postzygoticpostzygotic (those that reduce the viability or (those that reduce the viability or

hybrid offspring).hybrid offspring).


Examples of prezygotic isolating Examples of prezygotic isolating mechanisms:mechanisms: Different habitat choiceDifferent habitat choice Activity at different times of dayActivity at different times of day Differences in sexual advertisements: calls, Differences in sexual advertisements: calls,

displays, pheromones.displays, pheromones.


Examples of postzygotic isolating Examples of postzygotic isolating mechanisms:mechanisms: Failure of zygote to developFailure of zygote to develop Reduced viability of zygoteReduced viability of zygote Sterility Sterility

HybridizationHybridization

In many cases hybrid offspring have reduced In many cases hybrid offspring have reduced fitness and this maintains two distinct gene pools fitness and this maintains two distinct gene pools and incipient species.and incipient species.

However, in some instances, hybridization However, in some instances, hybridization appears to promote speciation, especially in appears to promote speciation, especially in plants, as some hybrids may obtain plants, as some hybrids may obtain combinations of genes from parental species combinations of genes from parental species that enable them to occupy habitat that neither that enable them to occupy habitat that neither parental strain can. parental strain can.

HybridizationHybridization

For example, For example, HelianthusHelianthus anomalousanomalous a a southwestern species of sunflower, southwestern species of sunflower, possesses a unique combination of genes possesses a unique combination of genes from from H. annuusH. annuus and and H. petiolarisH. petiolaris and is and is clearly the result of a hybridization event.clearly the result of a hybridization event.

Genetics of speciationGenetics of speciation

How much genetic differentiation is How much genetic differentiation is needed to separate populations enough needed to separate populations enough that two new species are produced?that two new species are produced?

Historically, it was considered that large Historically, it was considered that large differences would be necessary, but more differences would be necessary, but more recent thinking is that large differences are recent thinking is that large differences are not necessarynot necessary

Genetics of speciation: pea aphidsGenetics of speciation: pea aphids

Pea aphids are small sap-sucking insects.Pea aphids are small sap-sucking insects.

Via et al. have studied two populations one that Via et al. have studied two populations one that lives on red clover and the other on alfalfa.lives on red clover and the other on alfalfa.

They have shown that members of each They have shown that members of each population actively chooses its preferred plant population actively chooses its preferred plant and each does poorly if reared on the other and each does poorly if reared on the other plant.plant.


Crosses between the two populations Crosses between the two populations produce F1 hybrids that do poorer than produce F1 hybrids that do poorer than either parental population on both plants.either parental population on both plants.

Via et al. have identified alleles at several Via et al. have identified alleles at several locations in the genome that increase locations in the genome that increase fecundity on one plant, but decrease it on fecundity on one plant, but decrease it on the other.the other.


These data suggest that there is a genetic trade-These data suggest that there is a genetic trade-off and that alleles that lead to high fitness on off and that alleles that lead to high fitness on one plant lead to low fitness on the other.one plant lead to low fitness on the other.

In addition, alleles for plant preference and In addition, alleles for plant preference and success on that particular plant appear closely success on that particular plant appear closely related, which suggests the same allele may related, which suggests the same allele may have multiple effects or that alleles for success have multiple effects or that alleles for success and preference are closely linked.and preference are closely linked.


If it is common for the same genes or closely If it is common for the same genes or closely linked sets of genes to simultaneously alter linked sets of genes to simultaneously alter preference and increase success on host plants preference and increase success on host plants then mutations of these genes should lead to then mutations of these genes should lead to speciation on the basis of host plant use.speciation on the basis of host plant use.

Because there are millions of plant-feeding Because there are millions of plant-feeding insects, this may be an important mechanism of insects, this may be an important mechanism of speciation.speciation.