Origon of Species

8/14/2019 Origon of Species

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Copyright (c) The McGraw-Hill Com 1

CHAPTER 25

ORIGIN OF SPECIES

Prepared by

Brenda Leady, University of Toledo


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SpeciesA group of organisms that maintains a

distinctive set of attributes in natureMacroevolution

Evolutionary changes that create new speciesand groups of speciesOccurs by accumulation of microevolutionarychanges (changes in a single gene)


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Known number of species about 1.4million

Estimates of unidentified species rangefrom 2 to 100 millionDifficulty in identifying a species

A single species may exist in 2 distinctpopulations that are in the slow process of evolving into 2 or more different species


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Amount of separation time for 2

populationsShort time likely to be similar andconsidered the same speciesLong time more likely to show unequivocal

differencesMay find situations where somedifferences are apparent but difficult todecide if the 2 populations are trulydifferent species

Sometimes use subspecies classification


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Species concepts

PhylogeneticBiologicalEvolutionaryEcological


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Phylogenetic species conceptSpecies are identified by having a uniquecombination of traitsHistorically used physical traits

Now can use DNA sequencesAdvantage

Can be applied to all organisms

DrawbacksHow many traits to consider, traits that vary ina continuous way, choose degree of dissimilarity to use, and members of the samespecies can look very different while membersof a different species can look very similar


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Biological species concept

A species is a group of individuals whosemembers have the potential to interbreed

with one another in nature to produceviable, fertile offspring but cannotsuccessfully interbreed with members of other speciesReproductive isolation prevents breedingwith other species


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Evolutionary species conceptA species is derived from a single lineagethat is distinct from other lineages and hasits own evolutionary tendencies andhistorical fateLineage

Genetic relationship between an individual or

group of individuals and its ancestorsDrawback no easy way to identify aunique species because lineages difficultto examine and quantitate


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Ecological species concept

Each species occupies an ecological nicheUnique set of habitat resources that a speciesrequires, as well as its influence on the environment

and other speciesWithin their own niche, members of a givenspecies compete with each other for survivalIf two organisms are very similar, their needs will

overlap, which results in competitionSuch competing individuals are likely to be of thesame speciesUseful for bacterial species


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Reproductive isolating mechanisms

Consequence of genetic changes asspecies adapts to its environmentPrezygotic barriers

Prevent formation of zygote

Postzygotic barriersBlock development of viable, fertile individuals

Interspecies hybrid- when 2 species doproduce offspring


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Prezygotic barriers

Habitat isolationGeographic barrier prevents contact

Temporal isolationReproduce at different times of the day or year


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Behavioral

isolationBehaviorsimportant in matechoiceChanges in song


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Mechanical isolationSize or incompatible genitalia preventsmating

Gametic isolationGametes fail to unite successfullyImportant in species that releasegametes into the water or air


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Postzygotic barriersLess common in nature because they aremore costly in terms of energy andresources used

Hybrid inviability fertilized egg cannotprogress past an early embryoHybrid sterility interspecies hybrid viablebut sterile

Mule exampleHybrid breakdown hybrids viable andfertile but subsequent generations havegenetic abnormalities


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Patterns of speciation

AnagenesisSingle species transformed into a different

species over the course of many generationsCladogenesis

Division of a species into 2 or more species


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Allopatric speciation

Most prevalent method for cladogenesisOccurs when some members of a species

become geographically separated


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Can also occur when small populationmoves to a new location that is

geographically separated founder effectGenetic drift and natural selection may quicklylead to differences

Adaptive radiation single species evolvesinto array of descendents that differ greatly inhabitat, form or behavior


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Podos Found That an Adaptation to Feeding AlsoMay Have Promoted Reproductive Isolation

in FinchesDarwins finches have different beak sizesand shapes as adaptations to different

feeding strategiesPodos analyzed songs to see if beakmorphology birds with larger beaks had

more narrow frequency range and/or trillrateCould have played a role in reproductive

isolation


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Hybridization

Prior to complete reproductive isolation,the zones where two populations caninterbreed are known as hybrid zonesAs the two populations accumulatedifferent genetic changes, this maydecrease the ability of individuals fromdifferent populations to mate with eachother in the hybrid zoneOnce gene flow through the hybrid zone isgreatly diminished, the two populations

are reproductively isolated


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Sympatric speciation

Occurs when members of a species thatinitially occupy the same habitat within the

same range diverge into two or moredifferent speciesTends to involve abrupt genetic changesthat quickly lead to the reproductiveisolation of a group of individualsChanges in chromosome number


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Pace of speciation

Gradualismeach new species evolves continuously over longspans of time

large phenotypic differences that produce newspecies are due to the accumulation of many smallgenetic changes

Punctuated equilibrium

Tempo more sporadicSpecies in equilibrium for long periods and then shortrapid bursts of changes


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Evo-Devo

Evolutionary developmental biologyCompares the development of different

organisms in an attempt to understandancestral relationships between organismsand the developmental mechanisms thatbring about evolutionary changeInvolves the discovery of genes thatcontrol development


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Developmental genes are key

players in evolutionChanges in developmental genes affecttraits that can be acted on by natural

selectionCompare chicken and duck foot

Due to differences in expression of 2 cell-

signaling proteinsBMP4 causes cells to undergo apoptosis and dieGremlin inhibits the function of BMP4 and allowscell to survive


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Mutations on the expression of BMP4 and

gremlin provided variationIn terrestrial settings, nonwebbed feet arean advantage

Natural selection maintains nonwebbed feetIn aquatic environments, webbed feet arean advantage

Natural selection would have favored webbedfeet

Speciation may have been promoted bygeographical isolation of habitats


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Hox genes

Found in all animalsGenetic variation may have been criticalevent in the formation of new body plansNumber and arrangement of Hox genesvaries among different types of animalsIncreases in the number of Hox genesmay have led to greater complexity inbody structure


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Three lines of evidence support the idea

that Hox gene complexity has beeninstrumental in the evolution andspeciation of animals with different bodypatterns

Hox genes are known to control bodydevelopmentGeneral trend for simpler animals to havefewer Hox genes and Hox gene clustersComparison of Hox gene evolution andanimal evolution bear striking parallels


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Developmental genes that affect

growth rateAllometric growth different parts of thebody grow at different rates with respect to

each other Compare growth of head between humanand chimpanzee


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Changes in growth rates can also affectthe developmental stage at which onespecies reproduces compared to that of another speciesReproduction in the adult is observed atan earlier stage in one species thananother Paedomorphosis retention of juveniletraits in an adult organism


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The Study of the Pax6 Gene Indicates That DifferentTypes of Eyes Evolved from a Simpler Form

Explaining how a complex organ comes intoexistence is a major challenge

Researchers have discovered many differenttypes of eyesThought that eyes may have independently

arisen many different times during evolutionPax6 is a master control gene that controlsthe expression of many other genes and

influences eye development


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Eyes of Drosophila and mammals areevolutionarily derived from a modification of an eye that arose once during evolutionIf Drosophila and mammalian eyes hadarisen independently, the Pax6 gene frommice would not be expected to induce theformation of eyes in DrosophilaHypothesized that the eyes from manydifferent species all evolved from a commonancestral form consisting of, as proposed byDarwin, one photoreceptor cell and one

pigment cell


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Origon of Species

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