The Origin of Species

The Origin of Specieschapter 24Campbell and ReeceSpeciationprocess by which one species splits into 2 or more speciesSpeciation explains both the diversity of life and the unity of living things.

Speciation : forms bridge between:MICROEVOLUTIONEvolutionary change below species levelExample:change in allele frequencies in population over generationsMACROEVOLUTIONEvolutionary change above the species levelExamples:origin of new group of organismsimpact of mass extinctionsBiological Species ConceptSpecies: a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspringmembers of a species cannot produce viable, fertile offspring with other groupsemphasizes the separateness of species due to reproductive barriers

What holds the gene pool of a species together?Gene Flow: transfer of alleles between populations of same speciesexchange of alleles tends to hold populations together genetically

Reproductive Isolationexistence of biological barriers that keep members of 2 populations from interbreeding over long periods of time

Reproductive Isolationhybrids: offspring that result from the mating of individuals from 2 different species or from 2 true-breeding varieties of same species

Prezygotic Barriersblock fertilization from happening by:impeding members of different species from attempting to matepreventing attempted mating from being completed successfullyhindering fertilization if mating was completed successfullyPostzygotic Barriersreproductive barrier that prevents hybrid zygotes produced by 2 different species from developing into viable, fertile adultslethal developmental errorsinfertility in viable offspring

Types of Prezygotic Reproductive BarriersHabitat Isolation2 species that occupy different habitats w/in same area may rarely interactexample:

Types of Prezygotic Reproductive Barriers2. Temporal Isolation:species breed during different times of day, different seasons, or different years

Types of Prezygotic Reproductive Barriers3. Behavioral Isolation:Courtship rituals used to attract mates are effective barriers

Types of Prezygotic Reproductive Barriers4. Mechanical Isolation:morphological differences prevent successful completion even if attempted

Types of Prezygotic Reproductive Barriers5. Gametic Isolation:Sperm of 1 species may not be able to fertilize egg of another:Reproductive tract hostile to spermSperm does not have enzymes to penetrate zona pellicida of another species

Types of Postzygotic Reproductive BarriersReduced Hybrid Viability:hybrids development or survival is impaired

Types of Postzygotic Reproductive Barriers2. Reduced Hybrid Fertility:hybrids may develop and be healthy but they are not fertile

Types of Postzygotic Reproductive BarriersHybrid Breakdown:Some 1st generation hybrids are fertile but those offspring are feeble or sterile

Species There is no single, universally applicable species concept that can define, explain, and identify all species.There are multiple ways to think about & define species.

Biological Species Limitationsunable to use these characterisitics on fossils of extinct speciesonly applies to organisms that reproduce sexuallyonly applies where there is no gene flow

Other Definitions of SpeciesThese dfns emphasize the unity w/in a species.morphological species concept: characterizes a species by a structural featureapplies to species that reproduce sexually or asexuallyhow scientists distinguish most speciesdisadvantage: subjectiveMorphological Species ConceptProblems:domestic dogs may look very different but are still same speciesmouse lemurs look very similar but there are 18 species of themgrey mouse lemur lesser mouse lemur

Ecological Species Conceptviews species in terms of its nichethe sum of how members of the species interact with the nonliving & living parts of their environmentasexual or sexual speciesemphasizes role of disruptive NS as organisms adapt to different environmental conditionsPhylogenetic Species Conceptdefines species as smallest group of individuals that share a common ancestor, forming one branch on the tree of lifedetermining degree of differences is difficult

SpeciesThere are >20 other ways to define species

Speciation can take place with or w/out geographic separationSpeciation can occur in 2 main ways:Allopatric SpeciationSympatric Speciation

ALLOPATRIC SPECIATIONother countryGene flow is interrupted when population is divided into geographically isolated subpopulations

Allopatric Speciation ProcessOnce geographic separation has occurred, the separated gene pools will each have their own mutationsNS & genetic drift may alter allele frequencies in different ways in each subpopulation group

Allopatric Speciation

Evidence of Allopatric SpeciationThere are many studies & examples supporting this type speciationIndirect support: regions that are isolated or highly subdivided have more species than regions w/out those featuresDrosophila Experiment

Sympatric Speciationsame countryoccurs in populations in same geographic arealess common than allopatric occurs if gene flow is reduced by factors like:polyploidyhabitat differentiationsexual selectionPOLYPLOIDYmeans extra sets of chromosomescan occur in animalsgray tree frog (Hyla versicolor)around Great Lakes

Polyploidymuch more common in plantsestimate: 80% of todays plants species have ancestors that formed by polyploid speciation2 forms

1. AutopolyploidIndividual has >2 chromosome sets all derived from a single speciesPlant polyploidy

Plant Polyploidytetraploid plant can produce fertile tetraploid offspring by self-pollinating or mating with other tetraploids

2. Allopolyploid2 different species interbreed making a hybridhybrid reproduces asexuallyover generations sterile hybrid fertile polyploid (called an allopolyploid)Allopolyploids can breed with each other but not with either of their parents so are a new speciesrare: 5 new plant species since 1850 documentedMimulus peregrinus

Allopolyploids include many agricultural cropsTriticum aestivum (bread wheat) has 6 sets chromosomes (2 pair from each of 3 parents), an allohexaploid1st polyploidy event probably occurred ~8,000 yrs ago as spontaneous hybrid

Top 2 parentsbottom: Triticum aestivum

Allopolyploids plant geneticists create new polyploids making hybrids with desired characteristicsuse chemicals that induce meiotic & mitotic errors

Habitat DifferentiationSympatric speciation can occur when genetic factors enable a subpopulation to exploit a habitat or resource used by the parent populationRhagoletis pomonellaNorth American apple maggot flyOriginal habitat was the native hawthorn tree

Habitat DifferentiationAs apples mature faster than hawthorn fruit, NS has favored flies with rapid developmenthave an allele that benefits flies that feed off only 1 or the other not both (post-zygotic barrier to reproduction)The flies feeding on apple trees now show temporal isolation from flies still eating hawthorn fruit (prezygotic restriction to gene flow)

Sexual Selectioncan also drive sympatric speciation:cichlid fish Pundamilia pundamilia

Cichlid Fish>600 species found in Lake Victoriaoriginated in past 100 000 yrshypothesis : subgroups of original population adapted to different food sources genetic divergencefemale preference for mates may also be a factor: 1 species breeding males have blue back another species has orange backBreeding Cichlids Colors

Sexual Selection Studyplaced the 2 subspecies in same tank used monochromatic orange light so both appeared very similarfemales bred with eitherConclusion: mate choice by females is based on male coloration so its the main reproductive barrier (prezygotic behavior)Allopatric & Sympatric Speciation are the 2 main modes of speciationAllopatricgeographic isolationNSgenetic driftsexual selectionSympatricrequires emergence of a reproductive barrier that isolates a subgroupless commonpolyploidysexual selectionHybrid Zones