Ch 24 – Origin of SpeciesCh 24 – Origin of Species

Overview: The “Mystery of Overview: The “Mystery of Mysteries”Mysteries”

Darwin explored the Galápagos Darwin explored the Galápagos IslandsIslands And discovered plants and animals And discovered plants and animals

found nowhere else on Earthfound nowhere else on Earth

Figure 24.1

Concept 24.1: The biological species Concept 24.1: The biological species concept emphasizes reproductive concept emphasizes reproductive isolationisolation

SpeciesSpecies Is a Latin word meaning “kind” or Is a Latin word meaning “kind” or

“appearance”“appearance”

The Biological Species The Biological Species ConceptConcept The biological species conceptThe biological species concept

Defines a species as a population or group Defines a species as a population or group of populations whose members have the of populations whose members have the potential to interbreed in nature and potential to interbreed in nature and produce viable, fertile offspring but are produce viable, fertile offspring but are unable to produce viable fertile offspring unable to produce viable fertile offspring with members of other populationswith members of other populations

Reproductive IsolationReproductive Isolation

Reproductive isolationReproductive isolation Is the existence of biological factors Is the existence of biological factors

that impede members of two species that impede members of two species from producing viable, fertile hybridsfrom producing viable, fertile hybrids

Is a combination of various Is a combination of various reproductive barriersreproductive barriers

Prezygotic barriersPrezygotic barriers Impede mating between species or hinder Impede mating between species or hinder

the fertilization of ova if members of the fertilization of ova if members of different species attempt to matedifferent species attempt to mate

Postzygotic barriersPostzygotic barriers Often prevent the hybrid zygote from Often prevent the hybrid zygote from

developing into a viable, fertile adultdeveloping into a viable, fertile adult

Prezygotic and postzygotic barriersPrezygotic and postzygotic barriers

Figure 24.4

Prezygotic barriers impede mating or hinder fertilization if mating does occur

Individualsof differentspecies

Matingattempt

Habitat isolation

Temporal isolation

Behavioral isolation

Mechanical isolation

HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION

(b)

(a)(c)

(d)

(e)

(f)

(g)

Viablefertile

offspring

Reducehybrid

viability

Reducehybridfertility

Hybridbreakdown

Fertilization

Gameticisolation

GAMETIC ISOLATION REDUCED HYBRID VIABILITY

REDUCED HYBRID FERTILITY HYBRID BREAKDOWN

(h) (i)

(j)

(k)

(l)

(m)

(a) Allopatric speciation. A population forms a new species while geographically isolated from its parent population.

(b) Sympatric speciation. A smallpopulation becomes a new specieswithout geographic separation.

Figure 24.5 A, B

Concept 24.2: Concept 24.2: Speciation can Speciation can take place with or take place with or without without geographic geographic separationseparation

Speciation can Speciation can occur in two waysoccur in two ways Allopatric Allopatric

speciationspeciation Sympatric Sympatric

speciationspeciation

Allopatric (“Other Country”) Allopatric (“Other Country”) SpeciationSpeciation

In allopatric speciationIn allopatric speciation Gene flow is interrupted or reduced Gene flow is interrupted or reduced

when a population is divided into two or when a population is divided into two or more geographically isolated more geographically isolated subpopulationssubpopulations

Sympatric (“Same Country”) Sympatric (“Same Country”) SpeciationSpeciation

In sympatric speciationIn sympatric speciation Speciation takes place in geographically Speciation takes place in geographically

overlapping populationsoverlapping populations

An autopolyploidAn autopolyploid Is an individual that has more than two Is an individual that has more than two

chromosome sets, all derived from a chromosome sets, all derived from a single speciessingle species

Figure 24.8

2n = 64n = 12

2n

4n

Failure of cell divisionin a cell of a growing diploid plant afterchromosome duplicationgives rise to a tetraploidbranch or other tissue.

Gametes produced by flowers on this branch will be diploid.

Offspring with tetraploid karyotypes may be viable and fertile—a new biological species.

An allopolyploidAn allopolyploid Is a species with multiple sets of Is a species with multiple sets of

chromosomes derived from different chromosomes derived from different speciesspecies

Figure 24.9

Meiotic error;chromosomenumber notreduced from2n to n

Unreduced gametewith 4 chromosomes

Hybrid with7 chromosomes

Unreduced gametewith 7 chromosomes Viable fertile hybrid

(allopolyploid)

Normal gameten = 3

Normal gameten = 3

Species A 2n = 4

Species B 2n = 6

2n = 10

Adaptive RadiationAdaptive Radiation Adaptive radiationAdaptive radiation

Is the evolution of diversely adapted Is the evolution of diversely adapted species from a common ancestor upon species from a common ancestor upon introduction to new environmental introduction to new environmental opportunitiesopportunities

Figure 24.11

The punctuated equilibrium model The punctuated equilibrium model Contrasts with a model of gradual Contrasts with a model of gradual

change throughout a species’ existencechange throughout a species’ existence

Figure 24.13

Gradualism model. Species descended from a common ancestor gradually diverge more and more in their morphology as they acquire unique adaptations.

Time

(a) Punctuated equilibrium model. A new species changes most as it buds from a parent species and then changes little for the rest of its existence.

(b)