Species and Speciation

Biological Species Concept A group of organisms that can interbreed and produce viable, fertile offspring

Biological Species Concept Assumes –Common characteristics –Genetically compatible –Interbreed under natural conditions –Sexual Reproduction Iguana iguana

Drawbacks of Biological Species Concept Cannot be used with exclusively asexual organisms –Prokaryotes –Amoeba & some other protists –Some animals, plants & fungi Amoeba proteus

Drawbacks: Isolated Populations How do you test for interbreeding under natural conditions? Can a deer in Florida interbreed with one in Wisconsin?

Drawbacks Can a Great Dane interbreed with a Chihuahua?

Drawbacks Some species look and behave differently Can interbreed and have viable, fertile offspring Example: coyotes, wolves, dogs

Drawbacks: Horizontal Gene Transfer Vertical gene transfer Parents offspring Horizontal gene transfer Individual individual Not parent-offspring

Horizontal Gene Transfer Can occur between organisms: –Different species –Different kingdoms –Different domains

Horizontal Gene Transfer in Bacteria (1) Transformation

Horizontal Gene Transfer in Bacteria (2) Transduction

Horizontal Gene Transfer in Bacteria (3) Conjugation

Horizontal Gene Transfer E. coli 0157:H7 has picked up Shiga toxin gene from Shigella Up to 25% genetic difference among E. coli strains

Horizontal Gene Transfer Normally, 25% genetic diversity different species Human and chimp genomes differ by only 1.2%

Drawbacks: Fossil Species Cannot tell if fossil organisms were capable of interbreeding

Fossils Normally cant compare genes of different fossil species

Morphological Species Concept Group of individuals that share common characteristics Used for: Fossils Exclusively asexual Traditional method for sexual organisms

Biological Species Concept: Reproductive Isolation Barriers between species Prevent viable fertile offspring (Lesser Bird of Paradise)

Reproductive Isolating Mechanisms Prezygotic mechanisms –Before fertilization Postzygotic mechanisms –After fertilization

Prezygotic Mechanisms Prevents: –Prevent mating from occurring –Prevents gametes from forming a zygote

Prezygotic Mechanisms: Temporal Isolation Isolated by time May breed: –Different times of day –Different seasons –Different years

Temporal Isolation Late WinterLate Summer

Prezygotic Mechanisms: Habitat Isolation Same geographic area Different habitats

Prezygotic Mechanisms: Behavioral Isolation Different courtship rituals Blue-Footed Booby

Behavioral Isolation Eastern and Western Meadowlarks Isolated by Songs

Prezygotic Mechanisms: Mechanical Isolation Structural differences prevent mating Example: Genital openings (arrows) not aligned No Mating

Prezygotic Mechanisms: Gametic Isolation Sperm cannot fertilize eggs Very important in aquatic species (broadcast spawners) Example: Sea Urchins

Postzygotic Mechanisms Prevents hybrid zygotes from developing into viable, fertile adults

Postzygotic Mechanism: Reduced Hybrid Viability If Sheep and goats mate Hybrid zygotes Die before birth.

Postzygotic Mechanism: Reduced Hybrid Fertility Male donkey x Female horse Mule Mules: –Healthy (viable) –Sterile.

Postzygotic Mechanism: Hybrid Breakdown First generation hybrids are viable and fertile. Offspring of hybrids are feeble or sterile.

Speciation Process of species formation Two main mechanisms: –Allopatric speciation –Sympatric speciation

Allopatric Speciation Geographic separation Genetic exchange Populations become genetically different Reproductive isolation

Sympatric Speciation Populations not geographically separated Can be due to rapid genetic changes: –Alter morphology –Alter behavior –Alter habitat preferences

Example: Sympatric Speciation Euhadra snails Mutation in one gene Shell spirals in opposite direction Instant mechanical isolation Two species

Adaptive Radiation A common ancestor Many new species Relatively rapid Common in island chains –Isolated –Numerous habitats & resources

Galápagos Finches Ancestral finch from South America Several finch species –Different diets –Different beak shapes –Different beak sizes

Galápagos Finches

Hawaiian Silverswords Tarweed arrived from North America ~ 5 mya Common ancestor Silverswords

Hawaiian Silverswords