Chapter 24 The Origins of the Species

Biological Species Concept Species: population whose members can interbreed and produce viable, fertile offspring (reproductively compatible) These happy face spiders look different, but since they can interbreed, they are considered the same species: Theridion grallator

Speciation lineage-splitting event that produces two or more separate species Original species cannot produce viable offspring together or that they avoid mating with members of the other group

speciation and extinction Microevolution: changes within a single gene pool Macroevolution: evolutionary change above the species level Speciation explains similarities and differences among species. speciation video speciation and extinction

New species are formed by being isolated from each other New species are formed by being isolated from each other. Reproductive isolation barriers that prevent members of 2 species from producing viable, fertile hybrids

Reproductive Barriers/Isolation Limits gene flow between species and formation of hybrids

Types of Reproductive Barriers Prezygotic: impede mating or hinder fertilization if mating occurs

Habitat Isolation Ex: garter snakes Two species occupy different habitats rarely encounter each other Ex: garter snakes aquatic or terrestial

Temporal Isolation Species that breed during different times of day, different seasons, or different years cannot mix gametes. eastern spotted skunk western spotted skunk mates in late winter mates in late summer

Behavioral Isolation blue footed boobie Unique behavioral patterns / rituals isolate species - identifies members of species - attract mates of same species courtship rituals, mating calls blue footed boobie

Mechanical Isolation Mating is attempted, but morphological differences prevent its successful completion different direction spirals prevent genital allignment

Gametic Isolation Sperm of one species not able to fertilize eggs of another species Sea urching egg and sperm can’t fuse due to different proteins on gamete surface

Types of Reproductive Barriers Postzygotic: prevent a hybrid zygote from developing into viable, fertile adult

Reduced Hybrid Viability Genes of different parent species may interact in ways that impair hybrid’s development or survival in its environment. Hybrids don’t complete development and are frail

Reduced Hybrid Fertility Robust hybrids are sterile - parent species chrom. # differ - meiosis does not form normal gametes Horse + donkey = mule mules are sterile

Hybrid Breakdown Some first generation hybrids are viable and fertile, but when they mate with one another, offspring of next generation are feeble or sterile. Hybrid offspring of rice are vigorous and fertile Next generation are small and sterile Species have begun separation

Haldane’s Rule In the early stage of speciation, if in a species hybrid only one sex is sterile more likely to be the heterogametic sex. (one with two different sex chromosomes) major form of post-zygotic reproductive isolation

Other Definitions of Species Morphological : by body shape, size, and other structural features Ecological : by niche/role in community Phylogenetic : smallest group of individuals that share common ancestry, branch on tree of life * holds true for sexual and asexual species*

Two Main Modes of Speciation Allopatric Speciation “other” “homeland” Geographically isolated populations Caused by geologic events or processes Mutations occur- evolution by natural selection & genetic drift Reproductive isolation Eg. Squirrels on N/S rims of Grand Canyon Sympatric Speciation “together” “homeland” Overlapping populations within home range (new species within original area-not isolated) Gene flow between subpopulations blocked by: polyploidy sexual selection habitat differentiation Eg. polyploidy in crops (oats, cotton, potatoes, wheat)

Modes of Speciation

Allopatric Speciation by Geographical Isolation Population of wild fruit flies minding their business munching on their bananas Hurricane washes bananas and unhatched fruit flies out to sea. Bananas washes up on island off coast of mainland Fruit flies hatch and live on island Two population too far apart for gene flow between them

Populations diverge due to different selective pressures on populations Morphology Food preferences Courtship behaviors Rivers, mountains, continental drift, migration, etc.

Allopatric speciation of antelope squirrels

Sympatric Speciation by Reduction of Gene Flow Populations not totally isolated Populations live at distant locations in geographic range Mating throughout range is not random Causes reduced gene flow

Sympatric Speciation by Polyploidy Polyploidy: extra sets of chromosomes (2, 3n, 4n) More common in plants Types Autoploid: 2+ sets of chrom. from a single species - produce fertile offspring Alloploid: 2+ sets of chrom. from different species - produce sterile hybrids- propagate asexually - can become fertile and reproduce with each other but not parent species - represent new biological species

Sympatric speciation: autoploidy Same species Error in meiosis

Sympatric Speciation: alloploidy chromosomes not homologous (different species) can’t undergo meiosis undergo asexual reproduction new species - diploid number equal to two parent species

Sympatric speciation by Sexual Selection Under normal light, two cichlid species are different in coloration, and females mate only with males of their own species. Under orange light, the two species appear identical, and females mate with males of either species, yielding fertile hybrids. Intersexual selection by females based on coloration is the reproductive barrier

What happens when new or partially formed species come into contact with each other?

Notice difference in allele frequencies Hybrid Zone: region where different sp. mate and produce hybrid offspring - Incomplete reproductive barriers Notice difference in allele frequencies

Change in Hybrid Zones Over Time Result: 1. reinforcement: barriers strengthened (limit hybrid formation) 2. fusion: barriers weakened (formation of single sp.) 3. stability: barriers same (continuation of hybrids) 1. Three populations connected by gene flow barrier to gene flow established Separated population begins to diverge from other 2 populations Gene flow is re-established in a hybrid zone

Weakened barriers: grizzly and polar bears  hybrid

Rate of Speciation/Evolution Punctuated Equilibium Eldridge & Gould Long period of stasis punctuated by short bursts of rapid, significant change Gradualism Common ancestor Slow, constant change

Remember Evolution begins with small changes in a species that become more and more pronounced eventually leading to macroevolution.