The Origin of Species

 Two basic patterns of evolutionary change can be distinguished –Anagenesis –Cladogenesis

Origin of Species  How do we define species?  A population of organisms that produces viable fertile offspring in nature  When does this definition fall apart?  Asexual, extinct and blurred organisms blurred organismsblurred organisms  What definition is used in these cases?  Morphospecies concept

What is the driving factor behind speciation (what event must happen to produce a new species)?  Reproductive isolation –Pre-zygotic  Temporal isolation  Behavioral/sexual isolation  Mechanical isolation  Gametic isolation –Post-zygotic  Hybrid inviability  Hybrids may not be able to reproduce due to differences in courtship behaviors, sterility (chromosomes may be incompatible with meiosis)  Hybrid breakdown

Figure 24.4 Individuals of different species Mating attempt Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation HABITAT ISOLATION TEMPORAL ISOLATIONBEHAVIORAL ISOLATION MECHANICAL ISOLATION (a) (c) Pre-Zygotic Barriers

Viable fertile offspring Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown Fertilization Gametic isolation GAMETIC ISOLATION REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN (j) (l) Post-Zygotic Barriers

Which type of reproductive isolation mechanism is at work? –Firefly signals with specific flashes to attract mates –Male dragonfly has appendages that clasps female during mating –Brown trout breed in the fall and rainbow trout in the spring –1 type of garter snakes lives in the water and the other lives on land

What sort of reproductive barrier is this? Prezygotic Has reproductive isolation occurred?

 Horse and donkey produce a sterile mule

Biogeography of Speciation What is the difference between allopatric and sympatric speciation?  Allopatric - involves a geographical barrier between 2 groups  Sympatric - the result of a genetic isolation without a geographical barrier

Conditions for Allopatry  Long physical isolation and different selective pressures  Populations of a species become separated geographically and undergo genetic drift & natural selection  Genetic drift continues to change gene pool  The “fittest” traits among the new group survive

 Allopatric speciation may occur relatively rapidly  Kaibab squirrel is an example of allopatric speciation in progress –Separated by the Grand Canyon –Show marked phenotypic differences

Ring Species  Examples of various stages in the gradual divergence of new species from common ancestors –Pops are distributed around some geographic barrier –Pops that have diverged the most meet where the ring closes –Some populations can interbreed, others can’t

Ring Species

Adaptive Radiation  Islands are laboratories of speciation  Adaptive radiation is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities  Archipelagos are the often the home of adaptive radiations

Hawaiian Archipelago

Sympatric Speciation  Genetic alterations result in a reproductive barrier  Can occur in a single generation  More frequently seen in plants  Nondisjunction and selfing leads to polyploidy  Autopolyploid Autopolyploid  Alloployploid Alloployploid  Evolution of wheat Evolution of wheat Evolution of wheat

Sympatric Speciation in Animals normal lightmonochromatic light Cichlid fish - resulted from sexual selectionCichlid fish - resulted from sexual selection Genetic divergence between species is likely to be small, suggesting that speciation in nature has occurred relatively recentlyGenetic divergence between species is likely to be small, suggesting that speciation in nature has occurred relatively recently 200 species of cichlid fishes in Lake Victoria, Africa!

Genetic Mechanisms of Speciation  Adaptive Divergence:  Reproductive barriers evolve as secondary result of divergence  Barriers evolve to enhance reproduction within a group - not to eliminate reproduction b/t groups  Barriers occur as a side effect of the accumulated adaptive divergences

Tempo of Speciation  Gradualism  Punctuated Equilibrium Punctuated Equilibrium Punctuated Equilibrium

From Speciation to Macroevolution  Speciation – the boundary between micro and macroevolution  Cumulative change over vast amounts of time accounts for macroevoulution  How do evolutionary novelties evolve?

Eye Evolution Eye Evolution

Origin of Novelties  How do large scale novelties arise?  Exaptation - modifications of older structures  Panda’s thumb, stinger of bees, etc…  Gen. changes that lead to dev. changes: –Allometric growth – rel. rates of growth differ Allometric growth Allometric growth –Change one stage - big change –Alter timing of developmental events, (age of sexual activity) –Alteration in homeotic genes Alteration in homeotic genesAlteration in homeotic genes  Evolution, however, is not goal oriented Evolution, however, is not goal oriented Evolution, however, is not goal oriented

An individual that has more than two chromosome sets, all derived from a single species

A species with multiple sets of chromosomes derived from different species

 Tetrapod evolution –Fish; Hox gene leads to fin development –Chicken; same Hox gene leads to leg development