24 Speciation

So…what is a species? Biological species concept defined by Ernst Mayr population whose members can interbreed & produce viable, fertile offspring reproductively compatible Distinct species: songs & behaviors are different enough to prevent interbreeding Meadowlarks look so similar but are considered different species. Cryptic species—two or more species that are morphologically indistinguishable but do not interbreed. Meadowlarks Similar body & colorations, but are distinct biological species because their songs & other behaviors are different enough to prevent interbreeding Biological species concept: “Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” Ernst Mayr Eastern Meadowlark Western Meadowlark

Reproductive isolation is key to speciation Obstacle to mating or to fertilization if mating occurs geographic isolation ecological isolation temporal isolation Speciation requires the interruption of gene flow within a species whose members formerly exchanged genes. The evolution of reproductive isolation is important for understanding the origin of species. behavioral isolation mechanical isolation gametic isolation 2005-2006

Allopatric Speciation Ammospermophilus spp Geographic isolation Species occur in different areas physical barrier allopatric speciation “other country” Harris’s antelope squirrel inhabits the canyon’s south rim (Above). Just a few miles away on the north rim (L) lives the closely related white-tailed antelope squirrel Barriers can form when continents drift, sea levels rise and fall, glaciers advance and retreat, or climates change. The populations evolve through genetic drift and adaptation to different environments in the two areas.

Allopatric Speciation among Darwin’s Finches Finch species in the Galápagos Islands evolved from a single South American species that colonized the islands. The islands are far apart, and have different environmental conditions.

Sympatric Speciation Ecological isolation Species occur in same area, but occupy different habitats so rarely encounter each other 2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial Sympatric speciation—speciation without physical isolation. Gene flow has been reduced between flies that feed on different food varieties, even though they live in the same area

Temporal isolation Species that breed during different times of day, different seasons, or different years cannot mix gametes sympatric speciation “same country” Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer 2005-2006

Behavioral isolation Unique behavioral patterns & rituals isolate species identifies members of species attract mates of same species  courtship rituals, mating calls The most comedic species of the Galapagos Islands is the Blue Footed Booby, what a ridiculous outfit and expression! Their name is in fact taken from the Spanish 'bobo' which means clown. The Blue Footed Boobies above display part of their humorous courtship ritual whereby they raise their feet one at a time and then swivel their heads away from the prospective mate looking to the sky. Other interesting Booby features are the highly evolved airbag systems in their skulls which allow them to dive bomb into the sea for fish from great height, and the egg and hatchling nesting boundaries they make which are rings of Boobie poop. They aren't the only Booby on the island — there are also Masked and Red Footed Boobies about. Blue footed boobies mate only after a courtship display unique to their species

Recognizing your own species courtship songs of sympatric species of lacewings

Mechanical isolation Morphological differences can prevent successful mating

Mechanical isolation For many insects, male & female sex organs of closely related species do not fit together, preventing sperm transfer The selection is intense because it directly affects offspring production -- it is affecting sex itself

Gametic isolation sympatric speciation? Sperm of one species may not be able to fertilize eggs of another species mechanisms biochemical barrier so sperm cannot penetrate egg receptor recognition: lock & key between egg & sperm chemical incompatibility sperm cannot survive in female reproductive tract Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.

Dobzhansky-Muller model example: Speciation by Centric Fusion In bats of the genus Rhogeessa, chromosomal rearrangements in different lineages have led to speciation. Hybrids between some types will not be able to produce normal gametes in meiosis. As species diverge genetically, reproductive isolation increases. Development of reproductive isolation may take millions of years, or may develop in a few generations (e.g., the Rhogeessa bats).

Reduced hybrid fertility- example of postzygotic isolating mechanism Even if hybrids are vigorous they may be sterile chromosomes of parents may differ in number or structure & meiosis in hybrids may fail to produce normal gametes Mules are vigorous, but sterile What’s wrong with having 63 chromosomes? Odd number! Cannot pair up in meiosis. Postzygotic isolating mechanisms—reduce fitness of hybrid offspring. Postzygotic mechanisms result in selection against hybridization, which leads to reinforcement of prezygotic mechanisms. Horses have 64 chromosomes (32 pairs) Donkeys have 62 chromosomes (31 pairs) Mules have 63 chromosomes!

Hybrid breakdown Hybrids may be fertile & viable in first generation, but when they mate offspring are feeble or sterile In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile. On path to separate species

Fig. 24-14-4 Formation of a hybrid zone and possible outcomes for hybrids over time Isolated population diverges Possible outcomes: Hybrid zone Reinforcement OR Fusion Gene flow Hybrid OR Hybrid zones provide opportunities to study factors that cause reproductive isolation A hybrid zone is a region in which members of different species mate and produce hybrids When closely related species meet in a hybrid zone, there are three possible outcomes:---Strengthening of reproductive barriers---Weakening of reproductive barriers---Continued formation of hybrid individuals Barrier to gene flow Population (five individuals are shown) Stability