AP Biology Mom, Dad… There’s something you need to know… I’m a MAMMAL! Chapter 18 WHAT IS SPECIATION?

AP Biology So…what is a species?  defined by Ernst Mayr (Evolutionary Biologist)  A population whose members can interbreed & produce viable, fertile offspring  Being reproductively compatible is a key component Western Meadowlark Sturnella magna Eastern Meadowlark Sturnella neglecta Distinct species: songs & behaviors are different enough to prevent interbreeding

AP Biology How and why do new species originate? New species are formed by a series of evolutionary processes which cause reproductive isolation.  Reproductive Isolating Mechanisms  Pre-zygotic barriers are various forms of isolation  Post-zygotic barriers are generally related to fertility & viability of offspring

AP Biology  This means there is an obstacle to mating or to fertilization if mating occurs PRE-Reproduction (pre-zygotic) Barriers behavioral isolation geographic isolation ecological isolation temporal isolation mechanical isolation gametic isolation

AP Biology 1. Geographic isolation  New species occur in different areas  physical barrier separated ancestor species Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white- tailed antelope squirrel Ammospermophilus spp

AP Biology 2. Ecological isolation  Species occur in same region, but occupy different habitats so they rarely encounter each other 2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial lions & tigers could hybridize, but they live in different habitats:  lions in grasslands  tigers in rainforest lions & tigers could hybridize, but they live in different habitats:  lions in grasslands  tigers in rainforest

AP Biology 3. Temporal “time” isolation  Species that breed during different times of day, different seasons, or different years cannot mix gametes. Eastern spotted skunk (L) & Western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

AP Biology 4. Behavioral isolation  Unique behavioral patterns & rituals isolate species  identifies members of species  attract mates of same species  courtship rituals, mating calls Blue footed boobies mate only after a courtship display unique to their species That’s hot…

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firefly courtship displays courtship display of Gray-Crowned Cranes, Kenya courtship songs of species of lacewings (that live in the same area) Recognizing your own species

AP Biology 5. Mechanical isolation  Morphological differences can prevent successful mating Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen. Plants

AP Biology Mechanical isolation  For many insects, male & female sex organs of closely related species do not fit together, preventing sperm transfer  lack of “fit” between sexual organs: hard to imagine for us… but a big issue for insects with different shaped genitals! Damsel fly penises Animals Get that thing away from me!

AP Biology 6. Gametic isolation  Sperm of one species may not be able to fertilize eggs of another species  biochemical barrier: 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 are unable to fuse.

AP Biology POST-Reproduction (Post-Zygotic) Barriers  Prevent hybrid offspring from developing into a viable, fertile adult

AP Biology Post Zygotic: Reduced hybrid viability  Genes of different parent species may interact & impair the hybrid’s development Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.

AP Biology Mules are vigorous, but sterile Post Zygotic: Reduced hybrid fertility  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 Donkeys have 62 chromosomes (31 pairs) Horses have 64 chromosomes (32 pairs) Mules have 63 chromosomes!

AP Biology Post Zygotic: 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.

AP Biology Rate of Speciation  Current debate: Does speciation happen gradually or rapidly? Or both?  Gradualism  Punctuated equilibrium

AP Biology Gradualism  Gradual divergence over long spans of time  assume that big changes occur as the accumulation of many small ones

AP Biology Punctuated Equilibrium  Rate of speciation is not constant  rapid bursts of change  long periods of little or no change  species undergo rapid change when they 1 st bud from parent population Time

AP Biology MODELS OF SPECIATION 18.2 AND 18.3

AP Biology Allopatric Speciation  A physical barrier arises and prevents gene flow between populations  Reproductive isolating mechanisms evolve in the genetically diverging populations  Speciation occurs when the members of the divergent populations no longer interbreed

AP Biology Sympatric Speciation  A new species may form within the home range of an existing species (there is no physical barrier)  May be:  Temporal  Mechanical  Behavioral

AP Biology Parapatric Speciation Daughter species form a small proportion of individuals along a common border between 2 populations. Hybrid Zone is formed BULLOCK’S ORIOLE BALTIMORE ORIOLE HYBRID ZONE Figure Page 299

AP Biology The two types of plants are close enough that tolerant and non-tolerant individuals could potentially fertilize each other—so they seem to meet the first requirement of parapatric speciation, that of a continuous population. However, the two types of plants have evolved different flowering times. This change could be the first step in cutting off gene flow entirely between the two groups

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