Transcript AP Biology
Chapter 18 Mom, Dad… There’s something you need to know… I’m a MAMMAL! WHAT IS SPECIATION? AP Biology So…what is(Evolutionary a species? defined by Ernst Mayr Biologist) A population whose members can interbreed & produce viable, fertile offspring Being reproductively compatible is a key component Distinct species: songs & behaviors are different enough to prevent interbreeding AP Biology Eastern Meadowlark Western Meadowlark Sturnella neglecta Sturnella magna 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 PRE-Reproduction (pre-zygotic) Barriers This means there is an obstacle to mating or to fertilization if mating occurs geographic isolation AP Biology behavioral isolation ecological isolation temporal isolation mechanical isolation gametic isolation Ammospermophilus spp 1. Geographic isolation New species occur in different areas physical barrier separated ancestor species AP Biology 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 whitetailed antelope squirrel 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 AP Biology lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest 3. Temporal “time” isolation Species that breed during different times of day, different seasons, or different years cannot mix gametes. AP Biology Eastern spotted skunk (L) & Western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer 4. Behavioral isolation Unique behavioral patterns & rituals isolate species identifies members of species attract mates of same species That’s hot… courtship rituals, mating calls AP Biology Blue footed boobies mate only after a courtship display unique to their species AP Biology Recognizing your own species courtship songs of species of lacewings (that live in the same area) courtship display of Gray-Crowned Cranes, Kenya firefly AP Biology courtship displays 5. Mechanical isolation Morphological differences can prevent successful mating Plants 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. AP Biology Mechanical isolation Animals 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! Get that thing away from me! Damsel fly penises 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 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 Mules are vigorous, but sterile Horses have 64 chromosomes (32 pairs) AP Biology Mules have 63 chromosomes! Donkeys have 62 chromosomes (31 pairs) 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 AP Biology assume that big changes occur as the accumulation of many small ones 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 1st bud from parent population AP Biology Time 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 BULLOCK’S ORIOLE BALTIMORE ORIOLE Hybrid Zone is formed AP Biology Figure 18.10 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 AP Biology AP Biology