Macroevolution : How do species arise? How do we go from species A to Species B? Mom, Dad… There’s something you need to know… I’m a MAMMAL!
Speciation Tip! Use a simple flow chart to organize your information…. Barriers! Geographic Pre-zygotic Post-zygotic Allopatric Sympatric Types A-F Ex. Hybrids
What IS a Species? “Biological species concept” • population whose members can interbreed & produce viable, fertile offspring • reproductively compatible Distinct species:songs & behaviors are different enough to prevent interbreeding Eastern Meadowlark
What is the first step to speciation? • END Gene flow between groups! • (reproductive isolation )
Once gene flow ends.. Populations can: • Diverge genetically via mutation • Be subject to natural selection pressures • Experience genetic drift
1. Obviously…different types of geographic isolation can occur… • 2 different types of isolation can be… • allopatric • geographic separation between groups • sympatric • Group still live in same area
There are many other ways to keep groups apart including Reproductive isolating mechanisms Such mechanisms • Arise as populations move apart (diverge) • They are Heritable aspects of body form • They can be functions of body parts Or Behaviors (that prevent interbreeding among species)
There are two types of reproductive isoolating mechanisms • Pre-zygotic isolating mechanisms (prevents successful mating/pollination) • Post-zygotic isolating mechanisms (inability of new individuals to mate successfully)
geographic isolation ecological isolation temporal isolation gametic isolation mechanical isolation behavioral isolation 2. PRE- zygotic (reproduction) • Obstacles to mating or to fertilization if mating occurs
Ammospermophilus spp A. Geographic isolation Most common mode of speciation • physical barrier arises and ends gene flow • We call this: allopatric speciation (“other country”) 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
B. Ecological isolation • Species occur in same region, but occupy different habitats (microenvironments) so rarely encounter each other They are…reproductively isolated 2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial
Ex. lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest
C. Temporal isolation: timing of reproduction differs • Species that breed during different times of day, different seasons, or different years cannot mix gametes • Another form of reproductive isolation • WE call this: sympatric speciation : new species form within home range of an existing species and in the absence of a physical barrier! • Means: “same country” Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer
D. Behavioral isolation Unique behavioral patterns & rituals isolate species • identifies members of species • attract mates of same species (females of different species are not impressed!) • courtship rituals, mating calls • reproductive isolation Blue footed boobies mate only after a courtship display unique to their species
Recognizing your own species courtship songs of sympatricspecies of lacewings courtship display of Gray-Crowned Cranes, Kenya firefly courtship displays
sympatric speciation E. 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. (You have to have the RIGHT pollinator!)
For example: Black sage (left) v White Sage (right) Only smaller bees (honeybees) can pollinate Black Sage; larger bees cannot be supported by smaller flowers Filaments stick out above petals making it impossible for little honeybees to access anthers: larger bumble bees OK!
sympatric speciation F. Gametic isolation • Sperm of one species (or group) 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 Ex. Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple sea urchins —are unable to fuse.
3. POST-zygotic barriers (ex. Hybrids) These will prevent hybrid offspring from developing into a viable, fertile adult (3 ways) • reduced hybrid viability • reduced hybrid fertility • hybrid breakdown zebroid
sympatric speciation A. Reduced hybrid viability • Genes of different parent species may interact & impair the hybrid’s development • Or recent divergence of genes cause some missing or some extra genes; incompatible! Species of salamander genus, Ensatina, may interbreed, but most hybrids do not completely develop; those that do are frail.
B. 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) Donkeys have 62 chromosomes (31 pairs) Mules have 63 chromosomes!
sympatric speciation C. 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.
Current debate: Does speciation happen gradually or rapidly • Phyletic Gradualism • Charles Darwin • Charles Lyell • Punctuated equilibrium • Stephen Jay Gould • Niles Eldredge Niles Eldredge Curator American Museum of Natural History
Stephen Jay Gould (1941-2002) • Harvard paleontologist & evolutionary biologist • punctuated equilibrium • prolific author • popularized evolutionary thought
Phyletic Gradualism – Darwin and Lyelle • Gradual divergence over long spans of time • Species stay stable for long periods of time • assume that big changes occur as the accumulation of many small ones
Punctuated Equilibrium – Gould/Eldridge • Rate of speciation is not constant • rapid bursts of change • long periods of little or no change between • Occurs with drastic change in environments Time
Is Evolution goal-oriented? No! An evolutionary trend does not mean that evolution is goal-oriented. Surviving species do not represent the peak of perfection. There is compromise & random chance involved as well Remember that for humans as well! Evolution is not the survival of the fittest. Rather it is the survival of the just good enough.