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anagenesis.html: 24_02AnaVCladogenesisA.jpg
Anagenesis is the accumulation of heritable changes, altering the characteristics of a species.
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EXPERIMENT
Males and females of Pundamilia pundamilia and P. nyererei are placed together.
Under normal light, the two species are noticeably different in coloration; under monochromatic orange
light, the two species appear identical in color.
RESULTS Under normal light, females of each species mated only with males of their own species. But under orange light, females of each species mated indiscriminately with males of both species. The resulting hybrids were viable and fertile.
CONCLUSION
Mate choice by females based on coloration is the main
reproductive barrier that normally keeps the gene pools of these two species separate.
cladogenesis.html: 24_02AnaVCladogenesisB.jpg
Cladogenesis is branching evolution, in which a new species arises from a population that buds from a parent species.
Cladogenesis is the basis for biological diversity.
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The flightless cormorant (Nannopterum harrisi),
one of many new species that have originated on the isolated Galápagos Islands.
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eye.html: 24_14MolluscEyes.jpg
The simplest eyes (such as those of limpets
are patches of light–sensitive photoreceptor cells that can distinguish light from dark.
Over time, the layer of cells fold into a cup that allows for sense of direction, a "pinhole" and later a lens and cornea allow light to be focused.
Complex eyes of squids and octopuses and of vertebrates
evolved from an ancestral simple cluster of photoreceptor cells.
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postzygotic.html: 24_04ReproBarriersPos.jpg
| Postzygotic barriers prevent a hybrid zygote from developing into a fertile adult. | Gametes of red and purple sea urchins are released into the water, but are unable to fuse. | Some salamander subspecies of the genus Ensatina can hybridize, but hybrids do not complete development or are frail. | A mule is the robust but sterile hybridbetween a male donkey and a female horse. | Hybrids of two rice strains are vigorous and fertile, but the next generation may be sterile. |
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| Prezygotic barriers impede mating or hinder fertilization if mating does occur. | ||||||
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| Two species of garter snakes: one lives mainly in water while the other is mainly terrestrial. | The eastern spotted skunk mates in late winter; the western spotted skunk mates in late summer. | Blue-footed boobies of the Galapagos mate only after a courtship display unique to their species. (hint) | These 2 species of snails have opposite spirals in their shells so their genital openings are not aligned. | |||
speciation-allopatric.html: 24_06AllopatSpeciation.jpg
Allopatric speciation dur to geographic separation of the Grand Canyon.
Harris's antelope squirrel (Ammospermophilus harrisi) inhabits the canyon's south rim (left).
Just a few miles away on the north rim (right) lives the closely related white–tailed antelope squirrel
(Ammospermophilus leucurus).
Another example is the Tassel-eared Squirrel, where Abert's Squirrel (Sciurus aberti) lives on the south rim and the
Kaibab Squirrel (Sciurus aberti kaibabensis) lives on the north rim.
speciation.html: 24_05AllopatSympatSpeciat.jpg
| Allopatric speciation. A population forms a new species while geographically isolated from its parent population, reducing gene flow. | Sympatric speciation. A small population becomes a new species without geographic separation. |
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