THEORY OF EVOLUTION
THE EVOLUTION OF SPECIES
Species = a group of organisms that look alike and have the ability to interbreed and
produce fertile offspring in nature.
Speciation = evolution of a new species
Can occur only when either interbreeding or the production of fertile offspring is
somehow prevented.
GEOGRAPHIC ISOLATION
= a physical barrier separates a population into groups
Physical barriers can prevent interbreeding
Tree frogs in dense rainforest of Amazon Basin
Over time, periodic droughts begin to break up stretches of continuous forest into smaller
patches.
Separating the original tree frog population into smaller groups
Other environmental factors cause shifts in local river courses that further isolate
In the natural world, physical barriers frequently form and break up large populations into
smaller ones.
Volcanic eruptions cause lava flows
Sea-level changes along continental shelves (New Guinea) create island environments
with water barriers.
If small populations of tree frogs were geographically isolated by deforestation, they
could no longer be able to interbreed and gene exchange would cease.
Over time, each small population would adapt to the local environment through the
process of natural selection.
Eventually, gene pools of each group become so different that one would be considered a
new species.
REPRODUCTIVE ISOLATION
= when formerly interbreeding organisms are prevented from producing fertile offspring.
Geographic isolation can lead to differences in mating behavior
Gene pools are closed
Genetic material is not exchanged between the groups
Over time, as populations become more and more distant, reproductive isolation can
arise.
Many types.
One type: occurs when genetic material becomes to different that mistakes happen
during development of zygote, if zygote forms at all.
Another: one frog evolves mating in fall and the other in spring
POLYPLOID SPECIATION
= any species with any multiple of the normal set of chromosomes
Speciation can occur when chromosome numbers change
Geographical isolation is not the only factor
Plants in same geographical area = polyploids
Mistake occurs during meiosis
Fastest form of speciation because reproductive isolation is instantaneous (one
generation)
Estimate nearly half known flowering plants
Wheat, cotton, apples and bananas
GRADUALISM
= idea that species originates through a gradual buildup of new adaptation
Speciation can occur quickly or slowly
Darwin argued evolution proceeds at a slow, steady rate and that small, adaptive changes
gradually accumulate over time in populations.
Supported in fossil record: camel and horse
PUNCTUATED EQUILIBRIUM
= speciation occurs quickly in rapid bursts with long periods of stability in between
1971 another hypothesis
When environmental conditions change, such as warmer temperatures or introduction of
a new competitive species = rapid
Interrupting or punctuating genetic equilibrium
New species form in 10,000 years or less
PATTERNS OF EVOLUTION
Add further evidence to Natural Selection
Species diversify when introduced to new environment
ADAPTIVE RADIATION
= process of evolution of an ancestral species into an array of species that occupy
different niches.
Hawaiian Islands = diversity of plants and animals not seen elsewhere
Hawaiian Honeycreepers
Similar in body size and shape
Differ in plumage color and beak shape (most important)
Each species occupies its own niche and beak to the type of food it eats
Evolved from single common ancestor that invaded Hawaii long ago
Darwin’s finches in the Galapagos
300 varieties of cichlid fish in Lake Victoria, Africa
DIVERGENT EVOLUTION VS. CONVERGENT
EVOLUTION
Divergent = pattern of evolution in which species that once were all similar became
more and more distinct.
Occurs when species begin to adapt to different environmental conditions and to change,
becoming less and less alike.
Convergent = pattern of evolution in which distantly related organisms evolve similar
traits.
Dolphin and fish body shape similar
Both are vertebrates, but not closely related
Occurs when unrelated organisms occupy similar environments and face similar selection
pressures.
Streamline bodies needed for movement in water.