Download Chapter 24 The Origins of the Species

Chapter 24
The Origins of the Species
Biological Species Concept
Species: population whose members can interbreed
and produce viable, fertile offspring
(reproductively compatible)
These happy face spiders look different, but since they can interbreed,
they are considered the same species: Theridion grallator
Speciation
• lineage-splitting event that produces two or
more separate species
• Original species cannot produce viable offspring
together or that they avoid mating with members
of the other group
Speciation
Microevolution: changes within a single gene pool
Macroevolution: evolutionary change above the
species level
• Speciation explains similarities and differences among
species.
speciation video
speciation and extinction
New species are formed by being isolated
from each other.
Reproductive isolation
barriers that prevent members of 2 species
from producing viable, fertile hybrids
Reproductive
Barriers/Isolation
Limits gene flow between species
and formation of hybrids
Types of Reproductive Barriers
Prezygotic: impede mating or hinder fertilization if
mating occurs
Habitat Isolation
Two species occupy different habitats
rarely encounter each other
Ex: garter snakes
aquatic or terrestial
Temporal Isolation
Species that breed during different times of day,
different seasons, or different years cannot mix
gametes.
eastern spotted skunk
mates in late winter
western spotted skunk
mates in late summer
Behavioral Isolation
Unique behavioral patterns / rituals isolate species
- identifies members of species
- attract mates of same species
• courtship rituals, mating calls
blue footed boobie
Mechanical Isolation
Mating is attempted, but morphological
differences prevent its successful completion
different direction spirals prevent
genital allignment
Gametic Isolation
Sperm of one species not able to fertilize
eggs of another species
Sea urching egg and sperm can’t fuse due to
different proteins on gamete surface
Types of Reproductive Barriers
Postzygotic: prevent a hybrid zygote from developing into
viable, fertile adult
Reduced Hybrid Viability
Genes of different parent species may interact in ways
that impair hybrid’s development or survival in its
environment.
Hybrids don’t complete development and are frail
Reduced Hybrid Fertility
Robust hybrids are sterile
- parent species chrom. # differ
- meiosis does not form normal gametes
Horse + donkey = mule
mules are sterile
Hybrid Breakdown
Some first generation hybrids are viable and fertile,
but when they mate with one another, offspring of
next generation are feeble or sterile.
Hybrid offspring of rice are vigorous and fertile
Next generation are small and sterile
Species have begun separation
Haldane’s Rule
In the early stage of speciation, if in a
species hybrid only one sex is sterile
• more likely to be the heterogametic sex.
(one with two different sex chromosomes)
• major form of post-zygotic reproductive
isolation
Other Definitions of Species
Morphological : by body shape, size, and other
structural features
Ecological : by niche/role in community
Phylogenetic : smallest group of individuals that
share common ancestry, branch on tree of life
* holds true for sexual and asexual species*
Two Main Modes of Speciation
Allopatric Speciation
Sympatric Speciation
“other” “homeland”
“together” “homeland”
Geographically isolated
populations
Overlapping populations within
home range (new species within
original area-not isolated)
• Caused by geologic events or
processes
• Mutations occur- evolution by
natural selection & genetic drift
• Reproductive isolation
Gene flow between subpopulations
blocked by:
• polyploidy
• sexual selection
• habitat differentiation
Eg. Squirrels on N/S rims of
Grand Canyon
Eg. polyploidy in crops (oats,
cotton, potatoes, wheat)
Modes of Speciation
Allopatric Speciation by
Geographical Isolation
• Population of wild fruit flies
minding their business
munching on their bananas
• Hurricane washes bananas and
unhatched fruit flies out to sea.
• Bananas washes up on island
off coast of mainland
• Fruit flies hatch and live on
island
• Two population too far apart
for gene flow between them
• Populations diverge due to different
selective pressures on populations
• Morphology
• Food preferences
• Courtship behaviors
• Rivers, mountains, continental drift, migration,
etc.
Allopatric speciation of
antelope squirrels
Sympatric Speciation by
Reduction of Gene Flow
• Populations not totally isolated
• Populations live at distant locations in
geographic range
• Mating throughout range is not random
• Causes reduced gene flow
Sympatric Speciation by Polyploidy
Polyploidy: extra sets of chromosomes (2, 3n, 4n)
• More common in plants
• Types
– Autoploid: 2+ sets of chrom. from a single species
- produce fertile offspring
– Alloploid: 2+ sets of chrom. from different species
- produce sterile hybrids- propagate asexually
- can become fertile and reproduce with each other
but not parent species
- represent new biological species
Sympatric speciation: autoploidy
• Same species
• Error in meiosis
Sympatric Speciation: alloploidy
•
chromosomes not homologous
(different species)
• can’t undergo meiosis
• undergo asexual reproduction
• new species - diploid
number equal to
two parent species
Sympatric speciation by Sexual Selection
Under normal light, two cichlid species are different in
coloration, and females mate only with males of their
own species.
Under orange light, the two species appear identical, and
females mate with males of either species, yielding
fertile hybrids.
Intersexual selection
by females
based on coloration
is the reproductive barrier
What happens when new or
partially formed species come into contact
with each other?
Hybrid Zone: region where different sp. mate and
produce hybrid offspring
- Incomplete reproductive barriers
Notice difference in allele frequencies
Change in Hybrid Zones Over Time
Result:
1. reinforcement: barriers strengthened (limit hybrid formation)
2. fusion: barriers weakened (formation of single sp.)
3. stability: barriers same (continuation of hybrids)
1. Three populations
connected by gene flow
2. barrier to gene flow
established
3. Separated population
begins to diverge from
other 2 populations
4. Gene flow is re-established
in a hybrid zone
Weakened barriers: grizzly and polar bears  hybrid
Rate of Speciation/Evolution
Gradualism
• Common ancestor
• Slow, constant change
Punctuated Equilibium
• Eldridge & Gould
• Long period of stasis punctuated
by short bursts of rapid,
significant change
Remember
Evolution begins with small changes in a
species that become more and more
pronounced eventually leading to
macroevolution.