A. What is a species?
1. From Latin word for “kind” or “appearance”
2. There are different concepts for which species is defined.
The most accepted is the “biological species concept”
a. a species is a population or group of populations whose
members have the potential to interbreed (mate) in
nature and produce fertile offspring.
3. Reproductive isolation – the existence of biological
barriers that do not allow two species to produce viable,
fertile, offspring.
Prezygotic barriers – can’t mate or fertilization can’t happen
a. Habitat isolation – rarely encounter one another
b. Temporal isolation – breed at different times
c. Behavioral isolation – courtship rituals not understood
d. Mechanical isolation – mating occurs, but structurally they
don’t “fit”
e. Gametic isolation – sperm does not recognize egg or can’t
survive in other species environment
Postzygotic barriers – hybrid can’t develop into viable, fertile adult
a. Reduced hybrid viability – development incomplete
b. Reduced hybrid fertility – hybrid is sterile
c. Hybrid breakdown – offspring weak or not viable
B. Mechanisms of speciation: Speciation can take place with
or without geographic separation
1. Allopatric Speciation –
“Allo”= other; “patra”= homeland
a. gene flow is interrupted when
a population is divided
into geographically isolated
subpopulations.
2. Sympatric Speciation – “Same Country”
a. How can reproductive barriers between populations
evolve when the members remain in contact with each
other?
b. Switching habitat, food source, or other resource not
used by the parent population
c. When two species interbreed and produce a hybrid
that goes through asexual reproduction which can
lead to mutations that make them fertile with other
hybrids and not the parent.
d. Plant examples
• Polyploidy =
an organism that has more than 2 complete
sets of chromosomes
• Autopolyploidy =
• Allopolyploidy =
A polyploid organism that is fertile where the sets
of chromosomes come from a single species
A polyploid organsim that is fertile where the sets
of chromosomes come from 2 different species
Autopolyploid
Allopolyploid
Evolution
of
Wheat
C. Reproductive barriers may evolve as populations diverge
1. Fruit fly experiment in the lab:
2. Change in a single gene:
D. Hybrid Zones cause reproductive isolation
1. A Hybrid Zone is a region in which members of different
species meet and mate, producing at least some
offspring of mixed ancestry.
a. Narrow band pattern
 there is an obstacle to gene flow
obstacle is probably that hybrids have increased
rates of embryonic mortality and a variety or
morphological abnormalities (therefore, poor survival
and reproductive rates)
b. Complicated spatial patterns
2. Several possible outcomes for hybrids
a. Reinforcement = strengthening or reproductive
barriers…hybrids gradually stop forming
b. Fusion = weakening of reproductive barriers…the two
species fuse to be one.
c. Stability = continued production of hybrid individuals
E. Adaptive Radiation – the evolution of many diversely
adapted species from a common ancestor.
1. Ancestor was introduced to various new environmental
opportunities and challenges.
2. Natural Selection caused different traits to be selected
for based on environment and challenges ancestor
faced.
F. Tempo of Speciation (how fast/slow does it take?)
1. Punctuated Equilibrium model
2. Gradualism model