A population or a group of populations whose members have the potential to interbreed with one another in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species.
Key Ideas:
They produce offspring in a “natural environment.”
Fertile hybrids can sometimes be produced in a zoo but are never seen in the wild.
If hybridization occurs between two separate species the offspring are not fertile.
Example: horse + donkey → mule (infertile)
The Hallmark of the Species Concept: Reproductive Isolation
See pages 420-421 in Campbell's Biology in Focus for details on and examples for these types of reproductive isolation.
Prezygotic barriers help define species by impeding mating or hindering fertilization:
Habitat isolation
Two species living in different habitats, like water and land—even in the same geographic area—are not likely to mate.
Behavioral isolation
Special mate-attracting signals and behaviors are often exclusive to a species, so even closely related species that do not display such signals are incompatible.
Temporal isolation
Species that breed during different time periods cannot mate, even if they are very closely related.
Mechanical isolation
Even when attempted, mating may be anatomically impossible.
Gametic isolation
the gametes of different species may not be able to fuse to form a zygote, due to either chemical incompatibility or hostile female reproductive tracts.
Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult:
Reduced hybrid viability
Genetic incompatibilities may abort development of the hybrid at some embryonic stage.
Reduced hybrid fertility
Reproductive isolation remains intact if the hybrid is sterile.
Hybrid breakdown
In some cases, although the first-generation hybrids are viable and fertile, the hybrids of subsequent generations are feeble or sterile.
Modes of Speciation
Interrupting gene flow is the key to reproductive isolation and therefore to creating new species. There are two ways to do this:
Allopatric Speciation
Allo: other
Patric: country
Populations evolve into two new species in “other countries.”
A geographic barrier physically isolates gene flow between populations.
Example:
The north and south rims of the Grand Canyon each have a different species of squirrel (see below).
Presumably this speciation event occurred because of the geographic separation due to the appearance of the canyon.
What was once one species of squirrel is now two separate species.
Sympatric Speciation
Sym: together
Two new species evolve in the same geographic region (“together in the country”).
This occurs when an individual has more than two sets of chromosomes (4n), all derived from an original species (2n).
Word derivation:
Auto: self
Poly: many
Ploid: fold (chromosome)
Autopolyploid: having many chromosomes from their self (their species)
How does this occur?
During normal meiosis the chromosome number is reduced by half (2n to n).
In autopolyploidy, a failure in meiosis causes the chromosomes to fail to separate, resulting in gametes with twice as many chromosomes as normal (2n).
If this organism fertilizes itself then it offspring will be tetraploid (4n).
Because this offspring has twice as many chromosomes as its “parent” species, it is unable to interbreed with it and is therefore now a separate species.
This occurs when two different species contribute to a polyploid hybrid.
Word derivation:
Allo: other
Poly: many
Ploid: fold (chromosome)
Allopolyploid: having many chromosomes from others
How does this occur? It is not easy:
Interspecific hybrids (hybrids created from two different species) are usually sterile because the haploid set of chromosomes from one species cannot pair with the haploid set of chromosomes from the other species during meiosis.
However, the hybrids are usually very vigorous and can propagate asexually (assuming they're plants).
Eventually these sterile hybrids can be transformed into fertile polyploids through two different mechanisms: see the diagram below.
Accounts for 25–50 % of all new species of plants.
Allopolyploidy and you
Many of our food plants are polyploids.
So, while this means of speciation may sound complicated and rather bizarre, it is common in nature and the species that have been generated by it are important for our own survival.
