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Ch. 17 Notes: Origin of Species
Microevolution = the origin of new taxonomic groups
Speciation = the splitting of one species into two or more new species
Species = Latin term meaning “kind” or “appearance
Species Concepts = different ways in which a species is defined. These include:
1) Morphological Species Concept = defines species by one or more distinct physical
characteristics  diagnostic traits

Used by Linnaeus

Not helpful with most bacteria and microorganisms

Cryptic Species = look almost identical but are very different in other traits
2) Phylogenetic Species Concept = an evolutionary “family tree” (a phylogeny) is used to identify
species based on a common ancestor

Monophyletic = when a group of species can all be traced back to a single, common
ancestor

Uses morphology and biochemistry relationships to determine
3) Biological Species Concept = A population or group of populations whose members have the
potential to interbreed with one another in nature and to produce viable, fertile offspring, but
cannot do so with members of other species.

Ernst Mayr (1942)
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Is defined by reproductive barriers from other species in natural environments
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Breeding may be possible between two species in the lab or in zoos

Depends on the development of a REPRODUCTIVE BARRIER

Two Kinds: pre-zygotic and post-zygotic
Pre-zygotic Barriers = prevent mating between species or hinder fertilization of the ova should
members of different species attempt to mate (prevent a zygote from forming)
May be due to:

Habitat isolation

Behavioral isolation
o Species-specific signals or elaborate behavior for mating

Pheromones (distinctive chemical signals)

Temporal isolation = two species breed at different times of the day, seasons, or years

Mechanical isolation
o Anatomical incompatibility

Gametic isolation = Gamete recognition is based on the presence of specific receptor molecules
on the coats of the egg which adhere only to complementary molecules on sperm cells of the
same species
Post-zygotic Barriers = prevent the hybrid zygote from developing into a viable fertile adult

Hybrid Inviability = hybrid spontaneously aborts development at some embryonic stage
o Usually due to mismatched chromosome sets

Hybrid Sterility = hybrids are infertile
o Ex. the mule is produced by crossing a donkey and a horse


very rarely able to backbreed with either parent species
Hybrid Breakdown = the F1 generation hybrids are viable and fertile, but when these hybrids
mate with one another or with either parent species, offspring of the next generation are feeble
or sterile.
The Biological Species Concept Does Not Work in All Situations
Cannot be applied to:

Organisms that are completely asexual
1) Ex. protists and fungi, plants (bananas), and bacteria
2) Asexual reproduction effectively produces a series of clones

Extinct organisms represented only by fossils.

In some cases, clear determination of species is not possible. If members of a species have
been separated for a long time and then reintroduced, three things are possible:
1) If the populations may interbreed freely, then they are still the same species.
2) If the two populations cannot interbreed anymore, they are obviously different species
(and speciation has occurred).
3) A Hybrid Zone may be established.

Hybrid zone = A region where two related populations that diverged after
becoming geographically isolated make secondary contact and interbreed where
there geographical ranges overlap

Genotypic and phenotypic frequencies that distinguish the two
populations form steep clines into the hybrid zone.
o Cline = a gradual change in phenotype frequencies as you move
across a geological transect


Ex. Mammal size and latitude in N. America
Away from the hybrid zone, the two populations remain distinct and
cannot breed.

This is a problem for the Biological Species Concept