Species are evolutionary units within which gene flow occurs.
Natural populations belong to the same species only if they can
interbreed and leave fertile offspring. Different species are
reproductively isolated from each other. Most new species originate
geographically.
"Truth emerges more readily from error than from confusion"
. —Albert Einstein
Hugo DeVries and the Mutation Theory (1903)
Controversy, 1900-1940
Laboratory geneticists vs. Field naturalists
Resolution:
- Population genetics: Fisher (1930), Wright, Haldane
Fisher's book ("The Genetical Theory of Natural Selection") explained
that Darwin's Natural Selection and Mendel's genetics were compatible, as long as mutations were:
- Rare
- Harmful (usually)
- Small (usually), and
- Recessive (usually)
- Studies of geographic variation: Dobzhansky, etc.
- "Modern Synthesis": Huxley (1940, 1942), Mayr (1942)
Importance of species: Species characteristics are passed from parents to
offspring. Species were long considered real and important because these
characteristics seemed to be constant and unchanging.
Morphological (typological) species definition: Each species is
defined by characteristics thought to be "essential" or "typical".
Biological species definition: Species are groups of interbreeding
populations that are reproductively isolated from other species.
Biological species definition
- reproductive isolation
- reproductive isolating mechanisms (premating, postmating)
- example: mallards and pintails
- sibling species
Reproductive isolating mechanisms that act prior to mating:
- Ecological isolation: Potential mates do not meet because
they live in different habitats or breed at different times or seasons.
- Behavioral isolation: Mating calls or mating rituals differ.
- Mechanical isolation: "Lock and key" mismatch of genitalia.
Reproductive isolating mechanisms that act after mating:
- Gametic mortality: Gametes die before fertilization.
- Zygotic mortality: Fertilized eggs fail to divide properly.
- Embryonic or larval mortality: They die prematurely.
- Hybrid inviability: Hybrids never reach reproductive age.
- Hybrid sterility: Hybrids cannot reproduce, as in mules.
- F2 breakdown: Offspring of hybrids are inviable.
Geographic variation: Natural selection in different environments
causes populations to differ. Gene flow reduces the opportunities
for populations to differ; restricted gene flow allows enhanced differences.
Populations of some geographically widespread species may differ so much
that they may become unable to interbreed.
- If barriers to breeding accompany differences in visible traits,
the species may become divided into subspecies.
- Continuous geographic variation is usually described in terms
of clines (character gradients across a map).
- Geographic variation is usually the first step in species formation.
Geographic variation *
- Importance of geographic variation
- Occurence of (or absence of) geographic variation
- Gene flow at ecotones
example: mine entrances (Antonovics)
- Altitudinal "races"
- Clines
- Insular & continental patterns
- Subspecies and polytypic species; pocket gophers and other examples
What is speciation?
- Phyletic transformation (anagenetic; no new RIM)
- True speciation or splitting (cladogenetic;
new RIM must evolve)
How does this happen? Next topic
REVIEW:
Study guide and vocabulary
|