Matt Johnson
Lecture Notes
ORNITHOLOGY
(Humboldt State Univ. WILDLIFE 365)
LECTURE 10 – SPECIATION, BIOGEOGRAPHY, & HYBRIDIZATION
I.
II.
Speciation.
A. Characters that are used in phylogenetic analyses arise in taxa as they diverge
over time. Given enough time and isolation, such divergence can result in the
formation of new species, a process called speciation.
B. Modes of speciation
1. Allopatric - geographically disjunct population divergence over time.
2. Sympatric - overlapping population divergence.
3. Parapatric - contiguous population divergence.
4. Important to remember that in all cases, reproductive isolation must occur
in one way or another. It is just that it does not necessarily occur by
spatial, large scale, isolation (microhabitat isolation can happen in
sympatry, or behavior can result in isolation in sympatry by the
preferential mating of like types). The latter is called assortative mating.
Ex. Western and Clark's grebes -- Clear Lake.
Biogeography - the study of evolution and ecology within a geographical context.
A. Avian evolution and continental drift.
1. Cracraft conducted an extensive cladistic analysis of the ratites (large
southern hemisphere flightless birds). His results were based on the idea
of "parsimony." Again, this is the idea that the "true" evolutionary
relationship is the one with the fewest steps. It greatly simplified the
relationship from earlier theories.
2. But Bledsoe took another look, this time also examining the pattern of
continental drift of southern continents when the large southern landmass
of Gondwanaland broke up about 80 mya. He came up with a slightly
different pattern of relationships.
3. Recent molecular analyses have corroborated Bledsoe's hypothesis. Thus,
we probably can come closest to the truth by examining shared derived
characteristics (Cracraft's cladistic analysis), biogeography (Bledsoe's
geographical analysis), and molecular similarity (recent corroborative
work).
B. Geographical variation. Most birds show some degree of variation within
their range. These are due to environmental influences (selection pressures)
that over time accumulate into genetic variation, random chance, as well as
environmental phenotypic variation.
1. Fox sparrows show clear patterns in color and bill size throughout the
West OVERHEAD. These probably reflect differences in their diet and
foraging habitats.
2. We tend to think of this variation as being largely genetic, but James’
study of the red-wing blackbirds proves otherwise.
3. Gene flow and clines.
a. The evolution of geographical variation depends on two processes:
i.
natural selection - the process by which individuals become
adapted to their local environments, and thereby different from
others in other environments. This acts to promote divergence
within a species' range.
ii.
Gene flow - the movement of alleles between populations due
to dispersal. This acts to erode divergence within a species'
range.
b. The result of these opposing forces is often manifested in the gradual
change of phenotypes along a continuous range - a cline. Clines may
be static or dynamic.
i.. static clines - a cline that has reached an equilibrium. E.g. Eastern
Screech Owls OVERHEAD
ii.
dynamic clines - a cline that is still in the process of changing.
E.g., Bananaquits
C. Hybridization. A shift.
1. Secondary contact of populations that were previously isolated
(geographically or otherwise). After a recent speciation event has
occurred, often the incipient new species are reunited. This tests the
viability of the isolation, and the validity of the species status. The
resulting zone of overlap is called a hybridization zone. Several possible
outcomes are possible:
2. Hybridization dissolves previous isolation (free interbreeding; species
status not warranted). This is the case for eastern and western forms of
Yellow-rumped Warblers (myrtle and Audubon's respectively). They
were separated by Wisconsin glaciation. When reunited (7500 years ago)
they freely hybridized, as they do today. Their interbreeding is widening
the zone of overlap, and their differences are eroding. They are they same
species. OVERHEAD
3. Hybridization abates; the isolates persist. Silver-eyes in South Pacific
colonized an island and diverged to form a new species. When parental
stock re-colonized the island, the two species briefly hybridized, but the
hybrids did not persist, and now the two species co-exist. Hybrids often
fail to persist when the parental stocks are highly divergent because they
are less well-adapted than either parental species. This is called hybrid
inferiority. Often, hybrids are sterile, or if they aren't, their offspring are
(F2 breakdown).
4. Hybrids fuse into a third species and isolates persist. Sometimes, the area
of overlap is intermediate in environment between the environments of the
two reunited parental species. In these cases, hybrids are often superior in
the zone of overlap than either of the parental species. If this is the case,
the selection favors the isolation of the hybrids as well as the parental
lineages -- three species result. Red-shafted + Yellow-shafted = Gilded
Flickers.
5. Stable hybrid zones. Sometimes, the zone of overlap doesn't abate, widen,
or result in a new species but instead appears to reach an equilibrium.
These cases are called stable hybrid zones. Two theories exist to explain
the presence of stable hybrid zones:
a. Dynamic equilibrium (stable hybrid zone) - Here, the hybrid zone is a
zone of ecological inferior individuals (hybrid inferiority), but it is
maintained at some steady state by the continual immigration from the
two parental lineages. The hybrid population is a sink, meaning it only
persists by receiving immigrants from neighboring populations.
b. Bounded superiority (stable hybrid zone) - Here, the environment in
the hybrid zone is intermediate between the environments of the
parental species, and the hybrids are better adapted to their local
environments than are either of their parents. This process is exactly
like that which produced a third new species in the zone of overlap
above, except that here the hybrids develop no behavioral mechanism
such that they assortatively mate. They only persist by virtue of their
proximity to parental species.
C. Ecology of speciation.
1. When two divergent lineages are reunited, their degree of competition is
dependent on their level of divergence.
2. If they have diverged significantly, they will probably compete relatively
little, and will persist as distinct species.
3. However, if they have diverged relatively little, then even if they are
assortatively mating and could theoretically coexist, one may be a better
competitor than the other, and may drive the other to extinction. This is
the case for Blue-winged and Golden-winged warblers in the East. Blue
wings have expanded northward via forest loss, invading areas previously
occupied only by Golden-wings. Along their zone of overlap, Blue-wings
outcompete and replace golden-wings usually in about 50 years. The
Golden-wing is threatened with extinction if this continues, which it is
likely to do.