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Today: complete lecture 3 and begin lecture 4
Conclude lecture on speciation
Begin Lecture 4. Macroevolution
Readings for lecture 4:
Variation in speciation rates (ch 22:420-422)
Evolutionary radiations (ch 22: 422-423)
The significance of speciation (ch 22; 423-424)
Rates of Evolutionary change (ch 20: 390-392)
Patterns of evolutionary change (ch 20: 392-393)
The future of evolution (ch 20; 393-394)
Allopatric Speciation: Important Considerations
•Inference for the importance of allopatric speciation in animals;
•The effectiveness of a geographic barrier to impede or eliminate gene flow
depends on the locomotion and other characteristics of individuals
•We consider speciation to have ensued if and when two gene pools have
diverged to the point that, should secondary contact occur, the individuals of
each are reproductively isolated from each other; can no longer interbreed
and produce fertile offspring.
Sympatric Speciation
New species evolves withiin geographic range of parent species
•Common in plants, via polyploidy
(2n=28)
(2n=14)
Hugo DeVries with new species of primrose, Oenothera
gigas
Recall the breach between “Mendelism” and “Darwinism”…
Sympatric Speciation in Plants via Polyploidy
Polyploidy Chromosome complement with one or more extra sets of chromosomes; an
increase in the number of chromosomes
There are two distinct mechanisms by which polyploid species of plants arise
•Autopolyploid Species
•Allopolyploid Species
•Auto-polyploid species arise
from single parent species
•Allo-polyploid species arises through
interbreeding of two different species
•Can arise through a meiotic
error – a non-disjunction event
that constitutes a mutation -results in gametes with one or
more extra sets of chromosomes
compared to chromosome
complement in normal gametes
•A number of different routes to
allopolyploidy are known – all involve
arriving at a chromosome complement
that is functional in terms of meiotic
reduction division
•Regarded as being much more
common than autopolyploidy
Example of Autopolyploidy through nondisjunction and self-fertilization
Meiotic nondisjunction of a diploid (2n) cell results in gamete with unreduced
chromosome number of 6
Self-fertilization, as depicted below, by such an in individual gives rise to a new species;
individuals are capable of sexual reproduction with complete set of homologous
chromosomes –required for successful meoisis
Examples of Allopolyploidy
Modern Bread Wheat is a Hexaploid Plant that probably originated
about 8000 years ago as a spontaneous hybrid of a cultivated wheat
and a wild grass (Campbell 2000)
Sympatric Speciation in Animals
•Animals may become reproductively isolated if genetic factors cause them to
depend on different resources than parent population;
•The idea that sexual selection is an agent of sympatric speciation in animals is
gaining increasing support.
•(Polyploid speciation in animals is rare)
•Mechanisms not well-understood, but probably not common
Reproductive Isolation;Prezygotic and Postzygotic Barriers
that Isolate Gene Pools of Biological Species
•As an incipient new species diverge behaviorally, physiologically,
morphologically from the parent species, those very differences may preclude
the two from reproducing successfully; i.e., the two may become “good
biological species”, or not!!
•Reproductive barriers; Evolved traits that preclude production of fertile,
viable hybrid offspring
•Prezygotic Barriers; reproductive isolating mechanisms that operate
before fertilization, some before mating
Spatial, temporal, mechanical and gametic isolation
•Postzygotic Barriers; reproductive isolating mechanisms that operate
after fertilization
•Problems with hybrids – including developmental abnormalities,
infertility and low viability
•In “hybrid zones” or “areas of secondary contact” where hybridization takes
place, if there is selection against hybrids, we may expect evolution of
stronger prezygotic barriers (demonstrated in some laboratory populations,
not well-supported in observations of natural populations)
Habitat Isolation. Populations live in different habitats and do not meet
Temporal Isolation. Mating or flowering occurs at different seasons or
times of day
Behavioral Isolation. Little or no sexual attraction between males and
females
Barriers to reproduction can arise without having been favored directly
by Natural Selection, as a consequence of adaptive divergence
Adaptive divergence of two populations
•populations diverge evolutionarily (think in terms of the genetic structure of
each)
•divergence is consequence of populations experiencing different selective
forces; divergence is “adaptive” in that sense (can be true for sympatric or
allopatric)
•populations may diverge so much (morphologically, physiologically, behaviorally,
etc) that interbreeding is not possible; reproductive isolation
•complete reproductive isolation, but not as a consequence of selection for
isolation
Spatial Isolation = Habitat or Ecological Isolation
Behavioral Isolation-- Blue-footed Boobies on Galapagos Islands
Behavioral Isolation -- Song in Eastern and Western Meadowlarks
Distinct songs help prevent interbreeding among these sibling species
Reproductive Isolation can arise as a consequence sexual selection operating
within one or the other population, or both
•Sexual Selection is a form of natural selection; selection that occurs when individuals
vary in their ability to acquire mates (less successful individuals are “selected against”)
•In many species of animals, it’s the males that experience substantial sexual selection
•This selection pressure drives evolution of traits that make individuals more successful at
acquiring mates
Solomon 1999
Male great frigate bird shows “ornament”
that evolved through sexual selection
Raven and Johnson 1999
Male white-tail deer shows “armament” that
evolved through natural selection
Mouth-brooding Cichlids surrounded by swarms of
fry in Lake Tanganyika. Young are periodically
released to feed but gathered up into parents
mouth at first sign of danger.
Four species of
Haplochromis cichlids in
Lake Victoria that occupy
different ecological niches,
although they are similar
in appearance.
Non-Random mating in a polymorphic species may have
led to sympatric speciation the Genus Pundamilia in Lake
Victoria
two closely related species of Cichlids in the genus Pundamilia
Reproductively isolated in nature and in captivity under natural
light conditions – females only choose conspecific males.
Under monochromatic orange light, males look similar
(presumably) to females – and females mate indiscriminantly
with males of either species
Inference from experiment that speciation occurred relatively
recently and that color is the main, perhaps only “reproductive
barrier”
Hybridization and the Concept of “Biological Species”
Hybridization (one definition): interbreeding among individuals from two divergent
populations
Hybrid Zone: region where two related populations that diverged after becoming
geographically isolated make secondary contact and interbreed
Yellow-rumped (Audubon’s)
warbler
Yellow-rumped (Myrtle) warbler
What can we say about “speciation” when areas
of secondary contact exist where hybrids do not
have reduced fitness?
The yellow-rumped warbler diverged into two distinct
races: Eastern populations were separated from
Western ones during the Wisconsin glaciation, and
probably came into secondary contact about 7500
years ago. Populations are reproductively isolated
over most areas of secondary contact, except in
some regions in the Canadian Rockies, where
hybrids do not have reduced fitness; yellow-rumped
genes are introgressing west, and Myrtle genes are
migrating east.
Macroevolution; Evolutionary changes occurring over long time
spans and usually involving changes in many traits
Three major faunas have dominated animal life on Earth
Cambrian Explosion – all major animal lineages arose
Paleozoic and Triassic Explosions – many new families, genera and species, but not new
fundamentally new body plans
Reason for difference in pattern of diversification (no new phyla) may relate to the
ecological conditions; low competition and predation may have fostered evolution of
major body plans
millions of years ago
The size and complexity of
organisms have increased
Early Eukaryotes were larger and
substantially more complex than
Prokaryotes from which they arose
(and modern Prokaryotes as well)
Multicellularity allowed greatly
increased size, which facilitated
homeostasis, specialization….
Co-evolution among predators and
prey is probably partly responsible for
increasing complexity, paricularly in
the form of highly developed nervous
and muscular systems, and for
capture and avoidance traits in
general
Evolution of shell morphology indicates increasing
predation rates on snails over evolutionary time.
Most Evolutionary Novelties are Modified Versions of Older Structures
Descent with modification Extends to
major morphological transformations
limpet (Patella)
slit shell mollusk
(Pleurotomaria)
Complex structures often evolve
incrementally from simple ones
Exaptation Evolutionary novelty can arise
through gradual refinement of existing
structure for new function
marine snail (Murex)
Nautilus
squid (Loligo)
Homologous structures in the forelimbs of
mammals. Wing of a bat and flipper of a whale
are examples of exaptations of terrestrial
forelimbs.
Eye complexity in Molluscs
Complex eyes evolved from simple
ones many times in evolutionary
history
Significant evolutionary change leading to the origin of new species
may be gradual or may occur in spurts
•Fossil record does not bear
many forms transitional
between species; suggests
that significant morphological
(and underlying genetic)
change occurs quickly
relative to the life of a
species.
•Gould and Eldridge
developed and published this
idea in the 1970’s, referring
to the process as
“Punctuated Equilibrium”
•species undergo most
morphological change
shortly after diverging from
parent stock
•No reason to regard these
hypthotheses as mutually
exclusive among lineages
long periods of evolutionary
stasis puncuated by episode
of morphological change that
reflects speciation
Punctuated Equilibrium
•Speciation happens rapidly; most of the morphological differences evolve rapidly in
a new species, as that new species first buds from its parent species
•Support for theory in fossil record; Darwin acknowledged that fossil record didn’t
seem to show the gradual change he expected
•Allopatric speciation thought to occur relatively rapidly; natural selection and
genetic drift can cause significant change in a few hundred to a few thousand
generations
•If a species survives (leaves fossils!) for five million years, first 50,000 years of
its existance would be only 1% of its existance, 1% of its fossil-producing time
•Mutation in genes that regulate embryonic development may be associated with
changes that can generate new species..
Developmental genes have strong influence on basic body plans and
therefore, potentially, on macroevolutionary change
•Developmental genes control the
rate, the timing (eg onset) and spatial
pattern of changes in form as an
organism grows and develops
•The specific form a body takes on
depends in part on proportioning or
allometric growth (“other measure”);
growth rates of different body parts
relative to eachother
•Slight change in growth rate of one
body part relative to the others can
have substantial effect on adult form
•Heterochrony refers to evolution of
morphology through modification in
allometric growth; heterochrony is
driven by developmental genetics
Arms and legs grow faster than head and
trunk (different aged individuals all rescaled in
drawing to same height)
Comparison of chimp and human skull
growth. Fetal skulls are similar in shape.
Sloping skull of adult chimp compared to
human is due to faster growth of the
chimps jaw than other parts of the skull
(compared to humans)
Heterchrony may involve a change in the
timing of reproductive development of
reproductive relative to the timing of
somatic (non-reproductive) development
Paeodmorphosis If reproductive rate accelerates,
may contribute to evolution of new species that
when sexually matures, retains structures that
were characteristic of the juvenile form in the
ancestral species
Axolotl – a salamander that
retains certain larval (tadpole
characteristics, including gills,
after it has grown to full size
and is sexually mature
Mutations in genes that control spatial organization and location of body
parts can contribute to macroevolutionary change
Hox mutations and the origin of vertebrates