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Evolution Cont.
Macroevolution: The evolution of species
Dif than Microevolution in that we are looking at one spp. Diverging into another
(as opposed to microevolution...which means what?)
What do micro and macroevolution have in common?
I. Intro:
A. 2 types of speciation
1. anagenesis
2. cladogenesis
B. What is a species?
1. A spp is a group of populations that have similar morphology, function,
biochemistry, behavior and genetic makeup.
2. The Biological species concept defines a species as a pop. Or group
of pops. Whose members have the potential to interbreed and produce viable,
fertile offspring, but who cannot produce viable fertile offspring with other
species.
3. Because this definition depends on viable offspring, we say that 2 spp.
Are reproductively isolated (even if they are not geographically isolated).
II. Reproductive Barriers (Prezygotic Barriers)
A. Habitat Isolation: Isolation based on where they live (or how they occupy a
habitat)
B. Behavioral Isolation: mating rituals and mating plumage (reproductive
dances)
C. Temporal Isolation: reproduction at dif. Times of day, season, year, etc.
D. Mechanical Isolation: no chance of copulation (very common in insects)
E. Gametic isolation: sperm and egg can’t fertilize (improper environment,
no molecular markers for sperm penetration, etc.)
III. Reproductive Barriers (Postzygotic Barriers)
A. Reduced Hybrid Viability: spontaneous abortion, offspring dies before
maturity
B. Reduced Hybrid Fertility: can reproduce, but not successfully
C. Hybrid Breakdown:
IV. See Fig 24.5 for review
V. It’s hard to test the biological spp. Concept, so there are other models
available
A. Ecological spp. Concept: a spp. Is defined by the niche it occupies
B. Pluralist spp. Concept: maybe more than one definition of a spp. (e.g.
reproductive isolation and morphological diff. or biochem dif and
ecological dif.)
C. Morphological Spp. Concept
D. Geneological spp. Concept
MODES OF SPECIATION: TWO BIGGIES: ALLOPATRIC SPECIATION AND
SYMPATRIC SPECIATION
VI. Allopatric Speciation:
A. A geologic rift can occur between two species
B. Examples:
1. continental drift
2. island formation, separated by water (volcanic)
3. Mountain ranges
4. Floods, drawback and reflooding of lakes
5. Ring species and migration
6. Land bridges:
C. Drosophila Example (a prezygotic barrier set up by a “geologic barrier)
D. Mimulus (Monkey faced plants) Example (a post-zygotic barrier created
by genetic drift caused by distance between two species of flower.
E. Adaptive Radiation
VII. Sympatric Speciation: new species arise within the range of parent
populations
A. Polyploidy in plants
B. Can lead to hybrization between two species of plants in that a
polyploidy of one plant hybrizes with a normal gamete of another plant and
the two gamets create a new diploid organism.
C. Lake Victoria Ciclids:
Polymorphic populations that occupy the same habitat, but females
adhere to strict mating preferences
1. P. pundamilia (blue backed)
2. P. nyererei (red backed)
3. In natural light, females only prefer their “kind”
4. In monochromatic light, females mate non specifically and
produce viable, fertile offspring.
5. Hints at recent divergence of the two morphs
6. The question becomes, are they really different species?
D. Sympatric speciation required the emergence of some type of
reproductive barrier that isolates the gene pool of a subset of a population
without geographic separation of the parent population.
VIII. Punctuated Equilibrium
A. There is not a lot of evidence for gradualism in the fossil record.
B. What is seen is a decent with modification that is abrupt and lacks
easily recognizable transitional forms.
C. Punctuated Equilibrium, literally periods of abrupt speciation followed
by stasis (no change in species)
1. As a species diverges from a parent population it has a relatively
rapid change (mutation and inbreeding set up by geologic and
behavioral barriers) that is followed by a period of no change.
2. Change over 1000s of generations followed by millions of years
of stasis. Once agan, we’ve got to change our concept of time.
IX. From speciation to Macroevolution
A. Much like variable alleles and characteristics reflect the phenotype of a
species given “vast tracts of time”, natural selection can select for divergent
species over same amounts of time.
B. Does speciation reduce competition among individuals within a
population?
C. Does speciation allow for partitioning of resources and reduce
competition within a community?
D. Speciation is reflected in the fossil record and the fossil record reflects
the “vast tracts of time.”
E. The eye and down with “Intelligent Design”
1. Most eyes are clusters of photoreceptors.
2. The more fine-tuned the photoreceptor needs to be, the more
working parts it has (see fig. 24.18).
3. That’s not to say that the more primitive eyes are any less useful
to it’s owner (e.g. a planarian, or a snail)
4. The same argument can be made for our cockroach example of
gap junctions and negative phototaxis
5. The same argument can be made for a bats ear, for its ability to
use sonar to hunt prey in darkness. Our ears don’t need to be as sensitive
b/c we hunt and farm and gather by vision.
6. Even though we (vertebrates, mammals, primates, hominids,
humans) are increasingly cephalized, we (humans) have a decreased
sense of smell, hearing and to an extent, touch. We rely so heavily on
speech and vision to communicate and get what we want.
F. Exaptation: an evolutionary novelty that is adapted for one purpose but
is “co-opted” for another function. This is a case of “pre-adaptation”, but it
is not guided. The pre-adaptation happens as a “happy accident”
1. Hollow bones of small, agile reptiles were well suited for flight
2. Plumage and forelimb “decoration” used for mating rituals were
well suited for gliding and flight.
3. As flying reptiles became more prolific, they likely divereged from
their ground dwelling forebearers and became reproductively
isolated.
X. Evolution and Development are linked.
A. Allometry: a body’s specific shape or form based on developmental
rates of different parts.
1. The majority of human growth occurs in the limbs as opposed to
the trunk and head.
2. Conversely, the large head and eyes of an infant are appealing
to a parent, we want to protect our young.
3. Allometry can affect skull growth and the differential growth
affects adult function.
a. Chimpanzee vs human adult skull.
B. Heterochrony (different timing): the evolutionary change in the rate or
timing of developmental events.
1. Salamander hands and feet: decreased bone growth and
apoptosis leads to feet better suited for climing than crawling.
2. This creates a physical gap between two populations of
salamanders that can lead to reproductive isolation and speciation.
C. Homeotic genes: changes in genes that determine the placement and
development of body parts.
1. birds wings,
2. Birds’ legs
3. Flower parts
4. Hox genes: provide positional information in an animal
embryo and determine how certain skeletal features will
develop.
a. Hox genes may be responsible for the development of
phalanges in the limbs of lobe-finned fishes that gave rise to
tetrapods.
b.Hox genes may have been duplicated and lead to new
gene families that gave rise to vertebrae and eventually jaws.
i. in this example, there is an increase in genetic
material, and eventhough this material is duplicated, it is essentially
recruited for new function. (That is if there is environmental
pressure to utilize the new genes, and use of the new genes leads
to differential reproductive success.)
ii. How does a backbone and a jaw lead to differential
reproductive success?
XI. There is no finish line. Or Swedish Line, or Belgian Line…
A. Allogenesis, cladogenesis, allopatric/sympatric speciation isn’t a
straight line. (Remember that 95% of all species end up extinct. The fossil
record reclects this.)
B. An “evolutionary tree” showing Species divergence and diversity from a
common ancestor to a an extant species is more like a “bush” with
numerous branches. Some we can see, some we can’t.
C. What do we see?
D. One Idea is species selection: the species that endure the longest
and generate the greatest number of new species determine the
direction of major evolutionary trends.
E. Evolution (macro and micro evolution) is response to interactions
between the environment and organisms. If conditions change, an
evolutionary trend may cease or even reverse.