What are the Postulates of
Darwin’s Theory?
• Darwin’s Postulates (theory of natural selection as the
major cause of evolution – each postulate can be
tested; each potentially falsifiable)
1. Individuals within populations are variable
2. Variations among individuals are, at least in part, passed from
parents to offspring (Darwin was not aware of genetic
mechanisms)
3. In every generation, some individuals are more successful at
surviving and reproducing than others
• Most juveniles die before reproducing (note biotic potential)
4. The survival and reproduction of individuals are not random;
instead, they are tied to the variation among individuals. The
individuals with the most favorable variations, those who are better
at surviving and reproducing, are naturally selected
• Fitness: measurement of organism’s ability to survive and reproduce
What Factors Cause Evolution?
• Evolution (population genetics definition):
change in gene frequencies in a population
(changes in gene pool)
• Factors that can change the nature of a gene
pool:
1. Natural selection: a strong force in evolution
2. Migration: especially strong in island populations
3. Mutation: a weak force in evolution, but the ultimate
source of novelty; mutations are generally mildly
deleterious (due to second copy of gene)
4. Non-random mate choice: sexual selection
generally involves female choice (among competing
males)
5. Chance events: environmental changes and catastrophes; “random” evolution called genetic drift
What Evidence Supports the Modern
Theory of Evolution?
1. Direct observations of change through time
– Ex., changes in beak morphologies among Darwin’s
finches (long-term study at Galapagos Islands)
– Ex., change in beak lengths of soapberry bugs after
introduction of golden rain trees in Florida
2. Vestigial traits: functionless or rudimentary
version of functional feature in other, closely
related species or subspecies
– Examples: eye sockets in blind cave fishes; wings in
flightless birds; pelvic and leg bones (and spurs)
in snakes (similar situation with cetaceans);
reduced tailbone (coccyx) and arrector pili
muscles in humans ( goosebumps; lift hair in
other mammals)
What Evidence Supports the Modern
Theory of Evolution?
3. Evidence from the fossil record
– Extinction: in 1812, Cuvier provided strong evidence of
extinction with analysis of fossils (mammoths,
mastodons, and Irish elk)
– Law of Succession: general pattern of correspondence
between fossil and living forms from the same
locale; supported from wide variety of locations
and taxonomic groups (ex. marsupials of
Australia)
– Transitional forms: exhibit various characteristics seen
in ancestral species and other characteristics
seen in more recent descendents (the latter
often including important novel features)
•
Examples: Archaeopteryx; Basilosaurus; transitional tetrapods
What Evidence Supports the Modern
Theory of Evolution?
4. Homology: the study of likeness (modern meaning:
similarity due to inheritance of traits from a common
ancestor)
–
Structural and developmental homology
•
•
–
Ex., pattern of limb bones similar in all tetrapods
Ex., vertebrate embryos undergo similar developmental stages
before acquiring group-specific features (first noted by Karl
Ernst von Baer in 1828)
Molecular homology: shared genetic code for nearly all living
organisms; genes for critical enzymes with few
differences among groups; shared genetic flaws in
related species
5. Thousands of lab, field, and in silico studies that document
the importance of natural selection, sexual selection,
mutation, and migration in the evolution of populations
What are Adaptations?
• Adaptation: a feature used for some function that has
become prevalent or is maintained in a population
because of natural selection for that function
– Multiple functions of single traits: many traits have multiple uses
(ex. functions of fish swim bladder include buoyancy, oxygen
storage, and sound production)
– Trade-offs: single traits may have off-setting benefits and detriments (ex. fish swim bladder provides buoyancy, but is a good
target for dolphin echolocation)
– Key innovations: traits that are associated with large gains in
evolutionary success (ex. skeletal fin rays in bony fishes)
– Preadaptation: a feature already present in a population that
fortuitously serves a new function
• Examples: wings in ancestral insects likely selected for surfaceskimming performance; bird wings likely enabled uphill running,
gliding, and/or thermoregulation before birds obtained flight
How Does Speciation Occur?
• The Biological Species Concept: species are groups of
actually or potentially interbreeding populations, which
are reproductively isolated from other such groups
(Ernst Mayr, 1942); emphasizes reproductive isolation
(lack of gene flow); later modified to account for
existence of fertile animal hybrids (animal hybrids are
rare, and are typically sterile or exhibit low fitness)
• Mechanisms of Speciation
– Speciation: origin of new species (process vs. event)
– Allopatric Mechanisms (physical isolation triggers
reproductive isolation)
• Via dispersal and colonization (ex., islands, edge of range)
• Via physical split of original range (ex., new mountain range or
isthmus, change in river’s course)
– Sympatric Mechanisms
• Genetic mechanisms: polyploidy (ex., wheat), mutations in
regulator genes
• Behavioral mechanisms: temporal separation, courtship displays
What are Some Patterns of Macroevolution?
• Adaptive Radiation: ancestral species evolves into multiple descendent
species, with each exploiting a different available lifestyle in their
respective environment
– Darwin’s finches on Galapagos Islands
– African cichlids (very diverse family of fishes in African Great Lakes)
• Convergence: independent evolution of superficially similar traits (in
response to similar selection pressures)
– Streamlining in dolphins, penguins, tunas (reduces drag in water)
– Echolocation in bats and dolphins (swarmed, patchy food sources)
• Coevolution: reciprocal changes in two or more species in close
association with each other
– “Arms races” between predators and their prey
– Adaptations for pollination (insects/hummingbirds/bats and flowering plants)
• Gradualism: slow emergence of new species (Darwin emphasized)
• Punctuated Equilibrium: long periods of stasis interrupted by sudden
emergence of new species (Stephen J. Gould and Niles Eldridge,
1972)
What do Ecologists Study?
• Ecosystem: all interactions between living things
(community) and physical factors in a given area
– Biotic (living) vs. abiotic (non-living) factors (ex., floods, droughts)
• Habitat: place where organism lives; can be general or
specific (biomes are major climatic zones)
• Niche: organism’s way of life; multi-dimensional; in theory,
only one species can occupy a niche (ecological
species concept)
• Energy Flow: producers, autotrophs, phytoplankton;
consumers, heterotrophs, zooplankton, herbivores,
carnivores, omnivores, detritivores, decomposers
– Food Chains: ~90% energy loss each trophic step
– Food Webs: more realistic; note importance of krill in Southern
Ocean food web (shared resource, not necessarily limited)
– Food Pyramids: less biomass (and abundance) at higher levels;
decomposers act on all trophic levels
• Biogeochemical Cycles: hydrologic, carbon, nitrogen cycles
– Carbon cycle: related to global warming theory
Figures 4-20 and 4-21
What Relationships Exist Between
Organisms in Ecosystems?
• Predation and Anti-predation
– Diet Specialists/Generalists: specialists can have morphological, behavioral,
and physiological adaptations for capturing/assimilating prey; scarcity of
prey can lead to extinction of diet specialists
– Anti-predation: cryptic and warning colorations, mobbing, displays
• Competition: assumes a limited (not just shared) resource; removal
experiments used to test for effects on fitness
– Intraspecific: between members of same species; most intense is between
males for access to females
– Interspecific: between separate species; can lead to competitive exclusion
– Scramble: rare in nature; all may get less than needed
– Contest: mechanisms; ex. harems vs. sneakers (ex., wrasse, marine iguana)
• Symbiosis: evolved life-relationship between two or more species
– Mutualism: both species benefit (ex. anemone and clownfish)
– Parasitism: one benefits, other is harmed; endo- and ectoparasites
– Commensalism: one benefits, other with no effect; least common,
examples often debated (exs. whale shark with pilotfish; reef shark with
remora? – debatable, since remora may cause hydrodynamic drag)
• Facilitation: organism indirectly benefits others (ex., earthworms aerate
soil, nightly excretion of ammonium by blacksmith benefits algae)
Why is Biodiversity Important?
• Biodiversity: variation among living organisms
– Species diversity: number of species in an ecosystem; increases
with stability/uninterrupted evolution (ex., deep sea, tropical rain
forests), and available niches; decreases with isolation
– Genetic diversity: variation within a species
• If low, more vulnerable to catastrophic changes/extinction
• Importance of Biodiversity
– Ecosystem stability: keystone species are those with influence
disproportionate to their abundance (ex. sea otter in Alaska)
– Genetic reserves; esp. regarding agriculture; endemic species are
unique to particular habitat (ex. marine iguana in Galapagos Is.)
– Practical uses (ex. medicine, future foods)
– Aesthetic and ethical value: biophilia, Gaia Hypothesis
• Largest Threats to Biodiversity
1. Habitat loss and fragmentation: conservation incl. wildlife corridors
2. Introduced species (especially on islands)
3. Hunting/poaching; illegal trade  international treaty (CITES)