Description
BIOL005C Intro to Evolution & Ecology Final Study Guide
I. Biomes & Climate
A. Key Characteristics of TERRESTRIAL Biomes
1. A biome is a major type of biological assemblage
2. What determines a biome?
● On land…
○ Temperature and rainfall
● In water…
○ Temperature, depth, and salinity
Biome
Location
Rainfall
Temperature
Vegetation/Pr imary
Productivity
Biodiversity
Tropical
Forest
Equatorial/
Subequatorial
Rain forests: constant
Dry forests:
seasonal
High, minimal seasonal
variation
Vertically
layered, lots of
competition
for light; High “pp”
Very high -
home to 70% of land
animals and plants
Temperate Broadleaf
Forest
Mid-latitude
High rainfall
year-long
Cool winters, hot & humid summers
Vertically
layered trees, shrubs,
herbs;
moderate
Moderate
Northern
Coniferous Forest
(Taiga)
North America & Eurasia
*largest
terrestrial
biome
Varied
rainfall;
periodic
droughts,
coastal areas are wet
Cold winters, hot summers
Conifers -
pine, spruce, fir, hemlock; low
Low
Chaparral
Mid-latitude
coastal
Low rainfall
Cool & rainy winters, hot & dry summers
Many native plants
specific to
location; low
Low
Savanna
Equatorial/
Subequatorial
Seasonal w/ an extensive dry season
Warm,
varying w/ the season
Fire- and
drought
tolerant
plants -
grasses,
forbs, trees;
moderate
Moderate
We also discuss several other topics like What is not usually a characteristic of the developed countries in the contemporary world?
We also discuss several other topics like What are the 3 tenets of the cell theory?
Temperate Grassland
Central North America
(prairie) &
Asia
Low, usually occurring in
summer
Cold winters, hot summers
Grasses &
forbs are fire and drought tolerant;
moderate
Moderate
Desert
30°
North/South
gof equator & interiors of
continents
Low - less
than 30cm a year
Varies from extremities - can be very
hot or very
cold
Adapted for
extremely dry weather and low nutrition; low
Low but
variable
Tundra
Arctic
Alpine tundra: high
mountaintops, all latitudes
Arctic tundra: low
Alpine tundra: high(er)
Very cold
winters, cool summers
-permafrost: overs the
ground so
that no plants may
penetrate/gro w
Mosses,
grasses,
forbs, dwarf
shrubs and
trees, lichen; low
Low
Don't forget about the age old question of What is the meaning of foodborne illness?
B. Key Characteristics of AQUATIC Biomes
Biome
Location
Depth
Salinity
Intertidal Zone
Area of the shore
covered at high tide, & exposed at low tide
Shallow
Moderate
Rivers/Streams
Narrow &
meandering through mountains, toward ocean
Shallow
Fresh
Wetlands
Marshes or swamps
Shallow
Moderate
Lakes
Individual bodies of water
Moderate
Fresh
Estuaries
Area where
rivers/streams meet the ocean
Moderate
Moderate
Coral Reef
Mid-/shallow ocean
Moderate
Saline
Oceanic Pelagic
Open ocean
Deep
Saline
Oceanic Benthic
Deep ocean (ocean floor)
Deep
Saline
We also discuss several other topics like How do isotopes behave?
C. Climate
1. Climate vs. Weather?
● Climate: long-term weather conditions
● Weather: daily weather conditions
2. The SUN
● The sun drives the climate
● Latitude affects the intensity of the sunlight
○ More intense at the equator due to direct sunlight hitting
surface of earth
○ Less intense at North/South poles
3. Air Circulation
● The earth spins faster at the equator (1,670 km/hr), causing the
wind to bend
● Wind pushes water → ocean currents
4. Climate Change We also discuss several other topics like Attitude formation refers to what?
● Bodies of water moderate climates - coastal regions have less
fluctuations in temperature compared to areas like Riverside
● Greenhouse Effect: CO2 and other gases (mostly produced by car
emissions) trap heat from the sun, causing an increase in climate
temperature If you want to learn more check out Nonspontaneous process means what?
○ Consequences:
○ → Rising temperatures
○ → Loss of glaciers
○ → Loss of polar sea ice @ exponential rate
○ → Rising sea levels
○ → More severe storms
○ → Increase in ocean acidification
○ → Depletion of the ozone layer
II. Food Webs & Trophic Pyramids
A. Food chains: lists trophic levels of an ecosystem from producers (bottom) to carnivores (top)
1. Trophic levels always numbered starting w/ “1” (producers)
4 Tertiary Consumers
3 Secondary Consumers
2 Primary Consumers
1 Primary Producers
2. Decomposers: organisms that break down organic matter from ALL trophic levels
3. Green Food Web (Grazing): basal trophic level = primary producer
4. Brown Food Web (Decomposer): basal trophic level = dead plant/animal matter
B. Trophic Pyramids
1. Trophic transfers: transfer of energy b/w trophic levels
● Total energy available declines by 90% w/ each step up food
chain
2. What’s controlling the food chain?
● From BOTTOM-Up…
○ Producers
● From TOP-Down…
○ Predators - can cause “trophic cascade”, where predator
keeps suppressing prey until next trophic level below that
is freed of predation
III. Biogeochemical Cycles
A. Biogeochemical cycle: describes flux of atoms and molecules b/w biotic and abiotic reservoirs
1. Flux: rate of movement b/w reservoirs
2. Reservoir: place where types of atoms or molecules accumulate or are held for a relatively long period of time
● Lithosphere
● Hydrosphere
● Atmosphere
● Organisms
3. Earth is a closed system for elements and nutrients:
Inaccessible ⇄ Accessible ⇄ Organisms
B. Water Cycle
1. Water is primarily used in its liquid state
2. Reservoirs:
● Ocean = 97%
● Glaciers/Ice caps = 2%
● Lakes, rivers, groundwater = 1%
3. Movement by: evaporation, transpiration, condensation, precipitation, & surface/groundwater
C. Carbon Cycle
1. Plants convert CO₂ to organic molecules consumed by heterotrophs 2. Reservoirs:
● Fossil fuels
● Soil/sediment
● Atmosphere
3. Movement by: photosynthesis and cellular respiration
D. Nitrogen Cycle
1. N is component of amino acids, proteins, & nucleic acids
→ Often a limiting nutrient in terrestrial and aquatic ecosystems
2. Reservoir:
● Atmosphere (N₂)
→ N₂ must be converted to usable forms NH₄⁺ or NO₃⁻ in order
to be taken up by plant bacteria via nitrogen fixation
3. Nitrogen Pollution
● Human activities doubled rate of N entering biogeochemical cycles
due to: nitrogen fertilizers, fossil fuels (cars), over-cultivation of
legumes (which are useful for N fixation)
→ Consequences:
- Inc. in global concentration of nitrous & nitric oxide
- Contamination of drinking water
E. Phosphorus Cycle
1. Imp. for nucleic acids, phospholipids, & ATP
→ Most important inorganic form: PO₄³⁻
2. Reservoirs:
● Sedimentary rocks in ocean
● Soils
● Organisms
IV. Density, Dispersion, & Demographics
A. The Population and its Characteristics
1. Populations are dynamic
● Births & immigration => adding to population
● Deaths & emigration => subtracting from population
● 4 Properties of a Population:
→ Size (N): total # of individuals in population
→ Density: # individuals per unit area or volume
→ Dispersion: distribution of individuals in population over
space/volume
→ Rate of change in size over time: growth, decline, or stability
2. How do you measure density?
● Count standardized census methods (plots)
● Count subsamples & estimate (divide plot, count, multiply by #
plots)
● Use a proxy (nests, tracks)
● Mark - recapture (tag animals)
→ Use equation:
N = (n₁ - n₂)/m,
where...
N = pop. size
n₁ = # captured and marked
n₂ = # captured in 2nd sample
m = # recaptured
3. Patterns of Dispersion
● Random distributions: every point in space has equal and
independent probability of containing an individual
● Clumped distributions: Individuals more aggregated, more likely
to be found close to one another
● Uniform distributions: Individuals more evenly spaced, less
likely to be found near each other
B. Demographics
1. Demography: study of the vital stats of a population and how they vary w/ age
● Life Table: an age-specific summary of demographics
→ Data from life table can be used to graph survivorship curves
● Cohort: group of individuals in a population born around same
time
● Survivorship (Iₓ): proportion of individuals born that survive to
certain age (X)
● Reproductive Table: age-specific summary of reproductive rates
→ Only contains data for reproductive females & number of
female offspring produced
C. Life History Traits
1. Life History: the schedule of an organism’s life
● Age/size when ready for reproduction
● Allocation of energy for reproduction
● Size and # of offspring
→ Either produce more, smaller offspring (insects) or fewer, larger
offspring (bears)
● # of times of reproduction
→ Semelparous: reproduces once
→ Iteroparous: reproduces multiple times
● Life span
V. Population Growth & Age Structure
A. Regulated Population Growth
1. Logistic Population Growth
● Unregulated pop. growth cannot be sustained due to:
→ shortages in food & resources
→ greater aggression w/in the population
→ increased predation rates
→ higher risk of diseases
● More realistic growth model includes carrying capacity (K): the maximum population the environment can support
● Logistic Growth Model (CONTINUOUS MODEL)
dN/dt = rN x [(K - N)/K]
Where…
→ K - N = # of additional individuals that can still be supported
→ (K - N)/K = fraction of carrying capacity still available for
population growth
2. r-Selected vs. K-Selected
● r-selection: life history traits that maximize reproductive success at times of low population density
→ “Live fast, die young”
● K-selection: life history traits that are advantageous at times of high population density
→ “Live long and prosper”
3. Density Dependence
● In populations that are density-dependent, birth/death rates are regulated by pop. density
→ Only density-dependent factors can influence pop. size
→ Example of negative feedback that regulates pop. growth
● In density-independent populations, birth/death rates are
unaffected by pop. density
● Mechanisms to regulate growth of density-dependent populations: → Competition for resources: crowded populations intensify
competition for resources & lower birth rates
→ Territoriality: competition for space can disperse dense
populations
→ Disease: pathogens spread quicker
→ Predation: large populations are easier to spot & prey on
→ Intrinsic Factors: physiological factors
→ Toxic Wastes
B. Age Structure
1. Age-Structure Diagrams
● Age structure: the relative # of individuals of each age in a
population
● Age structure diagrams (pyramids): predicts a population’s growth trends
→ Illuminates social conditions and helps us plan for future
2. Global Carrying Capacity
● Predicted 8.1-10.6 billion people will live on Earth in 2050
→ The carrying capacity is uncertain
● Ecological footprint concept summarizes aggregate land and
water are needed to sustain a person, city, or nation
● Potential limiting resources for carrying capacity are food, space,
nonrenewable resources, and waste buildup
→ Humans have the power to regulate population growth through
social change
VI. Interspecific Interactions
Def: Interactions b/w species; classified by the direction of effects (+ / - / 0) A. Antagonism (-/-) or (+/-)
1. Competition (-/-)
● Occurs when species compete for limited resources that are
essential for survival and reproduction
● 2 mechanisms of competition:
○ exploitative: “first come, first serve” - individuals deplete
resources by using them up
○ interference: aggression b/w individuals
● Strong competition can lead to competitive exclusion: when 2
species competing for the same limited resources cannot coexist
permanently, and one is eventually eliminated from the local area
● Species can coexist if there are 1 or more significant differences in
their ecological niches...
→ fundamental niche: the niche that a species could potentially
occupy
→ realized niche: the niche that a species actually occupies
○ Resource partitioning: differentiation in ecological niches
that enables similar species to coexist in ONE community
→ spatial partitioning: different birds live in different parts
of the same tree
→ temporal partitioning: one species of mice looks for food
during the day, the other at night
2. Predation (+/-)
● An interaction in which one species (the predator) kills and eats
the other (the prey)
○ predation: predators kill live prey
○ scavenging: scavengers eat dead organisms
● Both predator and prey have specialized adaptations
Predator
Prey
➢ Camouflage
➢ Sensory adaptations to
➢ Camouflage
➢ Sensory adaptations to
detect prey
➢ Adaptations for
handling prey
detect predators
➢ Adaptations for
escaping capture
● Presence of predators may result in slower growth rate of prey ● Defense Mechanisms:
○ Aposematic Coloration: prey exhibit bright colors as a
warning signal to predators that it is dangerous
○ Mimicry: close resemblance of an organism (the mimic) to
some different organism (the model); the mimic benefits
from the mistaken identity
→ Batesian: an unprotected species evolves to look like a
protected species (only one is dangerous)
→ Müllerian: a chemically protected species evolves to
look like another protected species (both are dangerous)
→ Predators also mimic other species to get closer to prey
3. Herbivory (+/-)
● An interaction in which an herbivore eats part of a plant or alga ● Both the herbivore and the plant have adaptations
Herbivore
Plant
➢ Physiological
➢ Morphological
➢ Herbivore teeth
➢ Structural
➢ Chemical
4. Parasitism (+/-)
● One organism (parasite) derives nourishment from another
organism (host), which is harmed in the process
○ Ectoparasite: external; on outside of host
○ Endoparasite: internal; lives inside host
○ Parasitoids:
→ Special type of parasitism
→ Always kills host
→ Lays eggs inside host
→ Adult stage is free-living
B. Mutualism (+/+): interaction that benefits both species
1. Obligate: necessary for survival and reproduction of one or both species; sometimes symbiotic
2. Facultative: not necessary for survival of either species
3. Reciprocal Exploitation
● ex) Plant-pollinator mutualisms
○ Service/resource mutualisms
○ → An insect provides service of pollination
○ → Flower provides resource of nectar
● Flowers can exploit pollinators by…
○ restricted access to nectar
○ deception: resembling a female insect
○ visible/UV light: insects are attracted to colorful flowers
● Pollinators can exploit flowers by…
○ nectar robbing: extracting the resource w/o providing
service
C. Commensalism (+/0): interaction in which one species benefits and the other is neither harmed nor helped
● Commensal interactions can sometimes be mutualistic
● It is difficult to prove/disprove that the host is always unaffected
Interaction Type
Species A
Species B
Antagonism: Predation/Herbivory/Parasitism
+
-
Antagonism: Competition
-
-
Commensalism
+
0
Mutualism
+
+
VII. Species Diversity
Def: variety of organisms that make up a community
A. Components of Species Diversity
1. Species richness: # of different species in a community
2. Relative abundance: the proportion each species represents of all
individuals in a community
B. Calculating Species Diversity
1. Species-Area Curve
● The species-area curve quantifies the idea that a bigger
geographical area will have more species
S = cAz
→ S = # of species
→ c is a constant
→ A = area
→ z = how many more species should be found as area increases
C. Trends in Species Diversity
1. Area - Island Equilibrium Model
● Species richness represents a balance b/w immigration and
extinction, which in turn are affected by island size and distance
from mainland
○ Islands contain fewer species than a nearby mainland
○ Small islands contain less species than large islands
→ Immigration ↓, Extinction ↑
○ Islands closer to mainland contain more species than
similarly sized islands farther away
→ Immigration ↑, Extinction ↓
● Equilibrium: rate of immigration = rate of extinction
2. Latitude - Latitudinal Diversity Gradient
● Species richness is high near equator and slowly declines
approaching the poles
→ Gradient is affected by evolutionary history and climate
○ Evolutionary History
→ “Tropics as Cradle” model: tropical environments may
have greater species richness b/c there has been more
time for speciation to occur
→ “Tropics as Museums” model: temperate and polar
communities have “started over” multiple times after
glaciations
○ Climate
→ evapotranspiration: the evaporation of water from soil +
transpiration of water from plants (shaped by amounts
sunlight and precipitation)
→ Species richness is related to (potential)
evapotranspiration, which tends to be greater in tropics
VIII. Environmental Change and Selection
A. Directional Selection
1. Industrial Melanism in Peppered Moth
● Peppered moth, Biston betularia, sits on trees during the daytime
and is well camouflaged against lichen-covered bark
● ~1850: rapid increase of black (melanic) form of moth began
appearing in northern England
→ What caused change in color?
○ Genetic change: peppered (typica) and melanic
(carbonaria) morphs determined by a polymorphic gene
(dark allele dominant)
○ Increase in carbonaria morph coincided w/ dramatic
increase in coal pollution due to Industrial Revolution
(centered around northern England)
→ coal pollution => kills lichens on trees, soot blackens
tree branches
B. Selection
1. Rapid Directional Selection
● Types of Selection
○ Directional Selection: when distribution of seed size
changes (e.g. during drought)
○ Stabilizing Selection: when seed distribution remains fairly
constant, selection favors a fixed optimal beak depth
● Phenotypic selection of beak depth results in natural selection b/c
bill depth is highly heritable
● Heritability (h2) = slope of parent-offspring regression
○ Estimates the proportion of individual variation due to
genetic differences
2. Selection on an Environmental Gradient
● Selection on a gradient often results in a genetic polymorphism
that takes form of a gene frequency cline
→ cline: directional change w/ distance
→ often associated w/ some environmental gradient
○ Mytilis edulis (a mussel): a cline in lap (leucine amino
peptidase) alleles in Long Island Sound is related to
changes in salinity
→ Mussels at the entrance of the Sound experience
changes in salinity w/ every tide => enzyme lap controls
osmotic environment w/in the mussel
○ Fundulus heterclitus (small fish found on coast of eastern
US): cline in heart-type Ldh (lactate dehydrogenase)
alleles related to latitude and - hence - water temperature
→ the different homozygotes have faster swimming
speeds at the temperature extremities - high (a/a) and low
(b/b)
C. Positive vs. Negative Selection
1. Terms & Definitions
● Directional natural selection: where one end of a genetically based
phenotypic distribution is favored over the other
○ Positive selection: results in spread of new advantageous
alleles in a population
→ industrial melanism in the 1800s
○ Negative selection: results in removal of rare,
disadvantageous alleles
→ such selection acts to prevent accumulation of
mutations that may result in genetic disorders or lower
fitness
IX. Drift, Inbreeding, Nonrandom Mating, & Mimicry
A. A Founder Event
1. Variegate Porphyria
● Variegate Porphyria: an inherited skin disease due to a dominant allele that is (unusually) common in South Africa
○ Symptoms: areas of skin exposed to sun develop severe
blisters, scars, & changes in pigmentation and become
fragile & easily damaged; no symptoms in ~40% of cases
○ Estimated to affect >20,000 people in So. Africa
→ Origin of mutant allele traced back to 1688 when Dutch
settlers Gerrit Jansz van Deventer married Adriaantje
Ariens in Cape Town
● High frequency of Variegate Porphyria allele is an example of a founder event during establishment of Afrikaaner population
○ In Netherlands, frequency of the allele is <1/10,000
○ In So. Africa, frequency of the allele is 1/300
→ the frequency of a rare allele can increase exponentially
by the random inclusion of just a single copy in a small
population
2. Retinitis Pigmentosa
● 1814: Tristan da Cunha settled in the mid Atlantic w/ 15 other settlers from the UK => 30 gene copies
○ Any allele must initially be at frequency of ≥1/30 or 3.3%
● Among the 15 settlers, there existed ≥1 copy of a recessive
mutant allele that causes retinitis pigmentosa: progressive
blindness
● Late 1960s: 4/240 people had the disease
B. Random Sampling and Evolution
1. Population Bottleneck
● During a population bottleneck, strong selection may act, but random sampling always alters some allele frequencies
○ Changes are retained as the population recovers
2. Random Genetic Drift
● General term for the changes in gene frequency due to random sampling
● Random genetic drift occurs because…
○ If 2 individuals have same expected fitness, their actual
fitness may differ
→ one may die before reproducing while other may
successfully mate and produce offspring
○ Mendelian inheritance has a random element
● Genetic drift has biggest effect when 2 alleles give rise to
genotypes w/ same “wild-type” fitness (neutral alleles), but can
overcome selection and cause disadvantageous alleles to spread in very small populations
● Long-term effects of genetic drift are non-adaptive
→ Since genetic change isn’t caused by natural selection, it is
often called “neutral evolution”
C. Inbreeding & Hardy-Weinberg Ratios
1. Inbreeding
● Increased probability of mating b/w relatives
○ Can occur in small, isolated populations or through mate
choice
○ Most extreme form is selfing (self fertilization), made
possible only in organisms w/ both male and female
reproductive organs (i.e. Mendel’s pea plants)
→ Some plant species have high frequency of selfing,
resulting in reduced flowers
● Selfing guarantees fertilization, but at a cost of some inbreeding depression in offspring (i.e. loss of fitness)
2. Inbreeding Depression
● If there is more mating among relatives than expected, this is a form of non-random mating => population won’t be in H-W ratios ○ Inbreeding results in excess of homozygotes (relative to
H-W expectation)
○ Effect due to the fact that inbred families have higher freq.
of homozygotes than outbred (random mating) ones w/in
same population
→ Offspring of inbred matings more likely to be
homozygous for recessive deleterious alleles
3. Non-Random/Positive Assortative Mating
● A non-random choice of mate based on similarity of a heritable phenotype will lead to an increase in homozygotes at the loci that determines that phenotype
→ ex) mating based on height: short/short => short offspring, vs. tall/tall => tall offspring
● Effect on genotype frequencies:
○ Inbreeding: increases frequency of homozygotes at all loci
○ Positive assortative mating: increases freq. of
homozygotes at the loci determining the target phenotype
○ Non-random mating (by itself) does not change gene
frequencies bc in general it does not change mating
success
● Heliconius cydno alithea (a butterfly from Ecuador) is a Müllerian mimic, but in Ecuador, populations are polymorphic
→ Müllerian mimic: protected by toxins and evolved to look like another protected species
● Positive assortative mating helps maintain the 2 forms since color and pattern loci are unlinked
→ ex) yellow males prefer yellow females
D. Mimicry
1. Müllerian Mimicry
● 2 or more dangerous/toxic animals can coevolve to more closely
resemble each other if they share a common predator that learns
to avoid them, reducing fitness loss caused by naive predators
○ Often involves aposematic (warning) coloration
● Heliconius numata is a distasteful So. American butterfly that is
polymorphic and resembles other distasteful Melinaea species
○ The diff. forms often co-occur in one region and are
inherited as a “supergene” involving several polymorphic
genes inherited as a unit due to suppressed recombination
→ Intermediates are rare as a result
2. Batesian Mimicry - Evolutionary Cheating
● Harmless animal mimics/closely resembles a dangerous/toxic
animal
→ Fitness is gained because of reduced predation
● Papilio dardanus (an unprotected African butterfly)
○ In a single geographic region, females will mimic 1 of 3
protected species
X. Sexual Selection, Pollination, Local Adaptation
A. Sexual Selection
1. Natural Selection for Mating Success
● Most commonly affects male characteristics
● Can trade off w/ other aspects of fitness
2. Intrasexual Selection
● Intrasexual selection (“mate choice”): competition amongst
individuals of one gender (usually males) for mates of the opposite
gender
○ Occurs when individuals of one sex (usually the females)
are selective in choosing who they mate with
○ Usually results in sexual dimorphism for traits favored in
competition (e.g. color, sound, size, horns, etc)
● Lek mating: when males gather together at “leks” to display
○ Females visit leks, compare males, and choose one to
mate with
○ In lekking species (like grouse) there is no paternal care
3. “Runaway” Process
● In absence of parental care, females will likely choose mates that
have “good genes” - males that will produce high-fitness offspring
● This type of mate choice can lead to the “runaway” process
○ ex) If females favor mates w/ the longest tail: male
genotypes producing the longest tails are most successful
→ average tail length increases → females still favor
longest tails → average tail length continues to increase
4. Choosing “Good Genes”
● In lab tests, female gray tree frogs prefer long duration calls
○ Experimental crosses of frogs all caught in same vicinity
showed that offspring of long callers were (on avg) more fit
5. Experiments
● Long-tailed widow bird - Malte Andersson
○ Altered tail length in a natural population
→ Males w/ extra long tails had higher fitness
B. Pollination
1. Attracting Pollinators
● Flowering plants do not attract mates directly
→ they require an intermediary - a pollinator - unless they are
wind-pollinated like most grasses
○ Pollinators usually fly: moths, butterflies, birds, & bats
○ Plant provides a reward in return, usually nectar
● In some cases extreme coevolution may occur
○ ex) Darwin predicted existence of insect w/ extremely long
tongue that pollinated the Angraecum sesquipedale orchid
○ 1903: naturalists found Morgan’s sphinx moth Xanthopan
morgani
2. Pollinator Cues
● Plants provide different cues to attract different pollinators
○ Red tubular flowers = hummingbirds
○ Nocturnal flowers = bats and/or moths
○ Flowers that smell like rotting meat = carrion flies
● Generalist strategy: plant attracts a wide range of pollinators ● Specialist strategy: plants rely almost entirely on a single pollinator ● Each strategy has a trade-off for the plant
○ Specialist system maximizes chance that the pollinator
takes pollen from one plant to another of same species,
but minimizes chances of being visited bc there is a lack of
pollinators
○ Generalist has opposite problem
C. Local Adaptation
1. Disruptive Selection
● Selection that favors the extremes of a distribution, and selects against intermediates
● How can disruptive selection maintain adaptation?
○ 3 examples of polymorphic mimicry show it can be
resolved by:
→ “supergenes”
→ a regulatory switch that shifts the phenotype from one
extreme to the other, avoiding intermediates
→ positive assortative mating, changing the distribution of
genotypes to increase the extremes
● Most commonly arises when 2 neighboring environments select
for different traits
○ ex) the snail Cepaea nemoralis is subject to bird predation
=> it is advantageous to be obscure
→ In areas of mixed beech woodland/grassland habitats in
England, yellow/banded genotypes are favored on
grassland but brown are favored in the woodland
2. Gene Flow vs. Local Adaptation
● Local adaptation can occur over small distances (i.e. boundary
b/w a wood and a grassy field) provided that gene flow across the
boundary is limited
● Gene flow: the distance that gene copies move per generation,
either by individuals moving or by movement of gametes (e.g.
pollen dispersal)
○ Gene flow across a habitat boundary reduces local
adaptation by introducing maladapted alleles every
generation
XI. Species & Speciation (pt. 1)
A. What is a species?
1. Biological Species Concept
● A species is a population or group of populations whose members
have the potential to interbreed w/ one another in nature to
produce healthy, fertile offspring, BUT cannot produce healthy,
fertile offspring w/ members of other groups
● Appropriate mating tests are often impossible/impractical to apply
w/ asexual species and fossils. Alternatives:
○ Morphological species concept - physical differences
○ Paleontological species concept - physical differences
○ Ecological species concept - habitat differences
○ Phylogenetic species concept - genetic differences
● Problems arise bc speciation is a process involving the
development of reproductive isolation => must maintain different
ecological adaptations
○ Divergence of allopatric (physically separated) populations
of the same species often builds up slowly over time
→ As allopatric divergence increases, we divide a species
into geographical subspecies which may ultimately
become distinct species
2. Ring Species
● In CA, Ensatina eschscholtzii salamander found on both sides of central valley
○ Where the ring meets, the subspecies do not interbreed
● Greenish warbler Phylloscopus trochiloides is divided into
subspecies that differ in song; two Siberian subspecies do not
interbreed
● 2 mechanisms that drive reproductive isolation:
○ Prezygotic barriers
→ Habitat isolation
→ Temporal isolation
→ Behavioral isolation
→ Mechanical isolation
→ Gametic isolation
○ Postzygotic barriers
→ Reduced hybrid viability
→ Reduced hybrid fertility
→ Hybrid breakdown
3. Types of Speciation
● Allopatric
○ 2 populations of a single species are spatially separated,
preventing the 2 forms from mating
→ Allows the 2 populations to become genetically distinct,
often through local adaptation
● Sympatric
○ Disruptive selection favors 2 forms w/in a single population
→ Active mate choice/other prezygotic separation that
limits mating b/w the 2 forms is critical in maintaining the
effects of disruptive selection
● Hybrid Zones: if 2 closely related, allopatric species meet through the range expansion of one or both (secondary contact) they
may…
○ Fuse back together
○ Be so different that they already show prezygotic isolation
○ Form a hybrid zone at the point of contact
B. Hybridization & Isolation
1. Reinforcement: Strengthening Reproductive Barriers
● If hybrids have fitness disadvantage, there is selection for wider
range in of mating signals at zone of overlap
○ Selection can result in reinforcement of inter-specific
mating barriers => reduction in rate of hybridization
○ Reproductive barriers will be stronger in sympatric
populations, versus in allopatric populations
→ Pied and collared flycatcher males are more similar in
allopatric populations than sympatric populations
2. Pollinator-Mediated Prezygotic Isolation
● 2 closely related species of monkey flower have different
pollinators due to different flower shape & color
→ Pink Mimulus lewisii attract bumblebees
→ Red Mimulus cardinalis attract hummingbirds
○ In areas where they coexist, reproductive isolation is
maintained by this difference in pollinators
3. Adaptive Radiation on Island Chains
● Sequence of allopatric speciation events
○ ex) Drosophila fruit fly speciation in Hawaii
○ → Genetic data indicates that the 800+ species of
Drosophila-like flies on the islands are descended from
one invasion
XII. Speciation (pt. 2), Continental Drift, Fossils
A. Sympatric Speciation
1. Ecological Speciation
● Apple-maggot fly Rhagoletis pomonella occurs in eastern US
○ Natural larval host - hawthorn berries
○ 200 yrs ago - started to colonize introduced apple trees
● Host “races” developed b/c...
○ Two fruits ripen at different times
○ Flies are attracted to their own host fruits => positive
assortative mating by niche preference
○ Strong disruptive selection against hybrids (not adapted to
either host fruit)
2. Chromosomal “Instant” Speciation
● Autopolyploids: an increase in ploidy w/in a species
● Allopolyploids: arise by hybridization b/w species
● Polyploidy relatively common in agricultural crops
○ Often infertile (and still diploids) at first
○ Achieve chromosomal balance and fertility later by
diploidization of both chromosome sets
● Spartina anglica - fertile allotetraploid, a 4n hybrid of 2 different species
○ 1870 - originated as sterile hybrid (S. townsendii) that
reproduced asexually
→ Cross b/w the European native cordgrass S. maritima
and the introduced American species S. alterniflora
○ Common in so. England
○ Now an invasive species in western US marshes
● Interspecific hybridization in animals typically results in an asexual (i.e. parthenogenetic “unfertilized”) species
○ All-female whiptail lizard, Cnemidophorus neomexicanus
species, was formed by hybridization of C. inornatus and
C. tigris - two sexual species that have males
B. Allopatric Speciation
1. Vicariance & Continental Drift
● Vicariance: splitting of a species range by a geographical barrier ○ 9-13 MYA: 15 pairs of sibling species of Alpheus snapping
shrimp were separated by Isthmus of Panama, which
separates Atlantic and Pacific oceans
● Creation of the isthmus caused by slow movement of the
continents, joining North & South America
● Himalayas formed where India joined Eurasia
2. Plate Tectonics & Biogeography
● Earth’s crust is divided into plates
● Various forces like seafloor spreading cause plates to move ○ Subduction zone: where one plate goes under another;
location of volcanoes and earthquakes
● 65 mil yrs ago (MYA) - So. America, Antarctica & Australia all connected
○ Proof: Southern beech species are found in So. America
and Australia
→ Fossil impression of a southern beech leaf found in
Antarctica
3. Fossils
● Fossils record history of life on Earth
○ Formed in sedimentary rock: rock formed by deposition of
material over time
● How do we know age of fossils?
○ Stratigraphy: use layers (“strata”) of sedimentary and
volcanic rock
● Biostratigraphy: links strata in different areas based on
assemblage of fossils
○ This approach used in Darwin’s time
○ Seemingly no possible way to determine fossil age, but
possible to define geological time scale based on relative
ages
XIII. Dating the Geological Record, Mass Extinctions
A. Dating the Fossil Record
1. Radiometric Dating
● Obtaining actual age of fossils was impossible before early 1900s
→ Methods for dating old rock do not work on sedimentary rock;
instead age is based on presence of volcanic rock b/w
sedimentary strata
● Radiometric dating: the ratio of a radioactive isotope to its
breakdown product
○ The ratio of isotope/product at time of formation must be
known
● Some important radiometric methods:
* First 2 methods can indirectly date fossils in sed. rock located
right below a volcanic layer *
○ Uranium-lead: U235 → Pb207 and U238 → Pb206for volcanic
(igneous) rock
→ Half-life = 700my and 4500 my, respectively
→ Good for rock >1my before present (bp)
○ Potassium-argon: K40 → Ar40 generally in volcanic rock
→ Half-life = 1300my
→ Good for rocks >100,000yrs
○ Carbon dating: C14, created in upper atmosphere
→ Half-life = 5,730yrs
→ Can date organic matter back to 75,000ybp
B. Timescale of Geological Record
1. Organic Compounds on Early Earth
● Earth formed ~4.6 billion yrs ago (bya)
● Bombardment of earth by rocks and ice likely vaporized water and
prevented formation of seas until about 4.2-3.9bya
● Earth’s early atmosphere probably contained water vapor and
chemicals released from volcanoes: N, NO, CO2, CH4, NH3, H,
H2S
2. Self-Replicating RNA, Protocells, Natural Selection
● First genetic material was probably RNA, not DNA, b/c RNA
molecules called ribozymes have been found to catalyze RNA
replication
● Protocells: fluid-filled vesicles w/ membrane-like structure
○ Ultimately need protocells w/ RNA that code for functions
to be favored by selection
3. Earliest Evidence of Life
● Stromatolites: sedimentary particles that have been trapped,
bound, and cemented by the biofilms of microorganisms
○ Modern stromatolites found worldwide in lakes and
lagoons w/ high salinity
● Most atmospheric O2 created from photosynthesis of
cyanobacteria
● Oldest eukaryotic fossils date back to 2.1 bil. yrs
○ Eukaryotic cells have nuclear envelope, mitochondria, ER,
and a cytoskeleton
● Endosymbiont Theory: proposes that mitochondria and
chloroplasts were originally small prokaryotes living inside a larger
host cell
○ Endosymbiont: an organism that lives w/in host cell, and
critical support is given by DNA
● Wave of diversification occurred after complex body plans
appeared ~640mya, setting up “Cambrian explosion” about 100
mil. yrs later
4. Mass Extinctions
● # of species is a balance b/w formation of new species and
extinction of existing species
● Extinction typically caused by changes in a species’ environment
○ Average duration of a species = 1-15 million yrs
● Mass extinction: dramatic increase in rate of extinction as a result
of some disruptive global environmental change
● In each of the 5 mass extinction events, >50% of earth’s species
went extinct
○ Permian extinction: occurred 251mya, lasted <5 mil. yrs,
96% of marine animals and 70% of land vertebrates went
extinct
○ Cretaceous extinction: occurred 65.5mya, 75% of all
species went extinct due to asteroid impact that wiped out
dinosaurs; mammals & birds emerged as dominant land
vertebrates afterward
XIV. Radiation of Birds and Mammals, Phylogenetic Trees A. Extinctions
1. Consequences of Mass Extinctions
● Extinction events mark boundaries b/w eras of Phanerozoic eon, Paleozoic-Mesozoic, and Mesozoic-Cenozoic, and the ends of the Ordovician and Triassic periods
● Recovery of diversity after an extinction can take from 5-100 million yrs
● Mass extinctions allow for adaptive radiation
○ Permian period: reptile-like synapsids dominated
(300-250mya)
→ After Permian extinction: dinosaurs radiated in Triassic
period
→ Cretaceous extinction wiped out dinosaurs, birds
underwent major radiation
2. Origins of Birds
● Birds and predatory dinos share similar features
○ Skeleton: breast bone, collarbone, 4-toed foot supported
by 3 toes
○ Feathers: found in many dinos
○ Collagen: short amino acid sequences from T. rex match
closely w/ chickens
○ Egg shell microstructure
● Mammals existed when dinos were around, but radiated after Cretaceous extinction
○ Earliest known placental mammal dated to 160mybp
(middle of Mesozoic era & before Cretaceous extinction)
B. Phylogeny & Taxonomy
1. Taxonomy
● The division and naming of organisms
● 18th century - Linnaeus published system of taxonomy based on resemblances
○ Binomial names: Genus + species (e.g. Homo sapiens)
○ Hierarchical classification: domain, kingdom, phylum,
class, order, family, genus, species (from broad to specific)
2. Phylogeny
● Systematists can show evolutionary relationships by making phylogenetic trees
○ Branch lengths in phylogenetic trees can:
→ be arbitrary - mean nothing
→ represent time
→ represent genetic divergence
● Phylogenies are inferred from morphological and molecular data ○ Organisms w/ similar features or DNA are more closely
related than organisms w/ different features or sequences
● When comparing an organism w/ its ancestor, it will have both
shared and derived characteristics
○ Cladistics groups organisms based on shared derived
traits
○ Maximum parsimony uses a cladistic approach to create a
tree w/ the fewest evolutionary divergences possible
○ Maximum likelihood uses DNA sequence data to create a
tree
→ Bayesian method: computer-intensive approach based
on Bayes Theorem to create probability distribution of the
trees that best explain the data
XV. Phylogenetic Trees, Tree of Life, Human Ancestry
A. Tree of Life
1. Evidence of a Single Origin
● Genetic code is nearly identical for all known life forms
● All cells use ribosomes (rRNA) and tRNA to synthesize proteins
from polypeptide chains
● Biochemical processes only use L-amino acids
● Tree of Life can be rearranged as a molecular phylogeny using
very slowly evolving rRNA genes
○ Current position of the root suggests eukaryotes are more
similar to archaea than bacteria, or evolved from archaea
2. Primates
● Primates originated around 55mya, during early radiation of
mammals
○ Split b/w New World and Old World anthropoids occurred
35-40mya, a while after So. America separated from Africa
● Apes (Hominoids) originated 25mya
○ While most monkeys have tails, apes do not
○ Chimp-Human split occurred 7mya
● It is predicted that all modern humans have African ancestry
○ 1.8mya - Homo erectus was first ancestral humans to
leave Africa for Asia
○ Homo neanderthalensis have a Eurasian origin (350k-30k
mya)
→ Distinct Asian group of “Denisovans” based on DNA
○ Homo sapiens have African origin (200-300k yrs ago),
spread to Eurasia 100,000 yrs ago