Studying organisms in their environment
organism
population
community
ecosystem
Bio
biosphere
biosphere
Populations
Occupy niches in
ecosystems
Grow exponentially or
logistically (carrying
capacity)
A group of the same kind
of organisms living in the
same place at the same
time.
EVOLVE! Allele
frequencies can be
described by the
Hardy-Weinberg
Equilibrium:
p2 + 2pq + q2 = 1
Will remain the same if no
migration, no selection, random
mating. No evolution
Measured by
random sampling
Not realistic but, offers a
standard to compare rates of
evolution to – Null Hypothesis
Life takes place in populations
• Population
– group of individuals of same species in same
area at same time
 rely on same
resources
 interact
 interbreed
Population Ecology: What factors affect a population?
Population Size
• Changes to
population size
– adding & removing
individuals from a
population
•
•
•
•
birth
death
immigration
emigration
Population growth rates
• Factors affecting population growth rate
– sex ratio
• how many females vs. males?
– generation time
• at what age do females reproduce?
– age structure
• how females at reproductive age in cohort?
Population growth
change in population = births – deaths
Exponential model (ideal conditions)
dN = riN
growth increasing at constant rate
dt
N
r
ri
t
d
= # of individuals
= rate of growth
= intrinsic rate
= time
= rate of change
intrinsic rate =
maximum rate of growth
every pair has 4
offspring
every pair has 3
offspring
Exponential growth rate
• Characteristic of populations without
limiting factors
– introduced to a new environment or rebounding from a
catastrophe
Whooping crane
coming back from near extinction
African elephant
protected from hunting
Introduced species
• Non-native species
– transplanted populations grow
exponentially in new area
– out-compete native species
• loss of natural controls
• lack of predators, parasites, competitors
– reduce diversity
– examples
•
•
•
•
African honeybee
gypsy moth
zebra mussel
purple loosestrife
gypsy moth
kudzu
Zebra musselssel
~2 months


ecological & economic damage

reduces diversity
loss of food & nesting sites for
animals
economic damage
Purple loosestrife
1968
1978


reduces diversity
loss of food & nesting sites for
animals
Regulation of population size
marking territory
= competition
• Limiting factors
– density dependent
• competition: food, mates, nesting
sites
• predators, parasites, pathogens
– density independent
• abiotic factors
– sunlight (energy)
– temperature
– rainfall
competition for nesting sites
swarming locusts
Logistic rate of growth
• Can populations continue to grow
exponentially? Of course not!
no natural controls
K=
carrying
capacity
What happens as
N approaches K?
effect of
natural controls
– varies with
changes in
resources
What’s going
on with the
plankton?
10
8
6
4
2
0
1915
1925
1935
1945
Time (years)
500
Number of cladocerans
(per 200 ml)
• Maximum
population size
that environment
can support with
no degradation of
habitat
Number of breeding male
fur seals (thousands)
Carrying capacity
400
300
200
100
0
0
10
20
30
40
Time (days)
50
60
Population of…
China: 1.3 billion
India: 1.1 billion
Human population growth
Doubling times
250m  500m = y ()
500m  1b = y ()
1b  2b = 80y (1850–1930)
2b  4b = 75y (1930–1975)
What factors have contributed to this
exponential growth pattern?
Is the human
population reaching
carrying capacity?
adding 82 million/year
~ 200,000 per day!
20056 billion
Significant advances
in medicine through
science and technology
Industrial Revolution
Bubonic plague "Black Death"
1650500 million
Populations evolve
• Natural selection acts on individuals
– differential survival
• “survival of the fittest”
– differential reproductive success
• who bears more offspring
• Populations evolve
– genetic makeup of
population changes
over time
– favorable traits
(greater fitness)
become more common
Presence of lactate dehydrogenase
Mummichog
5 Agents of evolutionary change
Mutation
Gene Flow
Genetic Drift
Non-random mating
Selection
Variation & natural selection
• Variation is the raw material for natural
selection
– there have to be differences within population
– some individuals must be more fit than others
Where does Variation come from?
– random changes to DNA
Wet year
Beak depth
• Mutation
• errors in mitosis & meiosis
• environmental damage
– mixing of alleles
• recombination of alleles
– new arrangements in every offspring
• new combinations = new phenotypes
– spreads variation
• offspring inherit traits from parent
Dry year
1977
Dry year
1980
1982
1984
11
Beak depth of
offspring (mm)
• Sex
Dry year
10
9
8
Medium ground finch
8
9
10
11
Mean beak depth of parents (mm)
1. Mutation & Variation
• Mutation creates variation
– new mutations are constantly appearing
• Mutation changes DNA sequence
– changes amino acid sequence?
– changes protein?
• changes structure?
• changes function?
– changes in protein may
change phenotype &
therefore change fitness
2. Gene Flow
• Movement of individuals &
alleles in & out of populations
– seed & pollen distribution by
wind & insect
– migration of animals
• sub-populations may have
different allele frequencies
• causes genetic mixing
across regions
• reduce differences
between populations