Population Growth
Population growth

Population: a group of individuals of the same species in the same
place

Populations may display two types of growth:
o
Exponential: all or a fixed proportion of individuals can
reproduce indefinitely
o
Logistic
Exponential growth

Consider a case where we start with a single cell, and all cells can
reproduce

Exponential growth leads to rapid population changes
Exponential growth curve
N
time

Clearly not linear
 an exponential growth rate
Exponential growth formula
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Growth Rate = change in size divided by change in time (dN/dT)

Growth rate is slope of curve
o
dN/dT = rN
o
N = population size
o
r = the growth constant OR the increase in size per
generation
o
r is calculated as the difference in the birth rate and the
death rate
Exponential growth: by the numbers

Considering what is happening numerically
Time
0
1
2
3
4
5
N
1
2
4
8
16
32
dN/dt
1
2
4
8
16

dN/dt increases with increasing N

dN/dt is proportional to N (dN/dt = rN)
Implications

Even with lower r, dN/dt can be actually larger because of the
higher N
o
ex: r = 0.4, N = 1000 vs. r= 0.3, N = 1500
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 Fall '07
 Mason
 Genetics, Population Ecology, Evolution

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