GB 23- Population Genetics

Population:

group of potentially sexually interbreeding organisms of same species living in the same area at the same time - share gene pool

ecology

study of interactions between living organisms and physical environment

Individuals within populations...

• Rely on (and compete for) same resources

• Influenced by similar environmental factors

Populations Ecology:

considers number of individuals in an area and population dynamics

Population Dynamics:

study of change in populations

Population Size

• Number of individuals

• Scale matters

Population Density

• Number of individuals per unit area or volume at a given time

• Scale matters!

Change =

(births + immigration) - (deaths + emigration)

Change in pop over time

• Consider just births (B) and deaths (D)

• N= pop size

• t= time

• change in N/ change in t = B - D

b:

per capita birth rate; # offspring produced per unit time by average individual in pop

B = bN

expected number of births per year in pop size N

m

• per capita death rate (m for mortality); likelihood of death per unit time by average individual in pop

• D = mN

AN/At = B - D, so...

• AN/At = bN- mN

• AN/At = N (b - m)

• Simplify -> Per capita rate of increase (r): r= b - m, so...

• AN/At = rN

if r<0 —>

population is shrinking

r>0

population growing

r=0

no net change in population (dynamic equilibrium)

instantaneous population growth

• change to T =0

• can divide change in N by change in T

Instantaneous Growth Rate

• dN/dT = r inst N

• r inst : instantaneous rate of increase

• r inst x population size (N) = instantaneous change in population size

r max

• maximum rate at which population could increase under ideal conditions

• Abundant resources

• Low density

• Microorganisms have highest r max

• Large organisms tend to have low r max

Exponential Growth Model

• Every member of population reproduces at physiological capacity

• Population growing at r max

• r inst = r max

• dN/t = r inst N

• J-shaped curve

• not realistic for long-term population growth due to resource limitations.

• Population is always reproducing at R max (possible max rate of reproduction) 

Logistic Growth Model

• more realistic growth model, it goes up then it flattens out 

• carrying capacity (K) causes it to flatten out 

• If line is below carrying capacity it can grow if its above the carrying capacity its gonna shrink because thats the sustainable limit

• Considers environment resistance

• Growth rate slows as populations reaches limit

• Sigmoid or S-shaped curve

• Starts as exponential

• dN/dt = r inst N (K - N)/K)

Carrying Capacity (K)

• Largest population that can be maintained for an indefinite period by a particular environment

• Assumes no change in environment

• Most populations fluctuate around some mean

• Pop < K, pop grows

• Pop > K, pop shrinks

• Populations tend to fluctuate around K

Life history

• Ways in which organisms maximize survival and reproduction

• MANY different strategies

• It's all about trade-offs

• 3 main variables:

• Age at first reproduction

• Frequency of reproduction

• # offspring per reproductive episode

• Natural selection determines optimum

Semelparous

• only one big reproductive event and that's it 

• 1 large reproductive effort

• Insects, plants, fish ill. Favored if:

  1. Low offspring survival rate

  2. Highly variable environment

  3. High adult mortality

Iteroparous

• many reproductive events, you reproduce many times in your life 

• Most vertebrates, many plants in.

• Favored if:

  1. Stable environment

  2. Low adult mortality

  3. High competition

Reproduce early

1. E towards offspring instead of own growth

2. May reduce survival

Reproduce late

1. E towards growth -> higher survival

2. Less E towards offspring

r-selection

Strategy: produce many offspring fast

• High population growth rate, close to r max

• Tendency towards:

1. Short life span

2. Early maturation

3. Large broods

4. Little or no parental care

5. Small body size

• Variable and/or temporary environments

• Low probability of long-term survival

• Only works with a high r

K-selection

• Strategy: few offspring, high parental investment

• Population size close to K most of the time

• Tendency towards:

1. Long life

2. Slow development

3. Low reproductive rate (low r)

4. High parental care

5. Large body size

• Favored in stable environments

• Higher probability of long-term survival

• Maintains relatively stable N close to K