If success in passing on genes determines the evolution of a population, why shouldn't all organisms grow rapidly, reproduce shortly after birth, produce many large offspring, reproduce frequently, and take extensive care of young?

Beetle JPEG
 
 
 
 

The key to understanding 'life history patterns' is to understand the following diagram:
 
 

To express this mathematically (as stated in the previous lecture):

For a population that has existed over many generation,

mean R0 = · lama = 1

Timing of Reproduction
 

If a dog can produce between ~2-10 pups/liter, why hasn't greater liter size been selected for?
 

Does reproduction at the present time have future costs in terms of survival and future reproduction?
 
 

Profits of immediate reproductive investment are at the cost of prospects of future reproduction (which depends on survival)

     http://fig.cox.miami.edu/~schultz/fall02/bil235/6.html
 
 
  Roughly a trade off between mx (immediate reproductive investment) and Vx (prospects of future reproduction which depends on survival).
 
 

Organism display a continuum of strategies, defined along to extremes:

semelparity - one massive reproductive effort

Many organisms are semelparous, e.g. mayfly larvae, salmon, and the annual plant Cenchrus biflorus iteroparity - few offspring in a series of reproductive events What kind of environmental conditions might select for semelparity?

Parental investment

 
 

Can a plant allocate energy to parental investment?

Other examples:

 
 
 

For size of offspring, the trade off may be brood size:

 
 

http://www.umsl.edu/~biology/Bio220/LifehistLecture/lifehistory.html
 
 

Is a trait selected for if it improves the efficiency at which the genes for that trait are passed on to the next generation?
 
 
 
 

Some examples parental investment in relationship to environmental conditions:
 

David Lack (1948) examined how many eggs (Clutch size) should  starlings lay based on future success of offspring.

 
   

Clutch size	No. of nests w/  that clutch size	% surviving  after 3 mo.
1	65	none
2	328	1.8
3	1278	2.0
4	3956	2.1
5	6175	2.1
6	3156	1.7
7	651	1.5
8	120	0.8
9, 10	28	none

Because it physically possible for Swiss Starlings to produce clutchs of 10, why don't all female do this, thereby passing on as many genes as possible?

Why is the percent surviving low for the lowest clutch sizes?

Which clutch size has the most successful number of offspring? Which clutch size is most commonly laid?  Does this support the concept of life history patterns as 'the optimization of traits that maximize the passing of genes on to future generations'?

  Little parental care by species in open ocean where parental care would be difficult Mola mola, the ocean sunfish drift about in the open ocean and produce 200 million eggs/brood.
Would parental care be difficult in such an environment?

 
 

In highly unpredictable environments, what parental allocations of energy might increases the success of their offspring?

"Fugitive species"

 
 
 
 
 
 
 
 
 
   

 

Both time of first reproduction and size and number of offspring can be affected by size, especially in animals with indeterminant growth.

Gizzard shad:

Age	eggs/brood	% spawning
2	59,000	15%
3	215,000	80%

 
 

 
 

Delay of first reproduction in Albatroses attributed to time needed to learn sufficient foraging skills, feeding 10-100 miles offshore for up to 1 week.

A long delay in maturation must be associated with high survivorship:

 
 
 
 
 
 
 
 
 
 
 
 
 
 

The concept of r-K Selection:
Two extremes for reproductive strategies dealing with environmental predictabilty in which species lie along a continuum.
 

r-selection	K-selection
<-----------------	------------------->

 
 
 
 

Comparison of environments:

	r-selection	K-selection
variability	variable, unpredictable	stable, predictable
populations	uncrowded, variable	crowded
mortality	density-independent	density-dependent
resources	abundant	scarce
competition	lax or variable	keen and constant

Comparison of life history traits selected for in the above environments

	r-selection	K-selection
body size	small	large
competitive ability	low	keen
maturation	early	delayed
offspring	many small	few larger
parental care	minimal	greater
number of  reproductions	semelparity	iteroparity
length of life	short	long

 

Is this an example of a fundamental law of ecology?
 

Are fugitive species r or K?
Are salmon r or K?
 
 

Are Asiatic clams r or K?

Within an environment, are r and K species mutually exclusive?
 
 
 
 
 

Environmental predictabilty may not be a simple dichotomy.
Stearns (1976) identifies 6 types of environmental variability, combinations of:

• regular fluctuations (cycles)
• unpredictability
• generation time relative to period of fluctuations

e.g. predictable environmental in which period of cycles is short compared with generation time.  Such an environment might select for sychronizing breeding to optimal times or to releasing young to swamp predators

 
  Cicadas: insects whose nymphs live underground living off root juices emerge every 17 yrs (3 spp.) or 13 yrs (southern sp.) into adults to breed and lay eggs. Long life span is greater than life span of their predators, but Why 13 and 17?

Bottom line: selection will favor different life history "packages".  Reproductive-survival traits are not just a hodge-podge and packages are related to environmental predictability.