Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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I.  Why Evolutionary Tradeoffs Exist

The first law of thermodynamics tells us that energy can neither be created nor destroyed.  Thus, the amount of energy and resources available for a population is limited.

Evolution will often select between different strategies of how these resources will be used.

Some species may invest more of their limited resources into reproduction, increasing the number of offspring (and their intrinsic rate of population growth -- which is termed 'r' as we found out last lecture)

Other species may invest more of their limited resources into their own growth and health, making them better competitors for the resources.  This will allow more individuals to live at a given resource level, which is termed 'K' as we also found out last lecture.

Because resources are limited, it will not be possible for a species to do both of these at the same time.  Every resource which is used to maximize reproduction is not available for increased competitive ability, and vice-versa.

Some species (called generalists) may attempt to be both good reproducers and competitors at the same time, however, they will never reproduce as well as those that have invested most of their resources into reproduction, or will be as competitive as those  that have invested their resources into competition.

However species cannot put all their resources into just one of these categories.

To be able to reproduce, species must be able to compete successfully for some resources.  If all resources are put into reproduction, the species will not be able to get the resources necessary for reproduction, and will die before they can reproduce.

Also, if a species invests all of resources into competitive ability, and none into reproduction, the species will become extinct when that generation eventually dies from old age.

Thus,  a wide range of life-history options exist from those species which invest the least possible into their own competitive ability and the most into their reproductive potential (r-selected) to generalists (which try to be somewhat good reproducers and competitors) to those species which invest the least possible into reproduction while maximizing their competitive ability (K-selected).

II.  Characteristics of  r-selected species.

How can species increase their reproductive ability given limited  resources?

rapid growth rate                           small size of offspring
small body size                              many offspring per litter
early age to sexual maturity            little parental care
reproducing once in a lifetime         good dispersal ability
short life span

These factors lead to populations which:

(1) Have variable and unpredictable birth and mortality rates

(2) Have varying population sizes, usually below carrying capacity

III.  Characteristics of K-selected species.

How can species increase their competitive ability given limited resources?

slow growth rate                                larger offspring size
large body size                                   few offspring per litter
later age to sexual maturity                 much parental care
reproducing many times during life      poorer dispersal ability
long life span

These factors lead to populations which:

(1) Have more constant and predictable birth and mortality rates

(2) Have relatively constant population sizes at or near carrying capacity.

IV.  It is important to remember that these lists are ideal.

In the real world, most species will show some traits for each.  This can be easily seen in mammals.  Some (like mice) are clearly r-selected, while others (like elephants) are clearly K-selected.  However, some (like humans) show traits for both.  Look back over the above lists and consider which of these traits seem true for us.

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Created 2 September 2011, Last Update 02 September 2011

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