Lecture 6: Ecology and Evolution

Reading:  None.

Natural Selection does not have a goal.

Differential survival and reproduction simply occurs among the individuals in a given population. The outcome of selection depends on the specific environment at a given place and time, and the phenotypes present in a given population.

Natural selection is not the only means by which evolution can occur. Potential causes for evolutionary change are:

• Natural Selection
• Mutation (random process, but the source
  of all variation)
• Drift (random process)
• Migration (random process)

Natural selection is the principal guiding force in evolution.

• Altering the direction of selection alters the direction
  of change.
• Causes of mutation are independent of the causes for
  selection.
      Mutation does not guide change.
      Mutations do not respond to need.
• Only the causes for selection remain consistently
  directional for long time periods.

What is the purpose of life?

	Is there a singular goal? No
	Is there a singular consequence? Yes

If evolution by natural selection is the major force molding phenotypes then:

	All organisms must be striving to do one thing, maximize their genetic representation in future generations.
	There is no single best means of achieving this, but in general, organisms are selfish.

Natural Selection and Selfish Phenotypes

        "Natural Selection cannot possibly produce any modification in a species exclusively for the good of another species; though throughout nature one species incessantly takes advantage of, and profits by, the structures of others... If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced through natural selection." (Darwin’s Challenge)

Charles Darwin, 1859, The Origin of Species

Yet, organisms do not always seem to behave selfishly!

Swollen thorn acacia plants, Acacia collinsii and A. cornigera, in Central America, and acacia ants, Pseudomyrmex spp., found in association with these plants, have some unusual characteristics.

Swollen thorn acacias are trees and shrubs that:

	produce and hold leaves year-round, unlike closely related plants that are deciduous and drop leaves in the dry season (leaves provide year-round shelter for acacia ants) (see Ricklefs, 1996, p 415, Fig. 18.14)
	produce swollen thorns on stems, much larger than those found on other Acacia species (used as nesting sites by acacia ants)
	produce Beltian bodies, leaf tip protein and lipid structures which seem to serve no function for the plant (collected by acacia ants and fed to ant larvae)
	produce extrafloral nectaries year-round, nectar (carbohydrate and water) production at the base of leaf petioles, which are not important in attracting pollinators (nectar is used as food by the acacia ants) (see Ricklefs, 1996, p 416, Fig. 18.15)

These structures are produced by the swollen thorn acacia apparently for the exclusive good of the acacia ants.

Figure 18.14

Figure 18.15

 

Acacia ants live their entire lives associated with a given swollen thorn acacia plant. The ants provide the swollen thorn acacia plant with:

	competitor removal, seedlings that sprout under the canopy of a given acacia plant are cut-down by the ants from that acacia and branches of other plants that touch the acacia plant are cut-back
	herbivore protection, unlike other Pseudomyrmex species which are diurnal, acacia ants are active 24 hours and prevent and discourage herbivory (by both insects and vertebrates) on the acacia plant they occupy

This is an example of a mutualism. The swollen thorn acacia survive better when the ants are present than when the ants are absent. The swollen thorn acacia ants do not live anywhere but in acacia thorns. Each species is ultimately behaving selfishly because they benefit from the aid they give to the other species.

Are all organisms biologically selfish? Does this general consequence extend to the molding of individual phenotypes? Are individual organisms selfish?

How do individuals reproduce?

An individual may influence allele frequencies in future generations by:

Production of offspring by that individual (direct reproduction).

Influencing the survival and reproduction of individuals
carrying genes identical by descent (indirect reproduction).
These influences are called Inclusive Fitness Effects (W.D.
Hamilton, 1964) or Kin Selection.

Examples of Inclusive Fitness Effects:

	Parental care
	Helpers at the nest, cooperative breeding (birds and mammals)
	Eusocial insects (Hymenoptera, Isoptera)
	Aposematic coloration

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