7. Predators & sexual selection 

Of course, which traits are successful and which are not are determined in large part by the organism's lifestyle and its ecological niche – basically how and where it makes its living and how it reproduces. 

Each species is part of the environment of other species. Changes in one species can have dramatic impacts on the evolution of others.  


As the Red Queen said to Alice ... "Here, you see, it takes all the running you can do to keep in the same place" -Though the Looking Glass by Lewis Carroll


An obvious example is the interrelationship between predators, pathogens and prey. Which organisms survive to reproduce will be determined in large part by their ability to avoid their predators or recover from infection. 

Certain traits may make the prey less likely to avoid, elude, repulse, discourage or escape a predator's attack. 

As the prey population evolves in response to a specific predator, these changes will impact the predator – they will also have to adapt. 

The same type of evolutionary interplay occurs between pathogens and hosts, although the effects are more difficult to read in the fossil record.

way cool movie of the odrovician ocean here
Over time, as species appear and disappear, the selective landscape changes and populations of organisms are subject to different selective pressures. 

Sexual selectionThere is another type of selection, known as sexual selection. The importance of sexual selection was first recognized by Darwin in his book "The Descent of Man and Selection in Relation to Sex". 

Sexual organisms often do not mate randomly, and generally the larger parental investment in the production and rearing of offspring, the less random is mating [the evolution of love link].

In many types of organisms, and most vertebrates, females (the sex that produces the larger gametes) either choose their mating partners or males (the sex that produces the smaller gametes) contend with one another to control access to females. Under some conditions, females also compete with other females for access to males, particularly when male partnering makes a significant contribution to reproductive success.

Why is this? From the female's perspective, generating offspring involves a serious commitment of resources

Consider human pregnancy as an example.  A female can have at most one pregnancy a year, while a male could (in theory at least) make a rather large number of females pregnant.  

Moreover, the biological cost of generating offspring is generally substantially greater for the female, compared to the male. 

So, if the female is going to have offspring, it would be in their evolutionary interest that those offspring survive and reproduce. 

The more robust the mate, the more likely that the offspring will be genetically "well endowed" and likely to survive and reproduce, that is, the greater the return on the female's substantial investment.  

At the same time, the survival of offspring can be increased by the continued presence of the male, to protect the female and its (and their) offspring.

In many species the male's "investment" in the next generation is much less substantial.

Males can often maximize their reproductive success by mating with multiple females, or in some species, by helping the female to successfully produce and raise offspring.

Both males and females "know" these rules and may attempt to deceive each another.

A male may appear more robust than he actually is, a female may mate with multiple males in order to select the most robust sperm (sperm selection), or may mate with one male and bond with another, so that the second can help with raising the offspring.

In turn, males may respond by attempting to monopolize the female. This, of course, will require the male to spend energy in "guarding".

There are many variations to be found in the natural world and a full discussion is beyond the scope of this course. 


One of the most robust and reliable findings in the scientific literature on interpersonal attraction is the overwhelming role played by physical attractiveness in defining the ideal romantic partner (Hatfield & Sprecher, 1986; Jackson, 1992).

Both men and women express marked preference for an attractive partner in a noncommitted short-term (casual, one night stand) relationship (Buss & Schmitt, 1993).

For committed long-term relationships, females appear to be willing to relax their demand for a partner's attractiveness, especially for males with high social status or good financial prospects (for a review see Buss, 1999).

Males also look for various personality qualities (kindness, understanding, good parental skills) in their search for long-term mating partners, but unlike females, they assign disproportionately greater importance to attractiveness compared to other personal qualities (Buss, 1999).

The paramount importance of attractiveness in males' mate choices has been recently demonstrated by using the distinction between necessities (i.e., essential needs, such as food and shelter) and luxuries (i.e., objects that are sought after essential needs have been satisfied, such as a yacht or expensive car) made by economists.

Using this method, Li et al., (2002) reported that males treat female attractiveness as a necessity in romantic relationships; given a limited "mating budget," males allocate the largest proportion of their budget to physical attractiveness rather than to other attributes such as an exciting personality, liveliness, and sense of humor.

- from Mating strategies for young women by Devendra Singh (2004).

Sexual selection can lead to seemingly maladaptive traits. Consider the peacock's tail.  It takes a continuous supply of energy to generate a large, symmetrical and colorful tail.  It can also make the male more vulnerable to predators.

On the other hand, the presence of a gaudy tail implies that its owner is able to successfully compete with others for resources, and so is likely to be genetically "well endowed".

Once females notice tail size as a marker of success, they select as mates those males with the largest and most attractive tails.

This can lead to runaway' selection, the more prominent the male's tail the more likely he will find a mate, even though it may also have negative effects.  For example, the development of extremely large antlers, associated with male dominance and mate accessibility, as occurred in Megaloceros giganteous, can inhibit their ability to move through heavily wooded areas.

As their environment changed, their behavior and phenotypes could act to limit their ability to adapt fast enough to avoid extinction.

In the end, as with all adaptations, there is a balance between the positive effects, which lead to increased reproductive success (such as access to mates) and their negative effects, which can influence survival. 

The optimal form of a trait may not be stable over time, particularly if the environment is changing.


rhinocerus beetlepeacockFrench King

Questions to answer

  1. How does the adaptation of a prey species to the presence of a predator/pathogen influence the evolution of the predator/pathogen? 
  2. What does it mean to cheat, in terms of sexual selection - is the "cheating" organism actually being consciously deceptive?
  3. What types of "cheating" behavior do females use with males; or males with females?
  4. What are the costs involved when a male tries to monopolize one or more females? what are the advantages?
  5. What limits "runaway selection"?
Questions to ponder


revised 31-Aug-2012