Evolution  Lecture 22

Chapter 14

Topics for today:

  1. What factors promote cooperation among unrelated individuals?
    1. Evolutionary stable strategies (ESS)
  2. What is sexual selection?
  3. What factors drive selection on males?
  4. What factors drive female preference?

 

What allows cooperation among unrelated individuals?

Best strategy depends upon what others are doing

Evolutionary stable strategy (ESS)

Fig. 14.2

Payoff matrix

C = fitness cost

V = fitness value

Box 14A

Fig. 14.2

 

Under what conditions is each strategy an ESS?

w = fitness

wo = baseline fitness

p = frequency of Doves

1-p = frequency of Hawks

 

How are intentions assessed?

Honest signals

·        Body size

·        Antler size

·        Call volume

Dishonest signals

·        Claw size in fiddler crabs

o       Original claw is strong and honest signal

o       Replacement claw is same size but weaker

o       Males are duped by the replacement

Fig. 14.3

 

Why are the males fighting?

  • Members of the same sex compete for territories and mates

Sexual selection (Darwin)

  • Differences in reproduction resulting from differences in the ability to obtain mates

 

Why do males compete more than females?

Males produce many small gametes (sperm)

  • Mate with many females
  • Little negative consequence to his fitness if one mate has low fitness

Females produce few large gamete (eggs)

  • Fewer matings
  • Large negative consequence to her fitness if her single mate has low fitness

Fundamental drivers of sexual selection

o       Males compete for limited reproductive opportunities (eggs)

o       Females are choosey

 

Males vary in reproductive success

  • Previously thought that male seals dominate females in harem
  • Only 23.5% of pups are from the closest male
  • New study shows females move to find a good mate (35 m)
  • Males rarely move more than a body length (2 m)

 

Females compete more when gender roles are reversed

  • Males care for young
  • Females can produce more offspring than males can care for

Examples

1.      Red phalarope

2.      Seahorses and pipefish

 

What drives sexual selection in males?

1.      Contests produce selection on traits that improve success in confrontations

·        Large size

·        Strength

·        Weapons

·        Threat signals

·        Dove behavior around superior males

Examples

·        Canines of a tropical Asian pig

·        Antlers of Red deer

Fig. 14.6

Directional selection can cause trait exaggeration

·        Ultimately balanced by the other costs

·        More exaggerated than if there was selection only from other ecological factors (no sexual selection)

·        Lack of these traits in females suggests that they are ecologically disadvantageous

Example

·        Túngara frog

Fig. 14.8

 

2.      Mate preference by the opposite sex selects for attractive and stimulatory features

o       Larger, more intense, more exaggerated features

o       Attractive territory

 

Sexual selection on males

  • Sperm competition selects for mechanisms of assuring paternity
  • Defend female from other males after mating
  • Clasp females after mating until no longer receptive
  • Males insert copulatory plug
  • Seminal fluid itself reduces female attractiveness

 

3.  Sperm competition selects for mechanisms of assuring paternity

·        Produce more sperm to increase their chances of fertilization when there are multiple matings

Fig. 11.20

Sperm competition selects for mechanisms of assuring paternity

·        Produce more sperm to increase their chances of fertilization when there are multiple matings

·        Genitalia adapted to remove sperm of previous mates

Fig. 14.7

 

What drives female preferences?

Traits may be harmful to male but selected for anyway because:

  1.   Direct benefits to female

·        Nourishment

·        Superior territory

·        Better parental care

Example

·        House finch color varies

·        Bright red males bring food to nestlings at a higher rate

·        Females chose more brightly colored males

 

  1. Sensory bias

·        Traits are intrinsically stimulating

·        Stimuli outside normal range often attractive

·        Female preference may evolve before male trait

Fig. 14.9

 

  1. Indirect benefits

·        Male trait doesn’t directly increase female’s fitness

·        Only contributes genes

 

If there’s no benefit to the female, why is she selecting male traits?

Two ways indirect benefits can evolve

  1. Runaway sexual selection (sexy son hypothesis)

·        Females express a preference for a male trait

·        No benefit to her

·        Sons express the trait that mothers preferred increasing the son’s reproductive success

·        Linkage disequilibrium develops between genes that affect female preference and genes that affect male expression of the trait

Evidence for sexy son hypothesis

Fig. 14.12

Sand fly

·        Females prefer certain males

·        There is no fitness advantage to the offspring associated with male attractiveness

·        But sons of attractive males also have higher reproductive success

  1. Good genes hypothesis

·        Females chose males with high genetic quality

·        How can females detect good genes?

·        Some male traits only expressed if they are in good physiological condition

Hoffman et al. 2007 on web

                              Example

Evidence for good genes hypothesis

·        Stickleback Gasterosteus aculeatus

·        Females prefer redder males

·        Females mated to “red” fathers and “dull” fathers

·        Offspring of “red” fathers had higher resistance to tapeworms

·        Red males eat food with higher carotenoid  concentrations which is involved in immune system development