EEOB 400: Lecture 9

Sexual selection

Sexual dimorphism

Sexual dimorphism

Sexual dimorphism

“Considering that colors of chameleons often reflect their “mood”, one has to wonder why mating elicits such a different color response in males and females.”

- Lizards: Windows to the Evolution of Diversity (2003) L.J. Vitt & E.R. Pianka

Photo: Bill Love

Sexual dimorphism

Weapons

Sexual dimorphism

male malefemale

Weapons

Sexual dimorphism

Ornaments

Sexual dimorphism

Sexual dimorphism

Sexual size dimorphism

Bizarre sexual dimorphism

The peacock’s tail

Extravagant male ornaments

The peacock’s tail greatly impairs his mobility…how could such a trait evolve?

Sexual selection

Darwin’s second “major” book:

1871 On the Descent of Man, and Selection in Relation to Sex

Why a theory of sexual selection?

Darwin needed a theory to explain the many extravagant traits that seem to reduce survival

e.g. the peacock’s tail

What is sexual selection?

Sexual selection is distinguished from natural selection by the following criterion:

Sexual selection arises through variance inmating success

Sexual selection

Is sexual selection different from natural selection?

Darwin saw them as distinct - only sexual selection could produce traits that compromise survival

The basic principles are identical – selection favors whatever gets more genes into the next generation

In sexual selection, fitness is measured relative to members of the same sex

Two kinds of sexual selection

Intrasexual selection – mating success determined by within-sex interactions e.g., male-male combat

Intersexual selection – mating success determined by between-sex interactions e.g., female choice of males

(also referred to as epigamic selection)

Mating systems

Monogamy One male mates exclusively with one female

Polygamy Individuals mate with more than one partner

Polygyny Some males mate with more than one females

Polyandry Some females mate with more than one males

Monogamy Polygyny Promiscuity

Promiscuity Males mate with more than 1 female and vice versa

Mating systems influence sexual selection

Monogamy Moderate Polygyny Strong Polygyny

Strength of sexual selection

Variance in mating success

All males have same mating

success = 1 mate

Some males = 0 mates Some males = 1 mateSome males = 2 mates

Most males = 0 mates One male = 8 mates

Mating systems

Harem polygyny

Elephant seals (Mirounga) - breeding females cluster together on beaches

- allows males to defend a harem of many females at once

Bull male elephant seals engagein violent, bloody fights overfemales – large size confers an advantage in male combat

Male elephant seals weigh up to 3x more than females!

Male reproductive success is highly variable:

8 individual males inseminated 348 females in one study!

Lekking polygyny

Lekking in Black Grouse, Fallow Deer & Stalk-Eyed Flies

Lekking - males aggregate in particular areas called leks, display for females

Lek

Combination of male competition…

Males may fight for position in center of lek

…and female choice:Females choose a mate…often dominant male or male in the center

Territorial defense polygyny

Territory

An area that is defended for exclusiveuse of the defender against rivals

Territories may be defended by malesor females and for multiple purposes

What is being defended?

Sometimes territories are defended simply for resources:food, basking sites, dens or hiding places, etc.

This may still be important for sexual selection, e.g. if females “choose” male territories based on the resources within the territory

In many species, males set up territories around females (or vice versa) – in this case it becomes similar to a harem defense polygyny

Male combat

Red Deer (Cervus elaphus)

Reproductive success is morevariable in males than females

Many stags never reproduce, some may sire up to 24 offspring!

% fe

mal

es

% m

ales

# offspring surviving to 1 yr.

Why are these males fighting?

Alternative reproductive tactics

Remember…there may not be a “best” way to be a male (or female)

Sex roles

Macho males and choosy females?

Why do males and females fall into these “typical” sex roles?

In part, it reflects a bias in the species typically studied……but is there also some inherent biological reason?

Bateman’s principle:

The sex which invests the most in offspring will become a limiting resource over which the other sex competes ( = sexual selection)

Anisogamy: difference in the size of male and female gametes

Gametic investment: sperm are consequently cheaper than eggs

This predisposes females to a greater level of parental investment

Males can easily produce enough sperm to fertilize all of a female’s eggs,but the reverse is not true:

mating opportunity limits male reproductive success

fecundity limits female reproductive success

Polyandry

Jacanas

Most jacana species exhibit harem polyandry and “sex role reversal”

Males maintain small territories

Males perform all parental care

Females mate with multiple malesand then leave eggs with males

The number of males a female matesdetermines her reproductive success because she doesn’t care for eggs

Sexual dimorphism

Females larger than males by 60% in mass (unusual for birds)

Females aggressively fight other females and also kill their chicks (infanticide)

Polyandry

When is polyandry favored?

When males become limiting resource for reproduction

This often occurs when low offspring survival requires male parental care, so that males have the greaterReproductive effort and lower reproductive rate

Saddleback Tamarins,Spotted Sandpipers,and Red-necked Phalaropes are examples of polyandrousspecies

What is the consequence?

Sexual selection is stronger on females than males

Intra- and inter-sexual selection

Intrasexual selection

All of the examples thus far have involved interactions within a sex - male-male combat- sperm competition- female competition in polyandrous species

Darwin’s theory of intra-sexual selection was readily accepted, even in his time

Intersexual selection

Intersexual selection has always been controversial

We know mate choice occurs because we can observe it directly

However, it is debated if and how some sexually dimorphic traits evolve in response to mate choice

The peacock’s tail

Extravagant male ornaments

The peacock’s tail greatly impairs his mobility…how could such a trait evolve?

Andersson (1982) Nature 299:818

Male ornaments

Long-tailed widowbird

Experimental manipulation of tail length

Males with unnaturally long tails attract females away from the nests of “normal” males or males with shortened tails

Male ornaments

Barn swallow

Experimental manipulation of tail length

Males with elongated tails:

1. Obtain mates morequickly

2. Have greater reproductivesuccess

3. Experience a cost intail size the following year

From Moller (1994)

Hypotheses for male ornaments

Fisher’s “runaway” hypothesis

Mate choice originally evolved to facilitate adaptive choice for traits conferring a survival advantage

Once female preference evolved, any genes thatconferred survival advantage but compromisedattractiveness would not be passed on because surviving males would fail to mate

Zahavi’s “handicap” hypotheses

Extravagant male traits are costly to develop and maintain

Choosing a mate with “good genes” requires an honestsignal of genetic quality

Only males in good condition (those with good genes)will be able to fully develop and maintain an ornament

Amotz Zahavi

Ronald Fisher

Fisher’s runaway modelF

itnes

s

Tail length

Total male fitness(survival + mating)

Fitness due to survival

Survival Selection

Sexual SelectionFemale choiceadaptive for survival

Zahavi’s handicap hypothesis

Some candidate “handicaps”

Note that the handicap itself need not be heritable…it need only provide areliable index of fitness, and fitness must be heritable

Traits that encumber theowner are physiologicallycostly (exertion in flight)as well as being more expensive to develop

Bright color honestly signalsimmunocompetence and parasite/disease resistance

Asymmetry is indicativeof developmentalinstability and possibly“bad genes”. Symmetryis chosen in some species

Sexual vs natural selection

Guppies (Poecilia) - sexual selection can favor traits that reduce survival

- laboratory selection studies by John Endler

http://evolution.berkeley.edu/evolibrary/article/artificial_01

Natural selection (predation) favors spot patterns that match background

Sexual selection (mate attraction) favors male patterns that contrast background

Sexual vs natural selection

Marine iguanas (Amblyrhynchus cristatus)

- One of the few lekking reptiles

- Large males most successful at lekking

- Sexual selection favors large males

- Males 2x more massive than females

Martin Wikelski’s website: http://www.princeton.edu/~wikelski/

Cost of large size

- During El Nino years, food is scarce

- The largest iguanas are unable to meet their minimum energy demands and literally starve to death

- Most of the largest animals are males, so sexual and natural selection have conflicting effects on male body size

Good times, bad times

Sexual dimorphism and sexual selection

Sexual selection

- Selection arising from variance in mating success within a sex

- Not all sexual dimorphism arises from sexual selection

- Natural selection can also differ between sexes

Fecundity selection

- Selection on female fecundity = number of offspring produced

- Fecundity is different than mating success = number of mates

- In many species where females are larger than males, dimorphism is though to reflect fecundity selection favoring large femlae size, since larger females can produce more eggs

Sexual dimorphism and sexual selection

Niche divergence

- Males and females evolve to fill different ecological niches, adaptive if it reduces competition for limited resources (e.g., food)

- Another example of natural selection causing sexual dimorphism