Reproduction: The evolution of sex and gender differences

ZOL 313

June 5, 2008

Reproduction: The evolution of sex and gender differences

ZOL 313

June 5, 2008

Objectives:

1. Understand and be able to explain potential costs and benefits of sexual reproduction.

2. Understand why males and females use different mating strategies. Be able to explain why males usually compete and females usually choose.

3. Be able to predict whether we would expect sex role reversal in a given situation.

‘Evolutionary biologists are much teased for their obsession with why sex exists.  People like to ask, in an amused way, “isn’t it obvious?”

Joking apart, it is far from obvious....  Sex is a puzzle that has not yet been solved; no one knows why it exists’ ~Mark Ridley 2001

VS

Why reproduce sexually?

1. Making males

Costs of sexual reproduction

Organisms that reproduce sexually face a “two-fold cost” of making males.

Example: Imagine a population with both sexual (pink) and asexual (green)

females.

Each individual produces 2 offspring each generation.

If an organism’s genes are ideally suited to its environment:

Asexual reproduction

Sexual reproduction

Costs of sexual reproduction2. Sex can break up good genetic combinations

Finding a mate and the process of mating itself

It can also greatly increase

Hypothesis: Mating is energetically costly to male Himalayan tahrs.

Costs of sexual reproduction3. Time/energy/survival

Prediction:

Costs of sexual reproduction4. Sexually transmitted diseases

Example:

Example:

In an asexual organism,

(Like photocopying a blurry photocopy.)

Benefits of sexual reproduction1. DNA repair through recombination

In a sexual organism ,

(A copy for your offspring can be made from your mate’s correct copy.)

Benefits of sexual reproduction2. Lottery Hypothesis

Sexual reproduction provides

Asexual reproduction is like

Sexual reproduction is like

Sexual reproduction enables organisms

Pro

port

ion

mal

es

Benefits of sexual reproduction2. Red Queen Hypothesis

Prediction:

Example: New Zealand snails have some populations that are all female (asexual) and some with males (sexual).

In general:

Females:

Males:

Why do males and females use different mating strategies?

Example: Bowerbird courtship

What is the definition of a female vs a male?

1. Differences in gametes

Why do males and females use different mating strategies?

Male Female

Size:

Number:

Function:

Prezygotic investment:

Postzygotic investment:

Why do males and females use different mating strategies?

2. Differences in parental investment

Parental Investment: Costly parental activities that

Example: What is the maximum number of offspring a male vs a female human can have in his/her lifetime?

Why do males and females use different mating strategies?

3. Differences in reproductive rate

Females reproductive rate is limited by

Males reproductive rate is limited by

Example:

Why do males and females use different mating strategies?

4. Differences in levels of sexual activity

__________ tend to have higher levels of sexual activity than _________ .

This leads to a male biased operational sex ratio

Example: Number of matings in male and female katydids.

The Coolidge Effect:

Origin of the term “The Coolidge Effect”:During a tour of a poultry farm, Mrs. Coolidge asked the farmer

how his farm could produce so many fertile eggs with such a small number of roosters. The farmer proudly explained that his roosters performed their duty dozens of times each day.

"Perhaps you could point that out to Mr. Coolidge," replied the First Lady in a pointedly loud voice.

The President, overhearing the remark, asked the farmer, "Does each rooster service the same hen each time?"

"No," replied the farmer, "there are many hens for each rooster.""Perhaps you could point that out to Mrs. Coolidge," replied the President.

Why do males and females use different mating strategies?

4. Differences in levels of sexual activity

Example:

Prediction:

Why do males and females use different mating strategies?

Figure 10.8

When males make a larger parental investment and/or have a lower potential reproductive rate than females, we would expect the sex roles to switch.

Sex role reversals: The exception that proves the rule

Example:

Females mate with multiple males and lay eggs in their nests. Males incubate and care for young.

Males are limited by:

Females are limited by:

Which gender to you predict will compete and which will chose?

When males make a larger parental investment and/or have a lower potential reproductive rate than females, we would expect the sex roles to switch.

Sex role reversals: The exception that proves the rule

Example: Males have brood pouch where females place eggs.Females can make enough eggs to fill 2 pouches.

Males are limited by:

Females are limited by:

Which gender to you predict will compete and which will chose?

When males make a larger parental investment and/or have a lower potential reproductive rate than females, we would expect the sex roles to switch.

Sex role reversals: The exception that proves the rule

Example:

When food is scarce, males are only able to produce one spermatophore (nutrient rich nuptial gift).When food is abundant, males can produce multiple spermatophores.

Under which condition should male choice/female competition be greater? Why?

Sex role reversals: The exception that proves the rule

Figure 10.8