Sexual Selection + Speciation/Evolutionary Divergence

Unit 2 Chapter 3 + 4

Female

Produce larger gametes

Male

Produce smaller gametes

Anisogamy

Gametes of unequal size

Natural Selection

Occurs when fitness, # of gene copies contributed to next generation is correlated with traits that individuals have

Sexual Selection

Occurs when variation in the # of eggs fertilized is correlated with a trait

Fecundity

Reproductive capacity of an organism, # of eggs able to produce

Uncertain Paternity = Rare Male Parental Care

• Females have certain paternity

• Males have uncertain paternity

Operational Sex Ratio

Ratio of males to capable reproducing females at a given time

Which sex experiences stronger sexual selection?

Males - they exhibit greater variance in reproductive success

Why is there greater variance in # of eggs fertilized among males?

• Greater payoff for competition over mates in males

• Greater potential for sexual selection

Intrasexual Selection/Male-Male Competition

Traits make it possible to win contests over mates

• Large body size

• Weapons

• Aggressiveness

Intersexual Selection/Female’s Choice

Traits that attract potential mates

• Males flashier than females

Sexual Dimorphism

Traits that improve mating success are selected differently in males vs. females

Age-Dependent Male Success

Younger and older are weaker than those at maturity which leads to sexual selection

Selection for larger males with weapons

Positive correlation between trait & mating success

Sperm Competition

Population where females mate with multiple males:

• Increased sperm production

• Traits to prevent other male’s sperm from reaching egg

• Traits to make sperm that swim faster

Female Choice

• Males have low investment in reproduction, not choosy, male has trait, and trait is heritable.

• Females have high investment in reproduction, choosy, preference for trait that indicates “quality”, preference is heritable

Direct Benefits of Female Choice

• Protection

• Good territories

• Food

• Help raising young

• Reduced risk of disease

Indirect Benefits of Female Choice

• Offspring does better

• Offspring more attractive to other females

Good Genes Hypothesis

• High-quality males might produce high-quality sons… males advertise quality by having costly trait —> only high quality males can “afford” to carry trait

• Costly display traits are more reliable indicators of male quality

Sexual Cannibalism

Males selected to be eaten by females (red-backed spiders):

• Males rarely have chance to mate more than once

• Females can’t mate more than once

• Females that eat male often refuse to mate with another male

• Males that were eaten got to copulate 2x as long = 2x more offspring

Speciation

Process + mechanisms behind the birth of new species

• New species form when population stop exchanging genes (gene flow reduced/eliminated)

Morphological Species Concept

Species defined by observable physical traits

Phylogenetic Species Concept

Species are the smallest monophyletic groups on phylogenetic tree

Biological Species Concept

Species are groups of interbreeding natural populations that are reproductively isolated from other such groups

General Lineage Species Concept

Species are segments of evolutionary lineages that maintain distinct genetic identity over time

Species

End point of process of population divergence

Divergence

Reduced gene flow between populations

Speciation involves

Barriers to reproduction (gene flow)

Types of Barriers to Reproduction:

• Geographic barriers

• Can’t attract or mate

• Adaptation to different environments

• Sperm can’t fertilize eggs

  • Hybrids can’t grow to maturity/sterile/selected against

Allopatric Speciation

Population separated in space, geographic barriers lead to evolution of reproductive barriers

Sympatric Speciation

Population overlap in space, reproductive barriers evolve without geographic barriers

Step 1 of Allopatric Speciation

Geographic barrier arises & reduces gene flow between populations

Step 2 of Allopatric Speciation

Genetic differences accumulate in each population (divergence) & reproductive barriers evolve (reproductive isolation)

Step 3 of Allopatric Speciation

By the time geographic barrier goes away, the two populations are no longer able to hybridize

Polyploid Speciation

Some plants can evolve to form more than two sets of chromosomes

• Polyploid individuals cannot hybridize other form —> instant reproductive isolation —> new species