Sexual Selection

Natural selection

the association btwn phenotypic traits and fitness

Sexual selection

a form of nat selec. that acts on heritable traits that affect reproductive success via competition for mates and mate choice

Is reproductive success still favored even if there is some tradeoff w survival?

yes

primary sex characteristics

genitalia and reproductive organs shaped by sexual selection

primary sexual characteristic example

seed beetle penis

why is the seed beetle penis that specific sexual characteristic?

it is the organ that plays a direct role in reproduction

secondary sexual selection

morphological (size, shape, and physical str) differences btwn sexes that are NOT DIRECTLY involved in reproduction

name one secondary sex characteristic example(s)

the abdomen of the peacock M spider, feathers of M peacocks, and some male beetle species have larger mandibles (jaws) than female beetles

isogamy

form of sexual reproduction that involves gametes with similar morphology (size, shape, and str)

what term is most likely the first stage of sexual reproduction to evolve?

isogamy

anisogamy

form of sexual reproduction that involves the fusion of two gametes which differ in size and/or form

sexual conflict

conflict btwn sexes where what is selectively advantageous for one sex is not for the other

what does sexual conflict lead to?

evolution of adaptations and counter-adaptations (i.e. arms race) » since the evolutionary interests of Ms and Fs never align, expect some conflict in most systems

sex roles

all things associated to Ms and Fs

monomorphic

both sexes are phenotypically indistinguishable from e/o

sexual dimorphism

two sexes of the same species exhibit diff morphological characteristics, but does not only pertain to characteristics involved in reproduction

directional selection

form of natural selection in which individuals with traits at one extreme of the phenotypic distribution have better fitness than individ.s w intermediate or opposite extreme phenotypes

direction selection example

L gametes and S gametes have higher fitness b/c L gametes are have high survivorship and S gametes have numerical advantage, so intermediate gametes are forced to become one or the other due to not having an advantage over either gamete leading to low fitness

Why do we have 2 sexes in terms of gametes and sexual dimorphism?

natural selection favored 2 types of gametes: 1) small and quick (sperm) 2) large and non-mobile (egg)

What do we mean by having 2 sexes?

• it can mean separate individuals, separate gonads in the same individ. or separate in time

  • based on gametes

‘meaning by having 2 sexes’ examples discussed in lec?????

• separate gonads in the SAME individ. (i.e. flowering plants)

• sequential hermaphrodites (i.e clownfishes can transition from M to F when the breeding F in the group is removed)

• species have multiple phenotypes within a gametic sex (i.e melanzona guppy males all have diff phenotypes, but they are considered male)

Explain anisogamy in terms of Bateman-Darwin Paradigm.

Since F reproductive success is dependant on the # and success of eggs produced and M reproductive success is dependant on the # of mates, anisogamy causes stronger sexual selection in Ms, F-biased parental care, and M-biased sexual dismorphism

Explain anisogamy in terms of Trivers’ Parental Investment Theory.

• assumes F invest more time and energy per gamete/offspring, so…

• Ms need to max the # of individuals they mate w and performing less parental care

• Fs need to max the # of individ. they mate w to 1 or fewer and performing more parental care

  • leading to Fs being taken out of the mating pool » more comp for eggs (females) than sperm » Fs are highly choosy and Ms are indiscriminate to whoever they mate » more sexual selection on Ms

What drives the diversity of F and M form and function? How?

ecological context shapes how Ms and Fs compete for the opposite sex and drives the variety of mating systems

What effect does anisogamy have on Ms and Fs?

creates potential for different evolutionary trajectories or in other words, the first step in establishing sexual dimorphism

what can we expect causes stronger selection on sex?

variation in reproductive success

what can we (sometimes) expect causes more sexual dimorphism?

stronger sexual selection

Peacocks do not provide parental care – so males gain a lot from deserting females after mating. In which of these groups would we expect greatest sexual selection pressure on: A) Males B) Females?

more comp btwn Ms > stronger sexual selection on Ms

Albatross males provide some parental care- so more to lose by leaving partner. In which of these groups would we expect greatest sexual selection pressure on: A) Males B) Females?

morphologically similar or similar comp btwn the 2 sexes, so we expect sexual selection on both sexes

In red-necked phalaropes, males provide all the parental care. In which of these groups would we expect greatest sexual selection pressure on: A) Males B) Females?

intense comp btw Fs > stronger sexual selection on Fs