Diversity of modes of sexual reproduction in (multicellular) eukaryotes
- Anisogamy: sexual reproduction involving fusion of two ‘different’ gametes
- larger gamete (female egg) & smaller gamete (male sperm)
dioecious
Organisms with individuals with separate sexes
Organisms with individuals that can produce both male and female gametes
Some species have mixtures of individuals that produce both gametes and individuals that produce only one type of gamete (androdioecy: only produce one or gynodioecy: some can produce both, some can produce one )
Organisms with individuals that can change sex
Bluehead wrasse (Protogynous)
- female-> male
- first produce eggs and then one female turns male and produces sperm (once previous sperm producing male dies)
Clownfish (Protandrous)
- male-> female
Sexual selection
- “selection caused by competition for mates among individuals of the same sex”
Evolution, Futuyma & Kirkpatrick - framework for understanding evolution and incredible diversity of reproductive modes, mating systems, and traits related to reproduction (e.g. ornaments, weapons, mating displays)
Sexual selection is driven by what
differences in reproductive biology
fundamental differences between small and large gametes
Bateman’s principle: Individuals that produce many small gametes (usually males & sperm)…
can often fertilize eggs from many different individuals = selection for traits to increase mating success
Bateman’s principle: Individuals that produce fewer large gametes (usually females & eggs)…
can often fertilize eggs with a single mating
no selection for traits to increase mating success
Some predictions of Bateman’s principle:
- with repeated matings, male fitness often increases
more than female fitness - males often have greater variance in reproductive success than females (and greater opportunity for sexual selection)
exceptions and departures of Bateman’s principle:
- females often mate multiply
-> not mating as many times as males - sex role reversals
-> females may be competing to mate with males
-> pregnant male seahorse
Variation in mating success in elephant seals
Distribution is very different when comparing males and females
* So in males fewer than 10% of the males sired nearly all of the pups
- one single male was responsible for producing 80% of the pups and most of them didn’t produce any
- In females more than half of the
females successfully weaned at least one offspring- the one that had the most number of babies had 10
Two Major Modes of Sexual Selection
- Male-male competition
- Female choice
Sexual selection via male-male competition
- “scramble” – traits involved in early search and swift location of mates
- “contests and displays” - e.g. size, strength, threat signals, weaponry
- sperm competition: sperm being able to displace other sperm
- infanticide
Alternative male mating strategies
???
* 3 genetically determined male morphs in the marine isopod (crustacean) Paracerceis sculta: large 𝛂 guards, medium-sized 𝛃 mimics females, small 𝛄 sneaks (reproductive success of the 3 males are basically the same)
- so they’re all producing sperm and fertilizing females but with very different strategies
- large 𝛂
- large
- will guard territories where different numbers of females will accumulate - medium-sized 𝛃
- mimics females - small 𝛄
- sneak into the harem and escape detection and aggressive interactions from that alpha male
- many other examples (& can be due to genetics or condition)
Sexual selection via female mate choice examples
- peacock
- stalk-eyed fly
very long eyes- proxy for size
- important for male competition
- females will mate with males with longer eyes
Direct benefits
a potential mate may offer food or other resources (a territory, parental care)
Indirect benefits
a potential mate may offer “better genes” (as indicated by larger size, brighter colours, or other signals of vigour and good condition)
