the role of reproduction (part 6)
TRANSCRIPT
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The Role of Reproduction (Part 6)
Isogamy/Anisogamy
In many species, the gametes are highly specializedo The male gamete is small, containing little but the haploid nucleus
o The female gamete is big and highly differentiated
Why? Is the difference in size important? Is there always a difference
in size?
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Anisogamy
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Gametes are different sizes
Evolution has given rise in many species to very different roles for
the male and female. Organisms produce few, large female gametes,
and many small male gametes.
Gamete Allocation Gamete number
Human females: ~200,000/life
Female oysters: ~100 million/yr
Human males: ~180 million/ejaculation
Why does anisogamy confer an evolutionary advantagecompared to isogamy?
Selection has been for larger sex cells becauseo Increasing zygote size increases survival
Larger gametes have more nutrients for the zygote
Increasing male gamete number increases siring success
o Reduces the potential for conflict between cytoplasmic organelles
in the embryo
Sperm does not contribute to cytoplasm
The Evolution of Cues for Gamete Production
(Animals) Mobile and less dependent on the environment for mating However, environmental cues are still important
Ovarian cycles vary in duration mating needs to be cued to times
when the female is ovulating: pheromones
Seasonal breeders: light cycle, temperature, food supply
Reproductive Cycle of the Mare Estrus cycle (21 days)
Seasonal anestrus
Transitional
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Animals: Seasonal Reproduction The ability to control reproductive is one of the evolutionary
advantages achieved by putting reproduction under control of the
brain
o Environmental factors (food supply, predator stress, temperature)
can all be used in ensuring that young are born when conditions
are optimalo A common strategy is to use the same mechanisms used for
puberty to regulate seasonal reproduction
The Evolution of Cues for Gamete Production(Plants)
Plants: Commonly Adapted to External Cues Plants are sessile: they dont move around
Optimal flowering time is a critical reproductive strategy in
fluctuating environments
Dependence on external cues: temperature, photoperiod, pollination
mechanisms
Photoperiodism and Vernalization in Plants Photoperiodism: the regulation of processes such as flowering by
changes in length of day/night
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Vernalization: induction of flowering by low temperatures
o May require as many as 50 days of -2 to 12C
o Some plants require both vernalization and long photoperiod to
flower
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When even a single leaf is exposed to inductive conditions, a plant
hormone florigen travels to the entire plant, and even to other
plants in grafting experiments, inducing it to a flower
Photoperiodism Synchronizes the flowering of plants of the same species in the local
population to promote cross-pollination and successful reproduction
Sexual Selection (Animals) The reproductive success of the female, and therefore of the
species, depends primarily ono The quality of genes she receives from her mate
o The resources he controls
Indirect gamete dispersal
Direct food, shelter, and territory
o The amount of assistance he provides in the care of offspring
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Thus, females choose males based on these criteria, leading to the
evolution of exaggerated traits that signal male quality
Consequences of Adopting a Competitive Strategy Evolutionary pressures:
o Selection for male weapons to compete for femaleso Selection by females for male sexual attractiveness
(ornamentation)
Gamete Dispersal Competition Competition between males:
o Abundant sperm and scarce eggs drive mating behaviour:
Sperm are small and cheap to produce
Eggs are large and expensive to produce
o Males often initiate courtship, and fight for opportunities to mate
with females, seldom reject receptive females
Fighting does not need to result in death or injury He who fights then runs away
Lives to mate another day
Weapons need not be lethal
Dominant males have many females Dominance greatly affects male mating success; increase
reproductive success by increasing number of mates
Dominant males have the choice of females
Stored energy within large males favours strength and enduranceo High ranking males can lose ~40% of body mass
o Low ranking males lose as little as 30%
Does dominance matter for humans? Yes, in some societies, political or military dominance translated into
reproductive success
o Leading to genetically inbred human populations
Are the best ornaments always chosen? Sometimes the ornaments are nests
And other courting behaviours
Lecture Summary Anisogamy confers advantage over isogamy increasing size of female
egg, decreasing size of male gamete
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Environmental cues frequently adopted to link reproduction to
environmental conditions
Sexual selection frequently involves competition between males to
establish dominance over territory, or attractiveness to females.