The Role of Reproduction (Part 6)

8/2/2019 The Role of Reproduction (Part 6)

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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.

The Role of Reproduction (Part 6)

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