In the Light of Evolution Volume III: Two Centuries of Darwin (2009) / Chapter Skim
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11 Reproductive Decisions Under Ecological Constraints: It's About Time--Patricia Adair Gowaty and Stephen P. Hubbell
11 Reproductive Decisions Under Ecological Constraints: It's About Time--Patricia Adair Gowaty and Stephen P. Hubbell
Pages 213-242
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From page 213... ...
the distribution of fitness conferred, the w distribution, together affect average life time fitness, and induce adaptive switches in individual reproduc tive decisions. The switch point is the rank of potential mates at which focal individuals switch from accepting to rejecting potential mates, a decision rule that the SPT proves maximizes the aver age lifetime fitness of a focal individual under given values of ecological constraints on time.
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From page 214... ...
. Darwin distinguished sexual selection from natural selection as selection that arises from some individuals having a reproductive advantage over other same-sex, conspecific individuals, not from different "habits of life," but from reproductive competition with rivals.
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From page 215... ...
and hrdy (1986) offered an explanation for why, in the face of so much evidence, so many biologists seem invested in the "myth of the coy female." in response to the failures of the simpler versions of parental investment theory, new theory to explain reproductive decisions appeared (hubbell and Johnson, 1987; Crowley et al., 1991; Clutton-Brock and Parker, 1992)
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From page 216... ...
. This means that fitness variances can arise from demographic stochasticity, acting through chance effects on individual encounter probabilities with potential mates, individual survival probabilities, and latencies, which are variables that can then induce individual behavior (Fig.
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First, the studies had controls that eliminated the effects of intrasexual behavioral contests and intersexual coercion that often confound mate choice studies (Kingett et al., 1981)
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(ii) We introduce the concept of the fitness conferred by alternative potential mates.
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This means that for individuals with many opportunities, the costs of rejecting more potential mates are smaller, whereas, when focal individuals have fewer opportunities, the costs of rejecting potential mates are greater. however, relative opportunities predict only means and variances in number of mates; to predict adaptive, flexible reproductive decisions of an individual, one must also know the fitness that would be conferred if a focal individual were to mate with any given alternative potential mate.
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From page 220... ...
This matrix specifies what we mean by "fitness conferred by alternative potential mates." We assume that focal individuals have information about the distribution, which is a key com ponent of expected mean reproductive success. The fitness components one might consider empirically or theoretically include: fecundity (the number of eggs laid or offspring born)
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From page 221... ...
The sPT says that focal individuals should accept all potential mates whose rank (with 1 being the highest fitness rank) is less than or equal to the rank at which average lifetime fitness would be maximized (f *
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From page 222... ...
here, we describe the steps that allowed us to analytically solve for the number of potential mates that a focal individual should find acceptable to maximize the focal's average lifetime fitness. in step 1, we constructed a series of absorbing Markov chain mod els for each decision rule associated with n, under specified ecological constraints affecting values of e, s, l, and the w distribution.
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From page 223... ...
, it is unlikely that focal individuals actually mate with all potential mates who are acceptable to them. Therefore, we characterize the rule as the "switchpoint" at which focal individuals switch from accepting potential mates to rejecting them.
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RESULTS Fitness Distributions Affect the Switch Points of Focal Individuals Figs. 11.4 and 11.5 show how the switch point changes for a focal individual experiencing identical values of e, s, l, and n, when the w distribution varies.
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. This is because for a given e, s, or l a larger n allows focal individuals to sample a larger portion of the fitness distribution.
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As s → 1, the focal individual has a longer and longer mean life span, and the derivatives increase without bound. This is because total lifetime fitness at f *
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DISCUSSION What the SPT Does and Does Not Do The sPT makes no assumptions about underlying sex differences. We assume that individuals' time available for reproduction is finite, and we characterize variation in individual reproductive careers in terms of constraints on time available for mating.
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sensitivity of the derivative of lifetime fitness at the switch point f with respect to the encounter probability e, b. as a function of survival probability s (x axis)
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FiGUre 11.9 Contour plots of the marginal means in fraction of acceptable mates when 2 parameters vary.
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The sPT is silent about how many times a focal individual actually mates, so that the sPT is accurately characterized as a model of the decision rule that a focal individual would use, if it encounters a particular potential mate. Because the absorbing Markov chain model is stochastic, some focal individuals may be unlucky during their entire lives and never encounter a potential mate.
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From page 231... ...
This experimental procedure would vary the focal individual's encounter rate with potential mates and, if carried out, would provide a strong within-subject test of the prediction that focal individuals reject more potential mates as e increases. importantly, the sPT predicts that if e, s, l, n, and the w distribution are the same for all tested individuals, males and females will show no significant differences in their accept/reject behavior.
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From page 232... ...
nevertheless, the sPT suggests that selection should favor traits that increase a focal individual's encounters with potential mates. enhanced encounters increase reproductive opportunities, thereby reducing the opportunity costs of accepting potential mates who would confer low w.
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From page 233... ...
The sPT suggests that many previous failures to associate fitness rewards for female mate choice for fancy traits in males might be explained by experimentally uncontrolled variation in e, s, l, n, or w distribution. The sPT predicts that a focal individual exposed only to a small number of potential mates would fail to show a preference.
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From page 234... ...
Derivation of the Switch Point Theorem The switch point theorem is an analytical solution to the question of how many potential mates a focal individual should accept or reject to maximize lifetime fitness. Consider a focal individual with a finite reproductive life span, during which the individual searches for potential mates, accepting some and rejecting others, and having to take time out from searching and mating every time it mates and reproduces, a latency period from the onset of one copulation to receptivity to remating.
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From page 235... ...
or the focal individual may encounter a potential mate in the current time step, in which case there are 3 possible states: encountering potential mate number 1 (state S1) , encountering potential mate number 2 (state S2)
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in the "accept all 3" case, whenever the focal individual encounters a potential mate, it mates, with a probability simply equal to the survival probability (because the only behavior the individual exhibits next is to mate)
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s 0s S2 1 0 0 0 0 0 esp2 esp3 1 es 0 s 0 0 esp1 S3 1 s 0 0 esp1 esp2 esp3 1 es 0 SA 1 s 0 0 esp1 esp2 esp3 1 es 0 L1 1 note that when the focal individual encounters potential mate 1 (S1) it moves to the state of mating with individual 1 (M1)
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From page 238... ...
[12] E M1 SA es 1 s ll 1s 1 p1 p2 p3 1s es 1 s p1 p2 p3 one can find by induction the general solution for the lifetime mating success of a decision rule in which the focal individual mates with f individuals out of a total of n total potential mates, where f ≤ n.
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From page 239... ...
(14) is the analytically derived mean of cumulative lifetime fitness computed over an stochastic ensemble of focal individuals, each of whom assesses the fitness distribution of potential mates in the same way, experiences the same values of e, s, and l, and has the same mating decision rule.
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From page 240... ...
11.6 shows the effect of varying latency l and the population size of acceptable mates n on the fraction of mates acceptable, for a w distribution of β(1,1)
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From page 241... ...
note that a higher fraction of potential mates are acceptable when the fitness distribution is skewed high than when it is skewed low or is uniform.
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