19.4 Evolutionary Ecology
Key Takeaways
Life-history strategies trade off age at first reproduction, offspring number and size, and parental care.
Many small eggs with no care, and few eggs plus feeding of nestlings, are alternative budgets rather than one strategy.
Altruism costs the actor and benefits another individual. It is not a vague benefit to the species as a whole.
Kin selection can spread a helping allele when the benefit to the recipient, weighted by relatedness, exceeds the cost to the actor, even if the helper has no offspring.
Full siblings are related by about one half. Reciprocal altruism is different: help returned later, including among non-kin.
19.4 Evolutionary Ecology
Life histories are budgets
Evolutionary ecology asks how selection shapes the schedule of a life, not only the shape of a body. This section goes past the r and K labels from population growth. Those labels are a rough sketch. The exam traits are the budget. A life history is the schedule of growth, reproduction, and survival. Time, energy, and risk are limited, so a gain on one part is paid for on another.
Age at first reproduction is the timing of the first breeding attempt. Breeding early raises the chance of leaving offspring before death. The cost is a parent that may still be small or short of stored energy. Waiting can mean a larger body, and it can also mean dying with no offspring.
Number and size of offspring trade off inside one reproductive budget. The same effort can become many small young or a few large young. Many small offspring are many chances in an uncertain year. Few large offspring start with more stored food, so each has a better chance, and a failed year costs the parent more of its effort.
Parental care is investment after the egg or newborn exists. Feeding and guarding raise the survival of each young and use time that could have produced another clutch. Some life histories stop when the eggs are laid.
Two birds, two budgets
One bird lays many small eggs and provides no care. Quantity is the advantage, because a failed nest still leaves other eggs in play. The cost is high death among young that have little stored food and no parent to feed them.
The other bird lays few eggs and feeds the nestlings. Each young gets food and defense. The cost is the small clutch: a lost nest takes a large share of the season, and days spent feeding are not spent on another large brood. Uncertain juvenile survival often favors many untended eggs. Conditions in which care raises survival often favor the smaller clutch. Answer from that tradeoff, not from the r or K label alone.
| Trait | One allocation | The other allocation |
|---|---|---|
| Age at first reproduction | Breed early, before death is likely | Wait, and risk dying with no offspring |
| Offspring number and size | Many small young | Few large young |
| Parental care | No care after laying | Care that raises each young's chance and limits the clutch |
Altruism
Altruism is a behavior that costs the actor and benefits another individual. The cost is a loss of the actor's survival or reproduction. A ground squirrel's alarm call fits when it draws a predator toward the caller and lets neighbors escape. Help that benefits both parties at once is mutual benefit, not altruism. If the actor gains an immediate net benefit in survival or reproduction, the behavior is not altruism.
The explanation to reject is group selection told as a vague good done for the species. An allele for a costly act does not spread just because the species would be safer if everyone performed it. Unless the copies return through relatives or through later repayment, individuals that refuse the sacrifice leave more offspring, and the allele shrinks.
Kin selection
Kin selection sends those copies through relatives. William Hamilton's idea, stated in words, is the test to use. An allele for helping spreads if the benefit to the recipient, weighted by relatedness, exceeds the cost to the actor. Full siblings are related by about one half, the same relatedness as one's own offspring. Helping a sibling can therefore spread the shared allele. The helper does not need offspring of its own. Nieces, nephews, and siblings can carry the allele forward if they reproduce. Sterility of the helper is not a reason to reject kin selection.
A sterile worker
A sterile worker honey bee is the classic eusocial example. She forages, defends, and cares for the queen's brood instead of producing her own young. That cost benefits relatives, and her alleles can spread through the queen's offspring.
State haplodiploidy in the breeding system, not as a slogan. Males develop from unfertilized eggs and are haploid, while females develop from fertilized eggs and are diploid. Because a haploid father passes the same genes to every daughter, full sisters that share one father can be related by three quarters, which is higher than a female's relatedness of one half to her own offspring. The three-quarter figure assumes one shared father, and it is only part of why workers help. The decision rule remains cost, benefit, and relatedness.
Reciprocal altruism
Reciprocal altruism is help that is returned later, and the partner can be non-kin. It can spread when the same individuals meet again, recognize each other, and can refuse a partner who does not repay. Vampire bats show the pattern. A bat that fails to feed may receive regurgitated blood, and donors tend to give to bats that have given to them before. The first gift costs the donor. The later return can save that donor on a hungry night. A one-time gift to a relative, with no repayment expected, is kin selection rather than this later exchange.
Warning
Altruism costs the actor and benefits another individual. It is not a vague claim that the behavior exists for the good of the species. Kin selection can spread helping when the benefit to the recipient, weighted by relatedness, exceeds the cost to the actor, and the helper need not have offspring. Reciprocal altruism is help returned later, including among non-kin.
Sorting the stem
If the actor gains immediately, the behavior is not altruism. If the recipient is kin and the weighted benefit exceeds the actor's cost, kin selection explains it, including for a sterile worker. If the favor is repaid later, including by non-kin, name reciprocal altruism. On a life-history item, pair the gain with its cost: many untended eggs versus few eggs and fed nestlings.
Which pair shows a real life-history tradeoff?
Earlier breeding, more offspring, and more parental care can all increase together at no cost
Many small eggs and no care trade off against fewer eggs and feeding of nestlings
Many small eggs with no care are the same strategy as few eggs plus feeding of nestlings
Life-history strategy means only the r and K labels, not age, offspring size, or care
When can kin selection spread an allele for helping?
Only if full siblings are unrelated, because strangers share more genes than siblings do
Only if the helper also produces its own offspring in that same season
Whenever the act helps the species, even if it never helps relatives and is never repaid
When the benefit to the recipient, weighted by relatedness, exceeds the cost to the actor
Which statement about helping is accurate?
A sterile worker can spread shared genes by helping relatives, and reciprocal altruism is later repayment, including among non-kin
In honey bees, males and females both develop from fertilized eggs and both are diploid
Altruism benefits the actor and costs the recipient, so the actor's own reproduction rises
Reciprocal altruism is care given only to one's own newborn and never repaid by anyone else
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