Life Histories

Reading; S & S Chapter 13

Consider; The Super-Organism

• Indestructible• Immortal• Can subsist on anything• Produces millions of young• Produces young all the time• Young are indestructible• Is an excellent parent• First reproduction is 15 sec after birth

• This super-organism is impossible because of the metabolic, developmental, and ecological tradeoffs inherent to living things.– Growing quickly means that there is no energy for early

reproduction• Extended parental care means having fewer offspring.• Producing a clutch with many offspring means that each

one must be smaller because there is only so much metabolic energy set aside for reproduction.

• Setting aside more energy means not reproducing for a year or more.

– Ultimately, reproducing means not living as long, because the act of reproducing drains energy, and subjects the organism to risks.

An Economic Analogy

• Imagine you had 1000 dollars to spend on reproduction. – Large offspring cost more than small offspring, but

large offspring are more likely to survive.

– Parental care costs, but it means your offspring are more likely to survive.

– If you don’t spend it now, you can put it away and earn interest on the money, but you might be eaten or killed before you can withdraw it…

• How would you spend the 1000? • How do you get the most return from your investment?

Reproductive Value• Organisms allocate their resources in such a way as to

maximize their lifetime reproductive success.– Lifetime reproductive success is the total number of

offspring an organism produces over the course of their lifetime, (accounting for the proportion of an individual’s genes its offspring shares)

• Of course, that is only measurable after an organism’s life is OVER. At any given time during the life of an organism the expected amount of reproductive success in an organism’s future is called its reproductive value.

• At any given time during an organism’s life, the amount of reproduction they have in front of them can be expressed as follows;

• Vx={i=x to infinity}(ly/lx)my

• Where Vx= the reproductive value of the organism

• my =the expected reproduction at time interval y

• ly/lx=the probability of the organism living to time interval y

– Thus, to maximize its own fitness, an organism will be selected to have those life-history attributes that maximize its reproductive value. This may involve a comprimise between reproducing now and reproducing in the future:

• Vx=mx+{I=x+1 to infinity}(ly/lx)my

– Where mx is contemporary reproductive output, and the second term is future reproduction.

– Depending upon the ecology of the organism: energy expended reproducing now, mx, might affect the ability to reproduce in the future (my), or survive to the future (ly).

– Optimal fitness involves maximizing Vx.

– Which attributes maximize Vx depend upon the ecology of the organism.

This is an optimality modelit assumes that organismswill evolve to ultimatelymaximize their fitness,given the constraintsunder which they operate.In theory, it is a perfectmodel.

In practice?It is difficult to test But qualitatively, itseems to hold uppretty well.

Some Tradeoffs in Life-History Evolution• Life histories evolve. Traits that affect the life history of an

organism have an enormous potential to affect fitness.• The intensity and type of selective pressure on a life history

character depends upon the particular ecological circumstances of the organism

• MacArthur and Wilson coined the terms “r selection” and “K selection” to describe two general, and opposite trends in life history evolution.– R strategists are selected for rapid population growth– K strategists are selected for rapid competitive ability

• Though thought-provoking and useful, this description deals is stereotpyes. Most organisms have a mixture of R and K selected traits. – (Also, check out the R, C, S system in your textbook)

• Some Life-History Tradeoffs • Some of these have been studied extensively, some are

thought to represent evolutionary tradeoffs based on our current ideas of how their evolution works.

• Early reproduction vs. amassing resources to reproduce later• A few, large eggs vs. many smaller ones• Parental care vs. reproducing more often or having more

offspring• Sexual or asexual reproduction• Male or female offspring• Use all your energy in one bout of reproduction

(semalparous) or hold back, and live to reproduce later (iteroparous).

Clutch Size/Parental Care• One crucial life-history tradeoff concerns the

balance between clutch size (animals) or seed set (plants) vs. the size of each offspring. – Assume that the organism has a set amount of resources

that they are able to devote to a bout of reproduction.– Producing more offspring means producing smaller

offspring.• For a wide variety of organisms, smaller offspring have

reduced survivorship relative to larger ones.• It also limits options regarding parental care.• Lack of parental care also restricts survivorship.

Is this true?• It depends upon how you frame the question.

» The Australian plague locust, Chorotoicities terminifera provides no parental care. It produces about five clutches of fifty eggs each.

» The European earwig, Forficulata auricularia provivides extensive parental care, it produces about seven clutches of fifty eggs each.

» Just comparing species is not much of a test, however, the two species have different ecologies.

• It is better to ask whether, in the context of a given species, a larger clutch size means reduced survivorship.

The Lack Optimum• For a wide variety of birds, and a parasitoid wasp, it is fairly

well established that having more offspring translates into reduced survivorship for the offspring, OR, reduced opportunities to reproduce later.

• The idea of optimum clutch size was suggested by David Lack, who suggested that birds that lay the optimum have the highest fitness.– Bird clutch sizes vary a great deal among species, and for some

species (but not others), they vary among species.– Birds do not survive without parental care, though the amount of

parental care varies among species. For a given bout of reproduction, it is reasonable to expect that more eggs means 1) there is a little less for everyone, or 2) the parents have to bust a nut caring for the extra offspring.

• Thus, a bird faces a “decision” every time they build a nest. Lay too few, and their fitness is lower than it could have been. Lay too many, and their fitness is lower as well.

• The Lack optimum has been tested many times.

– In some studies, the larger broods produce more offspring (ie. Lessels 1991).

– In others, larger broods suffer increased mortality and yielded fewer offspring.

– In one particularly interesting study of collared flycatchers (Gustaffson and Sunderland), larger clutches yielded more young, but those offspring had reduced survivorship, and the parents that took care of those young had reduced future reproduction (they busted a nut).

• Parasitoid wasps face a very similar “decision” when they decide to lay eggs.

• Ian Hardy studied the bethylid parasitoid Goniozus nephantidus.

• This species attacks leaf-rolling caterpillars. It finds them in their rolled leaves and paralyzes them with its venom.

• Once it has tracked one down, and has it at its mercy, it must decide how many eggs to lay.

http://wwwhortnet.co.nz

• Hardy manipulated the number of eggs on the hosts.– Too few eggs, and the immune systems of the hosts actually

destroy the parasitoids– Too many eggs, and the female offspring (because of their

ecology, those are the ones that really affect the fitness of their mom), established that laying too many eggs led to females that were very small and had very low fitness.

• Optimum clutch size depends upon host size, however, larger hosts can support larger clutches.

• If you encounter an already-parasitized host, it is sometimes beneficial to “screw” the original egg layer by laying a few of your own. This impairs their fitness, but helps your own (better than laying no eggs).

• Thus, the Lack Optimum is fairly well established as an ecological and evolutionary phenomenon, but, like everything in the field, it isn’t as simple as the model originally proposed.

Clutch size vs. Latitude• For birds and lizards, there is an interesting ecological

relationship between clutch size and latitude.– Northern species lay more eggs per clutch than equatorial species.

– This pattern can be extended to insects, such as milkweed bugs (total number of eggs, not clutch size), pitcher plant mosquitoes, and the first clutch of eggs laid by queen ants.

• This pattern probably reflects the underlying effects of resource availability on life-history evolution.– High latitudes; winters kill many individuals, in spring, there is

less competition and more food to go around

– Low latitudes; competitive environments, where high parental investment in a few offspring is favored.

Sex• Another crucial life history decision concerns whether or

not to reproduce sexually, and if so, whether or not to have sex with one’s self.

• Check your your textbook, pp. 231

– This ties in with ecology a great deal as well, because the evolutionary utility of sexual reproduction is largely to produce variable offspring.

– If the organism is very well-adapted for the environment, this can be a disadvantage, because sex breaks up potentially useful combinations of alleles.

– In changeable or uncertain environments, sexual reproduction can be the key to the continued survival of a genetic lineage.

• A nearly ubiquitous in species that can either reproduce asexually, or sexually, is to reproduce asexually as long as conditions are good, and then, when conditions become unfavorable, to switch to sexual reproduction.

• This strategy is seen in aphids, many protozoa and algae, and many fungi.

• For example, coprophagic fungi such as Psilocibin cubansis, grow asexually as a mycellium within patches of manure. Once the food supply is exhausted, the mycellium will produce fruiting bodies that shed sexually-derived spores into the environment. A cowpie does not last long, and no two are precisely alike, so this strategy optimizes the long-term survival of an individual’s alleles.

• Although the vast majority of animals reproduce sexually every generation, there are animals that exhibit this life history pattern as well.

• Asexual reproduction is particularly common among so called “r selected” animals.– Presumably, the asexual option conveys a fitness advantage when colonizing an empty

patch of environment.

• The freshwater crustacean, Bythotrephes, is such a species.– It is an invasive exotic. One reason it has become so common is its high reproductive

rate.

http://www.seagrant.umn.edu/exotics

female male

The female broods from one to ten eggs in a pouch on its back(note the limited parental care). This takes about 10 days. Sex is determined environmentally, so under normal situations, the eggs develop directly into asexual females, which reproduce parthenogenically.When water gets cold, and conditions are bound to change because of the onset of winter, sexual males and females hatch from eggs.

Sexual loop

Asexual loop

Reproduce Now? Or Do It Later?

• This is probably the most ubiquitous life-history tradeoff organisms face.

• DO IT NOW!– Reproducing now means that an organism does not incur the

chance of dying before they get another opportunity to reproduce.– In growing offspring, there is a “compound interest effect”, an

organism’s offspring will produce their own offspring sooner if reproduction is early. Thus, rapidly growing populations favor early reproduction.

• Mom told me to wait…..DO IT LATER– Reproducing later means that the organism may be able to amass

more resources, so that the bout of reproduction, when it finally occurs, is more successful.

• For a wide variety of organisms, delayed reproduction means increased body size, and increased body size means greater reproduction.

• Example;– Gizzard Shad reproducing at two years of age

produce 59,000 eggs. Those delaying reproduction until 3 years of age produce 379,000 eggs.

– Interestingly, the population is polymorphic. About 15% of the population reproduce at 2 years of age, and about 80% reproduce at 3 years of age.

Question; If humans began fishing a population of shad, so that the chance of survival between 2 and three years were dramatically reduced, how would you expect the population to evolve?

• David Reznick studied life history evolution in guppies.– Guppies grow fastest before they begin reproduction. Larger

guppies produce bigger broods of young.– He hypothesized that if there was a high risk of predation

early on, but that risk disappeared later in life, then larger individuals and delayed reproduction should be favored, because by delaying reproduction, a nice reproductive payoff was in store for the predation-free older guppies if they delayed reproduction and amassed resources.

– Conversely, if the risk of predation was high across-the-board, then there was no advantage to delayed reproduction, so early reproduction should be favored.

• Reznick went to Trinidad and located a variety of river systems.– Guppies can’t always go upstream from waterfalls, so there were

areas where guppies lived downstream, but not upstream. The larger predators don’t get over waterfalls at all.

– Reznick divided the habitats into two varieties; “high predation” and “low predation”.

– “Low Predation” areas have only a killifish that preys on juvenile individuals but can’t eat the big ones.

– “High Predation” areas have the killifish, and also have three big predators that eat large guppies.

• Experiment;– He introduced guppies from high predation

environments downstream, into low predation environments upstream.

– The downstream environments served as a control.– He predicted; later age at first reproduction, lower

reproductive allocation

• Experiment;– He introduced large predators to a low-predation

environment upstream.– He predicted that the guppies upstream would evolve

earlier reproduction, increased reproductive allocation.

• The results of his experiment matched his predictions perfectly.– Over the course of 7 years, the guppies introduced from high

predation environments to low predation environments evolved to reproduce later and be larger as adults, with less reproductive allocation.

– The areas where predators were introduced saw a decrease in the size of adult guppies, and an evolutionary increase in reproductive allocation.

– The introductions were done in 1981, the evolution continues.• Conclusion. The life history model is supported. Also interesting

is the demonstration that evolution can happen so quickly.

Check it outhttp://www.esof2004.org/pdf_ppt/session_material/reznick.pdf