Consider, if you will, the lowly Planarian. It is a flatworm, a relatively simple form of animal life. However it has a number of features recognizable in more complex organisms such as ourselves - eyes of a sort, a nervous system with a simple 'brain', a gut, an excretory system.
Hopefully most of you have been fortunate enough the planarians larger and more exciting cousin, the tapeworm. Tapeworms are vastly larger than the tiny planaria. Tapeworm are intestinal parasites while the planaria are free-living aquatic animals. Tapeworm lack virtually all of the structures mentioned above. They are basically a series of segments, each of which is chock full of reproductive tissue and not much else. Why is the tapeworm so astoundingly simple in its body plan?
Rather than answering that question I'm going to jump to another example illustrated by this rather dramatic picture.
The hapless amphibian is the Tungara frog, a common species in tropical forests in parts of Central and South America. Intensive study of this frog began in the 1970s by Michael Ryan, then a graduate student at Cornell. Ryan discovered, among other things, that male Tungara frogs make two types of calls to attract females. One is a whine, the other a chuck. The chuck is much more attractive to females than the whine. However the chuck is also much more easily detected and located by frog-eating bats. So the frogs are either high risk/high gain 'chuckers' or or low risk/low gain 'whiners'
Both of these are examples of trade offs between survival and reproduction. The Planarian lives in a world requiring a nervous system, gut etc. So it invests energy in developing these structures. That energy does not go into making gonads. In contrast the tapeworm lives in a largely predator free environment and is constantly bathed in nutrients. So it invests almost all of its resources in reproductive tissue. Within a single species, the Tungara frog has the evolutionary 'choice' between a low survival/high reproduction strategy and a high survival/low reproduction strategy.
Trade offs are an obvious component of evolution. We don't see organisms that are immortal, grow to adulthood in five minutes, have millions of offspring each year, all of which survive, have the ability to roast ducks out of the air with their laser beam eyes and draw the ducks into their mouths with magnetic fields generated by their teeth that attract the iron in duck hemoglobin. The idea is that any given organism has a set amount of energy to invest in growth, maintenance, and reproduction. The more energy you put into one, the less you can put into another. Different species, and different individuals within a species vary in their pattern of investment.
The role and even the existence of trade offs has been contentious within evolutionary biology. Many nice examples of variation in investment are known within species. In some cricket species there is variation in male calling. Some males call to attract mates while others are 'satellite' males that hang out silently near calling males and attempt to intercept approaching females. Not only do the satellite males avoid the energetic costs of calling but they are at reduced risk of being parasitized by the fly Ormia which is attracted to cricket calls and deposits larvae on them, eventually killing the cricket. Work on this system was pioneered by the husband of kossack TexMex.
Another example, also in crickets, is wing dimorphism (this occurs in other insects as well such as water striders). The male cricket above is long-winged - his hind wings extend beyond the back of his abdomen. Long-winged crickets have enlarged wing muscles and are capable of flight. Other individuals are short-winged and cannot fly. Having long wings allows individuals to disperse over long distances to reach new habitat but the development of the wing muscles and the energetic expense of flight represent substantial costs and mean less energy can be spent in reproduction.
Despite all these nice examples there has been substantial theoretical confusion about the meaning and importance of trade offs among evolutionary biologists. Let's look at a couple.
I. Phenotypic vs. Genotypic tradeoffs.
Many field biologists went out looking for tradeoffs. For example, an ornithologist studying a population of a song bird might gather data on number of eggs laid, offspring survival, amount of parental care, and survival (or not) of each parent to the next breeding season. What might be expected is that a high investment in reproduction (lots of eggs laid, more care per offspring) might result in a lowered probability of survival. In other words a negative correlation between investment in reproduction and survival.
Most studies of this sort instead found positive correlations. The individuals that laid the most eggs and supplied the most care actually were the most likely to survive. A paradox!
Well not really. The problem is that this is a phenotypic correlation. The phenotype is the trait that the organism displays (number of eggs, etc.). The phenotype is typically affected by both the genotype and the environment. Organisms who got a good draw of cards from the environment (good parents, a good winter territory for feeding) might be in such good shape that they are good at both reproducing and surviving. The environment may be masking the underlying genetic relationship.
At the time this issue was raised (the 1980s) the link between genetics and phenotype was still a black box for most of the traits in question. To more rigorous test for the idea of tradeoffs two approaches were used.
1. Field experiments. In birds clutch size could be experimentally altered by adding or removing eggs from nests. This allowed researchers to 'break' the environmental correlation between clutch size and other traits. They saw that adding eggs decreased survival while removing eggs increased survival (of parents).
2. Studies of genetic correlation. Elaborate breeding studies measured reproductive and survival traits in large numbers of individuals of known relatedness in controlled lab conditions. Mostly these were done in fruit flies due to the large numbers of individuals required. What they were looking to see is if genes or gene combinations that gave high survival resulted in low reproduction (and the reverse as well). These studies turned out to be very difficult to do, and impossible in most organisms other than those that can be reared in large numbers under controlled conditions.
Nowadays it is possible to look much more directly for genes affecting particular traits and the expectation is that we will understand more directly about trade offs at the level of gene mechanism.
However even the definite existence of a genetic basis for a trade off doesn't eliminate the second source of confusion.
II. Trade offs and the maintenance of genetic variation.
Let's go back to the Tungara frog. One might suppose that the existence of bats is allowing the two types of males to coexist. The whiners don't mate as often so they make up for it by living longer. This is a commonly held assumption. It is intuitive and it is wrong. It is wrong for two reasons.
1. Different costs and benefits don't guarantee equal success. Imagine that the 'chuck' males have an average reproductive lifespan of one month and sire and average of 1 tadpole per day. Imagine a 'whine' male has an average of 0.1 offspring per day and a reproductive lifespan of six months. This means that the average chuck male has 30 offspring and the average whine male 18. I just made these figures up but they illustrate the basic point - the existence of two separate forms that vary in survival and reproduction doesn't mean that they will be equally successful.
2. Even if they do have equal fitness they won't persist on their own. Now imagine that the whiners and chuckers are, on average, exactly equally successful in siring tadpoles. Even if that is the case, over the long term one of the two forms will be eliminated by genetic drift. Drift is random changes in the frequency of genes. If you flip a coin ten times you are quite likely to get something other than five heads and five tails just due to random chance. Drift works the same way and in the absence of other forces drift will remove one of the two forms from the population given enough time.
So what's the answer? Most likely for the examples we are looking at here, the different forms (whiners vs chuckers, callers vs satellites, long-winged vs short-winged) are preserved through frequency dependent selection. Frequency dependence generally means that whichever form is more rare is more successful. This is easily imaginable in the cricket system. If there are hardly any non-calling satellites they are likely to be fairly successful at finding females and will not get attacked by flies as there are so many calling males. As satellites become more common they are going to get fewer mates. TexMex herself, I believe, has participated in work looking at this issue.