Life histories — the specific details of an organism's life cycle and reproductive strategy — differ greatly among species. To get a glimpse of this diversity, consider the differences between a penguin and a salmon.
A salmon swims up a river to spawn, struggling against the current, jumping waterfalls, and dodging hungry bears. After years of living in the ocean chasing prey, avoiding predators, and storing up enough energy, it makes this final trip to the place of its birth. When it arrives, it produces thousands of eggs and dies soon after.
An emperor penguin — the only terrestrial vertebrate (other than a few scientists) to winter in Antarctica — can live more than 20 years. Each year, it makes a perilous trip to its inland breeding grounds, walking almost 100 miles in tiny steps, only to produce a single egg.
Both of these species have been shaped by evolution, but in very different ways. What it takes to be a fit salmon is obviously very different from what it takes to be a fit penguin. And before you say well, sure, one's a fish and the other's a bird, remember that not all fish die after reproducing once, and not all birds lay one egg a year for 20 years. Life histories vary significantly even among the same types of animals.
Scientists understand enough of the underlying genetics to know that life-history characteristics are heritable and that, over time, they change, or evolve. The fact of life-history evolution is that like evolution of other traits, it happens. What evolutionary biologists want to understand is why. How does selection drive this process? And how can so many different life-history strategies exist?
Scientists have a theoretical framework that explains the facts they observe, and they can test these ideas both in the lab and in nature. Concepts that at first seemed confusing (such as death and aging) make sense now, in light of an understanding of the evolutionary process.
Organisms don't live in a vacuum. The selective pressures they experience are a combination of their biotic and abiotic environment — that is, the organisms they interact with and the physical factors (temperature and such) they contend with. These factors are different for each organism, and the variations are responsible for the corresponding diversity of life-history patterns. In other words, no single life-history strategy is best because no one pattern could be best across all the different environments. Fortunately, scientists have a good understanding of how specific environmental differences influence the evolution of life histories.
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