Evolving to Maximise Fitness
Populations evolve to maximise their reproductive fitness in the habitat in which they live. This reproductive fitness is also called Darwinian fitness, and a high fitness simply means a high r value — a high intrinsic rate of natural increase.
Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy for those conditions. There is no single 'best' way to reproduce; what counts as efficient depends entirely on the environment the population is facing.
Breed Once or Breed Many Times
One major axis along which reproductive strategies differ is how often an organism breeds in its lifetime. Some organisms breed only once in their lifetime and then die. Pacific salmon fish and bamboo are classic examples of this once-in-a-lifetime strategy.
Others breed many times over the course of their lives. Most birds and mammals follow this repeated-breeding pattern, spreading their reproduction across several seasons.
Many Small or Few Large Offspring
A second axis of variation is the number and size of offspring produced. Some organisms produce a large number of small-sized offspring. Oysters and pelagic fishes are good examples — they release enormous numbers of tiny young, most of which will not survive.
Others take the opposite route and produce a small number of large-sized offspring. Birds and mammals typically do this, investing heavily in a few well-provisioned young that have a better chance of surviving.
Shaped by the Habitat
So which strategy is desirable for maximising fitness? The answer is that there is no universal winner. Ecologists suggest that the life-history traits of organisms have evolved in relation to the constraints imposed by the abiotic and biotic components of the habitat in which they live.
In other words, the same trait can be an advantage in one environment and a liability in another. Working out exactly how these traits evolve in different species is currently an active and important area of research among ecologists.
Quick Recap
- Populations evolve to maximise reproductive (Darwinian) fitness, meaning a high r value, in their habitat.
- Under a given set of selection pressures, organisms evolve the most efficient reproductive strategy.
- Some breed only once (Pacific salmon, bamboo); others breed many times (most birds and mammals).
- Some produce many small offspring (oysters, pelagic fishes); others produce few large offspring (birds, mammals).
- Life-history traits evolve in relation to the abiotic and biotic constraints of the habitat, an active field of ecological research.
Solved Examples — Section 7
Q1. What do populations evolve to maximise?
Answer: Their reproductive fitness, also called Darwinian fitness, which corresponds to a high r value in their habitat.
Q2. Give two examples of organisms that breed only once in their lifetime.
Answer: Pacific salmon fish and bamboo.
Q3. Which organisms typically breed many times during their lifetime?
Answer: Most birds and mammals.
Q4. Name organisms that produce a large number of small-sized offspring.
Answer: Oysters and pelagic fishes.
Q5. What kind of offspring strategy do birds and mammals usually follow?
Answer: They produce a small number of large-sized offspring.
Q6. In relation to what have life-history traits evolved?
Answer: In relation to the constraints imposed by the abiotic and biotic components of the habitat in which the organisms live.