5.2 Applications and skills
5.2.2 Natural selection on Daphne Major
- It is difficult to imagine the vast timescales necessary for macroevolution.
- Microevolution can occur within human timescales. The emergence of antibiotic resistance is a good example.
- Remember that natural selection increases the frequency of characteristics favourable to the environment. Certain species are able to adapt very quickly to rapid environmental change, in as little as 20–30 generations.
Figure 5.2.2b – Microevolution in the Australian black snake
Populations of Australian black snakes (Pseudechis porphyriacus) that have been previously exposed to poisonous toads do not attempt to use them as prey. Since toads are an invasive species and did not exist in Australia 20 generations ago, scientists have concluded that toad-avoiding behaviour has evolved in certain snake populations in a short period of time. Source: Proc. Biol. Sci. 22 June 2006; 273(1593): 1545–1550
Natural selection on Daphne Major
Daphne Major is an island in the Galapagos archipelago (see Figure 5.2.2f), where Peter and Rosemary Grant (see Figure 5.2.2e) studied morphological variations in a species of ground finch, Geospiza fortis, looking for evidence of rapid adaptive change.
Daphne Major is a small island with very few natural predators, so the Grants were confident in assuming that food availability and weather were the most important environmental pressures for the population of birds.
Figure 5.2.2c – A: Population fluctuations of G. fortis on Daphne Major; B: Food abundance in the same time period
Their hypothesis was that Darwin, who had formulated his theory of natural selection while observing finches on the Galapagos Islands, was right about the mechanism of evolution but wrong about the pace of evolution. Darwin believed that slow, gradual changes would lead to speciation.
The Grants returned to Daphne Major every year for almost 30 years, which is about six generations of finch lifecycles. They measured the mean beak length and shape of their finches and correlated these to annual weather and food availability patterns.
Figure 5.2.2d – Mean beak length and shape of G. fortis on Daphne Major, 1971–2001
Activity: Changes in the beaks of Darwin's finches
Using the information provided in the text and graphs above:
- Discuss the hypothesis that food availability is the most important selective pressure on Daphne Major.
- Outline the trends in beak length and shape for the years 1971–2001.
- Suggest reasons for the sharp decline in beak length in 1975 and the slow decline between 1980 and 1995.
- Evaluate the following statement: ‘Evolution is reversible.’
Aim 8: Ethical implications of science
Most of the rapid environmental change occurring on Earth is caused by human activity. How could species adaptability be measured in order to form part of an environmental impact assessment?
Nature of Science – evidence and deduction
Snakes do not have the capacity to ‘learn’ to avoid toads or teach others to avoid them – exposure to the poison is almost always fatal. The logical deduction is that avoidance behaviour has a genetic basis.
The ‘punctuated equilibrium’ model of evolution, proposed by American palaeontologist Stephen Jay Gould (1941–2002) (Figure 5.2.2d), suggests that there are periods of stasis followed by bursts of evolution due to rapid environmental change. He based his theory mainly on the fossil record.
Figure 5.2.2d – Stephen Jay Gould
Figure 5.2.2f – Map of the Galapagos archipelago