Darwin, natural selection, adaptive radiation and convergent evolution
Charles Darwin saw that living things leave more young than can survive, and that the young differ. The ones whose differences suit their surroundings leave more descendants. This lesson follows the idea, shows it at work in moths, finches and marsupials, and separates adaptive radiation from convergent evolution.
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Before Darwin
Jean Baptiste de Lamarck, a French naturalist, gave an early idea. He said that organs that are used grow stronger, and organs that are not used become weak. He said that the changes gained in life pass to the young. In his story, the giraffe stretched its neck to reach high leaves and its young were born with longer necks.
This is called the use and disuse idea. It has not stood the test. A body change gained in life, such as the muscles of a weight lifter, is not passed on to the young through the genes.
Darwin and Wallace
Charles Darwin sailed round the world on the ship H.M.S. Beagle from 1831 to 1836. He saw the plants and animals of many places, and the living things of the Galapagos Islands. He also read an essay on populations by Thomas Malthus. Alfred Wallace, who studied the Malay Archipelago, came to similar ideas at the same time. In 1859 Darwin published his book On the Origin of Species.
Two key ideas form the Darwinian theory.
Branching descent. All species came from common ancestors, as the branches of a tree come from one trunk.
Natural selection. Nature selects those that are fitted to the surroundings.
The logic of natural selection
The idea follows from four observations.
Living things make more young than can survive. A single fish or a single weed makes thousands of offspring.
Resources are limited. The size of a population stays stable, apart from seasonal changes. So the young compete for food and space.
Individuals differ, and some differences are inherited.
Those with inherited features that suit the surroundings survive and leave more young. So their features become more common in the next generation.
This is natural selection. The measure of success is fitness. NCERT puts it as the end result of the ability to adapt and to be selected by nature. In biology, fitness means the number of fertile offspring that an individual leaves, not strength or speed.
Darwin thought that the useful variations were small and gradual. Hugo de Vries, who worked with the evening primrose, said that mutations are large, sudden and random, and could make a new species in one step. This is the idea of saltation. Today both are united in the modern theory, which you meet in the next course.
Natural selection in action: the moths of England
Before industry grew in England, around the 1850s, the white-winged form of the peppered moth Biston betularia was more common than the dark-winged form. The tree trunks were pale with lichen, and the pale moths were hard for birds to see. By about 1920, the dark form was more common than the pale. Smoke and soot from the factories had darkened the trunks. Now the pale moths were easy to see and the birds ate them, while the dark moths hid and survived.
Two tree trunks side by side with moths on them. On the left, before industry about the 1850s, the trunk is pale with patches of lichen and carries three pale moths and one dark moth. The text says more pale moths survive. On the right, after industry about 1920, the trunk is dark with soot and carries three dark moths and one pale moth. The text says more dark moths survive.
No moth changed its colour. The share of dark moths in the population changed, because they left more young. The population evolved.
The same logic explains the use of drugs and sprays. Microbes that resist an antibiotic and weeds that resist a herbicide are selected in a short time when the chemical is used too much. NCERT calls these examples of evolution caused by human action.
You should see that the dark moths are rare for many generations and then rise very fast, and that a larger ratio makes the rise quicker.
Adaptive radiation
When one ancestral group spreads into many habitats in an area and gives many species, each suited to its own way of life, the process is adaptive radiation.
Darwin's finches of the Galapagos Islands. A flock of seed-eating finches reached the islands from the mainland. They found different foods, and the beaks changed to suit them: strong beaks for big seeds, thin beaks for insects, long beaks for cactus, curved beaks for buds and leaves.
Australian marsupials. A number of marsupials, each different from the others, arose from one ancestral stock inside the island continent of Australia. A marsupial is a mammal whose young are born at an early stage and develop in a pouch.
A scheme of adaptive radiation. In the centre is the ancestral seed-eating finch from the South American mainland, with four arrows to four finches on the Galapagos Islands. The large ground finch has a stout strong beak for big seeds. The warbler finch has a thin pointed beak for small insects. The cactus finch has a long beak for cactus flowers and fruit. The vegetarian tree finch has a short curved beak for buds and leaves.
Adaptive radiation gives homologous features by divergent evolution.
Convergent evolution
In Australia there are also placental mammals, which are mammals whose young develop inside the mother. Animals in other continents, with a similar way of life, look like the Australian marsupials. The Tasmanian wolf, a marsupial, and the wolf, a placental mammal, are an example. The flying phalanger and the flying squirrel, and the marsupial mole and the mole, are other pairs. They arose from different ancestors but came to look alike, since they live in similar surroundings. This is convergent evolution, and the features are analogous.
Two columns of boxes joined by double headed arrows. On the left are marsupials of Australia and on the right are placental mammals of other continents, in pairs: Tasmanian wolf and wolf, flying phalanger and flying squirrel, marsupial mole and mole, numbat and anteater, spotted cuscus and lemur, Tasmanian tiger cat and bobcat, wombat and woodchuck, koala and sloth. A note says each pair has a similar way of life, which is convergent evolution.
Try it in Python
The program follows five generations of moths in a wood where dark moths survive better.
Practise with new numbers
Common mistakes
Saying that individuals evolve. Individuals are selected. Populations change.
Saying that the giraffe grew a longer neck by stretching. This is Lamarck's idea. In Darwin's view, the giraffes with longer necks left more young.
Saying that natural selection makes new variations. Variation comes from mutation and recombination. Selection chooses among them.
Thinking that fitness means strength. It means the number of offspring that grow up and breed.
Mixing up adaptive radiation and convergent evolution. Radiation: one ancestor, many forms. Convergence: many ancestors, similar forms.
Remember these
Darwin: branching descent and natural selection. Lamarck: use and disuse (not accepted). De Vries: mutation theory.
Natural selection: more young than survive, limited resources, inherited variation, differential survival and breeding. Fitness means the number of offspring.
Peppered moth: pale moths common in the 1850s, dark moths common in 1920 after the soot.
Adaptive radiation: Darwin's finches, Australian marsupials. Convergent evolution: placental mammals and the marsupials that match them.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
State two points of Darwin's theory that differ from Lamarck's.
What did the pale moths and the dark moths show about natural selection in a place with soot?
How are Darwin's finches an example of adaptive radiation?
The Tasmanian wolf and the wolf look alike but are not close relatives. What is this called?
In a population, dark moths are 20 per cent at the start, and each generation the ratio of dark to pale moths is multiplied by 2. What is the share of dark moths after one generation?
References
NCERT (2023). Biology, Textbook for Class XII, chapter 6.
Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 18, Evolution and the Origin of Species.
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