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Evolution: Class 12 Biology
Origin of life ideas, evidence for evolution, Lamarck's and Darwin's theories, natural selection, variation and adaptation, speciation, the Hardy Weinberg idea, and the evolution of humans.
Key points
Evolution is the gradual change in the characteristics of populations of organisms over many generations. It explains how the great variety of species came from earlier forms. Life is thought to have begun in the sea about 3.5 billion years ago. The Oparin and Haldane idea proposes that simple molecules in the early atmosphere formed organic molecules, and later the first cells. Miller and Urey made amino acids by passing sparks through a mixture of water vapour, methane, ammonia and hydrogen.
Evidence for evolution:
- Fossils: the remains of ancient organisms in rocks. Older rocks hold simpler forms, and series such as the horse show gradual change. Archaeopteryx has features of both reptiles and birds.
- Comparative anatomy: homologous organs have the same basic structure but different functions, for example the forelimb of a human, a bat and a whale. They show common ancestry. Analogous organs have different structure but the same function, such as wings of insects and birds. Vestigial organs, such as the human appendix, are reduced and have lost their use.
- Comparative embryology: the early embryos of fish, reptiles and mammals look alike.
- Biochemistry: all organisms use DNA, the same genetic code and similar proteins. The fewer the differences in the sequence of cytochrome c or haemoglobin between two species, the closer they are related.
- Geographical distribution: for example, the marsupials of Australia.
Lamarck proposed that the use and disuse of organs changes them in an individual, and that these acquired characters are passed on. A giraffe was said to stretch its neck and pass the longer neck to its young. This is not accepted, because characters acquired in life do not change the DNA of the gametes. Weismann cut the tails of mice for many generations and the young still had tails.
Charles Darwin, after his voyage on HMS Beagle and study of the Galapagos finches, published the theory of natural selection in 1859, and Alfred Russel Wallace reached the same idea independently. The theory is based on observations. Organisms produce more offspring than can survive. The offspring show variation. There is a struggle for existence, as resources are limited. The individuals best suited survive and breed, so favourable characters are passed on and become more common in later generations. This is survival of the fittest.
Variation is the raw material of selection. It arises from mutation, from crossing over and independent assortment in meiosis, and from random fertilisation. Only inherited variation matters for evolution. Variation due to the environment alone, such as a plant that is small because it lacks water, is not inherited.
Examples of natural selection. In industrial melanism the peppered moth had pale and dark forms. In polluted areas with soot covered trees the dark form was hidden from birds and became common, while in clean areas the pale form was more common. Antibiotic resistance in bacteria arises when a chance mutation lets a bacterium survive a drug. The survivors multiply and pass on resistance. Pesticide resistance in insects works in the same way.
Adaptation is a feature that helps an organism to survive in its habitat, such as the thick fat layer of polar bears or the spines and deep roots of a cactus. Adaptations may be structural, physiological or behavioural. They arise through natural selection acting over many generations.
A species is a group of organisms that can interbreed to produce fertile offspring. Speciation is the formation of new species. In allopatric speciation a population is split by a barrier such as a mountain or a sea. The two groups face different conditions, change in different ways, and finally cannot interbreed even if brought back together. Darwin's finches on different islands are an example.
The Hardy Weinberg principle states that in a large, randomly mating population with no mutation, migration or selection, allele frequencies stay constant from generation to generation. If p and q are the frequencies of two alleles, p + q = 1 and the genotype frequencies are p² + 2pq + q² = 1. Departure from these values shows that evolution is occurring.
Human evolution: humans and apes share a common ancestor, and humans did not come from modern apes. Major steps include Australopithecus (walked upright, small brain, about 4 to 2 million years ago), Homo habilis (made simple tools), Homo erectus (used fire, larger brain) and Homo sapiens, which appeared in Africa about 300 000 years ago. Trends include a larger brain, upright posture, a smaller face and jaws, and use of tools and language.
Study tip: Learn Darwin's theory as five linked steps (overproduction, variation, struggle, survival of the fittest, inheritance) and use a named example such as the peppered moth or antibiotic resistance to earn full marks. Keep a short list of evidence points with one example each.
Definitions and flashcards
Say the meaning first, then open the card to check.
Evolution
Change in the inherited characteristics of a population over successive generations.
Natural selection
The process by which better adapted individuals survive and reproduce more than others.
Homologous organs
Organs with the same basic structure and origin but different functions.
Analogous organs
Organs with the same function but different structure and origin.
Vestigial organ
A reduced organ that has lost most or all of its original function.
Fossil
The preserved remains or traces of an organism that lived long ago.
Adaptation
A feature that increases the chance of survival and reproduction in a habitat.
Speciation
The formation of new species from existing ones.
Industrial melanism
The increase of dark forms of an organism in polluted, dark habitats.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Differentiate between homologous and analogous organs with one example each. (2 marks)
Homologous organs have the same origin and structure but different functions, for example the forelimb of a human and the flipper of a whale. Analogous organs have different origin but the same function, for example the wing of a bird and the wing of an insect.
Why is Lamarck's theory not accepted? (2 marks)
Lamarck said that characters acquired during life are inherited. Such changes affect body cells and not the DNA of the gametes, so they cannot be passed on. Experiments such as cutting tails of mice for generations showed no effect on the young.
State the main points of Darwin's theory of natural selection. (3 marks)
Organisms produce more offspring than the environment can support. Members of a species show inherited variation. Because resources are limited there is a struggle for existence. Those with favourable variations survive and reproduce more, and pass these on to their young. Over many generations the favourable character becomes common and new species may form.
Explain how bacteria become resistant to an antibiotic. (3 marks)
In a large population a chance mutation may give a bacterium the ability to survive the drug. When the antibiotic is used, non-resistant bacteria die and the resistant one survives. It reproduces without competition and passes the resistance allele to its offspring. After some generations most of the population is resistant. This is natural selection.
Explain the evolution of the peppered moth. (3 marks)
Before industry, tree bark was pale and the pale moths were hidden from birds, while the dark moths were eaten. Soot from factories blackened the trees, so the dark moths were now hidden and the pale ones were eaten. The dark form, favoured by selection, survived, bred and became common in polluted areas. When air became cleaner the pale form increased again.
Describe the evidence for evolution from fossils and comparative anatomy. (5 marks)
Fossils are preserved remains in rocks. Older rock layers contain simpler organisms, and some fossils, such as Archaeopteryx, show features of two groups (reptile and bird), which suggests one group arose from the other. Fossil series, such as that of the horse, show gradual change in size and teeth. In comparative anatomy, homologous organs, such as the pentadactyl limbs of humans, bats and whales, have the same bone plan although they have different uses, which suggests a common ancestor and adaptation to different ways of life. Vestigial organs, such as the human appendix, show that organs became reduced as the way of life changed. Together these show that living things are related and have changed over time.
Common mistakes
- Saying that individuals evolve. Populations evolve over generations, individuals do not.
- Writing that humans evolved from chimpanzees. Both share a common ancestor.
- Describing natural selection as a deliberate need, such as 'giraffes needed longer necks'.
- Confusing homologous and analogous organs.
- Calling variation caused only by the environment a basis for evolution. Only inherited variation counts.