Evolution: common doubts, answered
The questions students ask most often about Evolution, each with a short answer. For the full chapter, read the Evolution notes.
About the chapter
Is evolution the same as natural selection?
No. Natural selection is one mechanism of evolution, in which variants better suited to the environment leave more offspring. Evolution is the broader change in the frequencies of heritable traits in populations over generations. It also involves mutation, gene flow, genetic drift and recombination, which provide variation or alter allele frequencies independently of the environment.
Early earth and early ideas
Read this section in the notes →How old is the universe and when did life appear on earth?
The universe is almost 13.8 billion years old, the earth formed about 4.5 billion years ago, and life appeared roughly 4 billion years ago. Life therefore arose some time after the earth cooled, first as non-cellular forms and later as cells. These figures are used in timeline questions.
What did Louis Pasteur prove about the origin of life?
Pasteur showed that life comes only from pre-existing life and not from non-living matter. In pre-sterilised flasks, killed yeast gave rise to no new life, while killed yeast in a flask left open to the air soon held new organisms, carried in from the air. This disproved spontaneous generation, the old belief that life arose from decaying and rotting matter.
Chemical evolution and the first life
Read this section in the notes →What was the Miller experiment?
Stanley Miller, in 1953, recreated conditions thought to resemble the early earth: he passed electric discharges through a closed flask holding CH₄, H₂, NH₃ and water vapour, and amino acids formed. This supported the Oparin-Haldane idea that the first life arose from non-living organic molecules through chemical evolution. Other workers later obtained sugars, nitrogen bases, pigments and fats in similar experiments.
What is the Oparin-Haldane theory?
It proposes that the first life arose from non-living organic molecules in a process of chemical evolution under the conditions of early earth. Simple organic molecules formed from inorganic ones and became more complex, eventually giving rise to the first cell-like forms. Miller's experiment gave experimental support.
Special creation and Darwin's theory
Read this section in the notes →What is the difference between Darwin's natural selection and Lamarck's theory?
Darwin said that variations already present in a population are acted on by natural selection, so individuals better suited to the environment leave more offspring. Lamarck said organs change by use and disuse and that these acquired changes are passed on, as in the giraffe's stretching neck. Natural selection, not inheritance of acquired traits, is what modern biology accepts.
What does fitness mean in Darwin's theory?
Fitness means reproductive fitness, that is, the ability to leave more offspring that survive. It does not mean strength, size or a long life. A smaller, weaker animal that leaves more surviving offspring is fitter in the Darwinian sense. Natural selection favours those variations that increase reproductive success.
Fossils and embryos as evidence
Read this section in the notes →How do fossils show that evolution has occurred?
Fossils are remains or impressions of organisms of earlier times preserved in rocks. Different rock layers, formed at different times, contain different life forms, and the layers show a gradual change in forms over time. Fossils of extinct organisms tell us what past life looked like and show that many forms have disappeared.
Why was Haeckel's embryological evidence rejected?
Haeckel claimed that an embryo goes through the adult forms of its ancestors, but Karl Ernst von Baer disproved this. He observed that embryos of different animals look alike early on, for instance in showing gill slits in vertebrates, yet no embryo ever passes through another animal's grown-up stage. Only the idea of repeating adult stages was rejected.
Homology and analogy
Read this section in the notes →What is the difference between homologous and analogous organs?
Homologous organs have the same basic structure and origin but different functions, which results from divergent evolution. Analogous organs have different origin and structure but similar functions, which results from convergent evolution. The forelimbs of whale, bat, cheetah and human are homologous, while the wings of a butterfly and a bird are analogous.
Are sweet potato and potato homologous or analogous?
They are analogous. Sweet potato is a modified root and potato is a modified stem, so they have different origins but perform the same function of storing food. Many students call them homologous because both are tubers, but similar function with different origin means analogy.
Are the thorn of Bougainvillea and the tendril of Cucurbita homologous?
Yes. Both are modified stem structures that arise from the same part of the plant, but one has become a defensive thorn and the other a climbing tendril. Same origin with different functions is homology, and the case shows divergent evolution in plants.
Natural selection seen in action
Read this section in the notes →How does industrial melanism support natural selection?
Before industrialisation in England most peppered moths were white-winged and camouflaged on lichen-covered trees. After industrialisation soot darkened the trees, and the dark-winged moths were harder for birds to see, so their share rose. The change in proportions shows selection acting on an existing variation, and no moth had to change colour.
How is antibiotic resistance an example of evolution?
Bacteria in a population vary in their sensitivity to a drug. When an antibiotic is used, the sensitive ones die and the resistant ones survive and multiply, so the population becomes mostly resistant. Resistance to herbicides and pesticides arises the same way, and the examples are evolution through selection caused by human action.
Adaptive radiation
Read this section in the notes →What is adaptive radiation, with examples?
Adaptive radiation is the evolution of different species from a common ancestor in a geographical area, adapted to different habitats. Darwin's finches on the Galapagos Islands, which developed varied beaks, and the Australian marsupials are the standard examples. It is a case of divergent evolution.
What is convergent evolution, and what example links to Australian marsupials?
Convergent evolution is when unrelated groups in similar habitats evolve similar features. The Tasmanian wolf, a marsupial, and the placental wolf resemble each other because they live similar lives, though they come from different lineages. It results in analogous structures and contrasts with adaptive radiation, which is divergent.
Mutation and the Hardy-Weinberg principle
Read this section in the notes →What does the Hardy-Weinberg principle say?
It states that allele frequencies in a population are stable and remain constant from generation to generation if nothing disturbs them. This stable condition is genetic equilibrium. It is expressed as p² + 2pq + q² = 1 for genotypes and p + q = 1 for alleles. A change in the frequencies indicates evolution.
In Hardy-Weinberg, why is the heterozygote frequency 2pq?
Because a heterozygote Aa can arise in two ways: the A allele can come from the father and a from the mother, or the other way round. Each combination has the frequency pq, so the total is 2pq. The homozygotes are p² for AA and q² for aa.
What disturbs the equilibrium
Read this section in the notes →What are the five factors that disturb Hardy-Weinberg equilibrium?
Gene flow (migration), genetic drift, mutation, genetic recombination and natural selection. Each one can alter allele frequencies and so break the equilibrium. Genetic drift is chance change in frequency, strongest in small populations, and the founder effect is a dramatic version of it when a few migrants start a new population.
What is the difference between stabilising, directional and disruptive selection?
In stabilising selection more individuals acquire the mean value of a trait. In directional selection individuals at one extreme are favoured, so the peak shifts to that side. In disruptive selection both extremes are favoured and the mean becomes less common, giving two peaks. The type is seen from the shape of the curve over time.
Origin and evolution of man
Read this section in the notes →How do the brain sizes of Homo habilis, Homo erectus and Neanderthal man compare?
Homo habilis had a brain of about 650 to 800 cc, Homo erectus about 900 cc and Neanderthal man about 1400 cc, so brain size rose through the sequence. Habilis probably did not eat meat, while Homo erectus probably did. Students often mix up the three numbers, so learn them in the order of appearance.
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