Evolution is the historical, irreversible development of living nature: species change, new ones arise, some die out, and organisms develop adaptations to their environment. Exam questions often cover the views of different scientists, the key ideas of Darwin’s theory, the forms of natural selection, adaptations and the evidence for evolution.
§1. Views before Darwin
C. Linnaeus classified living organisms and introduced binomial nomenclature (genus + species name). But he regarded species as unchanging and created (creationism).
G. Cuvier founded comparative anatomy and palaeontology; he explained the succession of fossil forms by the theory of catastrophes.
J. B. Lamarck ("Zoological Philosophy", 1809) wrote the first complete theory of evolution. He argued that species change. According to Lamarck, the causes of evolution were an inner drive of organisms towards perfection, the use and disuse of organs, and the inheritance of acquired characteristics.
Lamarck’s mistake: traits acquired during life (such as a muscle strengthened by exercise) are not inherited. But his idea that species change was correct.
§2. Darwin’s theory
C. Darwin sailed around the world on HMS Beagle (1831–1836), gathered a great many facts, and in 1859 published "On the Origin of Species". The key ideas of his theory:
Hereditary variation — Darwin called it indefinite (individual, heritable) variation; this is the raw material of evolution. Definite (group) variation appears in all individuals alike under the influence of the environment and is not inherited.
Struggle for existence — organisms reproduce in geometric progression, but food, space and other resources are limited, so most of those born die. Its forms are listed below.
intraspecific struggle — the most intense, because members of one species need the same resources;
interspecific struggle — predator and prey, parasite and host, competing species;
struggle against unfavourable conditions — cold, drought and so on.
Natural selection — the survival and greater reproduction of the best-adapted individuals. It is the directing factor of evolution.
Artificial selection is the choice by humans of individuals with traits useful to them; it produces breeds and varieties. Darwin distinguished unconscious and methodical selection. Artificial selection serves human interests; natural selection serves the survival of the organism itself.
Worked example
How would Lamarck and Darwin explain the giraffe’s long neck?
Lamarck: ancestors stretched their necks to reach leaves on high branches; the neck lengthened by exercise was passed on to offspring.
Darwin: among the ancestors there were individuals with longer and shorter necks (hereditary variation); when food was scarce, long-necked ones survived more often and left more offspring (natural selection).
Answer: modern science supports Darwin’s explanation.
§3. Forms of natural selection
Directional (driving) selection — when the environment changes, the average value of a trait shifts in one direction. The classic example is the peppered moth (industrial melanism): in England, when birch bark was blackened by soot, the formerly rare dark form became common, because birds spotted and ate the pale moths more often. Insect resistance to pesticides and bacterial resistance to antibiotics arise in the same way.
Stabilising selection — in constant conditions the average form is kept and the extremes are removed. For example, after a storm, birds with very long or very short wings died more often. Thanks to it, "living fossils" such as the coelacanth have changed little for a long time.
Disruptive selection — two or more extreme forms are kept in one population, while the average form disappears. For example, on windy islands, wingless insects and insects with very strong wings survive.
Sexual selection — competition among members of one sex for mates (the peacock’s tail, deer antlers).
§4. Adaptations and their relative nature
Adaptations arise through natural selection:
Protective colouration — matching the background: the white winter coat of the mountain hare, the green grasshopper.
Camouflage by shape — resembling objects in the environment: the stick insect (a twig), the Kallima butterfly (a dry leaf).
Warning colouration — bright colours of poisonous or stinging animals: ladybird, wasp.
Mimicry — an unprotected species resembles a protected one: some flies (hoverflies) look like wasps.
Physiological adaptations (camel fat, hibernation) and behavioural ones (nest building, care for offspring).
Every adaptation is relative: it is useful only in certain conditions. If snow falls late, a white hare stands out on dark ground; a swift with very long wings cannot take off from flat ground.
§5. Evidence for evolution
Comparative anatomy:
homologous organs — same origin and body plan, functions may differ: human arm, whale flipper, bat wing, mole forelimb;
analogous organs — similar function, different origin: wings of a butterfly and a bird, fish fin and whale flipper;
vestigial organs (rudiments) — organs that have lost their function: the human appendix, coccyx, ear muscles, remnant of a third eyelid; pelvic bones in whales;
atavisms — reappearance of ancestral traits in individual organisms: a human born with a tail, extra nipples, thick hair covering the face.
Embryology — K. Baer’s law: embryos of different vertebrates are similar at early stages (all have gill slits, a notochord and a tail). The biogenetic law (F. Müller, E. Haeckel): ontogeny is a brief repetition of phylogeny.
Palaeontology — fossils, transitional forms (Archaeopteryx — between reptiles and birds; lobe-finned fish — between fish and amphibians; therapsids, the beast-toothed reptiles — between reptiles and mammals), phylogenetic series (evolution of the horse, V. O. Kovalevsky).
Biogeography — the distinctive fauna of long-isolated continents such as Australia (marsupials).
Molecular biology — one genetic code in all organisms; proteins and DNA are more similar in closely related species.
Worked example
Sort the pairs into homologous and analogous organs: 1) a horse’s leg and a bat’s wing; 2) a bee’s wing and a pigeon’s wing; 3) cactus spines and barberry spines (both modified leaves); 4) a potato tuber and a carrot root.
Horse leg and bat wing — both are mammalian forelimbs → homologous.
Bee and pigeon wings — same function, different origin → analogous.
Both spines come from leaves → homologous.
The tuber is a stem, the carrot a root; both store food → analogous.
§6. The modern evolutionary synthesis
In the twentieth century Darwin’s theory was combined with genetics to form the modern synthesis (S. S. Chetverikov, R. Fisher, J. B. S. Haldane, T. Dobzhansky, E. Mayr and others). Its main principles:
the elementary unit of evolution is not an individual or a species but a population;
the elementary evolutionary material is mutation and combinative variation;
the elementary evolutionary event is a lasting change in a population’s gene pool (its allele frequencies).
Factors of evolution:
mutation — creates new alleles, but without direction;
population waves — periodic fluctuations in numbers; when numbers fall, some alleles are lost or become common by chance;
genetic drift — random change in allele frequencies in a small population;
isolation — geographical (mountains, rivers, seas) or biological (breeding season, behaviour, structure); it stops interbreeding between populations;
natural selection — the only directing factor: it preserves well-adapted genotypes.
Mutations supply the material, isolation separates it, and natural selection gives it direction.
The results of evolution are adaptations, the formation of new species (speciation) and the diversity of life. Changes within a species at the level of populations are called microevolution, and the origin of groups above the species level is called macroevolution.
Worked example
In good conditions a bacterium divides every 20 minutes. How many bacteria could arise from one cell in 10 hours? Which of Darwin’s ideas does this support?
10 hours = 600 minutes; number of divisions: 600 / 20 = 30.
Number of cells: 2^30 ≈ 1.07 × 10^9 — about a billion.
In nature such growth cannot go on for ever: food and space run out and waste builds up.
Answer: about 10^9 cells; this shows reproduction in geometric progression and why limited resources make the struggle for existence inevitable.
Check yourself
Who created the first complete theory of evolution?
Which form of the struggle for existence is the most intense?
The spread of the dark peppered moth is an example of which form of selection?
What kind of adaptation is a hoverfly’s resemblance to a wasp?
Is the human appendix a vestigial organ or an atavism?
What did Darwin regard as the directing factor of evolution?
According to the modern synthesis, what is the elementary unit of evolution?
Which factor of evolution is the only directing one?
Common mistakes
Calling a whale’s flipper and a fish’s fin homologous — they are analogous.
Mixing up Lamarck’s and Darwin’s ideas: inheritance of acquired traits is Lamarck’s idea.
Confusing vestigial organs and atavisms: everyone has vestigial organs; atavisms appear only in some individuals.
Treating Linnaeus as an evolutionist — he held species to be unchanging.
Linking stabilising selection to a changing environment — it acts in constant conditions.
Thinking adaptations are absolute — they are always relative.