Evolution and Speciation

Lessons

Senior 6 · Biology

Evolution and Speciation

5 lessons

Lesson 2 of 5

Theories of evolution and the causes of evolution

30 min

Fish without eyes

In the dark caves of north-eastern Mexico live pale fish with no working eyes. Their close relatives in the rivers outside, of the same species (Astyanax mexicanus), have normal eyes and colour. Why have the cave fish lost their eyes? One answer is that the eyes, never used, shrank in each generation and the shrinking was passed on. Another is that, in the dark, eyes gave no advantage and cost energy to build, so fish with smaller eyes did slightly better and left more offspring. These two answers are the theories of Lamarck and of Darwin. This lesson compares them, shows how the modern theory grew out of Darwin's, and lists the processes that actually cause evolution.

Definition

Evolution: a change in the inherited characteristics of a population over generations; at the genetic level, a change in allele frequencies.

Lamarckism: the theory that characteristics acquired by an organism during its life, through use or disuse of its organs, are passed to its offspring.

Darwinism: the theory that species change by natural selection acting on inherited variation ("descent with modification").

Modern synthesis (neo-Darwinism): the combination of Darwin's natural selection with Mendelian and population genetics, explaining evolution as changes in allele frequencies caused by mutation, selection, drift and gene flow.

Gene flow: the movement of alleles between populations when individuals or their gametes migrate.

Sexual selection: selection caused by differences in success at obtaining mates.

Lamarck's theory

Jean-Baptiste de Lamarck, a French naturalist, published the first full theory of evolution in 1809. He proposed that:

organisms have a tendency to become more complex over time;
use and disuse: organs that are used a lot grow larger and stronger, and organs that are not used shrink;
inheritance of acquired characteristics: changes acquired in this way during an organism's life are passed on to its offspring.

His best-known example is the giraffe: ancestors stretched their necks to reach leaves, their necks lengthened a little, and their young were born with slightly longer necks. Lamarck deserves credit for arguing that species change over time and are adapted to their environments. But his mechanism is wrong. In the 1880s August Weismann cut off the tails of mice for several generations, and their young were always born with normal tails. More fundamentally, characteristics acquired by body cells do not change the DNA of the gametes; this separation of body cells and germ cells is called the Weismann barrier. (Some chemical tags on DNA that affect gene activity, called epigenetic marks, can occasionally pass to the next generation, but they do not produce the lasting, cumulative adaptation that Lamarck described.)

Darwin and Wallace

Charles Darwin developed his theory after the voyage of HMS Beagle (1831 to 1836), during which he studied the fossils, animals and plants of South America and the Galápagos Islands. Alfred Russel Wallace reached the same idea independently while working in the Malay Archipelago. Their papers were presented together in 1858, and Darwin's book On the Origin of Species followed in 1859. Darwin drew on:

his observation of variation in wild species and in domestic animals, where pigeon breeders had produced very different forms by artificial selection;
Thomas Malthus's argument that populations grow faster than their food supply, so many individuals must die;
the patterns of species on islands, and the geology of a very old Earth.

The weakness of Darwin's theory in his own time was that he did not know how characteristics are inherited. Many people then believed in "blending" inheritance, which would dilute any new variation. Gregor Mendel's experiments (1866), which showed that inheritance is carried by particles (genes) that do not blend, were not widely known until 1900.

Worked example 1: testing Lamarck's idea

A pupil argues: "Blacksmiths develop strong arm muscles, so their children are born strong. This proves Lamarck was right." Evaluate the argument.

What Lamarck would predict. If acquired characteristics were inherited, children of blacksmiths would be stronger at birth than other children, even if raised identically.
Problems with the evidence. Children of blacksmiths may share genes for a strong build (inherited, not acquired), and they often grow up doing similar physical work (environment). Neither needs Lamarck's mechanism.
A fair test. Compare children of people who built muscle by training with children of genetically similar people who did not, raised in the same way, as Weismann did with mice. Such tests have not supported inheritance of acquired characteristics.
The mechanism. Exercise changes muscle cells, not the DNA in sperm or eggs, so it cannot change the alleles passed on.

Conclusion. The observation does not separate genetic and environmental causes, so it does not support Lamarck.

The modern synthesis and later ideas

Between about 1918 and 1950, biologists combined Darwin's natural selection with Mendelian genetics and the mathematics of population genetics (lesson 5 of Unit 11). In this modern synthesis, variation comes from mutation and from the reshuffling of alleles in meiosis and fertilisation; natural selection, genetic drift and gene flow change allele frequencies; and the accumulation of such changes, together with isolation, produces new species. Since then:

molecular biology has shown the same genetic code in nearly all organisms and allows evolution to be studied directly in DNA (lesson 4);
the neutral theory (Motoo Kimura, 1968) showed that many changes in DNA have no effect on fitness and spread or disappear by drift;
the fossil record suggests that the pace of change varies. In gradualism, species change slowly and steadily. In punctuated equilibrium (Niles Eldredge and Stephen Jay Gould, 1972), species stay almost unchanged for long periods, and most change happens in short bursts, often when new species form.

The causes of evolution

Cause	How it changes allele frequencies	Example
Mutation	Creates new alleles; the ultimate source of all variation. Usually too slow on its own to change frequencies much, but essential as raw material.	The allele for sickle-cell haemoglobin arose by a single base substitution.
Natural selection	Alleles that increase survival and reproduction become more common; the only cause that consistently produces adaptation.	Antibiotic resistance; deeper finch beaks after drought.
Genetic drift	Random changes, strongest in small populations; can fix or lose alleles regardless of their effect.	Loss of variation in elephant seals after a bottleneck.
Gene flow (migration)	Brings alleles into a population or takes them out; makes populations more similar.	Pollen carried between fields; animals moving between reserves.
Non-random mating and sexual selection	Some individuals mate more than others, so their alleles increase, even if they do not survive better.	In Kenya, male long-tailed widowbirds whose tails were experimentally lengthened attracted more females than those whose tails were shortened.

Worked example 2: how much new variation does mutation supply?

Approximate values. The human mutation rate is about $1 \times 10^{-8}$ new mutations per base per generation, and each person has two copies of a genome of about $3 \times 10^{9}$ bases. (a) Estimate the number of new mutations in one person. (b) Estimate the number in one generation of a population of one million people. (c) Why does this not make populations change rapidly?

(a) $2 \times 3 \times 10^{9} \times 1 \times 10^{-8} = 60$ new mutations per person.
(b) $60 \times 1 \times 10^{6} = 6 \times 10^{7}$ new mutations in the population each generation.
(c) Each particular new allele starts as a single copy among two million, so mutation alone changes the frequency of any one allele very little. Most new mutations are neutral and many are lost by drift; it is selection and drift acting on this huge supply of variation that change allele frequencies.

Warning

Common mistakes

Lamarckian wording in Darwinian answers. Do not write that organisms "adapt in order to survive" or "develop" features because they need them. Write that individuals with a pre-existing advantageous variation survived and passed on their alleles.
"Evolution is just a theory, meaning a guess." In science a theory is a well-tested explanation supported by many lines of evidence, as the next two lessons show.
"Evolution always means progress." Evolution produces adaptation to current conditions; cave fish losing eyes is evolution too.

FuseSchool: "Theories of Evolution: Lamarck vs Darwin". Compare the explanations in the video with Figure 1.

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Practice: theories and causes of evolution

State the two main ideas of Lamarck's theory.
Explain how a Darwinian would explain the long necks of giraffes.
Describe Weismann's experiment and explain why the Weismann barrier makes Lamarck's mechanism impossible.
What was the main weakness of Darwin's theory in 1859, and how was it overcome?
List five causes of evolution, and state which one is the only source of new alleles.
Distinguish between gradualism and punctuated equilibrium.
Using Worked example 2, estimate the number of new mutations in a population of 50 000 people in one generation.
In the widowbird experiment, males with lengthened tails attracted more females. Explain how this could lead to the evolution of long tails even if long tails make males slightly more likely to be caught by predators.

Show answersHide answers

Use and disuse (organs used grow; unused ones shrink) and the inheritance of acquired characteristics.
Ancestral giraffes varied in neck length because of different alleles. When food was scarce, those with longer necks could reach more leaves, survived and reproduced more, and passed on alleles for long necks; over many generations the average neck length increased.
He cut off the tails of mice for several generations; their offspring always had normal tails. Changes to body cells do not change the DNA of the germ cells that make gametes, so acquired characteristics cannot be passed on.
He had no mechanism of inheritance, and blending inheritance would dilute new variation. Mendel's particulate inheritance, rediscovered in 1900 and combined with selection in the modern synthesis, solved it.
Mutation, natural selection, genetic drift, gene flow, non-random mating (sexual selection). Mutation is the only source of new alleles.
Gradualism: slow, steady change over long periods. Punctuated equilibrium: long periods of little change (stasis) broken by short periods of rapid change, often when new species form.
$60 \times 50\,000 = 3 \times 10^{6}$ new mutations.
Long-tailed males obtain more mates and father more offspring, passing on alleles for long tails. If the extra reproduction outweighs the extra risk of death, the alleles increase in frequency: sexual selection can favour a characteristic that natural selection alone would not.

Key points

Lamarck: use and disuse, and inheritance of acquired characteristics; rejected because acquired changes do not alter the DNA of gametes (Weismann).
Darwin and Wallace (1858–59): natural selection acting on inherited variation produces descent with modification.
Modern synthesis: natural selection plus genetics; evolution is a change in allele frequencies.
Causes: mutation (new alleles), natural selection, genetic drift, gene flow, non-random mating and sexual selection.
The pace may be gradual or punctuated.

Sources and credits

18.1 Understanding Evolution and 19.2 Population Genetics, from Biology 2e by Mary Ann Clark, Matthew Douglas and Jung Choi, OpenStax (Rice University), licensed under CC BY 4.0. Used for the history of evolutionary theory and the causes of evolution. Adapted by Ganzaa.
Video: "Theories of Evolution: Lamarck vs Darwin", FuseSchool, embedded from YouTube.

Figures 1 and 2, the table, the worked examples and the practice questions were written by Ganzaa.

1Fish without eyes

2Lamarck's theory

3Darwin and Wallace

4The modern synthesis and later ideas

5The causes of evolution

Fish without eyes

Lamarck's theory

Darwin and Wallace

The modern synthesis and later ideas

The causes of evolution