The Development of Understanding of Genetics and Evolution¶

Part of 4.6 Inheritance, Variation and Evolution.

This topic is about how scientific ideas changed over time. It shows that theories of evolution and inheritance were built gradually as new evidence and better technology became available.

Learning Objectives¶

ID	Official specification wording	Main teaching sections
`4.6.3-lo-1`	4.6.3.1 Charles Darwin, as a result of observations on a round the world expedition, backed by years of experimentation and discussion and linked to developing knowledge of geology and fossils, proposed the theory of evolution by natural selection. 4.6.3.1 • Individual organisms within a particular species show a wide range variation for a characteristic. 4.6.3.1 • Individuals with characteristics most suited to the environment are more likely to survive to breed successfully. 4.6.3.1 • The characteristics that have enabled these individuals to survive are then passed on to the next generation. 4.6.3.1 Darwin published his ideas in On the Origin of Species (1859). There was much controversy surrounding these revolutionary new ideas. 4.6.3.1 The theory of evolution by natural selection was only gradually accepted because: 4.6.3.1 • the theory challenged the idea that God made all the animals and plants that live on Earth 4.6.3.1 • there was insufficient evidence at the time the theory was published to convince many scientists 4.6.3.1 • the mechanism of inheritance and variation was not known until 50 years after the theory was published. 4.6.3.1 Other theories, including that of Jean-Baptiste Lamarck, are based mainly on the idea that changes that occur in an organism during its lifetime can be inherited. We now know that in the vast majority of cases this type of inheritance cannot occur. 4.6.3.1 A study of creationism is not required.	Darwin and the Theory of Natural Selection, Lamarck's Theory (Now Disproved)
`4.6.3-lo-2`	4.6.3.2 Students should be able to: 4.6.3.2 • describe the work of Darwin and Wallace in the development of the theory of evolution by natural selection 4.6.3.2 • explain the impact of these ideas on biology. 4.6.3.2 Alfred Russel Wallace independently proposed the theory of evolution by natural selection. He published joint writings with Darwin in 1858 which prompted Darwin to publish On the Origin of Species (1859) the following year. 4.6.3.2 Wallace worked worldwide gathering evidence for evolutionary theory. 4.6.3.2 He is best known for his work on warning colouration in animals and his theory of speciation. 4.6.3.2 Alfred Wallace did much pioneering work on speciation but more evidence over time has led to our current understanding of the theory speciation. 4.6.3.2 Students should be able to describe the steps which give rise to new species.	Alfred Russel Wallace and Speciation
`4.6.3-lo-3`	4.6.3.3 Students should be able to: 4.6.3.3 • describe the development of our understanding of genetics including the work of Mendel 4.6.3.3 • understand why the importance of Mendel’s discovery was not recognised until after his death. 4.6.3.3 In the mid-19th century Gregor Mendel carried out breeding experiments on plants. One of his observations was that the inheritance of each characteristic is determined by ‘units’ that are passed on to descendants unchanged. 4.6.3.3 In the late 19th century behaviour of chromosomes during cell division was observed. 4.6.3.3 In the early 20th century it was observed that chromosomes and Mendel’s ‘units’ behaved in similar ways. This led to the idea that the ‘units’, now called genes, were located on chromosomes. 4.6.3.3 In the mid-20th century the structure of DNA was determined and the mechanism of gene function worked out. 4.6.3.3 This scientific work by many scientists led to the gene theory being developed.	Gregor Mendel and the Origins of Genetics
`4.6.3-lo-4`	4.6.3.4 Students should be able to describe the evidence for evolution including fossils and antibiotic resistance in bacteria. 4.6.3.4 The theory of evolution by natural selection is now widely accepted. 4.6.3.4 Evidence for Darwin’s theory is now available as it has been shown that characteristics are passed on to offspring in genes. There is further evidence in the fossil record and the knowledge of how resistance to antibiotics evolves in bacteria. 4.6.3.7 Bacteria can evolve rapidly because they reproduce at a fast rate. 4.6.3.7 Mutations of bacterial pathogens produce new strains. Some strains might be resistant to antibiotics, and so are not killed. They survive and reproduce, so the population of the resistant strain rises. The resistant strain will then spread because people are not immune to it and there no effective treatment. 4.6.3.7 MRSA is resistant to antibiotics. 4.6.3.7 To reduce the rate of development of antibiotic resistant strains: 4.6.3.7 • doctors should not prescribe antibiotics inappropriately, such as treating non-serious or viral infections 4.6.3.7 • patients should complete their course of antibiotics so all bacteria are killed and none survive to mutate and form resistant strains 4.6.3.7 • the agricultural use of antibiotics should be restricted. 4.6.3.7 The development of new antibiotics is costly and slow. It is unlikely to keep up with the emergence of new resistant strains.	Evidence for Evolution
`4.6.3-lo-5`	4.6.3.5 Fossils are the ‘remains’ of organisms from millions of years ago, which are found in rocks. 4.6.3.5 Fossils may be formed: 4.6.3.5 • from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent 4.6.3.5 • when parts of the organism are replaced by minerals as they decay 4.6.3.5 • as preserved traces of organisms, such as footprints, burrows and rootlet traces. 4.6.3.5 Many early forms of life were soft-bodied, which means that they have left few traces behind. What traces there were have been mainly destroyed by geological activity. This is why scientists cannot be certain about how life began on Earth. 4.6.3.5 We can learn from fossils how much or how little different organisms have changed as life developed on Earth. 4.6.3.5 Students should be able to extract and interpret information from charts, graphs and tables such as evolutionary trees. 4.6.3.6 Extinctions occur when there are no remaining individuals of a species still alive. 4.6.3.6 Students should be able to describe factors which may contribute to the extinction of a species.	Evidence for Evolution, Extinction

Darwin and the Theory of Natural Selection¶

Before the 19th century, the dominant explanation for the diversity of life was creationism — the idea that all species were created separately and do not change. From the 18th century onwards, naturalists began to collect observations that challenged this view.

Charles Darwin (1809–1882) spent years observing nature, including a famous voyage to the Galapagos Islands. He noticed that individuals within the same species vary in their characteristics, and that not all individuals survive to reproduce. In 1859 he published On the Origin of Species, proposing the theory of natural selection:

Individuals within a species vary in their characteristics.
Some of this variation is heritable (passed from parent to offspring).
More offspring are born than can survive (competition for resources).
Individuals with advantageous characteristics are more likely to survive and reproduce.
These advantageous characteristics become more common in the next generation.
Over many generations, this can cause significant changes in the population — evolution.

Darwin's ideas were controversial. They conflicted with religious beliefs and, at the time, the mechanism of inheritance was not known.

Lamarck's Theory (Now Disproved)¶

Jean-Baptiste Lamarck (1744–1829) proposed an earlier theory of evolution: that characteristics an organism acquires during its lifetime can be passed on to offspring. For example, if a giraffe stretched its neck throughout its life, its offspring would be born with a longer neck.

This theory is now known to be incorrect. Acquired characteristics cannot be inherited because changes to the body during a lifetime do not alter the DNA in an organism's gametes. Lamarck is remembered as a stepping stone — he recognised that species change over time, but had the wrong mechanism.

Alfred Russel Wallace and Speciation¶

Alfred Russel Wallace independently developed a theory of natural selection at the same time as Darwin. His contribution prompted Darwin to publish his work. Wallace is also known for his work on speciation.

Theory of Speciation¶

A new species can form when a population is split by a barrier (e.g. a mountain range, river, or ocean), preventing interbreeding. Over time: 1. Different mutations arise in each isolated group. 2. Natural selection acts differently in each environment. 3. The two groups accumulate different genetic changes. 4. Eventually they become so different that they cannot interbreed even if the barrier is removed — they have become two separate species.

This process is called allopatric speciation (isolation-driven speciation).

Gregor Mendel and the Origins of Genetics¶

Gregor Mendel (1822–1884), a monk working in the mid-19th century, used breeding experiments with pea plants to study how characteristics are inherited. He studied traits such as seed shape (smooth vs. wrinkled) and discovered that they are passed on in discrete "units."

His key findings: - Some characteristics are dominant (always expressed when present) and some are recessive (only expressed when two copies are present). - Characteristics are inherited independently of one another. - His results could be expressed as clear numerical ratios (e.g. 3 dominant : 1 recessive).

Mendel's findings were ahead of their time. His work was not recognised during his lifetime because it was published in an obscure journal and the concept of genes was not yet established. Only after his death, when chromosomes were observed, did scientists realise his "units" were genes. Watson and Crick's discovery of DNA's double-helix structure in 1953 finally provided the molecular explanation for Mendel's observations.

Evidence for Evolution¶

Fossils¶

Fossils are the preserved remains or traces of ancient organisms in rock. They provide evidence for evolution by:

Showing that simpler life forms appear in older rocks and more complex organisms in newer rocks.
Revealing intermediate forms between ancient and modern species.
Tracing lineages — e.g. human evolution through hominid fossils.

Hominid fossil examples: - Ardi (Ardipithecus ramidus, ~4.4 million years old): brain similar to chimpanzees; walked upright but also climbed trees. - Lucy (Australopithecus afarensis, ~3.2 million years old): slightly larger brain than Ardi; feet more suited to walking than climbing. - Turkana Boy (~1.6 million years old, found by Richard Leakey): brain much closer to modern humans; fully adapted to upright walking.

Pentadactyl limbs (five-digit limbs found in mammals, birds, reptiles and amphibians) suggest all these groups share a common ancestor — the basic bone structure is the same across species despite being adapted to different uses (human hand, whale flipper, bat wing).

Limitations of fossil evidence: fossilisation is rare (soft-bodied organisms rarely fossilise); many fossils have been destroyed; the record is incomplete.

Antibiotic Resistance in Bacteria¶

Antibiotic resistance is direct, observable evidence of evolution by natural selection happening within human lifetimes:

A population of bacteria contains random variation (due to mutations in DNA).
Antibiotics are applied — most bacteria die, but any with a mutation that confers resistance survive.
The resistant bacteria reproduce rapidly, passing on the resistance gene.
The next generation is predominantly resistant.

MRSA (Methicillin-resistant Staphylococcus aureus) is a well-known example of a bacterium that evolved resistance to a powerful antibiotic.

Measures to reduce the spread of antibiotic resistance: - Only prescribe antibiotics when necessary (not for viral infections). - Patients should complete the full course so all bacteria are eliminated. - Reduce use of antibiotics in agriculture (e.g. routine use to promote animal growth). - Developing new antibiotics, though this is costly and time-consuming.

Extinction¶

Extinction occurs when every individual of a species has died and none remain. Causes include:

A new predator, disease or competitor that the species cannot withstand.
Environmental change (climate, habitat loss) that removes the conditions the species needs.
Catastrophic events (e.g. asteroid impacts, volcanic eruptions).

Examples of extinct species: woolly mammoth, dodo, passenger pigeon, quagga.

Once extinct, a species is lost permanently — its unique genetic information cannot be recovered.

Common Confusions¶

Darwin vs Lamarck: Darwin's natural selection is the accepted theory; Lamarck's inheritance of acquired characteristics is wrong. Do not mix them up.
Fossil record completeness: students sometimes say fossils "prove" evolution fully. In fact, the fossil record is incomplete. Fossils support evolution but gaps remain.
Antibiotic resistance: bacteria do not "choose" to become resistant — resistance arises from pre-existing random mutations. Antibiotics provide the selection pressure that makes resistance spread.
Mendel and DNA: Mendel did not know about DNA. He described "units" of inheritance; scientists later identified these as genes on chromosomes.

Key Terms¶

Evolution: the change in inherited characteristics of a population over time.
Fossil: the preserved remains or traces of an ancient organism.
Extinction: the permanent loss of a species when no individuals remain.
Antibiotic resistance: the ability of some bacteria to survive antibiotic treatment due to a resistance mutation; spreads by natural selection.
Natural selection: the process by which individuals with advantageous inherited characteristics are more likely to survive and reproduce.
Speciation: the formation of a new species when populations become reproductively isolated and diverge over time.
Lamarck's theory: the now-disproved idea that characteristics acquired during an organism's lifetime can be inherited by offspring.
Mendel: the 19th-century scientist whose pea plant experiments laid the foundations of genetics; discovered dominant and recessive inheritance.
Hominid: a member of the human lineage; includes extinct relatives such as Australopithecus.
Pentadactyl limb: a limb with five digits, found across many vertebrate groups; evidence of common ancestry.
MRSA: Methicillin-resistant Staphylococcus aureus; an example of a bacterium that has evolved antibiotic resistance.