The Development of Understanding of Genetics and Evolution¶

Part of 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.

What You Need to Learn¶

Trace how observations, breeding experiments and new evidence changed explanations of inheritance and evolution. Compare the ideas of Darwin, Wallace and Lamarck, follow Mendel’s contribution to genetics, and use fossils and resistant bacteria as evidence. Explain how populations can separate into species and why species sometimes disappear.

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 many scientists considered the available evidence insufficient. The mechanism of inheritance was also unknown, so acceptance grew gradually as evidence accumulated.

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. Most acquired changes, such as increased muscle size through exercise, do not alter the inherited DNA in gametes and are not passed on in that form. 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. Their ideas were presented jointly in 1858, prompting Darwin’s book the following year. Wallace gathered evidence internationally and investigated warning colouration: conspicuous colours can deter predators from attacking harmful or unpalatable animals. He also did pioneering work on speciation. These ideas made shared ancestry and selection central explanations in biology, with later evidence refining them.

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:

Different mutations arise in each isolated group.
Natural selection acts differently in each environment.
The two groups accumulate different genetic changes.
Eventually they become so different that they cannot interbreed to produce fertile offspring 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).
The different pea traits he studied could be inherited independently; this does not mean all genes always assort independently.
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. Observation of chromosomes during cell division in the late nineteenth century revealed behaviour resembling Mendel’s units. Early twentieth-century work linked genes to chromosomes. Determining DNA’s structure in 1953, followed by research on gene function, helped explain inheritance at the molecular level. This understanding developed through the work of many scientists.

Evidence for Evolution¶

Fossils¶

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

Showing changes in organisms through sequences of rocks of different ages; evolution does not require every lineage to become more complex.
Revealing intermediate forms between ancient and modern species.
Tracing lineages — e.g. human evolution through hominid fossils.

Fossils form in several ways. Remains may survive without decay when water, oxygen or a suitable temperature is missing. Minerals may replace tissues as they decay, preserving their shape. Footprints, burrows and root traces can also be preserved. Early soft-bodied organisms left few traces, and geological activity destroyed many others, making the earliest history of life uncertain.

When using a fossil-age table or evolutionary tree, compare dated features and identify shared ancestors at branch points. A gap in the record limits the detail of a conclusion; it does not show that populations stopped evolving.

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 take antibiotics for the prescribed duration, reducing the chance that surviving bacteria continue to reproduce and spread.
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 advantageous inherited characteristics become more common over generations.
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 great-ape family, including humans and their extinct relatives.
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.