Welcome to your study notes on Evolution and evidence. This topic is a cornerstone of biology, explaining how all life on Earth is connected and how species change over time.
What you'll learn
- The step-by-step mechanism of Charles Darwin's theory of natural selection.
- How antibiotic resistance in bacteria and fossil hominids provide proof of evolution.
- How stone tools and comparative anatomy (the pentadactyl limb) are used as evidence.
- Why genetic analysis caused scientists to update the classification system from five kingdoms to three domains.
Natural Selection: The Engine of Evolution
To understand how species change, we must first look at the mechanism driving it. This process is called natural selection, famously described by Charles Darwin.
Evolution
Evolution is the gradual change in the inherited characteristics of a population over many generations.
Natural Selection
Natural selection is the process by which organisms with characteristics better suited to their environment are more likely to survive, reproduce, and pass on their advantageous alleles to their offspring.
How Natural Selection Works, Step-by-Step
Natural selection does not happen because an organism "tries" to adapt. Instead, it is a passive, multi-step process that you can remember using the acronym V-S-S-R-G:
- Variation: Within any population, there is genetic variation. This is caused by random genetic mutations that create new alleles (different versions of genes), resulting in different physical characteristics (phenotypes).
- Struggle for Existence: Organisms face environmental challenges or selection pressures. These include competition for food, mates, and space, as well as threats from predators and diseases.
- Survival: Individuals with alleles that give them characteristics better suited to their environment are more likely to survive this struggle.
- Reproduction: The surviving individuals reproduce and pass on their advantageous alleles to their offspring.
- Generations: Over many generations, this process is repeated. The frequency of the advantageous allele increases in the population's gene pool, causing the overall characteristics of the species to change.
Acquired Characteristics
A very common mistake is stating that an individual organism changes its own genes or body to adapt during its lifetime and passes that change on. For example, a giraffe cannot stretch its neck during its life and pass on a longer neck. The variation must already exist in the genes (DNA) as a mutation before it can be selected for and inherited.
Darwin and Wallace: Shaping Modern Science
(Note: This section is for Separate (Triple) Biology students only.)
While Charles Darwin is a household name, another British naturalist named Alfred Russel Wallace independently came up with the very same theory of evolution by natural selection at the same time.
- The Collaboration: In 1858, Wallace sent Darwin a paper describing his ideas. Finding their work nearly identical, they presented their papers together to the scientific community. Darwin then published his famous book, On the Origin of Species, in 1859.
- Wallace's Contribution: Wallace is particularly famous for his work on warning colouration in animals (how bright colours warn predators of toxicity) and his research on how geographic barriers lead to the formation of new species (speciation).
Impact on Modern Biology
The work of Darwin and Wallace completely changed how we study life today:
- Common Ancestry: We now understand that all life is linked through evolution. Scientists draw evolutionary trees to show how different species branched off from common ancestors.
- Conservation: Understanding how species adapt to specific environments helps us protect threatened biodiversity.
- Molecular Biology: Modern DNA sequencing allows us to track evolutionary changes at the molecular level.
Evidence for Evolution 1: Resistant Organisms
One of the most rapid and visible examples of natural selection occurring today is the emergence of antibiotic resistance in bacteria.
Bacteria are excellent models for studying evolution because they reproduce incredibly quickly—some species double their population every 20 minutes!
- When a person takes antibiotics, most of the target bacteria are killed.
- However, due to random mutations, a tiny fraction of the bacterial population may possess a mutated allele that makes them resistant to the antibiotic.
- If the patient stops taking their antibiotics too early, or if the dose is not strong enough, these resistant bacteria survive.
- With the non-resistant bacteria wiped out, the survivors have less competition for nutrients. They reproduce rapidly via binary fission, passing the resistance allele to all their offspring.
- Soon, the entire infection is made up of resistant bacteria, making the antibiotic useless.
The Lesson of Resistance
The emergence of antibiotic-resistant bacteria directly supports Darwin's theory. It proves that when a selection pressure (the antibiotic) is applied, the individuals with the advantageous genetic trait (resistance) survive, reproduce, and dominate the population over time.
Evidence for Evolution 2: Fossil Hominids
Fossils provide a physical timeline of how organisms have changed over millions of years. Hominids are humans and our immediate, extinct ancestors.
The Edexcel specification requires you to know three crucial fossil discoveries that chart human evolution.
| Fossil Specimen | Scientific Name | Approximate Age | Key Features & Evolutionary Clues |
|---|---|---|---|
| Ardi | Ardipithecus ramidus | 4.4 million years ago | • Ape-like feet: Had a divergent (grasping) big toe for climbing trees. • Human-like pelvis & legs: Structure suggests she walked upright and did not use her hands for balance. |
| Lucy | Australopithecus afarensis | 3.2 million years ago | • No divergent toe: Feet were arched and adapted for walking upright. • Brain size: Small, similar to a modern chimpanzee's. |
| Leakey's Fossils | Homo erectus (e.g. 'Turkana Boy') | 1.6 million years ago | • Highly human-like: Tall, slim build with leg bones fully adapted to long-distance walking. • Large brain: Cranial capacity was significantly larger than Lucy's. |
Trends in Human Evolution
By studying these fossils in chronological order, scientists have identified two major evolutionary trends:
- Bipedalism: Transitioning from tree-climbing apes to fully upright-walking humans.
- Encephalisation: A substantial increase in brain size (cranial volume) over time.
Evidence for Evolution 3: Stone Tools
As hominids evolved larger brains, their intelligence and manual dexterity improved. This is visible in the stone tools they left behind.
The Development of Stone Tools Over Time
- Early Stone Tools (approx. 2.5 million years ago): Very simple pebble tools made by knocking a few chips off a stone to create a rough cutting edge. Used mainly for scraping meat off bones.
- Later Stone Tools (approx. 1.6 million years ago): More sophisticated hand axes, shaped with multiple flakes removed to create symmetrical, sharper edges.
- Recent Stone Tools (approx. 10,000–40,000 years ago): Highly advanced, delicate, and specialized tools like arrowheads, spear points, and needles made from flint.
Dating Stone Tools from Their Environment
Stone tools cannot be directly dated using radiometric carbon dating because they are made of rock, not organic material. Instead, scientists date them using their environment:
- Stratigraphy (Rock Layers): Over time, sediment settles in layers, with the oldest layers at the bottom and the youngest at the top. Tools found in deeper layers of rock are older than tools found in shallower layers.
- Associated Organic Material: If a stone tool is found in the same layer as organic material (like charcoal from an ancient fire, wood, or animal bones), scientists can radiocarbon date that organic material to determine the age of the tool.
Calculating the rate of cranial volume increase
This worked example demonstrates how to interpret fossil data and calculate evolutionary trends.
Data:
- Lucy (Australopithecus afarensis): lived 3.2×1063.2 \times 10^63.2×106 years ago, cranial volume of 400 cm3400\text{ cm}^3400 cm3.
- Homo erectus (Turkana Boy): lived 1.6×1061.6 \times 10^61.6×106 years ago, cranial volume of 900 cm3900\text{ cm}^3900 cm3.
Calculate the average rate of increase in cranial volume per million years between these two species.
- Calculate the time difference between the two fossils:
This is exactly 1.6 million years1.6\text{ million years}1.6 million years.
- Calculate the difference in cranial volume:
- Calculate the rate of increase per million years:
Evidence for Evolution 4: The Pentadactyl Limb
(Note: This section is for Separate (Triple) Biology students only.)
If you look at the limbs of a human, a bat, and a whale, they perform completely different functions: grasping, flying, and swimming. However, their internal bone structures are incredibly similar.
All of these vertebrates possess a pentadactyl limb—a limb ending in five digits (fingers or toes).
Why is this evidence for evolution?
The shared structure of the humerus, radius, ulna, carpals, and phalanges across these diverse animals is highly unlikely to have occurred by chance.
Instead, it suggests that all these organisms evolved from a common ancestor that possessed this basic five-digit limb template. Over millions of years, natural selection adapted this bone structure to suit different environments and lifestyles. This is an example of a homologous structure.
Evolution and Classification: From Five Kingdoms to Three Domains
As our understanding of genetics has advanced, so too has the way we classify living things.
The Five Kingdoms System
Historically, classification was based purely on visible physical traits and anatomy (morphology). Under this system, all life was divided into Five Kingdoms:
- Plants (multicellular autotrophs with cellulose cell walls)
- Animals (multicellular heterotrophs without cell walls)
- Fungi (multicellular/unicellular saprotrophs with chitin cell walls)
- Protists (unicellular eukaryotes)
- Prokaryotes (all unicellular organisms without a nucleus, like bacteria)
The Shift to the Three Domains System
In the late 20th century, scientist Carl Woese introduced genetic analysis (specifically looking at differences in ribosomal RNA sequences and cell membrane biochemistry).
This analysis revealed that the prokaryote kingdom was actually made up of two vastly different groups of organisms. This led to the introduction of a classification tier above kingdoms, called Domains:
- Archaea: Unicellular organisms that look like bacteria but have genetic sequences and biochemical pathways that are much closer to eukaryotes. They are often extremophiles (living in hot springs or salty lakes).
- Bacteria: "True" bacteria, containing standard prokaryotic cellular chemistry.
- Eukarya: All organisms with eukaryotic cells containing a nucleus (including plants, animals, fungi, and protists).
Classification is Dynamic
The shift from Five Kingdoms to Three Domains is a perfect example of how scientific theories are updated when new technology (like DNA and RNA sequencing) provides more precise evidence than old techniques (like visual observation).
In the exam
- Memorise the chronological order of the fossils: Ardi (4.44.44.4 mya) →\to→ Lucy (3.23.23.2 mya) →\to→ Leakey's Homo erectus (1.61.61.6 mya). Make sure you can state the trends: increasing brain size and increasing adaptation to walking upright.
- Define "stratigraphy" clearly: When explaining how stone tools are dated, use the term "stratigraphy" to describe the relative age of rock layers, and explain that deeper layers contain older artifacts.
- Use the V-S-S-R-G structure: If asked to write a 4-to-6-mark question explaining natural selection, structure your answer chronologically starting with genetic variation, then selection pressures, then survival, reproduction, and the passing on of alleles.
Check yourself
- Why are bacteria particularly useful organisms for demonstrating natural selection in action?
- Explain how the structural differences in the feet of Ardi and Lucy demonstrate human evolution.
- What key tool did Carl Woese use to split the prokaryotes into Bacteria and Archaea?
