Evolution and Natural Selection

Within any population, individuals show variation — differences in their characteristics.

Types of Variation

• Genetic variation — caused by differences in DNA/alleles (inherited from parents). Examples: eye colour, blood group
• Environmental variation — caused by the environment and lifestyle. Examples: scars, language, tanning
• Both — many characteristics are influenced by both genes AND environment. Example: height (genes determine potential, but nutrition/health affect actual height)

Sources of Genetic Variation

• Mutations — random changes in DNA that create new alleles
• Sexual reproduction — combines alleles from two parents, creating unique offspring
• Meiosis — crossing over and independent assortment shuffle alleles

Darwin's theory of natural selection explains how evolution occurs:

The Steps of Natural Selection

1. Variation exists within a population — individuals have different characteristics (due to genetic variation)
2. Competition for resources — organisms compete for food, water, mates, and space (there are more offspring than the environment can support)
3. Survival of the fittest — individuals with characteristics better suited to the environment are more likely to survive and reproduce
4. Inheritance — the advantageous alleles are passed on to the next generation
5. Over many generations, the proportion of individuals with the advantageous characteristic increases in the population

"Survival of the fittest" means the organisms best adapted to their environment, not necessarily the physically strongest.

Example: Antibiotic Resistance in Bacteria

1. A population of bacteria shows variation — some have a random mutation that makes them resistant to an antibiotic
2. When the antibiotic is used, non-resistant bacteria are killed
3. The resistant bacteria survive and reproduce
4. They pass the resistance gene to their offspring
5. Over time, the entire population becomes antibiotic-resistant

This is natural selection in action and explains why antibiotic resistance is a growing problem.

Example: Peppered Moths

• Before the Industrial Revolution: pale-coloured moths were camouflaged on lichen-covered trees; dark moths were eaten by birds
• During the Industrial Revolution: soot darkened the trees; now dark moths were camouflaged and pale moths were more visible to predators
• The proportion of dark moths increased because they had better survival and reproduced more
• After the Clean Air Act (less pollution), pale moths increased again as trees became lighter

Fossils

• Fossils are the preserved remains or traces of organisms from millions of years ago
• They are found in sedimentary rock — older fossils are in deeper layers
• The fossil record shows how organisms have changed over time
• Fossils show gradual changes in characteristics (transitional forms)
• Example: Horse evolution shows gradual changes in size, number of toes, and teeth structure

Limitations of the Fossil Record

• Many organisms did not form fossils (soft-bodied organisms decay completely)
• Geological activity (erosion, plate tectonics) has destroyed many fossils
• The record is incomplete — there are gaps

Other Evidence

• DNA analysis — comparing DNA sequences between species; more similar DNA = more closely related
• Comparative anatomy — similar bone structures in different species (homologous structures) suggest a common ancestor. Example: pentadactyl limb (5-fingered structure) in humans, bats, whales, and dogs
• Antibiotic resistance — observed evolution happening in real time
• Selective breeding — shows that organisms can change significantly when certain traits are selected

Speciation is the formation of new species from an existing species.

It occurs when populations of the same species become reproductively isolated — they can no longer breed to produce fertile offspring.

How Speciation Occurs

1. A population is separated by a physical barrier (e.g., a river, mountain range, or ocean) — this is called geographical isolation
2. The two isolated populations experience different environmental conditions (different food, predators, climate)
3. Natural selection acts differently on each population — different characteristics are advantageous in each environment
4. Over many generations, the populations become genetically different
5. Eventually, the populations are so different that they can no longer interbreed to produce fertile offspring
6. They are now two separate species

Organisms are classified into groups based on shared characteristics:

Linnaean Classification (traditional)

$\text{Kingdom} \rightarrow \text{Phylum} \rightarrow \text{Class} \rightarrow \text{Order} \rightarrow \text{Family} \rightarrow \text{Genus} \rightarrow \text{Species}$

Memory aid: King Philip Came Over For Good Spaghetti

• A species is a group of organisms that can interbreed to produce fertile offspring
• Organisms are named using binomial nomenclature (two Latin names): Genus + species. Example: Homo sapiens

Three-Domain System (modern)

• Proposed by Carl Woese based on genetic analysis (RNA sequences)
• Three domains: Archaea, Bacteria, Eukarya
• Uses biochemical and genetic evidence rather than just physical features

Species become extinct when no living members of that species remain.

Causes of Extinction

• Environmental changes — climate change, habitat destruction
• New predators — introduction of new species
• New diseases — pathogens to which the species has no resistance
• Competition — a new species out-competes the existing one for resources
• Catastrophic events — volcanic eruptions, asteroid impacts
• Human activity — deforestation, hunting, pollution

Question: Explain how a population of insects might evolve resistance to a pesticide through natural selection. (4 marks)

Solution:

Within the insect population, there is genetic variation — some individuals have a random mutation that gives them resistance to the pesticide. When the pesticide is applied, non-resistant insects are killed, but the resistant individuals survive. These survivors reproduce and pass the resistance allele to their offspring. Over many generations, the proportion of resistant insects in the population increases until most of the population is resistant. This is evolution by natural selection.

• Natural selection does NOT mean organisms "choose" to adapt — variation arises randomly through mutations, and the environment selects those best suited.
• Always describe natural selection as a step-by-step process: variation → competition → survival of best-adapted → reproduction → inheritance.
• Speciation requires reproductive isolation — make sure you include this key term.
• Know the difference between Darwinian evolution (natural selection) and Lamarck's theory (inheritance of acquired characteristics — now discredited).

• Evolution is the gradual change in inherited characteristics over generations through natural selection.
• Natural selection: variation → competition → survival of the fittest → inheritance → population change.
• Evidence includes fossils, DNA analysis, and comparative anatomy.
• Speciation occurs when populations become geographically isolated and evolve separately until they can no longer interbreed.
• Extinction occurs when a species can no longer survive in its environment.