Darwin's Observations (OCR A-Level Biology A): Revision Notes

Darwin's Observations

Introduction to convergent evolution

Before examining Darwin's key observations, it helps to understand how similar environments can shape organisms in comparable ways. Around 50 million years ago, marsupials migrated to Australasia and evolved to occupy various ecological niches, including underground habitats. The marsupial moles of western and northern Australian deserts demonstrate remarkable similarities to European placental moles, despite not sharing a recent common ancestor.

These shared features arose through convergent evolution – the independent development of similar characteristics in unrelated species facing comparable environmental challenges. The species differ in some respects: marsupial moles possess a hardened head region and uniquely fused neck vertebrae, whilst European moles have elongated snouts.

Darwin's four key observations

When developing his theory of natural selection, Charles Darwin identified four fundamental observations about living organisms:

1. Overproduction of offspring

All organisms produce far more offspring than will ultimately survive to adulthood. Darwin calculated that a single pair of elephants could theoretically produce $19$ million descendants over approximately $740$–$750$ years if all offspring survived and reproduced.

2. Population stability

Whilst population numbers fluctuate, they generally remain relatively constant over time. Populations do not show the exponential growth that would occur if all offspring survived.

3. Intraspecific variation

Individuals within a population show variation in many characteristics. This intraspecific variation arises during sexual reproduction through processes such as:

• Crossing over during meiosis
• Independent assortment of chromosomes
• Random fusion of gametes

4. Inheritance of characteristics

Offspring resemble their parents because features are transmitted from one generation to the next. Although Darwin did not understand the mechanism (genes and alleles were unknown at the time), he recognised that gametes must carry information determining an organism's characteristics.

Darwin's inferences from his observations

From these observations, Darwin drew several important conclusions:

1. Competition for limited resources

With more organisms produced than can survive, there exists what Darwin termed a "struggle for life". Individuals within the same population compete for finite resources such as:

• Food and water
• Space and shelter
• Mates
• Light (for plants)

2. Differential survival based on adaptation

Organisms possessing features that make them better adapted to their environment have a higher chance of surviving long enough to reproduce. Those less well adapted face greater risk of death before reaching reproductive age.

3. Inheritance of advantageous characteristics

Successful organisms pass their inherited characteristics to offspring. The next generation therefore contains a higher proportion of individuals with advantageous features compared to those less well adapted.

4. Gradual population change

This unequal survival and reproduction leads to gradual shifts in population characteristics, with certain adaptations becoming more frequent across generations.

Competition and environmental pressures

Resources in any environment exist in limited quantities. When more organisms are present than the environment can support, many individuals die, maintaining relatively stable population sizes across generations.

Poorly adapted individuals have reduced survival chances and higher mortality rates before reproduction.

Natural selection

Natural selection describes how environmental factors affect a population such that individuals best adapted to prevailing conditions have a higher chance of surviving, reproducing, and passing their advantageous alleles to future generations. It represents the primary mechanism driving evolutionary change.

Organisms better equipped to:

• Obtain resources efficiently
• Avoid lethal diseases
• Escape predation

are more likely to survive to reproductive age, find mates, and transmit their alleles to subsequent generations. These individuals are "selected" by environmental conditions.

Natural selection in stable versus changing environments

When environmental conditions remain stable, natural selection maintains existing species characteristics. However, if conditions change, selection pressures shift. Features previously disadvantageous may become beneficial, allowing organisms with these traits to compete more successfully.

In changing environments, individuals at one extreme of the natural variation range often show highest survival. For example, in colder climates, larger mice with smaller surface area to volume ratios survive better.

Understanding "survival of the fittest"

Darwin's theory is often reduced to the phrase "survival of the fittest", which many misinterpret as "survival of the strongest". This oversimplification misses the subtlety of natural selection.

In biology, fitness has a specific meaning: the ability of organisms to survive to adulthood and reproduce successfully. By this definition, the fittest organisms are simply those that survive and reproduce – making the phrase somewhat circular and less meaningful than the detailed explanation of natural selection mechanisms.

Worked example: beach mice evolution

The oldfield mouse (Peromyscus polionotus) typically lives in overgrown agricultural fields across the USA and possesses dark brown fur. However, populations colonising light-coloured sand dunes and barrier islands along the Florida and Alabama coasts have evolved much lighter fur colouration. These are known as beach mice.

Beach mice feed primarily at night on seeds and construct extensive burrow systems. Their main predators include owls, foxes, and domestic cats. Predation experiments demonstrated that owls (night-hunting predators) captured conspicuously coloured mice most frequently.

Why beach mice remain the same species

Beach mice and oldfield mice are classified as the same species because they:

• Can interbreed to produce fertile offspring
• Share many common features
• Have very similar DNA sequences

Potential for speciation

Beach mice could evolve into a separate species through:

1. Reproductive isolation: Geographical barriers separating beach and inland populations
2. Mutation accumulation: New mutations arising independently in the beach population
3. Natural selection: Beneficial mutations becoming "fixed" through selection for beach adaptations
4. Genetic divergence: Accumulation of differences over time until interbreeding becomes impossible