Introduction to Natural Selection

Ever wondered why some animals survive whilst others don't? Natural selection is evolution's main driving force, and it's surprisingly straightforward once you get it.

Darwin's theory shows us that when resources are limited, organisms with better traits are more likely to survive and reproduce. Think of it like a never-ending competition where the winners pass on their advantages to their offspring. Evolutionary fitness isn't about being the strongest - it's measured by reproductive success.

Here's what trips up many students: natural selection acts on phenotypes (what you can see), not genotypes (the genetic code). Also, evolution has no intentions - it's not trying to create the "perfect" organism, just responding to environmental pressures.

Key Point: "Survival of the fittest" doesn't mean the strongest survives - fitness changes depending on the environment!

How Natural Selection Works in Practice

Your environment is constantly putting selective pressures on populations, and this is where evolution gets interesting. These pressures can be anything from climate change to new predators, and they determine which traits become advantageous.

Remember that adaptations are changes that boost an organism's survival chances. What's crucial to understand is that organisms with different genotypes don't always have different phenotypes - thanks to dominant and recessive alleles.

Artificial selection proves evolution works, but with humans in control instead of nature. Darwin actually used examples like dog breeding to support his theory because people could easily see how traits change over generations.

Quick Check: Can you think of an example where artificial selection has created dramatic changes in a species? (Hint: think about the difference between wolves and pugs!)

Population Genetics and Random Events

Evolution isn't just about survival of the fittest - random events play a massive role too! Genetic drift occurs in small populations where chance events can dramatically shift allele frequencies.

Two major types of genetic drift you need to know are the bottleneck effect (when disasters drastically reduce population size) and the founder effect (when a few individuals start a new population). Both reduce genetic diversity, which can be dangerous for long-term survival.

Gene flow happens when individuals move between populations, bringing new alleles with them. This migration can introduce beneficial traits or dilute local adaptations.

Don't forget about mutations - they're completely random but provide the raw material for evolution by creating new genetic variations.

Real-world example: Cheetahs went through a severe bottleneck thousands of years ago, which is why they have such low genetic diversity today.

Types of Natural Selection

There are three main patterns of natural selection, and recognising them is essential for your exams. Directional selection favours one extreme (like giraffes developing longer necks), whilst stabilising selection favours the average (like human birth weight).

Disruptive selection is the odd one out - it favours both extremes whilst selecting against the middle. This can lead to populations splitting into two distinct groups.

Sexual selection is a special type that focuses on traits that help organisms attract mates, even if these traits might reduce survival chances. Think peacock tails - they're flashy but impractical!

Understanding these patterns helps you predict how populations might change over time. Each type responds to different environmental pressures and produces different evolutionary outcomes.

Exam tip: Draw simple bell curves to visualise each selection type - it'll help you remember the differences!

Hardy-Weinberg Equilibrium

The Hardy-Weinberg model is your tool for detecting evolution in action. It describes what happens to allele frequencies when evolution isn't occurring - which almost never happens in real life!

The magic equations are p² + 2pq + q² = 1 and p + q = 1, where p is the dominant allele frequency and q is the recessive allele frequency. These let you calculate expected genotype frequencies in a non-evolving population.

For Hardy-Weinberg equilibrium to exist, you need five impossible conditions: no mutations, random mating, no natural selection, extremely large population size, and no gene flow. Since these conditions are never met, the model serves as a null hypothesis.

When real populations deviate from Hardy-Weinberg predictions, you know evolution is happening. The bigger the deviation, the stronger the evolutionary forces at work.

Study hack: Remember the five conditions by thinking about what disrupts them - it's easier than memorising the ideal conditions!

Evidence for Evolution

Evolution has overwhelming support from multiple scientific fields, making it one of the most well-supported theories in science. Fossil records provide direct evidence of change over time, with dating methods like Carbon-14 giving us precise timelines.

Comparative morphology reveals relationships through homologous structures (similar due to common ancestry) and analogous structures (similar due to convergent evolution). Vestigial structures are particularly compelling - they're leftover features that no longer serve their original purpose.

Molecular evidence is incredibly powerful because DNA and protein sequences show evolutionary relationships with remarkable precision. The more similar the sequences, the more closely related the species.

Embryonic development also provides evidence - many species look surprisingly similar in their early stages, suggesting common ancestry.

Think about it: Your arm, a bat's wing, and a whale's flipper all have the same basic bone structure - that's homology in action!

Common Ancestry and Phylogeny

All life shares common ancestors, and we can trace these relationships using phylogenetic trees and cladograms. These diagrams show how species are related, with nodes representing common ancestors.

The key difference is that phylogenetic trees show the amount of evolutionary change over time, whilst cladograms focus on the branching pattern of relationships. Both use shared derived characteristics to determine relationships.

Understanding tree structure is crucial: sister taxa emerge from the same node, whilst the outgroup represents the least related lineage. Monophyletic groups include an ancestor and all its descendants.

These trees are constantly being revised as new evidence emerges, particularly from DNA sequencing. What seemed like close relatives based on appearance might actually be distant cousins!

Key insight: Physical similarity doesn't always indicate close evolutionary relationships - molecular evidence often reveals surprising connections!

Evolution Never Stops

Evolution is happening right now, all around you. Antibiotic resistance in bacteria is perhaps the most dramatic example - these organisms evolve so quickly that new treatments become ineffective within years.

Emergent diseases arise when pathogens evolve new ways to infect hosts or become more virulent. This ongoing evolution explains why we need new flu vaccines each year and why COVID-19 variants keep appearing.

The fossil record shows continuous change throughout Earth's history, with no periods where evolution simply stopped. Genomic studies reveal that all species are constantly accumulating genetic changes.

Even humans continue to evolve, though it's harder to see because of our long generation times. Some populations have evolved lactose tolerance, high-altitude adaptations, and resistance to certain diseases within just a few thousand years.

Current example: Think about how quickly SARS-CoV-2 has evolved new variants - that's evolution in real-time!