Evolution (Junior Cert Science): Revision Notes
📚Revision Notes
Evolution
Understanding evolution is essential to understanding how life on Earth has developed and changed over millions of years. This topic explores how species change over time, the role of Charles Darwin in developing evolutionary theory, and the mechanism of natural selection.
Variation and diversity
What is biodiversity?
Biodiversity describes the enormous variety of different living things found on Earth. From tiny bacteria to huge whales, from desert cacti to rainforest trees, our planet is home to millions of different species. This incredible range of life forms is what makes Earth so special.
Having high levels of biodiversity is crucial for maintaining healthy ecosystems. When many different species live in an environment, the ecosystem becomes more stable and better able to cope with changes. This benefits all life on Earth, including humans.
Note
Ecosystems with high biodiversity are more resilient because they have many different species that can perform similar roles. If one species disappears, others can often fill its place, helping the ecosystem continue functioning.
Understanding variation
Variation refers to the differences we see between members of the same species. Even though all humans belong to one species, we come in different heights, have different eye colours, and possess different abilities. These differences exist in all living things.
Think about dogs - they're all the same species, but compare a tiny Chihuahua with a massive Great Dane! This variation within a species is completely normal and actually very important for the survival of that species.
Note
Variation is the raw material for evolution. Without differences between individuals, natural selection would have nothing to work with, and species could not adapt to changing environments.
What are mutations?
A mutation is a change that occurs in an organism's DNA (the genetic code that controls how living things develop and function). Mutations happen randomly and can be passed down from parents to offspring.
Important
Not all mutations are harmful!
While many people think of mutations as dangerous or negative, the reality is different:
- Some mutations make no difference at all to the organism
- Some mutations are harmful and reduce survival chances
- Some mutations are actually beneficial and help organisms survive better in their environment
It's these helpful mutations that play a key role in evolution.
Evolution
How does evolution work?
Evolution describes how the characteristics that organisms inherit change gradually over many generations. This process happens very slowly - usually over thousands or millions of years.
Evolution occurs through changes in DNA called mutations. When these mutations help an organism survive, they can be passed on to future generations. Over time, these small changes can add up, eventually creating entirely new species.
Example: The evolution of horses
Example
Worked Example: The Evolution of Horses Over Time
The horse provides an excellent example of how evolution works over long time periods.
Starting Point ( million years ago):
- The earliest horse ancestor (Mesohippus) was quite small - only about metres tall
- Had three toes on each foot
- Lived in forest environments
Changes Over Millions of Years:
- Body size: Became much larger and taller
- Feet structure: Developed from having three separate toes into having a single hoof
- Speed: Became faster runners
Why These Changes Occurred:
- The three-toed feet worked well for small, slow horses living in forests
- As environments changed and horses needed to run faster across open grasslands, having larger bodies and single hooves became advantages
- Horses with these features survived better and had more offspring
- Over time, these traits became common in the population
Charles Darwin
Who was Charles Darwin?
Charles Darwin (-) was an English scientist who dedicated his life to studying different species of organisms. He carefully observed how they had developed and adapted to their environments.
Important
Common Misunderstanding About Darwin's Theory
Darwin never claimed that humans descended directly from apes. Instead, he suggested that humans and apes share a common ancestor - meaning they both evolved from the same ancient species, but followed different paths of development.
Darwin developed the concept of the "Tree of Life" to show how all species are related. Just like branches on a tree split off from the main trunk, different species evolved by branching off from common ancestors.
Darwin's voyage on HMS Beagle
Between and , Darwin travelled around the world on a ship called HMS Beagle. This five-year voyage took him to many different countries and gave him the opportunity to study wildlife in various environments.
Note
This voyage was crucial to developing Darwin's theory. By seeing how species varied across different environments and locations, Darwin began to understand the relationship between organisms and their surroundings.
Studies of Galápagos finches
Darwin's most famous observations came from studying finches (small birds) on the Galápagos Islands, off the coast of Ecuador. He noticed something remarkable: finches on different islands had developed different types of beaks.
Darwin discovered that:
- On islands with lots of seed-producing plants, finches had thick, strong beaks perfectly shaped for cracking open seeds
- On islands with more fruit, finches had beaks better suited for eating fruit
- Some finches had developed thin, pointed beaks ideal for catching insects
This led Darwin to understand adaptation - the process by which changes occur in a species to make them better suited to their environment.
Natural selection
Darwin's theory
In November , Charles Darwin published his groundbreaking book "On the Origin of Species by Means of Natural Selection". This book explained his theory of how species change over time.
Darwin's observations and conclusions
Darwin made careful observations about living things and drew important conclusions from them:
Important
Observation 1: Living organisms produce far more offspring than could possibly survive to adulthood.
Observation 2: Despite producing many offspring, population numbers usually stay fairly stable over time.
Observation 3: There are differences (variations) between individual members of the same species.
From these observations, Darwin reached two key conclusions:
Important
Conclusion 1: There must be a struggle for existence. Organisms constantly compete with each other for food, shelter, mates, and space. Because of this competition, many individuals die before reproducing.
Conclusion 2: Because of variations between individuals, some organisms are better adapted to survive in their environment than others. These well-adapted organisms survive longer, reproduce more successfully, and pass their advantageous genes to their offspring. Meanwhile, less well-adapted organisms die before reproducing, so their less favorable genes don't get passed on.
Survival of the fittest
People sometimes describe natural selection as "survival of the fittest". However, "fittest" doesn't mean the strongest or most athletic - it means the organisms that are best equipped to survive in their particular environment.
To understand natural selection:
- Individuals that are well adapted to their environment will survive to breeding age
- These successful individuals pass their genes to their offspring
- Their offspring inherit these helpful traits and are also more likely to survive and breed
- Individuals that are not well-adapted to their environment are less likely to survive long enough to reproduce
- Therefore, they don't pass their genes to the next generation
Note
The Key Mechanism
Over many generations, this process means that helpful traits become more common in a population, while unhelpful traits gradually disappear. This is the fundamental mechanism driving evolutionary change.
New species
How do new species form?
A species is a group of organisms that can breed with each other and produce fertile offspring (offspring that can themselves have babies).
New species form through a combination of mutations and natural selection:
- Random mutations occur in the genes of individuals within a species
- Sometimes these mutations make no significant difference, and the individual survives normally and passes the mutation to future generations
- Sometimes mutations make an individual less able to cope with their environment - these individuals usually die before breeding, so the unfavorable gene doesn't get passed on
- Sometimes mutations make an individual better suited to their environment - these individuals are more likely to survive and breed, passing on the favorable gene
Note
When many small changes accumulate over numerous generations (usually over very long time periods), a population can change so much that it becomes a completely new species. This process typically takes thousands or millions of years.
Extinction
Extinction occurs when there are no longer any living members of a particular species. This usually happens as a result of changes in the environment that make it unsuitable for that species' survival.
Example
Example: Climate Change and Extinction
If the climate becomes much colder and a species hasn't evolved features to keep warm (like thick fur), that species may die out.
Meanwhile, a better-adapted species with features suited to cold weather will survive and thrive.
This demonstrates how natural selection and environmental changes work together to shape which species survive.
Remember!
Summary
Key Points to Remember:
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Variation describes the natural differences between members of the same species, caused by differences in their genes
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Evolution is the gradual change in inherited characteristics that happens in species over many generations
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Charles Darwin developed the theory of natural selection after studying wildlife during his voyage on HMS Beagle, particularly finches in the Galápagos Islands
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Natural selection explains how organisms with favorable traits are more likely to survive and reproduce, passing these traits to future generations
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New species form when accumulated changes through mutation and natural selection create organisms different enough that they can no longer breed with the original species
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Extinction happens when environmental changes make it impossible for a species to survive