Evolution

In 1831, a 22-year-old Englishman named Charles Darwin embarked on a voyage that would change the course of science.

Aboard the HMS Beagle, Darwin was hired as a naturalist to survey the coasts of South America. The purpose of the expedition was to chart the coastline and collect specimens that could help in understanding the natural world.

Four years after the journey began, the Beagle reached the Galápagos Islands: a remote archipelago located about 1,000 kilometers off the coast of Ecuador.

The islands held an array of unique animals and plants that immediately fascinated Darwin.

Over the course of five weeks, he observed the islands' unusual inhabitants, meticulously collecting and documenting specimens.

As Darwin moved from island to island, he noticed that many of the species were similar, yet subtly different.

For instance, he observed that finches on each island had distinct beak shapes and sizes. Some finches had long, narrow beaks ideal for picking insects out of tree bark, while others had short, thick beaks designed for cracking seeds.

Why did the finches on each island have different beak shapes?

What could explain the variation in the shells of the giant tortoises, which seemed to vary depending on the island they inhabited? On one island, tortoises had dome-shaped shells, while on another, they had saddleback shells, allowing them to stretch their necks higher to reach vegetation.

Similarly, the mockingbirds on one island differed in subtle ways from those on another. Why did mockingbirds on one island differ from those on another?

These observations led Darwin to wonder: could these differences have something to do with the environment of each island?

Upon his return to England, Darwin studied his findings with even greater detail.

He found that the birds he thought were just varieties of finches were actually different species, each uniquely adapted to its specific island environment.

The mockingbirds and tortoises followed a similar pattern.

At that time, the prevailing view in Europe was that species were immutable, meaning they were unchanging and had been created in their current form by a divine creator.

This concept was rooted in the belief that every species had been designed for a specific purpose and that they had remained the same since their creation.

However, Darwin's observations in the Galápagos challenged this long-held belief.

Darwin began to theorize that certain traits, like the beak shapes of finches or the shell forms of tortoises, might give some individuals an advantage in survival and reproduction, particularly in the challenging environments of the Galápagos Islands.

For example, a finch with a beak well-suited for cracking seeds might be more likely to survive in an environment where seeds are the primary food source.

Over time, these advantageous traits would become more common in the population, because those individuals with favorable traits were the ones to survive long enough to reproduce.

In other words, the individuals of that species ‘fittest’ to survive in a given environment were the ones most likely to pass those traits on to the next generation. This process is often called “survival of the fittest”.

Over long periods, “survival of the fittest” can lead to the development of new species as populations with different advantageous traits become distinct from each other.

This process, Darwin theorized, was a key mechanism behind the changes in species over time and would later become known as the theory of natural selection.

In 1859, more than two decades after his visit to the Galápagos, Darwin published his ideas in the seminal work On the Origin of Species.

The theory he proposed—natural selection—suggested that species evolve over time through the gradual accumulation of small, inherited changes that increase an organism's chances of survival and reproduction.

The rest of the tile will focus on how Darwin’s theory of evolution actually works. For instance, how do traits like beak shape or shell form get inherited?

Why do these variations arise in the first place?

How do populations of the same species diverge over time to become entirely new species?

Together, these sections form a comprehensive picture of evolution, a process that has shaped the diversity of life on Earth and continues to influence the living world today.

The observations Darwin made in the Galápagos Islands—such as the differing beak shapes of finches and the varied shell forms of tortoises—led him to question how such variations could arise and persist in isolated populations.

Genetic variation is the differences in DNA sequences that exist within a population. Even in small populations, genetic variation can be surprisingly high. But how is genetic variation actually generated, maintained, and expressed? This orb will examine this question more closely.

First, let's take a look at mutations, a key driver of genetic variation.

Mutations occur when there are errors in DNA replication or due to external factors like UV radiation or chemicals. Even in small populations, such as those on isolated islands, mutations occur continuously.

While most mutations are neutral or deleterious, a small fraction can be beneficial. For instance, in a population of pocket mice living on lava flows in the American Southwest, a single mutation in the Mc1r gene caused darker fur, giving these mice better camouflage against the dark volcanic rock.

Over time, this mutation spread through the population, despite its small size, highlighting how even isolated groups can harbor significant genetic diversity.

This principle could explain how certain traits in Galápagos species, like the finches' beak shapes, might have arisen in response to their specific environments.

Genetic drift is another critical mechanism, especially in small populations like those on the Galápagos Islands. Genetic drift refers to random fluctuations in allele frequencies due to chance events. In small populations, these random changes can have outsized effects.

Take, for example, what’s known as a population bottleneck: a sudden reduction in population size, often due to events like natural disasters or disease, drastically reduces the number of individuals.

These kinds of events lead to a loss of genetic diversity.

The cheetah population experienced such a bottleneck thousands of years ago, reducing its genetic diversity. Despite this, cheetahs have managed to survive and even thrive, though with consequences like increased susceptibility to disease due to reduced genetic variation.

Gene flow—the movement of genes between populations—also plays a pivotal role in maintaining genetic diversity. In small, isolated populations, gene flow can introduce new genetic material, preventing the loss of variation that might otherwise occur due to genetic drift or inbreeding.

For example, when a single wolf from the mainland crossed the ice bridge to Isle Royale, it introduced new alleles into the small wolf population there, which had been suffering from inbreeding depression.

Inbreeding depression occurs when closely related individuals breed, leading to an increased chance of harmful genetic traits being expressed, which can reduce the population's overall fitness.

Therefore, the single event of a wolf crossing from the mainland eventually significantly boosted the genetic diversity of the entire population of the wolves in Isle Royale, demonstrating how even minimal gene flow can have a substantial impact.

There are other mechanisms that also contribute to maintaining genetic diversity, that we’ll look at a little more closely in our later orb on ‘adaptation’.

These include balanced polymorphism, which helps maintain certain genetic traits when they offer advantages in different conditions.

A well-known example is the sickle cell trait in humans. In regions where malaria is common, individuals with one copy of the sickle cell gene have a survival advantage because they are more resistant to malaria.

Epistasis involves complex interactions between genes, influencing traits in ways that aren't always obvious from individual genes alone.

And finally, hidden genetic variation refers to genetic traits that remain unnoticed until changes in the environment reveal them. For instance, certain plants might carry genetic variants that do not affect their growth under normal conditions but become advantageous under drought stress.

The questions Darwin pondered during his time in the Galápagos Islands—such as why so many organisms produced far more offspring than could possibly survive—touch upon a fundamental aspect of evolutionary biology: overproduction.

Overproduction of organisms refers to the tendency of living organisms to produce more offspring than their environment can support.

This is observed across a wide range of species, from plants and insects to birds and mammals.

The sheer number of offspring ensures that, even in the face of high mortality rates, some will survive to adulthood and reproduce.

And this phenomenon plays a critical role in the process of natural selection.

Consider the finches of the Galápagos Islands that Darwin observed.

Each finch might lay multiple eggs, resulting in many chicks. However, the islands' resources—such as food and nesting sites—are limited.

Not all of these chicks will survive to maturity. Some will die due to predation, disease, or simply because they cannot compete successfully for food.

Those that do survive are typically the ones best suited to their environment. For example, a finch with a beak shape that allows it to efficiently access food sources will have a better chance of surviving during times of scarcity.

These advantageous traits, which can arise from genetic variation and mutations, are then passed on to the next generation.

An example of overproduction can be seen in sea turtles.

A single female sea turtle may lay hundreds of eggs in one nesting season. For instance, a loggerhead sea turtle may lay around 100-126 eggs per clutch and can nest several times a season. However, the survival rate from egg to adulthood is very low. Typically, only about 1 in 1,000 to 1 in 10,000 hatchlings survive to reach maturity.

The high mortality rate among sea turtle hatchlings is due to predators, environmental hazards, and competition.

Those few that survive are often the ones with traits that give them a slight edge, whether it’s a stronger shell, faster swimming speed, or better camouflage.

Overproduction actually amplifies the effects of genetic variation within a population.

With more individuals, there is a greater chance for different genetic combinations to be expressed. This increases the likelihood that some individuals will possess advantageous traits, which can then be selected for through the pressures of natural selection.

This process is particularly important in dynamic environments where conditions may change rapidly, as it allows populations to adapt to new challenges.

Adaptation, a key mechanism in evolution, is the process by which organisms become better suited to their environments.

Having earlier explored the sources of variation—such as mutation, genetic drift, hidden variation, and gene flow—it's important to now examine how these mechanisms drive and contribute to specific adaptations in different organisms.

In this section, we'll look more closely at this through several examples, from animals to simpler organisms like bacteria and plants.

Mutations, as we've discussed, are changes in the DNA sequence that can lead to new traits. While most mutations are neutral or harmful, those that confer an advantage in a particular environment can become the basis for adaptation.

A prime example of adaptation driven by mutation is seen in bacteria, particularly in the context of antibiotic resistance. When a population of bacteria is exposed to an antibiotic, most of the bacteria are killed, but those with mutations that confer resistance survive and reproduce.

In plants, mutations can also lead to important adaptations. The development of C4 photosynthesis in certain plants is an adaptation to hot, arid environments.

Over time, plants with these mutations, rather than the standard C3 photosynthesis, thrived in dry climates where water was scarce, leading to the spread of C4 photosynthesis in species like maize and sugarcane.

Hidden genetic variation, which we touched on briefly in our orb on ‘genetic variation’, refers to genetic differences that do not manifest in the phenotype unless triggered by specific environmental conditions. This hidden variation can become a reservoir of potential adaptations, ready to be revealed when the environment changes.

A classic example of hidden variation contributing to adaptation can be seen in yeast populations.

Yeast cells carry many genetic variants that do not affect their phenotype under normal conditions.

However, when exposed to environmental stressors—such as changes in temperature or pH—some of these hidden variants become advantageous.

For instance, in acidic environments, certain yeast strains express previously silent genes that help them survive in low pH conditions. This ability to tap into hidden variation allows yeast populations to quickly adapt to fluctuating environments.

Gene flow, as we saw in the example of the Isle Royale wolf, is the movement of genes between populations. It plays a crucial role in spreading adaptive traits across populations and maintaining genetic diversity, which can enhance a species' ability to adapt to new challenges.

Another striking example of gene flow facilitating adaptation is seen in the interbreeding between wild and domesticated species.

In the case of wolves and domestic dogs, gene flow has led to the exchange of adaptive traits. For instance, in regions where wolves and dogs coexist, wolves have occasionally acquired traits such as increased disease resistance or behavioral adaptations through gene flow from dogs. This exchange of genes can enhance the wolves' ability to survive in environments where they face new challenges, such as exposure to human-related diseases or changes in prey availability.

In aquatic environments, gene flow between populations of the threespine stickleback fish has led to the spread of adaptive traits that enable these fish to survive in both marine and freshwater environments.

When marine sticklebacks colonized freshwater habitats, gene flow allowed the exchange of traits such as reduced armor plating, which is advantageous in the absence of marine predators.

This adaptation has enabled sticklebacks to thrive in diverse aquatic environments, demonstrating how gene flow can drive adaptation in response to different ecological pressures.

Epistasis, the interaction between genes, can create complex adaptations by modifying the effects of individual genes. These interactions can lead to the emergence of new traits that enhance an organism's ability to survive and reproduce in its environment.

In the case of maize (corn), epistasis plays a significant role in the plant's adaptation to different environmental conditions. The interaction between multiple genes involved in flowering time has allowed maize to be cultivated across a wide range of latitudes and climates.

These gene interactions enable maize to adapt its growth cycle to the local growing season, which is crucial for maximizing yield in various environments. The complexity of epistatic interactions in maize highlights how adaptation can result from the interplay of multiple genes, rather than changes in a single gene.

Adaptation is a dynamic and multifaceted process that arises from the interplay of various genetic mechanisms.

Whether through mutation, genetic drift, hidden variation, gene flow, or epistasis, organisms continually evolve traits that enhance their survival and reproductive success in specific environments.

The examples discussed—from antibiotic-resistant bacteria to the diverse adaptations of plants and animals—illustrate the diverse ways in which adaptation manifests across the tree of life.

A species is defined as a group of organisms that can interbreed and produce fertile offspring under natural conditions.

Speciation, the process by which new species arise, occurs when populations that once belonged to the same species stop being able to interbreed.

In terms of the causes of speciation, two major types are often distinguished: allopatric and sympatric speciation.

Allopatric speciation happens when populations are geographically isolated by physical barriers like mountains or rivers, leading to genetic divergence as they adapt to different environments.

A classic example is the snapping shrimp populations on either side of the Isthmus of Panama. Once the land bridge formed, these populations became geographically separated and evolved independently, eventually becoming so distinct that they could no longer interbreed.

On the other hand, sympatric speciation occurs without geographic isolation, for example, due to random chromosomal changes, or niche behavioral changes within a subset of a population.

But what actually prevents these species from breeding? What are the mechanisms?

Diverging populations of species are prevented from breeding, and therefore prevented from exchanging their genes, due to mechanisms of reproductive isolation.

As reproductive isolation mechanisms take hold, populations accumulate genetic, behavioral, and morphological differences, leading to the formation of distinct species.

We’re going to explore some of these isolation mechanisms in this orb.

But the first thing to note, however, is that reproductive isolation can occur at different stages in the reproductive process, and it’s typically categorized into prezygotic and postzygotic isolation mechanisms.

A zygote (you may remember) is the cell formed when a sperm fertilizes an egg.

So prezygotic mechanisms prevent this from happening, while postzygotic mechanisms act after a zygote has formed.

Essentially, prezygotic mechanisms prevent mating or fertilization from occurring in the first place. These mechanisms act before the formation of a zygote.

Meanwhile, postzygotic mechanisms operate after fertilization and typically result in the offspring being inviable or infertile.

Prezygotic isolation mechanisms can take several forms: temporal, behavioral, mechanical, and ecological.

Temporal isolation occurs when populations breed at different times.

For example, in the grass Anthoxanthum odoratum, populations in contaminated soils have adapted to flower at different times than those in uncontaminated soils. This timing difference prevents interbreeding because the two populations are reproductively active at different periods, even though they are geographically close.

Behavioral isolation arises when differences in mating behaviors or rituals prevent populations from mating.

The apple maggot fly (Rhagoletis pomonella) provides a clear example. Originally, these flies laid eggs on hawthorn fruits, but a portion of the population shifted to apples. Over time, this difference led to reproductive isolation despite living in the same geographic area.

Mechanical isolation occurs when physical differences in reproductive structures prevent successful mating.

In many insect species, the shape of the genitalia is species-specific. Even slight morphological differences can make mating impossible, ensuring that different species do not interbreed even if they come into contact.

Ecological isolation happens when populations adapt to different habitats within the same geographic area, reducing the likelihood of encounters and mating. Heliconius butterflies, for example, occupy distinct microhabitats within the same region. These ecological differences prevent interbreeding between species, contributing to reproductive isolation.

When prezygotic barriers fail and mating occurs, postzygotic isolation mechanisms come into play.

These mechanisms operate after fertilization and typically result in the offspring being inviable or infertile. This means that even if a sperm fertilizes an egg, the resulting zygote either does not develop into a viable offspring, or if it does, the offspring cannot reproduce.

Hybrid inviability is an example of postzygotic isolation, where hybrid offspring fail to develop properly or are too weak to survive. This often results from genetic incompatibilities that arise as populations diverge. For instance, when different subspecies of the European house mouse interbreed, their offspring often suffer from developmental issues due to chromosomal differences, leading to inviability.

Hybrid sterility occurs when hybrid offspring are viable but unable to reproduce. A well-known example is the mule, a hybrid between a horse and a donkey.

Mules are almost always sterile because horses and donkeys have different chromosome numbers, which prevents the proper formation of gametes (sperm or eggs) in the hybrids. This sterility stops gene flow between the parent species, reinforcing their separation.

Chromosome changes, such as what is known as polyploidy, a condition where an organism has more than two complete sets of chromosomes, are also significant drivers of speciation, particularly in plants.

In some cases, the first-generation hybrids are viable and fertile, but their offspring, the second generation, suffer from hybrid breakdown—they may be sterile or inviable.

You’ve now reached the end of the pathway ‘The Core Concepts of Biology’. Congratulations!

We started off the pathway asking the question ‘What is Life?’. So let's end by reflecting on how far we have answered that question.

What have we learned?

Well, firstly, we have learned that the concept of "life", while intuitively understood, is surprisingly difficult to define precisely.

At its most basic level, life refers to entities that exhibit certain characteristics, including growth, reproduction, response to stimuli, metabolism, homeostasis, and evolution over time. Organisms like bacteria, plants, and animals share these features, distinguishing them from non-living things such as rocks or machines.

We have also learned that the field of biology's core concepts provide a framework for understanding the principles underlying life.

The development of these ideas, such as cell theory, gene theory, and evolution, marked important historical milestones in our scientific understanding.

Understanding these features and core concepts has been the result of centuries of scientific progress, shaped by discoveries that continually deepen our view of what it means to be alive.

Cell theory, for example, showed that all living organisms are composed of cells, the smallest units capable of carrying out the functions essential for life.

Whether we’re talking about a single bacterium or a human being, the cell remains the fundamental building block of life.

Gene theory added another layer by explaining the flow of genetic information through DNA, the molecule that carries the instructions for life’s processes. The discovery of metabolism explained how organisms convert energy to sustain themselves, while homeostasis showed how they maintain stable internal conditions amidst changing external environments.

Finally, Darwin’s theory of evolution by natural selection provided the unifying explanation for life’s diversity and the adaptability of organisms to their environments. Evolution remains the thread that ties all these concepts together, explaining how life changes over time, driven by variation, inheritance, and selection.

Yet, even as these core concepts continue to guide our understanding, biology—like life itself—is evolving.

While these foundational ideas still hold, new discoveries keep expanding our understanding.

A perfect example of this is the endosymbiotic theory, which bridges the gap between evolution and cellular biology. It suggests that complex cells, like those in plants and animals, arose through a symbiotic relationship between ancient single-celled organisms.

This theory not only deepens our understanding of cellular structures but also ties it to the grand narrative of evolution, showing how collaboration at the microscopic level shaped the complexity of life we see today.

Similarly, fields like epigenetics have revealed that the environment can influence how genes are expressed, challenging earlier notions of inheritance.

And the exploration of the microbiome has shown us that no organism exists in isolation, with vast networks of microbes influencing the health and survival of larger organisms.

It's clear that life’s complexity continues to offer new mysteries to explore, and the core concepts you’ve learned in this pathway should provide the foundation for further building on your understanding.