An Introduction to Evolution

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What Is Evolution?

Photo © Brian Dunne / Shutterstock.

Evolution is change over time. Under this broad definition, evolution can refer to a variety of changes that occur over time—the uplifting of mountains, the wandering of riverbeds, or the creation of new species. To understand the history of life on Earth though, we need to be more specific about what kinds of changes over time we're talking about. That's where the term biological evolution comes in.

Biological evolution refers to the changes over time that occur in living organisms. An understanding of biological evolution—how and why living organisms change over time—enables us to understand the history of life on Earth.

They key to understanding biological evolution lies in a concept known as as descent with modification. Living things pass on their traits from one generation to the next. Offspring inherit a set of genetic blueprints from their parents. But those blueprints are never copied exactly from one generation to the next. Little changes occur with each passing generation and as those changes accumulate, organisms change more and more over time. Descent with modification reshapes living things over time, and biological evolution takes place.

All life on Earth shares a common ancestor. Another important concept relating to biological evolution is that all life on Earth shares a common ancestor. This means that all living things on our planet are descended from a single organism. Scientists estimate that this common ancestor lived between 3.5 and 3.8 billion years ago and that all living things that have ever inhabited our planet could theoretically be traced back to this ancestor. The implications of sharing a common ancestor are quite remarkable and mean that we're all cousins—humans, green turtles, chimpanzees, monarch butterflies, sugar maples, parasol mushrooms and blue whales.

Biological evolution occurs on different scales. The scales on which evolution occurs can be grouped, roughly, into two categories: small-scale biological evolution and broad-scale biological evolution. Small-scale biological evolution, better known as microevolution, is the change in gene frequencies within a population of organisms changes from one generation to the next. Broad-scale biological evolution, commonly referred to as macroevolution, refers to the progression of species from a common ancestor to descendent species over the course of numerous generations.

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The History of Life on Earth

Jurassic Coast World Heritage Site.
Jurassic Coast World Heritage Site. Photo © Lee Pengelly Silverscene Photography / Getty Images.

Life on Earth has been changing at various rates since our common ancestor first appeared more than 3.5 billion years ago. To better understand the changes that have taken place, it helps to look for milestones in the history of life on Earth. By grasping how organisms, past and present, have evolved and diversified throughout the history of our planet, we can better appreciate the animals and wildlife that surround us today.

The first life evolved more than 3.5 billion years ago. Scientists estimate that the Earth is some 4.5 billion years old. For nearly the first billion years after the Earth formed, the planet was inhospitable to life. But by about 3.8 billion years ago, the Earth's crust had cooled and the oceans had formed and conditions were more suitable for the formation of life. The first living organism formed from simple molecules present in the Earth's vast oceans between 3.8 and 3.5 billion years ago. This primitive life form is know as the common ancestor. The common ancestor is the organism from which all life on Earth, living and extinct, descended.

Photosynthesis arose and oxygen began accumulating in the atmosphere about 3 billion years ago. A type of organism known as cyanobacteria evolved some 3 billion years ago. Cyanobacteria are capable of photosynthesis, a process by which energy from the sun is used to convert carbon dioxide into organic compounds—they could make their own food. A byproduct of photosynthesis is oxygen and as cyanobacteria persisted, oxygen accumulated in the atmosphere.

Sexual reproduction evolved about 1.2 billion years ago, initiating a rapid increase in the pace of evolution. Sexual reproduction, or sex, is a method of reproduction that combines and mixes traits from two parent organisms in order to give rise to an offspring organism. Offspring inherit traits from both parents. This means that sex results in the creation of genetic variation and thus offers living things a way to change over time—it provides a means of biological evolution.

The Cambrian Explosion is the term given to the time period between 570 and 530 million years ago when most modern groups of animals evolved. The Cambrian Explosion refers to an unprecedented and unsurpassed period of evolutionary innovation in the history of our planet. During the Cambrian Explosion, early organisms evolved into many different, more complex forms. During this time period, nearly all of the basic animal body plans that persist today came into being.

The first back-boned animals, also known as vertebrates, evolved about 525 million years ago during the Cambrian Period. The earliest known vertebrate is thought to be Myllokunmingia, an animal that is thought to have had a skull and a skeleton made of cartilage. Today there are about 57,000 species of vertebrates that account for about 3% of all known species on our planet. The other 97% of species alive today are invertebrates and belong to animal groups such as sponges, cnidarians, flatworms, mollusks, arthropods, insects, segmented worms, and echinoderms as well as many other lesser-known groups of animals.

The first land vertebrates evolved about 360 million years ago. Prior to about 360 million years ago, the only living things to inhabit terrestrial habitats were plants and invertebrates. Then, a group of fishes know as the lobe-finned fishes evolved the necessary adaptations to make the transition from water to land.

Between 300 and 150 million years ago, the first land vertebrates gave rise to reptiles which in turn gave rise to birds and mammals. The first land vertebrates were amphibious tetrapods that for some time retained close ties with the aquatic habitats they had emerged from. Over the course of their evolution, early land vertebrates evolved adaptations that enabled them to live on land more freely. One such adaptation was the amniotic egg. Today, animal groups including reptiles, birds and mammals represent the descendants of those early amniotes.

The genus Homo first appeared about 2.5 million years ago. Humans are relative newcomers to the evolutionary stage. Humans diverged from chimpanzees about 7 million years ago. About 2.5 million years ago, the first member of the genus Homo evolved, Homo habilis. Our species, Homo sapiens evolved about 500,000 years ago.

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Fossils and the Fossil Record

Photo © Digital94086 / iStockphoto.

Fossils are the remains of organisms that lived in the distant past. For a specimen to be considered a fossil, it must be of a specified minimum age (often designated as greater than 10,000 years old).

Together, all fossils—when considered in the context of the rocks and sediments in which they are found—form what is referred to as the fossil record. The fossil record provides the foundation for understanding the evolution of life on Earth. The fossil record provides the raw data—the evidence—that enables us to describe the living organisms of the past. Scientists use the fossil record to construct theories that describe how organisms of the present and past evolved and relate to one another. But those theories are human constructs, they are proposed narratives describing what happened in the distant past and they must fit with fossil evidence. If a fossil is discovered which does not fit with current scientific understanding, scientists must rethink their interpretation of the fossil and its lineage. As science writer Henry Gee puts it:

‎"When people discover a fossil they have enormous expectations about what that fossil can tell us about evolution, about past lives. But fossils actually don't tell us anything. They are completely mute. The most the fossil is, is an exclamation that says: Here I am. Deal with it." ~ Henry Gee

Fossilization is a rare occurrence in the history of life. Most animals die and leave no trace; their remains are scavenged soon after their death or they decompose quickly. But occasionally, an animal's remains are preserved under special circumstances and a fossil is produced. Since aquatic environments offer conditions more favorable to fossilization than those of terrestrial environments, most fossils are preserved in freshwater or marine sediments.

Fossils need geological context in order to tell us valuable information about evolution. If a fossil is taken out of its geological context, if we have the preserved remains of some prehistoric creature but don't know what rocks it was dislodged from, we can say very little of value about that fossil.

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Descent with Modification

A page from one of Darwin's notebooks depicting his first tentative ideas about the branching system of descent with modification.
A page from one of Darwin's notebooks depicting his first tentative ideas about the branching system of descent with modification. Public domain photo.

Biological evolution is defined as descent with modification. Descent with modification refers to the passing on of traits from parent organisms to their offspring. This passing on of traits is known as heredity, and the basic unit of heredity is the gene. Genes hold information about every conceivable aspect of an organism: its growth, development, behavior, appearance, physiology, reproduction. Genes are the blueprints for an organism and these blueprints are passed from parents to their offspring each generation.

The passing on of genes is not always exact, parts of the blueprints may be copied incorrectly or in the case of organisms that undergo sexual reproduction, genes of one parent are combined with the genes of another parent organism. Individuals that are more fit, better suited for their environment, are likely to transmit their genes to the next generation than those individuals that are not well-suited for their environment. For this reason, the genes present in a population of organisms is in constant flux due to various forces—natural selection, mutation, genetic drift, migration. Over time, gene frequencies in populations change—evolution takes place.

There are three basic concepts that are often helpful in clarifying how descent with modification works. These concepts are:

  • genes mutate
  • individuals are selected
  • populations evolve

Thus there are different levels at which changes are taking place, the gene level, the individual level, and the population level. It is important to understand that genes and individuals do not evolve, only populations evolve. But genes mutate and those mutations often have consequences for individuals. Individuals with different genes are selected, for or against, and as a result, populations change over time, they evolve.

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Phylogenetics and Phylogenies

The image of a tree, for Darwin, persisted as a way to envision the sprouting of new species from existing forms.
The image of a tree, for Darwin, persisted as a way to envision the sprouting of new species from existing forms. Photo © Raimund Linke / Getty Images.

"As buds give rise by growth to fresh buds ..." ~ Charles Darwin In 1837, Charles Darwin sketched a simple tree diagram in one of his notebooks, next to which he penned the tentative words: I think. From that point on, the image of a tree for Darwin persisted as a way to envision the sprouting of new species from existing forms. He later wrote in On the Origin of Species:

"As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications." ~ Charles Darwin, from Chapter IV. Natural Selection of On the Origin of Species

Today, trees diagrams have taken root as powerful tools for scientists to depict relationships among groups of organisms. As a result, an entire science with its own specialized vocabulary has developed around them. Here we'll look at the science surrounding evolutionary trees, also known as phylogenetics.

Phylogenetics is the science of constructing and evaluating hypotheses about evolutionary relationships and patterns of descent among organisms past and present. Phylogenetics enables scientists to apply the scientific method to guide their study of evolution and assist them in interpreting the evidence they collect. Scientists working to resolve the ancestry of several groups of organisms evaluate the various alternate ways in which the groups could be related to one another. Such evaluations look to evidence from a variety of sources such as the fossil record, DNA studies or morphology. Phylogenetics thus provides scientists with a method of classifying living organisms based on their evolutionary relationships.

A phylogeny is the evolutionary history of a group of organisms. A phylogeny is a 'family history' that describes the temporal sequence of evolutionary changes experienced by a group of organisms. A phylogeny reveals, and is based on, the evolutionary relationships among those organisms.

A phylogeny is often depicted using a diagram called a cladogram. A cladogram is tree diagram that reveals how lineages of organisms are interconnected, how they branched and re-branched throughout their history and evolved from ancestral forms to more modern forms. A cladogram depicts relationships between ancestors and descendants and illustrates the the sequence with which traits developed along a lineage.

Cladograms superficially resemble the family trees used in genealogical research, but they differ from family trees in one fundamental way: cladograms do not represent individuals like family trees do, instead cladograms represent entire lineages—interbreeding populations or species—of organisms.

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The Process of Evolution

There are four basic mechanisms by which biological evolution takes place. These include mutation, migration, genetic drift, and natural selection.
There are four basic mechanisms by which biological evolution takes place. These include mutation, migration, genetic drift, and natural selection. Photo © Photowork by Sijanto / Getty Images.

There are four basic mechanisms by which biological evolution takes place. These include mutation, migration, genetic drift, and natural selection. Each of these four mechanisms are capable of altering the frequencies of genes in a population and as a result, they all are capable of driving descent with modification.

Mechanism 1: Mutation. A mutation is a change in the DNA sequence of a cell's genome. Mutations can result in various implications for the organism—they can have no effect, they can have a beneficial effect, or they can have a detrimental effect. But the important thing to keep in mind is that mutations are random and occur independent of the organisms' needs. The occurrence of a mutation is unrelated to how useful or harmful the mutation would be to the organism. From an evolutionary perspective, not all mutations matter. The ones that do are those mutations that are passed on to offspring—mutations that are heritable. Mutations that are not inherited are referred to as somatic mutations.

Mechanism 2: Migration. Migration, also known as gene flow, is the movement of genes between subpopulations of a species. In nature, a species is often divided into multiple local subpopulations. The individuals within each subpopulation usually mate at random but might mate less often with individuals from other subpopulations due to geographic distance or other ecological barriers.

When individuals from different subpopulations move easily from one subpopulation to another, genes flow freely among the subpopulations and the remain genetically similar. But when individuals from the different subpopulations have difficulty moving between subpopulations, gene flow is restricted. This may in the subpopulations becoming genetically quite different.

Mechanism 3: Genetic Drift. Genetic drift is the random fluctuation of gene frequencies in a population. Genetic drift concerns changes that are driven merely by random chance occurrences, not by any other mechanism such as natural selection, migration or mutation. Genetic drift is most important in small populations, where the loss of genetic diversity is more likely due to their having fewer individuals with which to maintain genetic diversity.

Genetic drift is controversial because it creates a conceptual problem when thinking about natural selection and other evolutionary processes. Since genetic drift is a purely random process and natural selection is non-random, it creates difficulty for scientists to identify when natural selection is driving evolutionary change and when that change is simply random.

Mechanism 4: Natural selection. Natural selection is the differential reproduction of genetically varied individuals in a population that results in individuals whose fitness is greater leaving more offspring in the next generation than individuals of lesser fitness.

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Natural Selection

The eyes of living animals provide hints about their evolutionary history.
The eyes of living animals provide hints about their evolutionary history. Photo © Syagci / iStockphoto.

In 1858, Charles Darwin and Alfred Russel Wallace published a paper detailing the theory of natural selection which provides a mechanism by which biological evolution occurs. Although the two naturalists developed similar ideas about natural selection, Darwin is considered to be the theory's primary architect, since he spent many years gathering and compiling a vast body of evidence to support the theory. In 1859, Darwin published his detailed account of the theory of natural selection in his book On the Origin of Species.

Natural selection is the means by which beneficial variations in a population tend to be preserved while unfavorable variations tend to be lost. One of the key concepts behind the theory of natural selection is that there is variation within populations. As a result of that variation, some individuals are better suited to their environment while other individuals are not so well-suited. Because members of a population must compete for finite resources, those better suited to their environment will out-compete those that are not as well-suited. In his autobiography, Darwin wrote of how he conceived this notion:

"In October 1838, that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed." ~ Charles Darwin, from his autobiography, 1876.

Natural selection is a relatively simple theory that involves five basic assumptions. The theory of natural selection can be better understood by identifying the basic principles on which it relies. Those principles, or assumptions, include:

  • Struggle for existence - More individuals in a population are born each generation than will survive and reproduce.
  • Variation - Individuals within a population are variable. Some individuals have different characteristics than others.
  • Differential survival and reproduction - Individuals that have certain characteristics are better able to survive and reproduce than other individuals having different characteristics.
  • Inheritance - Some of the characteristics that influence an individual's survival and reproduction are heritable.
  • Time - Ample amounts of time are available to allow for change.

The result of natural selection is a change in gene frequencies within the population over time, that is individuals with more favorable characteristics will become more common in the population and individuals with less favorable characteristics will become less common.

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Sexual Selection

While natural selection is the result of the struggle to survive, sexual selection is the result of the struggle to reproduce.
While natural selection is the result of the struggle to survive, sexual selection is the result of the struggle to reproduce. Photo © Eromaze / Getty Images.

Sexual selection is a type of natural selection that acts on traits related to attracting or gaining access to mates. While natural selection is the result of the struggle to survive, sexual selection is the result of the struggle to reproduce. The outcome of sexual selection is that animals evolve characteristics whose purpose do not increase their chances of survival but instead increases their chances of reproducing successfully.

There are two kinds of sexual selection:

  • Inter-sexual selection occurs between the sexes and acts on characteristics that make individuals more attractive to the opposite sex. Inter-sexual selection can produce elaborate behaviors or physical characteristics, such as the feathers of a male peacock, the mating dances of cranes, or the ornamental plumage of male birds of paradise.
  • Intra-sexual selection occurs within the same sex and acts on characteristics that make individuals better able to outcompete members of the same sex for access to mates. Intra-sexual selection can produce characteristics that enable individuals to physically overpower competing mates, such as the antlers of an elk or the bulk and power of elephant seals.

Sexual selection can produce characteristics that, despite increasing the individual's chances of reproducing, actually diminish the chances of survival. The brightly colored feathers of a male cardinal or the bulky antlers on a bull moose might make both animals more vulnerable to predators. Additionally, the energy an individual devotes to growing antlers or putting on the pounds to outsize competing mates can take a toll on the animal's chances of survival.

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The relationship between flowering plants and their pollinators can offer a classic examples of coevolutionary relationships.
The relationship between flowering plants and their pollinators can offer a classic examples of coevolutionary relationships. Photo courtesy Shutterstock.

Coevolution is the evolution of two or more groups of organisms together, each in response to the other. In a coevolutionary relationship, changes experienced by each individual group of organisms is in some manner shaped by or influenced by the other groups of organisms in that relationship.

The relationship between flowering plants and their pollinators can offer a classic examples of coevolutionary relationships. Flowering plants rely on pollinators to transport pollen among individual plants and thus enable cross-pollination.

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What Is a Species?

Shown here are two ligers, male and female. Ligers are the offspring produced by a cross between a female tiger and a male lion. The ability of large cat species to produce hybrid offspring in this manner blurs the definition of a species.
Shown here are two ligers, male and female. Ligers are the offspring produced by a cross between a female tiger and a male lion. The ability of large cat species to produce hybrid offspring in this manner blurs the definition of a species. Photo © Hkandy / Wikipedia.

The term species can be defined as a group of individual organisms that exist in nature and, under normal conditions, are capable of interbreeding to produce fertile offspring. A species is, according to this definition, the largest gene pool that exists under natural conditions. Thus, if a pair of organisms are capable of producing offspring in nature, they must belong to the same species. Unfortunately, in practice, this definition is plagued by ambiguities. To begin, this definition is not relevant to organisms (such as many types of bacteria) that are capable of asexual reproduction. If the definition of a species requires that two individuals are capable of interbreeding, then an organism that does not interbreed is outside of that definition.

Another difficulty that arises when defining the term species is that some species are capable of forming hybrids. For example, many of the large cat species are capable of hybridizing. A cross between a female lions and a male tiger produces a liger. A cross between a male jaguar and a female lion produces a jaglion. There are a number of other crosses possible among the panther species, but they are not considered to be all members of a single species as such crosses are very rare or do not occur at all in nature.

Species form through a process called speciation. Speciation takes place when the lineage of a single splits into two or more separate species. New species can form in this manner as a result of several potential causes such as geographic isolation or a reduction in gene flow among members of the population.

When considered in the context of classification, the term species refers to the most refined level within the hierarchy of major taxonomic ranks (though it should be noted that in some cases species are further divided into subspecies).