Evolution Accounts for the Unity and Diversity of Life

Evidence Supports Evolution and the Theory

An abundance of evidence of different types supports the occurrence of evolution and the theory that describes how it takes place. To quote one of the founders of modern evolutionary theory, Theodosius Dobzhansky, "Nothing in biology makes sense except in the light of evolution."

Evolution Was Recognized by Charles Darwin

An evolutionary view of life came into sharp focus in November 1859, when Charles Darwin published one of the most important influential books ever written, On the Origin of Species by Means of Natural Selection.

Descent with Modification Outcomes

As a result of descent with modification, two species share certain traits (unity) simply because they have descended from a common ancestor. Furthermore, we can account for differences between two species (diversity) with the idea that certain heritable changes occurred after the two species diverged from their common ancestor.

Unity Between Organisms

As diverse as life is, it also displays remarkable unity. Consider, for example, the similar skeletons of different animals and the universal genetic languages of DNA (the genetic code). In fact, similarities between organisms are evident at all levels of the biological hierarchy. For example, unity is obvious in many features of cell structure, even among distantly related organisms.

Tree Diagrams Express Evolutionary Relationships

Biologists' diagrams of evolutionary relationships generally take treelike forms, though the trees are often turned sideways. Tree diagrams make sense: Just as an individual has a genealogy that can be diagrammed as a family tree, each species is one twig of a branching tree of life extending back in time through ancestral species more and more remote.

Unequal Reproductive Success Leads to Adaptation

By making inferences from these three observations, Darwin arrived at his theory of evolution. He reasoned that individuals with inherited traits that are better suited to the local environment are more likely to survive and reproduce than less well-suited individuals. Over many generations, a higher and higher proportion of individuals in a population will have the advantageous traits. Evolution occurs as the unequal reproductive success of individuals ultimately leads to adaptation to their environment, as long as the environment remains the same.

Natural Selection

Darwin called this mechanism of evolutionary adaptation natural selection because the natural environment consistently "selects" for the propagation of certain traits among naturally occurring variant traits in the population. We see the the products of natural selection in the exquisite adaptations of various organisms to the special circumstances of their way of life and their environment. --> Wings of a bat

The Theory of Natural Selection Came from 3 Observations

Darwin developed his theory of natural selection from observations that by themselves were neither new nor profound. However, although others had described the pieces of the puzzle, it was Darwin who saw how they fit together. He started with three observations from nature.

Descendant Species from Natural Selection

Darwin proposed that natural selection, by its cumulative effects over long periods of time, could cause an ancestral species to give rise to two or more descendant species. This could occur, for example, if one population fragmented into several subpopulations isolated in different environments. In these separate arenas of natural selection, one species could gradually radiate into multiple species as the geographically isolated populations adapted over many generations to different sets of environmental factors.

(2nd Point) The Origin of Species - Natural Selection

Darwin's second main point was his proposal that "natural selection" is a primary cause of descent with modification.

Each Species is Given a Two-Part Name

Diversity is a hallmark of life. Biologists have so far identified and named about 1.8 million species of organisms. Each species is given a two-part name: The first part is the name of the genus to which the species belongs, and the second part is unique to the species within the genus. (For example, Homo sapiens is the name of our species)

Life has Demonstrated Diversity and Unity in Organisms

Evolution is the one idea that makes logical sense of everything we know about living organisms. As the fossil record clearly shows, life has been evolving on Earth for billions of years, resulting in a vast diversity of past and present organisms. But along with the diversity there is also unity, in the form of shared features. For example, while sea horses, jackrabbits, hummingbirds, and giraffes all look very different, their skeletons are organized in the same basic way.

1st Observation of Natural Selection

First, individuals in a population vary in their traits, many of which seem to be heritable (passed on from parents to offspring).

All Organisms are Divided into Three Groups

Historically, scientists have classified the diversity of life-forms into species and broader groupings by careful comparisons of structure, function, and other obvious features. In the last few decades, new methods of assessing species relationships, such as comparisons of DNA sequences, have led to a reevaluation of the classification of life. Although this reevaluation is ongoing, biologists currently divide all organisms into three groups called domains: Bacteria, Archaea, and Eukarya.

The Passage of Time

How can we account for life's dual nature of unity and diversity? The process of evolution illuminates both the similarities and differences in the world of life. It also introduces another important dimension of biology: the passage of time. The history of time, as documented by fossils and other evidence, is the saga of a changing Earth billions of years old, inhabited by an evolving cast of living forms.

Shared Mammalian Anatomy comes from a Prototype

Indeed, all mammalian forelimbs are anatomical variations of a common architecture. According to the Darwinian concept of descent with modification, the shared anatomy of mammalian limbs reflects inheritance of the limb structure from a common ancestor -- the "prototype" mammal from which all other mammals descended.

2nd Observation of Natural Selection

Second, a population can produce far more offspring than can survive to produce offspring of their own. With more individuals than the environment is able to support, competition is inevitable.

Finches Share Ancient Ancestor with Humans

Species that are very similar, such as the Galapagos finches, share a relatively recent common ancestor. Through an ancestor that lived much farther back in time, finches are related to sparrows, hawks, penguins, and all other birds. Furthermore, finches and other birds are related to us through a common ancestor even more ancient.

Galapagos Finches Display Process of Radiation of New Species

The Galapagos finches are a famous example of the process of radiation of new species from a common ancestor. Darwin collected specimens of these birds during his 1835 visit to the remote Galapagos Islands, 900 kilometers (km) off the Pacific coast of South America. These relatively young volcanic islands are home to many species of plants and animals found nowhere else in the world, though many Galapagos organisms are clearly related to species on the South American mainland.

Galapagos Finches Adapted to Different Food Sources

The Galapagos finches are believed to have descended from an ancestral finch species that reached the archipelago from South America or the Caribbean. Over time, the Galapagos finches diversified from their ancestor as populations became adapted to different food sources on their particular islands. Years after Darwin collected the finches, researchers began to sort out their evolutionary relationships, first from anatomical and geographical data and more recently with the help of DNA sequence comparisons.

(1st Point) The Origin of Species - Descent with Modification

The Origin of Species articulated two main points. The first point was that contemporary species arose from a succession of ancestors that differed from them. Darwin called this process "decent with modification" This insightful phrase captured the duality of life's unity and diversity--unity in the kinship among species that descended from common ancestors and diversity in the modifications that evolved as species branched from their common ancestors.

Mammalian Descent from a Common Ancestor

The diversity of mammalian forelimbs results from modification by natural selection operating over millions of years in different environmental contexts. Fossils and other evidence corroborate anatomical unity in supporting this view of mammalian descent from a common ancestor.

Protists are the Most Diverse Eukaryotes

The most numerous and diverse eukaryotes are the protists, which are mostly single-celled organisms. Although protists were once placed in a single kingdom, they are now classified into several groups. One major reason for this change is the recent DNA evidence showing that some protists are less closely related to other protists than they are to plants, animals, or fungi.

Prokaryotic vs Eukaryotic Domains

The organisms making up two of the three domains--Bacteria and Archaea--are prokaryotic. All the eukaryotes (organisms with eukaryotic cells) are in the domain Eukarya. This domain includes four subgroups: kingdom Plantae, kingdom Fungi, kingdom Anamalia, and the protists.

Evolution

The scientific explanation for the unity and diversity of organisms--as well as for the adaptation of organisms to their particular environments--is evolution: the concept that the organisms living on Earth today are the modified descendants of common ancestors.

The Three Kingdoms are Distinguished by Modes of Nutrition

The three kingdoms are distinguished partly by their modes of nutrition: Plants produce their own sugars and other food molecules by photosynthesis, fungi absorb nutrients in dissolved form from their surroundings, and animals obtain food by eating and digesting other organisms.

Bat Wings Have Similarities to Other Mammals

The wings of a bat are not like those of feathered birds; the bat is a mammal. The bat's forelimbs, though adapted for flight, actually have all the same bones, joints, nerves, and blood vessels found in other limbs as diverse as the human arm, the foreleg of a horse, and the flipper of a whale.

3rd Observation of Natural Selection

Third, species generally are suited to their environments--in other words, they are adapted to their circumstances. For instance, a common adaptation among birds that eat mostly hard seeds is an especially strong beak.

The Variety of Organisms Gives Biology a Broad Scope

To date, known species include at least 100,000 species of fungi, 290,000 plant species, 57,000 vertebrate species (animals with backbones), and 1 million insect species (more than half of all down forms of life)--not to mention the triad types of single-celled organisms. Researchers identify thousands of additional species each year. Estimates of the total number of species range from about 10 million to over 100 million. Whatever the actual number, the enormous variety of life gives biology a very broad scope. Biologists face a major challenge in attempting to make sense of this variety.

All Life is Connected

Trace life back far enough, and we reach the early prokaryotes that inhabited Earth over 3.5 billion years ago. We can recognize their vestiges in our own cells--in the universal genetic code, for example. Indeed, all of life is connected through its long evolutionary history.