In either case, species require several thousand generations – which may require several thousand years – to form. Relative to the species’ “lifespan” and the modest precision of our ability to pinpoint time within the geologic record, the differences between the two models may be minimal. Major characteristics of species could have evolved gradually over thousands of years, and still we would not be able to detect those changes on the geologic time scale. What we do detect are long periods of relative stability, when stabilizing selection tends to maintain equilibrium. Changes in physiology and internal anatomy could have continued during the long periods of apparent equilibrium without our seeing them; the fossil record cannot reveal these types of changes. The debate is not about the mechanism of evolution – only its tempo.
Proponents of either model admit that the pace of evolution probably changes from one set of conditions to another. Lake Victoria in East Africa, for example, is just a million years old, and yet home to more than 300 species of cichlid fish found nowhere else (return to Figure above). Genetic analysis shows that all of them descended from a single ancestral “founder” species – relatively rapid evolution. Without competition, that species expanded across the huge Lake, and natural selection suited individual populations to exploit the many different food sources and habitats present in the Lake. Cichlid evolution is an example adaptive radiation similar to that of Darwin’s finches and Hawaiian honeycreepers, discussed in the Evolutionary Theory chapter. The fossil record shows similar opportunities for accelerated evolution after mass extinctions. The extinction of the dinosaurs opened an abundance of ecological niches, soon occupied by radiating birds and mammals. Speciation may accelerate not only with a major increase in available niches, but also with the appearance of new adaptation, such as legs or wings, which allows a group to enter a variety of new niches. In contrast, long periods of environmental stability may slow the pace of speciation.
If you look back at the opening questions for this lesson, you can see that both evolution and our investigations into how evolution works are ongoing - works in progress. We have come a long way in understanding “the origin of species” since Darwin, but questions remain. Exactly what is selected - genes, genotypes, phenotypes, or individuals – is still debated; some, such as Stephen Jay Gould, accept that selection acts at multiple levels. Exactly how fast speciation occurs also remains under discussion; both gradualism and punctuated equilibrium models have merit. Several definitions of a species today serve different purposes; eventually, they should merge into a single clear picture of this basic unit of evolution.
Behind these disagreements, however, lies a powerfully reinforced theory of evolution and an increasingly detailed story of life on Earth. By any definition, humans form a single species, increasingly united by mobility, common genes, and common resources. We and the millions of species with whom we share the Earth today all arose from a single common ancestor billions of years ago – through deep time and myriad instances of allopatric and sympatric speciation. Amazing adaptive logic was most often a result of: directional, stabilizing, and disruptive environmental forces selected for favorable variations and against unfavorable ones. Chance played a major role – through mutation, gene flow, genetic drift, and much environmental change. The pace of speciation probably varied – from gradual change, to rapid change, to long periods of stability. Both speciation and extinction continue today – and will continue into the future, although we cannot predict the direction, because chance determines so much of the direction evolution takes. As a unique species, we are significantly influencing the amount of extinction – and may consequently affect the rate of speciation, as well. That, however, is a topic for a later chapter.
Lesson Summary
Ever since Darwin’s publication of The Origin of Species, biologists have continued to work out the details of how species originate – the foundation of evolutionary process.
A species is the smallest group of organisms into which biologists classify living things.
A biological species is a group of organisms that can interbreed to produce fertile offspring under natural conditions. This is probably the most widely accepted definition of a species.
A morphological species groups organisms based on extensive structural and biochemical similarities. This is probably the most practical definition for use in the field.
A genealogical or evolutionary species includes organisms which share a recent, unique common ancestor. This is the goal of all classification; the only question is how to recognize members.
An ecological species groups organisms together if they share a unique set of adaptations to a particular set of environmental conditions.
Ideally, all four definitions would merge to recognize a true species.
All humans are members of the same biological species, because all races and cultures can and do intermarry, and our DNA is 99.9% identical.
For populations to become separate species, they must experience isolation and genetic divergence.
Isolation can be physical or environmental, but it must be accompanied by heritable reproductive isolation, which requires genetic divergence.
The result of speciation is usually two groups of individuals, each more closely adapted to local environmental conditions than the larger parent population.
Because genetic drift and environmental change increase the influence of chance, adaptation is seldom “perfect” – but over time, natural selection tends to increase fitness.
Allopatric speciation involves geographic barriers, which physically isolate populations.
Barriers to mating and barriers to development of zygotes can both cause reproductive isolation.
Experiments with fruit flies support the possibility that geographic isolation can lead to reproductive isolation.
Sympatric speciation involves the emergence of a new species within the geographic range of the parent population.
In plants, polyploidy is a common sympatric form of “instant speciation.”
Hybridization together with polyploid has formed many vigorous crop species.
In animals, environmental complexity may lead to sympatric speciation.
Cichlid fish in Lake Victoria illustrate both adaptive radiation – a form of allopatric speciation – and more recent sympatric speciation due to heritable changes in mating preference.
The Gradualism model of speciation holds that small changes accumulate to form big changes.
The Punctuated Equilibrium model suggests that rates of change accelerate over short periods in small peripheral populations, and then stabilize for long periods in large, central populations.
Differences between the two may be minimal, because gradual change can occur over thousands of years without our ability to detect it in the fossil record.
Conditions that may accelerate speciation are mass extinctions and the formation of new habitat (e.g. volcanic islands) because both create a sudden availability of many “empty” niches.
Evolution of a major new adaptation, such as legs or wings, may also accelerate speciation by suiting a species to multiple new habitats.
Long periods of environmental stability may slow the rate of speciation.
Review Questions
Define the biological species concept and analyze its usefulness.
Compare the biological species concept to morphological, genealogical, and ecological concepts.
Analyze the reasons why biologists consider all humans to be members of the same species.
Describe two conditions that can lead to speciation.
Explain the results of speciation in terms of adaptation, chance, and changes in the environment.
Distinguish allopatric from sympatric speciation.
Describe two general types of reproductive isolation.
Describe the use of hybridization and polyploidy to form new crop species.
Analyze the importance of environmental complexity to sympatric speciat
ion for animals.
Compare and contrast the gradualist and punctuated equilibrium models of evolutionary change. Give examples which support each.
Further Reading / Supplemental Links
Dawkins, Richard. The Ancestor’s Tale: A Pilgrimage to the Dawn of Life. 2004. Boston: Houghton-Mifflin, ISBN 0618005838 Punctuated Equilibrium
http://www.geocities.com/ginkgo100/pe.html
http://www.pbs.org/wgbh/evolution/library/03/5/l_035_01.html
Phylogenetic Tree
http://itol.embl.de/
Eastern and Western Meadowlark information, with songs - Cornell Lab of Ornithology
http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Western_Meadowlark.html
http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Eastern_Meadowlark.html
Adaptive Radiation of Cichlids
http://www.current-biology.com/content/article/fulltext?uid=PIIS0960982207017046
http://www.nature.com/hdy/journal/v99/n4/full/6801047a.html
Vocabulary
allopatric speciation
The evolution of a new species from a closely related population isolated by geographic barriers.
biological species concept
A group of organisms similar enough to interbreed and produce fertile offspring under natural conditions.
ecological niche
The set of environmental conditions and resources used or required by a species; the role a species plays in its ecosystem.
ecological species concept
A group of organisms which share a unique set of adaptations to a particular set of environmental conditions.
evolutionary species concept
See genealogical species concept.
genealogical (evolutionary) species concept
A group of organisms which share a recent, unique common ancestor – common ancestry without divergence.
gradualism
The idea that the tempo of evolution is slow and constant, with small changes accumulating to form big changes.
hybrid
Offspring of cross-breeding between two different but closely related species.
morphological species concept
A group of organisms which share extensive structural and biochemical similarities.
polyploidy
The duplication of chromosome sets, often resulting in “instant speciation.”
punctuated equilibrium
The idea that species diverge in bursts of relatively rapid change and then remain stable for relatively long periods.
reproductive barrier
A condition which prevents mating or prevents the development of offspring.
reproductive isolation
The separation of closely related populations by barriers to producing viable offspring.
speciation
The process which results in new, separate and genetically distinct groups of organisms (species).
species
See biological, ecological, genealogical, and morphological species concepts.
sympatric speciation
The evolution of new species from closely related populations located in the same area.
Points to Consider
Which definition of species – biological, morphological, ecological, or genealogical – do you prefer?
To what extent do you think stabilizing, directional, and disruptive selection affect humans today?
What effects might genetic engineering have on speciation?
Do you find the evidence for sympatric speciation (the more disputed of the two forms) convincing?
Are gradualist and punctuated equilibrium models mutually exclusive?
Why don’t disagreements about speciation threaten the theory of evolution by natural selection?
Chapter 14: Classification
Lesson 14.1: Form and Function
Lesson Objectives
Define taxonomy, and understand why scientists classify organisms.
Describe Linnaean taxonomy and binomial nomenclature.
Introduction
Billions of years of evolution on Earth have resulted in a huge variety of different types of organisms. For more than two thousand years, humans have been trying to organize this great diversity of life. The classification system introduced by the Swedish botanist Carolus Linnaeus in the early 1700s has been the most widely used classification for almost 300 years.
Taxonomy
Scientific classification is a method by which biologists organize living things into groups. It is also called taxonomy. Groups of organisms in taxonomy are called taxa (singular, taxon). You may already be familiar with commonly used taxa, such as the kingdom and species. A kingdom is a major grouping of organisms, such as plants or animals. A species includes only organisms of the same type, such as humans (Homo sapiens) or lions (Panthera leo). The modern biological definition of a species is a group of organisms that are similar enough to mate and produce fertile offspring together. In a classification system, kingdoms, species, and other taxa are typically arranged in a hierarchy of higher and lower levels. Higher levels include taxa such as kingdoms, which are more inclusive. Lower levels include taxa such as species, which are less inclusive.
This type of hierarchical classification can be demonstrated by classifying familiar objects. For example, a classification of cars is shown in Figure below. The highest level of the classification system includes all cars. The next highest level groups cars on the basis of size. Then, within each of the size categories, cars are grouped according to first one and then another trait. Higher taxa (for example, compact cars) include many different cars. Lower taxa (for example, compact cars that are blue and have two doors and cloth seats) contain far fewer cars. The cars in lower taxa are also much more similar to one another.
Figure 14.1
Cars can be classified, or grouped, on the basis of various traits. In this classification, the most inclusive groups are the size categories, such as all compact cars or all mid-size cars. The most exclusive groups in this classification share several additional traits, including color, number of doors, and type of seats. Note that just one group for each trait is further divided as an example.
Why do biologists classify organisms? The major reason is to make sense of the incredible diversity of life on Earth. Scientists have identified millions of different species of organisms. Among animals, the most diverse group of organisms is the insects. More than one million different species of insects have already been described. An estimated nine million insect species have yet to be identified. A tiny fraction of insect species is shown in the beetle collection in Figure below.
Figure 14.2
Only a few of the more than one million known species of insects are represented in this beetle collection. Beetles are a major subgroup of insects. They make up about 40 percent of all insect species and about 25 percent of all known species of organisms.
As diverse as insects are, there may be even more species of bacteria, another major group of organisms. Clearly, there is a need to organize the tremendous diversity of life. Classification allows scientists to organize and better understand the basic similarities and differences among organisms. This knowledge is necessary to understand the present diversity and the past evolutionary history of life on Earth.
Early Classification Systems
One of the first known systems for classifying organisms was developed by Aristotle. Aristotle was a Greek philosopher who lived more than 2,000 years ago. He created a classification system called the “Great Chain of Being” (See Figure below). Aristotle arranged organisms in levels based on how complex, or “advanced,” he believed them to be. There were a total of eleven different levels in his system. At the lower levels, he placed organisms that he believed were less complex, such as plants. At higher levels, he placed organisms that he believed were more complex. Aristotle considered humans to be the most complex organisms in the natural world. Therefore, he placed them near t
he top of his great chain, just below angels and other supernatural beings.
Figure 14.3
The Great Chain of Being was Aristotles way of classifying organisms. The basis of Aristotles classification was the presumed complexity of organisms. On that basis, Aristotle placed plants near the bottom of the classification and humans near the top.
Aristotle also introduced two very important concepts that are still used in taxonomy today: genus and species. Aristotle used these two concepts in ways that are similar to, but not as precise as, their current meanings. He used the term species to refer to a particular type of organism. He thought each species was unique and unchanging. He used the term genus (plural, genera) to refer to a more general grouping of organisms that share certain traits. For example, he grouped together in the same genera animal species with similar reproductive structures.
As early naturalists learned more about the diversity of organisms, they developed different systems for classifying them. All these early classification systems, like Aristotle’s, were based on obvious physical traits of form or function. For example, in one classification system, animals were grouped together on the basis of similarities in movement. In this system, bats and birds were grouped together as flying animals, and fishes and whales were grouped together as swimming animals.
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