Organizing Life into Species

One of the most important tasks in biology is identifying and grouping living organisms.

The basic unit used for this is the species.

A species is a group of organisms that share important characteristics and form a distinct evolutionary lineage.

For many sexually reproducing organisms, a species is commonly defined as:

A group of organisms that can reproduce with one another under natural conditions and produce fertile offspring.

This definition is known as the biological species concept.

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The Biological Species Concept

The biological species concept focuses on reproduction.

Members of the same species:

• Can potentially reproduce with one another.
• Share a common gene pool.
• Can produce viable offspring.
• Can normally produce fertile offspring.
• Are reproductively isolated from other species.

For example, two individuals do not have to actually reproduce to belong to the same species. They must belong to populations that are potentially capable of exchanging genes under natural conditions.

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Fertile Offspring

Fertile offspring are offspring that can themselves reproduce.

This is important when applying the biological species concept.

Consider two organisms that mate and produce offspring.

If those offspring can successfully reproduce, this can provide evidence that the parents belong to the same species.

However, if hybrid offspring are sterile, this indicates a reproductive barrier between the parental species.

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A Familiar Example: Horses and Donkeys

A horse and a donkey are different species.

Horse:

Equus caballus

Donkey:

Equus asinus

They can sometimes reproduce together and produce a mule.

Most mules are sterile.

This means they cannot normally produce offspring of their own.

The sterility of the hybrid is evidence of reproductive isolation between horses and donkeys.

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Species Are Not Defined Only by Appearance

Organisms that look similar are not necessarily members of the same species.

Two species may appear almost identical but be:

• Genetically different.
• Behaviorally different.
• Reproductively isolated.

These are sometimes called cryptic species when distinct species are difficult to distinguish by appearance alone.

Scientists therefore use more than appearance when identifying species.

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Different Appearance Does Not Always Mean Different Species

The opposite can also occur.

Members of the same species can look very different.

Differences may result from:

• Sex.
• Age.
• Genetic variation.
• Environmental conditions.
• Seasonal changes.

For example, males and females of some species have dramatically different appearances.

Appearance alone is therefore not always enough to determine whether organisms belong to the same species.

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How Scientists Identify Species

Scientists can use several types of evidence to identify species.

These include:

• Physical characteristics.
• Anatomy.
• Behavior.
• Reproduction.
• DNA sequences.
• Biochemistry.
• Ecology.
• Geographic distribution.
• Evolutionary relationships.

Modern species identification often combines several lines of evidence.

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Morphological Evidence

Morphology refers to the form and structure of an organism.

Scientists may compare:

• Body shape.
• Skeletons.
• Flowers.
• Leaves.
• Reproductive structures.
• Teeth.
• Feathers.
• Scales.
• Internal anatomy.

Morphological evidence is particularly useful when organisms can be directly observed.

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The Morphological Species Concept

The morphological species concept identifies species based largely on consistent differences in physical characteristics.

This can be useful when:

• Reproductive behavior cannot be observed.
• Fossils are being studied.
• Organisms reproduce asexually.

However, morphology has limitations.

Members of different species can look very similar, while members of the same species can look quite different.

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DNA Evidence

Modern scientists frequently compare DNA when identifying species.

Closely related organisms usually have many similarities in their DNA.

Scientists can compare particular DNA sequences to determine whether populations are genetically similar or distinct.

DNA evidence is particularly useful for identifying organisms that are difficult to distinguish by appearance.

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DNA Barcoding

DNA barcoding uses a selected region of DNA to help identify an organism.

The DNA sequence can be compared with sequences from known species.

This technique can help scientists:

• Identify unknown specimens.
• Detect cryptic species.
• Identify immature organisms.
• Study biodiversity.
• Monitor wildlife trade.
• Identify organisms from small tissue samples.

DNA barcoding does not replace all other evidence, but it can be a powerful identification tool.

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Behavioral Evidence

Behavior can help separate species.

Closely related animals may differ in:

• Courtship behavior.
• Mating calls.
• Songs.
• Displays.
• Chemical signals.
• Breeding times.

These differences may prevent individuals from recognizing one another as suitable mates.

Behavior can therefore contribute to reproductive isolation.

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Species and Populations

A species and a population are not the same thing.

A species may include many populations.

A population is a group of organisms of the same species living in the same area at the same time.

For example, a species of bird might occur across an entire continent.

Birds living in one forest could form one population.

Birds of the same species living hundreds of kilometers away could form another population.

Both populations can still belong to the same species.

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Worked Example: Species Versus Population

Imagine a deer species living across a large country.

One group lives in a northern forest.

Another group lives in a southern forest.

These groups are geographically separated.

Northern group:

Population A

Southern group:

Population B

If they remain members of the same reproductively connected species, both populations belong to the same species.

Therefore:

Population ≠ Species

A species can contain multiple populations.

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What Is a Gene Pool?

A gene pool is the total collection of genetic variants present within a population or reproductively connected group.

When individuals reproduce, genes move between individuals and populations.

This movement of genetic information is called gene flow.

Gene flow tends to keep populations genetically connected.

If gene flow stops for long periods, populations may begin evolving independently.

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Reproductive Isolation

Reproductive isolation occurs when populations cannot successfully exchange genes through reproduction.

Reproductive isolation is central to the biological species concept.

Barriers to reproduction can occur:

Before fertilization

or

after fertilization.

These are called:

• Prezygotic barriers
• Postzygotic barriers

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Prezygotic Barriers

Prezygotic barriers prevent fertilization from occurring.

"Pre" means before.

These barriers operate before a zygote forms.

Examples include:

• Geographic separation.
• Different habitats.
• Different mating behaviors.
• Different breeding seasons.
• Incompatible reproductive structures.
• Gametes that cannot successfully fuse.

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Geographic Isolation

A physical barrier can separate populations.

Examples include:

• Mountains.
• Rivers.
• Oceans.
• Deserts.
• Glaciers.
• Islands.

Once populations are separated, gene flow between them may decrease or stop.

Over many generations, the populations may accumulate genetic differences.

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From Isolation to New Species

Consider one population of organisms.

A new river forms and divides the population.

The two groups can no longer regularly reproduce with one another.

Different:

• Mutations.
• Selection pressures.
• Environmental conditions.
• Genetic drift.

cause the populations to change over generations.

Eventually, the differences may become large enough that they could no longer successfully interbreed even if they came back into contact.

At this point, reproductive isolation has evolved.

This process can result in the formation of new species.

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Speciation

The formation of new species is called speciation.

A simplified sequence is:

One population

↓

Isolation

↓

Reduced or absent gene flow

↓

Genetic differences accumulate

↓

Populations diverge

↓

Reproductive isolation develops

↓

Separate species

Geographic separation is one important pathway through which this can occur.

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Behavioral Isolation

Populations may become reproductively isolated because of different mating behaviors.

For example, two closely related bird species may have different:

• Songs.
• Courtship dances.
• Visual displays.

Individuals may only respond to the mating signals of their own species.

Even if the species live in the same area, they may rarely mate.

This is behavioral isolation.

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Temporal Isolation

Temporal isolation occurs when populations reproduce at different times.

For example:

Species A breeds in early spring.

Species B breeds in late summer.

Even if they occupy the same habitat, they are unlikely to reproduce with one another.

Differences could also involve:

• Time of day.
• Season.
• Yearly reproductive cycles.

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Habitat Isolation

Two populations may occupy different habitats within the same geographic area.

For example:

One insect population lives and reproduces on one host plant.

Another closely related population uses a different host plant.

If individuals rarely encounter one another for reproduction, gene flow can decrease.

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Mechanical Isolation

Mechanical isolation occurs when reproductive structures are incompatible.

This can occur in both animals and plants.

For example, the shapes of flowers may be adapted to particular pollinators.

Structural differences can therefore reduce reproduction between populations.

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Gametic Isolation

Sometimes mating or pollen transfer occurs, but the gametes cannot successfully fuse.

For example:

• Sperm may be unable to fertilize an egg.
• Pollen may fail to fertilize an ovule.

This is called gametic isolation.

It prevents formation of a zygote.

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Postzygotic Barriers

Postzygotic barriers operate after fertilization has occurred.

A hybrid zygote may form, but problems occur later.

Examples include:

• Hybrid embryos failing to develop normally.
• Hybrid offspring having reduced survival.
• Hybrid offspring being sterile.
• Later generations of hybrids having reduced fitness.

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Hybrid Sterility

The horse and donkey example demonstrates hybrid sterility.

A horse and donkey can produce a mule.

However, most mules cannot reproduce.

Therefore, genes cannot normally continue flowing between the two parental species through the hybrid.

This maintains reproductive isolation.

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Prezygotic Versus Postzygotic Isolation

A useful distinction is:

Prezygotic barrier

Prevents fertilization.

Examples:

• Different mating behavior.
• Different breeding season.
• Mechanical incompatibility.

Postzygotic barrier

Acts after fertilization.

Examples:

• Hybrid does not survive.
• Hybrid is sterile.
• Hybrid has reduced reproductive success.

Both types of barriers can maintain separation between species.

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Worked Example: Frog Calls

Two frog populations live around the same pond.

Population A males produce one mating call.

Population B males produce a different mating call.

Females respond only to males producing the call associated with their own population.

This is:

Behavioral isolation

It is also:

Prezygotic isolation

because it prevents mating and fertilization from occurring.

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Worked Example: Flowering Times

Two closely related plant populations live in the same field.

Population A flowers in March.

Population B flowers in August.

Their reproductive periods do not overlap.

This is:

Temporal isolation

It acts as a prezygotic barrier.

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Worked Example: Hybrid Offspring

Two animal species mate.

Fertilization occurs and healthy offspring develop.

However, the offspring cannot produce functional gametes.

This is:

Hybrid sterility

It is a postzygotic reproductive barrier.

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Reproductive Isolation and Evolution

Reproductive isolation allows populations to evolve independently.

Without regular gene flow, populations can accumulate different:

• Mutations.
• Alleles.
• Adaptations.

Natural selection may also act differently if the populations experience different environments.

Over many generations, their genetic differences may increase.

This connects reproductive isolation directly with evolution and speciation.

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A Classic Example: Island Populations

Imagine birds from a mainland population colonize two separate islands.

Island A has:

• Large, hard seeds.

Island B has:

• Small, soft seeds.

Variation already exists in beak characteristics.

Different feeding conditions create different selection pressures.

Over many generations:

• Allele frequencies change.
• Populations become genetically different.
• Courtship behavior may also change.
• Reproductive isolation may eventually develop.

The populations may ultimately become separate species.

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Species Concepts

The biological species concept is useful, but it does not work perfectly for every organism.

Scientists therefore use several different species concepts.

These include:

• Biological species concept.
• Morphological species concept.
• Phylogenetic species concept.
• Ecological species concept.

Each emphasizes different evidence.

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Limitations of the Biological Species Concept

The biological species concept is difficult to apply to some organisms.

Asexual Organisms

Bacteria and many other organisms reproduce without mating.

We cannot test whether they can interbreed.

Fossils

We cannot observe whether extinct organisms reproduced with one another.

Geographically Separated Populations

Two populations may never naturally encounter one another.

It can therefore be difficult to determine whether they would successfully interbreed.

Hybridization

Some distinct species can occasionally produce hybrids.

Species boundaries are not always completely simple.

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The Morphological Species Concept

The morphological species concept identifies species using differences in physical form and structure.

This is particularly useful for:

• Fossils.
• Museum specimens.
• Organisms whose reproduction cannot easily be observed.

However, appearance can sometimes be misleading.

Cryptic species may look almost identical despite being genetically distinct.

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The Phylogenetic Species Concept

The phylogenetic species concept focuses on evolutionary relationships.

Scientists use evidence such as:

• DNA.
• Shared characteristics.
• Evolutionary ancestry.

A species can be recognized as a distinct evolutionary lineage.

This approach has become increasingly useful as DNA sequencing technology has improved.

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The Ecological Species Concept

The ecological species concept emphasizes an organism's ecological role or niche.

Populations occupying different ecological niches may experience different selection pressures and evolve independently.

Ecological information can therefore contribute to decisions about species boundaries.

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Why Do We Need Different Species Concepts?

Nature does not always fit perfectly into simple categories.

Different organisms reproduce and evolve in different ways.

For example:

The biological species concept works well for many sexually reproducing animals.

The morphological concept can be useful for fossils.

Genetic and phylogenetic approaches can be especially useful for microorganisms and cryptic species.

Scientists choose evidence appropriate to the organisms being studied.

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Species Concepts and Classification

Correctly identifying species is essential for biological classification.

The taxonomic hierarchy includes:

Domain

Kingdom

Phylum

Class

Order

Family

Genus

Species

Species form the most specific major level in this hierarchy.

Scientific names identify species using binomial nomenclature.

For example:

Panthera leo

identifies the lion.

Panthera tigris

identifies the tiger.

Both belong to the same genus but are separate species.

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Species Concepts and Biodiversity

Species definitions are also important when measuring biodiversity.

Suppose scientists believe a forest contains one widespread frog species.

DNA analysis later reveals that the frogs actually belong to four distinct evolutionary lineages recognized as separate species.

The estimated species richness of the forest changes.

Correct species identification therefore affects biodiversity measurements.

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Species Concepts and Conservation

Conservation programs need to know what species exist and where they occur.

Imagine that one apparent species actually contains two distinct species.

Species A:

Large population and wide distribution.

Species B:

Small population restricted to one forest.

If scientists incorrectly treat them as one species, the conservation risk to Species B could be overlooked.

Species concepts therefore have practical consequences for conservation.

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Species and Evolution

Species are not permanent, unchanging categories.

Populations evolve.

Over evolutionary time:

• Species can split into new species.
• Species can become extinct.
• Populations can diverge.
• Hybridization can occur.
• Geographic distributions can change.

The boundaries between species can therefore sometimes be complex.

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Species Versus Population

Remember the distinction:

Species

A broader biological group containing organisms belonging to the same evolutionary and reproductive lineage.

Population

Members of one species living in the same area at the same time.

For example:

All members of a particular bird species worldwide belong to the species.

The members living on one island may form a particular population.

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Population Versus Community

Another useful distinction is:

Population

One species in an area.

Community

All the populations of different species living and interacting in an area.

For example, a forest might contain populations of:

• Oak trees.
• Deer.
• Foxes.
• Beetles.
• Fungi.

Together, these populations form part of the forest community.

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Common Mistakes

Thinking Organisms Must Look Identical to Belong to the Same Species

Members of one species can show considerable variation.

Thinking Similar-Looking Organisms Must Be the Same Species

Cryptic species can look extremely similar while being genetically and reproductively distinct.

Thinking a Population Is the Same as a Species

A species may contain many populations.

Thinking Two Species Can Never Produce Offspring

Different species can sometimes produce hybrids. The fertility and reproductive success of those hybrids are important considerations.

Thinking Geographic Separation Automatically Creates New Species

Geographic isolation can begin the process, but populations must diverge sufficiently for reproductive isolation to develop.

Thinking All Reproductive Barriers Prevent Mating

Some barriers act after fertilization.

Confusing Prezygotic and Postzygotic Barriers

Prezygotic = before fertilization.

Postzygotic = after fertilization.

Thinking the Biological Species Concept Works for Every Organism

It cannot easily be applied to asexual organisms or fossils.

Thinking Species Are Completely Fixed Categories

Species and populations change through evolutionary time.

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Check Your Understanding

1. Define a species using the biological species concept.

2. What does "fertile offspring" mean?

3. Why does appearance alone not always identify a species?

4. Give four types of evidence scientists can use to identify species.

5. What is DNA barcoding?

6. Define a population.

7. Explain the difference between a species and a population.

8. What is gene flow?

9. Define reproductive isolation.

10. What is the difference between prezygotic and postzygotic barriers?

11. Give three examples of prezygotic reproductive barriers.

12. Give two examples of postzygotic reproductive barriers.

13. Two frog populations use different mating calls. What type of reproductive isolation could this produce?

14. Two plant populations flower during different months. What type of reproductive isolation is this?

15. Why are horses and donkeys considered different species even though they can produce mules?

16. Explain how geographic isolation can eventually contribute to speciation.

17. Why is the biological species concept difficult to apply to bacteria?

18. Why are fossils difficult to classify using the biological species concept?

19. How can identifying species incorrectly affect biodiversity measurements?

20. Explain why scientists sometimes need more than one species concept.

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Key Terms

• Species – distinct biological lineage; under the biological species concept, a group capable of interbreeding under natural conditions and producing fertile offspring.
• Population – members of the same species living in the same area at the same time.
• Biological species concept – species concept based largely on reproductive compatibility and reproductive isolation.
• Morphology – physical form and structure of an organism.
• Morphological species concept – identification of species based largely on consistent structural differences.
• Phylogenetic species concept – species concept emphasizing distinct evolutionary lineages.
• Ecological species concept – species concept emphasizing ecological roles and niches.
• Gene pool – collection of genetic variants within a population or reproductively connected group.
• Gene flow – movement of genetic information between populations through reproduction.
• Reproductive isolation – barriers preventing successful gene exchange between populations.
• Prezygotic barrier – reproductive barrier acting before fertilization.
• Postzygotic barrier – reproductive barrier acting after fertilization.
• Behavioral isolation – reproductive separation caused by differences in mating behavior.
• Temporal isolation – reproductive separation caused by differences in reproductive timing.
• Hybrid – offspring resulting from reproduction between genetically distinct populations or species.
• Speciation – evolutionary formation of new species.
• DNA barcoding – use of selected DNA sequences to help identify species.
• Cryptic species – distinct species that are difficult to distinguish using appearance alone.

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Key Takeaways

• A species is a fundamental unit of biological classification.
• Under the biological species concept, members of a species can potentially interbreed and produce fertile offspring.
• Scientists use morphology, behavior, reproduction, ecology, and DNA to identify species.
• Appearance alone is not always reliable for identifying species.
• A species may contain many different populations.
• A population consists of members of the same species living in the same area at the same time.
• Gene flow connects populations genetically.
• Reproductive isolation reduces or prevents gene flow between populations.
• Prezygotic barriers act before fertilization.
• Postzygotic barriers act after fertilization.
• Geographic, behavioral, temporal, mechanical, and gametic isolation can reduce reproduction between populations.
• Hybrid sterility is an example of a postzygotic barrier.
• Reproductive isolation can allow populations to evolve independently.
• The formation of new species is called speciation.
• The biological species concept has important limitations.
• Asexual organisms and fossils cannot easily be classified using reproductive compatibility.
• Scientists therefore use several different species concepts.
• Species identification is important for classification, biodiversity studies, evolutionary biology, and conservation.
• Species are evolutionary lineages, and their boundaries can change or become clearer as new evidence is discovered.