Speciation
Characterization of Species
Biologists employ multiple approaches to identify and characterize species, recognizing that no single method works universally. The
characteristics used to identify a species depend largely on the organisms being studied and the available evidence.
Morphological Traits
Physical characteristics and structural
features of organisms
Ability to Interbreed
Whether organisms can successfully
reproduce and produce fertile offspring
Molecular Features
DNA sequences, gene order, and
chromosomal characteristics
Ecological Factors
Habitat preferences, resource use, and
environmental adaptations
Evolutionary Relationships
Common ancestry and phylogenetic history
Each approach has strengths and limitations, which is why modern biologists typically use a combination of these methods for accurate
species identification. The most appropriate characteristics depend on whether the organisms are extant or extinct, sexual or asexual, and
how much information is available about their biology.
Morphological Traits
Morphological traits refer to the physical characteristics
and structural features of an organism. These observable
features have traditionally been the primary method for
species identification.
Drawbacks for Determining Species
• Trait selection: Determining which physical
characteristics to compare and how many traits should be
considered
• Continuous variation: Many traits vary along a
continuum, making it difficult to establish clear boundaries
between species
• Intraspecific variation: Members of the same species can
look dramatically different (sexual dimorphism,
developmental stages, seasonal variations)
• Convergent evolution: Members of different species can
look remarkably similar due to adaptation to similar
environments
Despite these limitations, morphological analysis remains valuable, especially when combined
with other identification methods.
Reproductive Isolation Can They Interbreed?
Reproductive isolation prevents one species from successfully interbreeding with other
species in nature. Sometimes organisms that appear similar are actually different species
because they cannot breed with each other, while other times visually distinct organisms
can interbreed.
Advantages of This Approach
• Focuses on the fundamental biological
process of reproduction
• Identifies functional boundaries
between populations
• Helps explain how species remain
distinct in nature
Four Main Limitations
1. Observation difficulty: May be
challenging to determine breeding
compatibility in nature, especially for
species in non-overlapping geographic
regions
2. Partial compatibility: Some
populations can interbreed yet maintain
distinct characteristics
3. Asexual organisms: Cannot be applied
to species that reproduce asexually,
such as many bacteria
4. Extinct species: Impossible to test
reproductive compatibility in organisms
known only from fossils
Molecular Features
Modern molecular biology has revolutionized species identification by allowing scientists to compare
genetic information at the molecular level. This approach examines multiple aspects of genetic material to
identify similarities and differences among populations.
DNA Sequences
Comparing nucleotide
sequences within specific
genes
Gene Order
Analyzing the arrangement
of genes along
chromosomes
Chromosome Structure
Examining physical
organization and
modifications
Chromosome Number
Counting and comparing
total chromosome sets
The Challenge of Drawing Boundaries
When DNA sequences are highly similar, populations may be judged to be the same species. However, this
raises a critical question: How much genetic difference is needed to designate separate species? Should we
use a 2% difference threshold? 5%? 10%? The answer varies depending on the organisms being studied,
their generation time, and their evolutionary history. This ambiguity represents one of the main challenges
in molecular species identification.
Ecological Factors
A variety of factors related to an organism's habitat and ecological niche
can be used to distinguish one species from another. This approach
focuses on how organisms interact with their environment and utilize
available resources.
Examples in Practice
Warblers provide a classic example. Different warbler species can be
distinguished by where they forage for food - some species feed primarily
on the ground, others in shrubs, and still others high in trees. These
distinct foraging behaviors reflect ecological specialization.
Bacterial species are frequently categorized based on their ecological
characteristics, including the types of nutrients they can metabolize, their
temperature preferences, and their pH tolerance.
Limitations of Ecological Approaches
Similarity in conditions: Different groups of bacteria sometimes display very similar growth characteristics, making them difficult to
distinguish. Even members of the same species may show great variation in the environmental conditions they can tolerate, including
temperature ranges and pH levels.
Evolutionary Relationships
Evolutionary trees (phylogenies) describe the relationships between ancestral species and modern
species, providing a historical perspective on how organisms are related through evolution. These
relationships are reconstructed using two primary sources of evidence.
By integrating information from
both fossils and modern genetics,
scientists can construct detailed
evolutionary trees that show when
species diverged from common
ancestors. This approach is
particularly valuable because it
reveals not just whether organisms
are related, but how and when
those relationships formed.
Fossil Record
Physical evidence of ancient organisms preserved in rock layers,
showing morphological changes over geological time
DNA Sequences
Molecular data from living organisms, revealing genetic similarities
and differences that reflect evolutionary history
Reproductive Isolation
Reproductive isolation is the inability of two populations to interbreed successfully. This isolation is maintained by reproductive barriers
that serve to isolate the gene pools of species, prevent interbreeding, and maintain species as distinct biological entities.
Single Population
Population with ongoing
gene flow
Barrier Formation
Reproductive barriers
begin to develop
Speciation
Two reproductively
isolated species exist
This diagram illustrates how reproductive barriers transform a single interbreeding population into two distinct species over time.
Prezygotic Barriers
Prevent fertilization from occurring - barriers that act before the formation of a zygote
Postzygotic Barriers
Reduce viability or fertility of hybrid offspring - barriers that act after zygote formation
When organisms from two different species do manage to produce offspring, the resulting individuals are called interspecies hybrids. The
success or failure of these hybrids plays a crucial role in maintaining species boundaries.
Prezygotic Barriers
Preventing Mating or Fertilization
Five types of prezygotic barriers prevent mating or fertilization between species. These barriers act before a zygote forms, effectively preventing the mixing of genetic material between different species.
Habitat Isolation
Species occupy different habitats and rarely encounter each other
Temporal Isolation
Species breed at different times of day or different seasons
Behavioral Isolation
Species fail to send or receive appropriate courtship signals
Mechanical Isolation
Reproductive structures are physically incompatible
Gametic Isolation
Eggs and sperm (or pollen and stigma) are molecularly incompatible
Temporal Isolation Example: These two related frog species
exhibit temporal reproductive isolation. Rana aurora breeds
earlier in the year than Rana boylii, preventing interbreeding
despite overlapping habitats.
Cricket Species Example
Spring field cricket (Gryllus veletis) and Fall field cricket (Gryllus pennsylvanicus) are reproductively isolated by breeding season.
a) Spring field cricket (Gryllus
veletis)
b) Fall field cricket (Gryllus
pennsylvanicus)
Prezygotic Barriers Continued
Behavioral, Mechanical, and Gametic Isolation
Behavioral Isolation
Species-specific courtship behaviors and mating signals ensure that individuals
recognize and mate only with members of their own species. Changes in
courtship songs, dances, pheromones, or visual displays can prevent mating
between closely related species.
Mechanical Isolation
Physical incompatibility of reproductive
structures prevents successful mating.
This is particularly important in insects
and flowering plants.
Gametic Isolation
Even if mating occurs, molecular incompatibility between gametes, meaning the
sperm cannot recognize or bind to the egg, or pollen cannot successfully interact
with the stigma.
The shape of the male reproductive organ varies
among male damselfly species, and is only
compatible with the female of that species.
Reproductive organ incompatibility keeps the species
reproductively isolated.
Changes in song
Some flowers have evolved to attract certain
pollinators. The (a) wide foxglove flower is adapted
for pollination by bees, while the (b) long, tube-
shaped trumpet creeper flower is adapted for
pollination by humming birds.
Postzygotic Barriers
Three types of postzygotic barriers operate after hybrid zygotes have formed. These barriers reduce the success of hybrid offspring even
after fertilization has occurred.
Reduced Hybrid Viability
Interaction of parental genes impairs
the hybrid's development or survival.
Some salamander species can
hybridize, but their offspring do not
develop fully or are frail and will not
survive long enough to reproduce.
Reduced Hybrid Fertility
Hybrids are vigorous but cannot produce
viable offspring. A mule is the classic
example - the sterile hybrid offspring of
a horse and a donkey. Mules are healthy
and strong but cannot reproduce.
Hybrid Breakdown
First-generation hybrids are viable and fertile,
but their offspring (F2 generation) are feeble
or sterile. Some rice hybrids are fertile, but
plants of the next generation are sterile or
weak.
Summary: Reproductive Isolating Mechanisms
This comprehensive table summarizes all the reproductive barriers that maintain species boundaries. Understanding these mecha nisms is crucial for
understanding how species remain distinct in nature.
Mechanism Description
PREZYGOTIC ISOLATING MECHANISMS
Ecological isolation Species occur in the same area, but they occupy different habitats and rarely encounter each other
Temporal isolation Species reproduce in different seasons or at different times of the day
Behavioral isolation Species differ in their mating rituals
Mechanical isolation Structural differences between species prevent mating
Prevention of gamete fusion Gametes of one species function poorly with the gametes of another species or within the reproductive
tract of another species
POSTZYGOTIC ISOLATING MECHANISMS
Hybrid inviability or infertility Hybrid embryos do not develop properly, hybrid adults do not survive in nature, or hybrid adults are
sterile or have reduced fertility
Darwin's Finches
A Classic Example of Adaptive Radiation
The Galápagos Islands currently host 14 species of closely related finches, collectively known as Darwin's finches because Charles Darwin collected specimens during his
voyage on the HMS Beagle. These birds played a crucial role in shaping Darwin's ideas about evolution and natural selection.
Characteristics of Darwin's Finches
• Shared ancestry: All species share many finch-like traits, indicating descent
from a common ancestor
• Beak diversity: Species differ dramatically in their feeding habits and
corresponding beak shapes
• Ecological specialization: Each beak type is specialized for what that
particular species eats
• Adaptive radiation: All 14 species arose through adaptive radiation from a
single ancestral species
Some finches have massive, powerful beaks for cracking hard seeds. Others have
long, slender beaks for probing flowers for nectar. Still others have sharp beaks
adapted for catching insects. This remarkable diversity evolved because different
food sources were available on the islands, and natural selection favored different
beak shapes for exploiting these resources.
Diversity of Darwin's Finches
The finches can be broadly categorized by their feeding strategies:
Seed Eaters
Large ground finches like Geospiza magnirostris have massive beaks for crushing hard
seeds
Insect Eaters
Species like Camarhynchus pallidus have medium-sized pointed beaks for catching
insects
Nectar Feeders
Certhidea olivacea has a thin, curved beak ideal for sipping nectar from flowers
Figure 22.16
The Pace of Speciation
The rate at which new species arise varies dramatically across different types of organisms. Understanding these differences is crucial for
comprehending evolutionary timescales and their practical implications.
Generation Time Matters
Large animal species with long generation times evolve much
more slowly than microbial species with short generations. An
elephant, which may take 15 years to reach reproductive maturity
and produce offspring every few years, evolves at a vastly
different pace than bacteria, which can reproduce every 20
minutes.
Important Consequence: Many new species of bacteria will come
into existence during our lifetime, while new species of large
animals arise on a much longer timescale - often millions of years.
Public Health Implications
The rapid speciation rate of bacteria has profound
environmental and public health effects. Bacteria can quickly
evolve:
• Antibiotic resistance
• New metabolic capabilities
• Pathogenic properties
• Adaptations to new environments
This rapid evolution makes bacterial speciation particularly
important in the public health field, where new disease-causing
strains can emerge in relatively short timeframes.
Sympatric Speciation
Sympatric speciation occurs when a new species arises within the same geographic area as its parent species. Unlike allopatric speciation, no phys ical barrier
separates the diverging populations. This raises an intriguing question: How can reproductive isolation develop when members of sympatric populations remain in
contact with each other?
Chromosomal
Change Reproductive
e IsolationHabitat Shift Mate
Preference
This diagram illustrates how a single population can split into two distinct species without geographic separation.
Aneuploidy
Changes in chromosome number
create instant reproductive barriers
Habitat Differentiation
Specialization to different
microhabitats within the same area
Sexual Selection
Evolution of distinct mate
preferences within a population
Gene flow between populations may be reduced by any of these mechanisms, allowing divergence even in the absence of geographic barriers.
mechanisms of sympatric speciation
Sympatric Speciation Through Aneuploidy
Many organisms, particularly plants, can survive and even thrive with abnormal numbers of chromosomes - a condition called
aneuploidy. This tolerance for chromosomal changes can lead directly to the formation of new species.
How Aneuploidy Leads to Speciation
When errors during cell division result in offspring with different chromosome numbers, these
individuals may be unable to successfully breed with the parent population. If the aneuploid
individuals can reproduce with each other, they instantly form a reproductively isolated group.
Having extra copies of certain genes can result in offspring that are "bigger," "faster," "more
drought tolerant," or possess other advantageous traits. If these traits provide a survival
advantage, natural selection can favor the aneuploid individuals.
The diagram shows how nondisjunction during meiosis - the failure of chromosomes to separate properly - results in gametes with too
many or too few chromosomes. The resulting offspring will have chromosome numbers of 2n+1 or 2n-1, potentially creating reproductive
isolation from the parent population.
Other Mechanisms of Sympatric Speciation
Beyond chromosomal changes, two additional mechanisms can drive sympatric speciation by reducing gene flow between population s that remain in geographic
contact.
Adaptation to Local Environments
A geographic area may contain variation in environmental conditions, creating micro-environments within a
continuous landscape. Some members of a population may diverge and become specialized to occupy these
different local environments.
For example, insects on a single plant species might specialize to feed on different parts - some preferring leaves,
others stems, and still others flowers. Over time, these feeding preferences could lead to reproductive isolation if
insects preferentially mate on their preferred plant part.
Sexual Selection
Sexual selection can drive speciation when populations develop different mate preferences. In some species, certain females prefer males with one color pattern or
song type, while other females prefer males with different traits.
If these preferences strengthen over time, the population may split into distinct groups that rarely interbreed, even though they occupy the same geographic area.
This mechanism is particularly important in species like cichlid fish, where female mate choice based on male coloration can drive rapid speciation.
Hybrid Zones
Where Species Meet and Interbreed
What happens when separated populations of closely related species come back into contact with each other? Biologists study this
question by examining hybrid zones - regions where members of different species meet and mate, producing at least some hybrid
offspring.
Hybrid zones provide natural laboratories for studying reproductive isolation and the
speciation process. They reveal what happens when the reproductive barriers between species
are incomplete.
The outcome of hybridization in these zones depends on the fitness of hybrid offspring relative
to the parent species. Three main scenarios can occur: reinforcement of barriers, stability of the
hybrid zone, or fusion of the species.
The geographic distribution and genetic composition of hybrid zones can change over time,
providing insights into the dynamic nature of speciation and the strength of reproductive
barriers.
Reinforcement in Hybrid Zones
Reinforcement occurs when hybrid offspring are less fit than
members of both parent species. In this scenario, natural selection
favors individuals that avoid mating with the other species, thereby
strengthening reproductive barriers.
The Flycatcher Example
The closely related collared flycatcher and pied flycatcher provide
an excellent example of reinforcement. These bird species
demonstrate how reproductive barriers can be strengthened where
species overlap.
When these species are allopatric (geographically separated), male
pied flycatchers have less distinct plumage. However, where the
species are sympatric (overlap), male pied flycatchers have
evolved more distinctive black and white coloration.
Why Does This Occur?
Natural selection has favored more distinctive male coloration in areas where both species occur
together. This helps females avoid mating mistakes that would produce less-fit hybrid offspring. The
result is that reproductive barriers are stronger in sympatric populations than in allopatric
populations.
Allopatric
populations
Sympatric
populations
Male
collared
flycatcher
Male pied
flycatcher
Pied flycatcher
from allopatric
population
Pied flycatcher
from sympatric
population
Fusion Through Hybridization
When Species Blur Together
What happens when reproductive barriers between species are not strong and extensive gene flow occurs in a hybrid zone? Somet imes fusion occurs - so
much interbreeding happens that the speciation process reverses, causing two hybridizing species to fuse back into a single s pecies.
Pundamilia nyererei
Males have red coloration
Pundamilia pundamilia
Males have blue coloration
Cichlids in Lake Victoria
A striking example of fusion has been occurring
among cichlid fish species in Lake Victoria, Africa.
These closely related species were originally kept
separate by female mate choice based on male
coloration.
Pundamilia nyererei males are red, while P.
pundamilia males are blue. Females typically
choose mates based on these color differences,
maintaining reproductive isolation between the
species.
However, pollution from development along the
lake's shores has turned the water murky, making
it difficult for females to distinguish between red
and blue males.
The Consequence of Pollution
Species Fusion in Real Time
The case of Lake Victoria cichlids provides a sobering example of how human activity can reverse millions of years of evolution. Water pollution has created
conditions that are causing distinct species to fuse back together.
Clear Water
Females easily distinguish red from blue males, maintaining species
boundaries
Murky Water
Female mate choice breaks down - can't tell red from blue males
Hybridization
Extensive interbreeding produces viable hybrid offspring
Fusion
Gene pools combine into single hybrid species
The Result: Pundamilia "turbid water"
When P. nyererei or P. pundamilia females cannot distinguish between red and blue males in murky water, the behavioral barrier that maintained species separati on
crumbles. Many viable hybrid offspring are produced through interbreeding.
The once-isolated gene pools of the parent species are now combining, with the two species fusing into a single hybrid species s ometimes called Pundamilia "turbid
water." This represents a tragic loss of biodiversity occurring in real time due to human environmental impact.
Beak Morphology and Reproductive Isolation
Jeffrey Podos's Research on Darwin's Finches
Biologist Jeffrey Podos conducted fascinating research exploring whether an adaptation to feeding may have inadvertently promoted
reproductive isolation in Darwin's finches. His work reveals how natural selection for one trait (feeding) can have unexpected consequences for
another (mating).
Darwin's finches have evolved different beak sizes and shapes as adaptations to different feeding strategies - large, powerful beaks for crushing
hard seeds versus smaller, more delicate beaks for handling small insects or probing flowers.
The Research Question
Podos analyzed the songs of different finch species to determine if beak morphology affected song characteristics. Since finc h songs play a crucial
role in attracting mates and defending territories, changes in song could impact reproductive success and species boundaries.
The Discovery
Podos found that birds with larger beaks produced songs with a more narrow frequency range and/or slower trill rate. The physical constraints
of the beak limited the types of sounds birds could produce.
Evolutionary Implication: This finding suggests that selection for particular beak shapes (driven by food availability) could have
inadvertently contributed to reproductive isolation by altering the songs used in mate attraction and recognition
Chicken vs Duck Feet
Gene Expression and Pattern Formation
The difference between chicken feet (non-webbed with separated digits) and duck feet (webbed with connected digits) provides an elegant example of how changes
in gene expression during development can produce major morphological differences.
The Molecular Difference
This dramatic difference in foot morphology results
from differences in the expression of just two cell-
signaling proteins during embryonic development:
BMP4 (Bone Morphogenetic Protein 4) - A signaling
molecule that causes cells in the interdigit regions
(spaces between developing toes) to undergo
apoptosis (programmed cell death) and die.
Gremlin - A protein that inhibits the function of
BMP4, preventing cell death and allowing cells in the
interdigit regions to survive.
Chicken Development
BMP4: Expressed in interdigit regions
Gremlin: NOT expressed between digits
Result: Cells die, creating separated toes
Duck Development
BMP4: Similar expression to chicken
Gremlin: Strongly expressed between digits
Result: Cells survive, creating webbed feet
Evolution of Webbed vs Non-Webbed Feet
Natural Selection Acts on Developmental Variation
The chicken versus duck foot example beautifully illustrates how natural selection acts on developmental gene expression to p roduce adaptive differences
between species.
Genetic Variation
Mutations changed expression patterns of BMP4 and Gremlin
Selection on Land
Nonwebbed feet advantageous for walking, running, perching
Selection in Water
Webbed feet advantageous for swimming and paddling
Speciation
Geographic isolation of habitats promoted divergence
Natural selection maintained nonwebbed feet in terrestrial environments where they provide advantages
for walking and perching. In aquatic environments, natural selection favored webbed feet that enhance
swimming ability. Speciation may have been further promoted by geographical isolation of these different
habitat types.
Hox Genes: Master Control Genes
Hox genes are often called "master control genes" because they orchestrate the development of major body structures. These remarkable
genes are found in all animals and play a fundamental role in determining body plans.
Key Characteristics of Hox Genes
• Universal presence: Found in all animals from simple
sponges to complex mammals
• Body plan development: Control where major structures
develop along the body axis
• Variation drives diversity: Changes in Hox genes have
spawned the formation of many new body plans
• Number matters: The number and arrangement of Hox genes
varies among different types of animals
Evolutionary Significance
Increases in the number of Hox genes appear to have led to
greater complexity in body structure throughout animal
evolution.
Simple animals like sponges have fewer Hox genes and simpler
body plans. More complex animals like mammals have more Hox
genes, allowing for the development of intricate body structures
with specialized regions.
Even small changes in how Hox genes are expressed can produce
dramatic changes in body form, making them key players in
evolutionary innovation.
Hox Gene Complexity Across Animals
• Sponges are the simplest animals, with bodies that are not organized along a body
axis.
• Anemones have a primitive body axis, showing radial symmetry.
• The other animals shown in this figure have a more complex form of symmetry called
bilateral symmetry, meaning that their bodies are organized along a well-defined
anteroposterior axis, with right and left sides that show a mirror symmetry. Such
organisms are called bilaterians. Flatworms are very simple bilaterians.
• Invertebrates such as insects are structurally more complex than flatworms, but less
complex than organisms with a spinal cord.
• Animals with spinal cords are known as chordates. The simple chordates lack bony
vertebrae that enclose the spinal cord.
• The vertebrates, such as mammals, have vertebrae and possess a very complex
body structure.
• *Sponges are early diverging anim als with no true tissues. They do not have true Hox genes, though they have an evolutionarily related gene called an NK-like gene.
Evidence for Hox Genes in Evolution
Three compelling lines of evidence support the idea that Hox gene complexity has been instrumental in the evolution and speciation
of animals with different body patterns.
Control of Body Regions
Hox genes are known to control the
developmental fate of regions along the
anteroposterior (head-to-tail) axis of
animals. Experiments show that
changing Hox gene expression can
cause legs to grow where antennae
should be, or other dramatic body plan
alterations.
Correlation with Complexity
There is a general trend for more
complex animals to have more Hox
genes and more Hox gene clusters.
Simple animals have fewer Hox genes;
complex animals have undergone Hox
gene duplications resulting in multiple
copies.
Parallel Evolution
Comparison of Hox gene evolution and
animal body plan evolution shows
striking parallels. The timing of Hox
gene duplications in evolutionary
history corresponds with the
appearance of new, more complex body
plans in the fossil record.
Together, these lines of evidence demonstrate that changes in Hox genes - through gene duplication, changes in gene sequence, or changes
in when and where genes are expressed - have been major drivers of evolutionary innovation in animal body plans.
Heterochrony: Differential Growth Rates How Timing Changes Shape Evolution
Slight changes in the relative growth rates of different body
parts can dramatically change an organism's adult form. This
phenomenon is called heterochrony - when one region of the
body grows faster or slower than another among different
species.
The Human-Chimpanzee Example
Skulls of humans and chimpanzees are
remarkably similar as fetuses, but quite
different as adults. This difference
results from different rates of growth in
various skull regions during
development.
In chimpanzees, the jaw and face grow
rapidly after birth, while the braincase
grows more slowly. In humans, the
braincase continues rapid growth while
facial growth is relatively slower.
Chimpanzee infant Chimpanzee adult
Chimpanzee fetus Chimpanzee adult
Human adultHuman fetus
Key Concepts: Study Guide Part 1
1
Identification of a Species
• Outline the characteristics that biologists use to distinguish different species
• Describe different species concepts (Biological, Ecological, Evolutionary Lineage, General Lineage)
2
Reproductive Isolating Mechanisms
• Compare and contrast prezygotic and postzygotic isolating mechanisms
• Explain how adaptation to feeding may promote reproductive isolation (Darwin's finches example)
3
Allopatric and Sympatric Speciation
• Describe how allopatric speciation occurs and leads to adaptive radiation
• Define hybrid zones and explain possible consequences of interbreeding
• Outline four mechanisms of sympatric speciation (aneuploidy, polyploidy, habitat differentiation, sexual selection)
Key Concepts: Study Guide Part 2
1
The Pace of Speciation
• Compare and contrast the concepts of gradualism and punctuated equilibrium
• Explain how generation time affects the rate of speciation
• Describe evidence from the fossil record for different tempos of evolution
2
Evo-Devo: Evolutionary Developmental Biology
• Describe how spatial expression of genes (BMP4 and Gremlin) affects pattern formation
• Explain the relationship between the number of Hox genes and body pattern complexity
• Outline how differences in growth rates of body parts (heterochrony) change characteristics of species
• Define paedomorphosis and provide examples