Chapter 22: The Origin of Species
The Nature of Species and the Biological Species Concept
Definition of Species Coexistence:
Species inhabit the same locale but remain distinct.
Species that occur together are:
Phenotypically different.
Utilize different parts of the habitat.
Behave differently.
Occasionally, two species appear nearly identical, necessitating examination beyond visual similarities (e.g., genetic analysis, pheromones).
Geographic Variation within Species:
Populations of a species can exhibit geographic variation.
Individuals in populations from different areas may be distinct.
If populations are geographically close, individuals may exhibit combinations of features characteristic of both populations, forming 'intergrades'.
Example: Geographic variation in the eastern rat snake, Pantherophis alleghaniensis.
Populations at the eastern, western, and northern ends of its range are phenotypically distinctive.
These distinct populations are connected by 'intergrade' populations that are phenotypically intermediate.
Ernst Mayr's Biological Species Concept (BSC):
Proposed by evolutionary biologist Ernst Mayr in 1942.
Defines species as groups of populations that possess the potential to interbreed and are reproductively isolated from other such groups.
Describes populations whose members do not mate with each other or cannot produce fertile offspring.
Populations reproductively isolated from each other are considered members of different species.
Reproductive Isolation:
This is the key to speciation under the BSC.
Reproductive isolating mechanisms are barriers to successful reproduction, preventing genetic exchange between species.
Two main types of barriers:
Prezygotic Isolating Mechanisms: Prevent the formation of zygotes.
Postzygotic Isolating Mechanisms: Prevent the proper functioning of zygotes after they form (e.g., hybrid inviability, infertility).
Table: Reproductive Isolating Mechanisms
Mechanism | Description |
|---|---|
PREZYGOTIC MECHANISMS | |
Ecological isolation | Species occur in the same area but occupy different habitats and rarely encounter each other. |
Behavioral isolation | Species differ in their mating rituals or signals. |
Temporal isolation | Species reproduce in different seasons or at different times of the day. |
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 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. |
Detailed Explanation of Reproductive Isolating Mechanisms
Prezygotic Isolating Mechanisms
These mechanisms prevent the formation of a zygote.
Ecological Isolation:
Species occur in the same geographical area but utilize different portions of the environment.
Result: They typically do not hybridize in the wild.
Example: Lions and tigers.
Their ranges overlap in India, but lions inhabit grasslands while tigers prefer forests.
They are ecologically isolated and do not hybridize in nature.
Hybrids (e.g., tiglons) can be successfully produced in captivity, demonstrating that isolation occurs in the wild due to ecological factors.
Behavioral Isolation:
Sympatric species avoid mating with members of the wrong species through various communication modes.
These include:
Visual Signals: Differences in courtship displays or appearance.
Sound: Differences in mating calls or songs.
Example: Sympatric species of lacewings have distinct courtship songs produced by abdominal vibrations.
Pheromones: Chemical substances released by an organism that influence the behavior or physiology of another organism of the same species.
They serve as attractants, trail markers, or alarm signals.
Sympatric species often produce different pheromones.
Electrical Discharges: Produced and used for communication in social interactions.
Example: African and South Asian electric fish have independently evolved specialized organs and electroreceptors for this purpose.
Temporal Isolation:
Sympatric species reproduce in different seasons or at different times of the day.
Example 1: Two species of wild lettuce (Lactuca graminifolia and L. canadensis).
They grow together in the southeastern U.S.
Hybrids can be easily made experimentally and are fertile.
However, L. graminifolia flowers in early spring, while L. canadensis flowers in summer, typically preventing natural hybridization.
Occasional hybrids form when their blooming periods overlap.
Example 2: Frogs (genus Rana).
Occur together in most of the eastern U.S.
Each species has a different peak breeding time.
Mechanical Isolation:
Structural differences prevent mating between some related species, particularly concerning male and female copulatory organs.
Common in many arthropod groups, where sexual organs are highly diverse and serve as a primary basis for distinguishing species.
Prevention of Gamete Fusion:
Eggs and sperm from different species may not attract or fuse with one another.
Observed in animals that spawn, releasing gametes directly into water.
In many animals with internal fertilization, the sperm of one species may function very poorly within the female reproductive tract of another species, preventing fertilization.
Postzygotic Isolating Mechanisms
These mechanisms prevent normal development into reproducing adults, even if hybrid matings occur and zygotes are produced.
Hybrid Inviability or Infertility:
Development is a complex process; genetic information from two different species may be too divergent, disrupting embryonic development.
Example 1: Hybridization between sheep and goats.
Embryos usually die in the earliest developmental stages.
Example 2: Leopard frogs.
Four species resemble one another closely externally.
Their status as separate species was first suspected when laboratory crosses between some pairs produced defective embryos.
Subsequent research revealed differences in mating calls, indicating both pre- and postzygotic isolating mechanisms are involved.
Hybrid Sterility:
Even when hybrids survive, they may not develop normally or are infertile.
Example: Mules, a hybrid between a female horse and a male donkey.
Mules are vigorous and strong but sterile.
Parents (horses and donkeys) have different numbers of chromosomes (2n{ ext{horse}} = 64, 2n{ ext{donkey}} = 62). A mule has 2n = 63 chromosomes.
A mule's chromosomes cannot align properly during meiosis due to the differing chromosome numbers and structures, leading to non-functional gametes and infertility.
Criticisms of the Biological Species Concept (BSC)
Despite its utility, the BSC does not explain all observations and faces several criticisms:
Underestimation of Hybridization: There is much greater interspecific hybridization observed than previously thought.
Example: Balsam poplars and cottonwoods have been phenotypically distinct for 12 ext{ million years} but have routinely produced hybrids throughout this period.
Applicability to Plants: Many botanists argue the concept is misnamed, as it primarily applies to animals and less effectively to plants, which often hybridize successfully.
Extent of Reproductive Isolation: New evidence suggests hybridization is not uncommon in animals.
Almost 10 ext{%} of the world's bird species are known to have hybridized in nature.
However, distinctions among species are often maintained by natural selection, as each species is adapted to its own specific environment.
Alleles introduced into one species' gene pool from another species through hybridization are often quickly eliminated if they reduce fitness in that specific niche, minimizing the long-term effect of hybridization.
Laboratory vs. Wild Conditions: Many species that coexist without interbreeding in nature will readily hybridize in the laboratory or zoo, raising questions about whether they would
interbreed naturallyif they encountered one another under different circumstances.Asexually Reproducing Organisms: The BSC does not apply to organisms that reproduce asexually, as there is no mating or gene flow to assess.
Lack of a Single Explanation: Given the tremendous diversity of life, there may be no single, universal explanation for what maintains the identity of species.
A variety of other species concepts have been developed, often specific to particular types of organisms.
Adaptive Radiation and Biological Diversity
Definition:
Adaptive radiation is the rapid speciation that produces a cluster of closely related species with diverse ecological adaptations.
Common Scenarios for Adaptive Radiation:
New Environments with Few Competitors: Common when a species colonizes an environment with few other species and many available resources.
Examples:
Creation of new islands: Through volcanic activity, such as the Hawaiian and Galápagos Islands.
Catastrophic events: Leading to the extinction of most other species.
Example: The mass extinction of dinosaurs 65 ext{ million years ago} led to the adaptive radiation of mammals, which then diversified to fill newly available ecological niches.
Evolution of a New Trait: Can also occur when a new trait evolves within a species.
This trait allows individuals to use resources or other aspects of the environment that were previously inaccessible.
Examples:
Evolution of wings in birds and insects, opening up aerial niches.
Process of Adaptive Radiation (Illustrated Steps from Transcript):
An ancestral species flies from the mainland to colonize one island.
The ancestral species spreads to different islands.
Populations on different islands evolve to become different species (allopatric speciation).
Species evolve different adaptations in allopatry.
Alternatively (after re-colonization/sympatry): Colonization of islands followed by species evolving different adaptations to minimize competition with other species (character displacement).
Adaptation to New Habitats:
Can occur either:
During the allopatric phase: As species respond to different environments on the different islands before contact.
After two species become sympatric: Adaptation may be driven by selective pressures to minimize competition for available resources, leading to character displacement.
Character Displacement:
Occurs when two reproductively isolated but ecologically similar species come into contact.
These two species initially use the same resources.
Natural selection in each species favors individuals that use different resources, leading to greater fitness for those individuals.
Traits that cause differences in resource use will increase in frequency.
Over time, the species will diverge in resource use, further reducing competition.
Examples of Adaptive Radiation and Character Displacement:
Three-Spined Sticklebacks in British Columbia:
Evolution of differences in two populations to minimize competition for food in the same lake.
One stickleback adapted to using open water, developing more effective gill structures and a streamlined body.
The other adapted to foraging margins and the bottom of the lake, developing a stouter body and an altered mouth shape.
Darwin's Finches (Galápagos Islands):
A classic example of adaptive radiation, with different species evolving specialized beaks and diets (e.g., Ground Finches, Cactus Finches, Vegetarian Tree Finches, Tree Finches, Warbler Finches).
Lake Victoria Cichlid Fishes:
Diversified very rapidly into numerous species with highly specialized feeding adaptations.
Examples include scale scrapers, leaf eaters, fish eaters, zooplankton eaters, snail eaters, algae scrapers (e.g., Labeotropheus fuelleborni, Maylandia zebra), and insect eaters.
These adaptations often involve specific snout lengths and jaw structures (e.g., short vs. long snout).
The Pace of Evolution
Gradualism:
A theory that evolutionary change occurs slowly through time.
Involves the accumulation of small changes, each nearly imperceptible from generation to generation.
Over thousands to millions of years, these small changes can lead to major evolutionary transformations.
Punctuated Equilibrium:
A theory proposing that species experience long periods of little or no evolutionary change (stasis).
These periods of stasis are 'punctuated' by bursts of rapid evolutionary change, occurring over geologically short time intervals.
Stasis: One factor that may enhance stasis is the ability of species to shift their geographic ranges in response to environmental changes, delaying or preventing strong selective pressures for local adaptation.
Speciation and Extinction Through Time
Long-Term Trend of Biodiversity:
Biological diversity has increased vastly over the last 600 ext{ million years}.
In general, the rate of speciation has surpassed the rate of extinction over much of Earth's history.
Mass Extinctions:
Despite the overall increase in species diversity, this trend has been interspersed with mass extinctions.
These are relatively sudden, sharp declines in the number of species across the globe.
Represent periods where extinction rates drastically exceed speciation rates.