Lesson 22: Macroevolution and Speciation
22.1: Species Concepts
Defining a species is a complex task, with no single, universally applicable concept. Any species concept must address two key phenomena:
Distinctiveness of Sympatric Species: How species living together in a single location remain separate.
Connections within Allopatric Populations: How populations of a single species living in different geographic locations maintain their unity.
Categories of Species by Geographic Location:
Sympatric Species:
Live in the same geographic area and often utilize different parts of the habitat.
Typically display visible morphological differences (e.g., various butterfly species in a garden).
May look morphologically similar but possess distinct mating calls or behaviors (e.g., certain frog species).
Allopatric Species:
Live in different geographic locations, separated by physical barriers (e.g., bodies of water, mountain ranges).
Subspecies:
Populations of the same species found in different areas often exhibit distinct differences.
If closely located, intermediate individuals may be observed.
Members of different subspecies must be able to produce fertile offspring to be considered part of the same species.
22.2: Reproductive Isolation
Reproductive Isolation is a critical aspect of speciation, occurring when individuals:
Will not mate with each other.
Cannot mate with each other.
Produce sterile offspring.
Disruption in gene flow is a prerequisite for reproductive isolation.
Mechanisms of Reproductive Isolation:
Pre-zygotic Mechanisms: Prevent the formation of a zygote.
Ecological Isolation: Species in the same area use different habitats and rarely encounter each other (e.g., lions and tigers in India, despite overlapping ranges, utilize distinct habitats).
Behavioral Isolation: Differences in courtship rituals or mating calls prevent inter-species mating (e.g., blue-footed boobies' elaborate dances, lacewing species with unique mating calls).
Temporal Isolation: Species have different growing or breeding seasons.
Mechanical Isolation: Structural differences in reproductive systems prevent mating (e.g., specialized insect reproductive organs, unique flower structures/pollinators limiting pollen transfer).
Prevention of Gamete Fusion: Gametes from different species are unable to fuse (relevant for species releasing gametes into water or internal fertilization where sperm/pollen cannot fertilize heterospecific eggs).
Post-zygotic Mechanisms: Impact hybrid zygotes if pre-zygotic mechanisms fail, occurring after fertilization.
Hybrid Inviability: Hybrid embryos are not viable and do not survive.
Hybrid Infertility: Hybrids survive but are sterile (e.g., mules).
22.3: Species Concepts Revisited
Biological Species Concept (BSC)
Proposed by: Ernst Mayr in 1942.
Definition: "Groups of actually or potentially inter-breeding natural populations which are reproductively isolated from other such groups."
Core Principle: Individuals must be capable of producing fertile offspring to be considered the same species.
Limitations:
Hybridization: Occurs commonly (especially in plants), where classified separate species interbreed and produce fertile offspring (e.g., milk snakes forming fertile intermediate morphotypes where populations overlap).
Observational Challenge: Difficult to apply to geographically isolated (allopatric) populations as interbreeding cannot be observed.
Asexual Organisms: Not applicable to species that reproduce asexually.
Phylogenetic Species Concept (PSC)
Proposition: A species should represent groups of populations that have been evolving independently.
Basis: Phylogenies (evolutionary trees) - hypotheses of relationships among species based on shared traits.
Advantages over BSC:
Does not require observing interbreeding in allopatric populations.
Applicable to both sexually and asexually reproducing species.
Drawbacks:
Could lead to classifying every slightly different population as a new species, even if they can produce fertile offspring.
Species may not always form a single clade, which contradicts strict definitions.
Summary of Species Definition
Ongoing Debate: Defining a species remains challenging due to continuous evolutionary processes.
Partial Reproductive Isolation: Populations may be only partially reproductively isolated during speciation.
If isolation mechanisms are incomplete, interbreeding can occur, leading to the loss of accumulated differences and preventing speciation.
If isolation mechanisms are advanced, breeding will not be successful, and populations will remain separate species.
Reinforcement: Where hybrids are partially sterile, selection favors alleles that prevent hybridization, leading to the continuous improvement of pre-zygotic isolation mechanisms (e.g., European flycatchers in sympatric regions evolving distinct coloration to prevent accidental hybridization).
22.4: Mechanisms of Speciation
Reinforcement Outcomes: Reinforcement doesn't always result in complete speciation, especially if fertile hybrids facilitate gene flow.
Adaptive Change in Mating Signals: Environmental selective pressures can drive changes in mating signals, leading to reproductive isolation.
Example: Anoles' dewlap coloration adapts to new environments. Female preference for specific dewlap colors promotes reproductive isolation and speciation.
Genetic Drift: Random changes in allele frequencies can cause reproductive isolation.
Founder Effects and Population Bottlenecks are contributing factors.
Over time, isolated populations diverge due to genetic drift, potentially leading to reproductive isolation and speciation.
Allopatric Speciation: Occurs when populations become geographically isolated, ceasing gene flow.
Causes: Geographic changes such as oceans separating populations, rivers cutting through populations, or extinction of intermediate populations.
Example: Little paradise kingfisher species show significant variation on isolated islands compared to mainland populations, indicating that geographic isolation can lead to speciation if gene flow stops.
22.5: Adaptive Radiation and the Pace of Evolution
Adaptive Radiation
Definition: The rapid evolution of a group of species from a common ancestor, adapting to different environmental niches, thereby increasing diversity.
Process (e.g., Island Archipelago):
An ancestral species colonizes an island.
Members gradually colonize other islands, leading to allopatric speciation as each new species adapts to distinct environmental variables.
Newly specialized species, upon re-encountering on other islands, can coexist due to reduced competition from utilizing different resources and habitats.
Example: Cichlids in Lake Victoria:
Evidence of recent and rapid evolution over approximately 200,000 years.
Water-level fluctuations led to isolated populations, fueling rapid evolutionary bursts.
Possession of a unique secondary set of functional jaws provided evolutionary flexibility, enabling diverse modifications for different habitats and accelerating radiation.
Pace of Evolution
Two primary hypotheses describe the relationship between speciation and evolutionary change:
Gradualism: Small, continuous changes accumulate very slowly over thousands to millions of years, leading to significant evolutionary changes.
Punctuated Equilibrium: Species experience long periods of stasis (little to no evolutionary change), "punctuated" by rapid bursts of evolutionary change occurring over relatively short periods.
Relationship: Gradualism and punctuated equilibrium are not mutually exclusive but represent two ends of a spectrum. Evidence suggests that different groups, or even the same group at different times, may exhibit patterns consistent with gradualism, punctuated equilibrium, or a combination of both.