Origins of Species and Macroevolution

Macroevolution

  • Occurs at/above the species level.

 

Speciation

  • The process of species formation.

 

Identification of Species

  • Species: A group of organisms that maintains a distinctive set of attributes in nature.

 

Number of Species on Earth

  • Currently identified: About 2 million species.

  • Estimates of total species: Range from 5 to 50 million.

  • Difficulty in identification:

    • Subspecies: Groups of the same species with some differences, but not enough to be called separate species.

    • Ecotypes: Genetically distinct bacterial populations adapted to local environments.

 

Characterization of Species

  • The characteristics used to identify species depend on the species in question.

  • Commonly used traits:

    • Morphological traits (morphological species concept).

    • Ability to interbreed (biological species concept).

    • Molecular features.

    • Ecological factors.

    • Evolutionary relationships.

 

Morphological Species Concept

  • Individuals of a species share measurable traits that distinguish them from other species.

  • Based on Linnaeus’ classification system (binomial nomenclature):

    • Example: Panthera leo (Panthera = Genus, Leo = Specific epithet).

 

Biological Species Concept

  • A species is a group of interbreeding (or potentially interbreeding) individuals that is reproductively isolated from other such groups.

  • Reproductive isolation means they can produce viable, fertile offspring among themselves but not with other species.

 

Problems with the Biological Species Concept

  • Reproductive isolation may be difficult to determine in nature.

  • Some species may be able to interbreed but do not.

  • Does not apply to:

    • Asexual organisms.

    • Extinct organisms.

 

Other Species Concepts Based on Criteria

  • Molecular Traits: DNA sequences, gene order, chromosome structure/number to identify similarities and differences.

  • Ecological Factors: Factors related to habitat can distinguish species (especially in bacteria), but these can be variable.

  • Evolutionary Relationships: Based on phylogenies (evolutionary trees) derived from fossil records or DNA sequences.

 

Reproductive Isolating Mechanisms

  • Prevent interbreeding between species.

  • Result from genetic changes as species adapt to their environments.

  • Interspecies Hybrid: Offspring from two different species, usually with reduced fitness.

 

Reproductive Isolation

  • Prevents gene pools from mixing.

  • Prezygotic Isolating Mechanisms: Occur before zygote formation.

  • Postzygotic Isolating Mechanisms: Occur after zygote formation.

 

Prezygotic Isolating Mechanisms

  1. Ecological Isolation: Geographic barriers prevent contact.

  2. Temporal Isolation: Species reproduce at different times of day or year.

  3. Behavioral Isolation: Differences in mating signals or sexual selection (e.g., changes in song).

  4. Mechanical Isolation: Incompatible reproductive structures (e.g., size mismatch or shell shape).

  5. Gametic Isolation: Incompatible gametes (gametes fail to unite).

 

Postzygotic Isolating Mechanisms

  • Prevent the development of viable, fertile individuals after fertilization.

 

Postzygotic Isolating Mechanisms

  • Interspecies offspring: Reproductively isolated if less fit than intraspecies offspring.

  • Hybrid Inviability: Species hybrids do not survive.

  • Hybrid Sterility: Species hybrids cannot produce functional gametes.

  • Hybrid Breakdown: Reduced fitness in F2 generation (offspring of hybrid).

 

Allopatric and Sympatric Speciation

  • The underlying cause of speciation is the accumulation of genetic changes that promote enough differences to define a population as a unique species.

  • Speciation can occur due to abrupt events (e.g., changes in chromosome number) that cause reproductive isolation.

  • Species arise as a consequence of adaptation to different ecological niches.

 

Patterns of Speciation

Cladogenesis

  • Division of a species into two or more species.

  • Requires interruption of gene flow between populations.

Allopatric Speciation

  • Occurs when some members of a species become geographically separated.

  • Most common method of cladogenesis.

 

Allopatric Speciation

  • Takes place when a physical barrier subdivides a large population or when a small population becomes separated from the species’ main geographical distribution.

  • Species Cluster: A group of closely related species.

    • Caused by founder effects and genetic drift.

  • Hybrid zones: Can form when populations come into secondary contact.

  • Occurs when: A small population moves to a new, geographically separated location, and natural selection may rapidly alter its genetic composition, leading to adaptation to the new environment.

  • Adaptive Radiation: A single species evolves into a range of descendants that differ greatly in habitat, form, or behavior.

 

Genetic Divergence during Allopatric Speciation

  • Genetic differences in allopatric populations can lead to speciation.

  • In the absence of gene flow, geographically separated populations inevitably accumulate genetic differences through mutation, genetic drift, and natural selection.

 

Reproductive Isolation

  • Postzygotic reproductive isolation (3 mechanisms):

    • Hybrid Inviability

    • Hybrid Sterility

    • Hybrid Breakdown

  • Prezygotic reproductive isolation (5 mechanisms):

    • Ecological isolation

    • Temporal isolation

    • Behavioral isolation

    • Mechanical isolation

    • Gametic isolation

  • Sexual selection: Mate choice influences reproductive isolation.

 

Sympatric Speciation

  • Occurs when members of a species within the same range diverge into two or more species, despite no physical barriers to interbreeding.

  • Mechanisms include:

    1. Polyploidy

    2. Hybrid speciation

    3. Adaptation to local environments

    4. Sexual selection

 

Mechanisms of Sympatric Speciation

Polyploidy

  • Organisms have more than two sets of chromosomes.

  • Plants are more tolerant of polyploidy than animals.

  • Autopolyploidy: Increased chromosome sets from the same parental species.

  • Allopolyploidy: Chromosomes from two or more different species (interspecies breeding).

  • Allopolyploid formation can lead to reproductive isolation and speciation.

Hybrid Speciation

  • Interspecies hybrid plants that are polyploid may be fertile and reproductively isolated from the parent populations.

  • Interspecies hybrids may also become a new species without being polyploid.

Adaptation to Local Environments

  • Variations in geographic areas can lead to population divergence into different local environments, even if the populations are continuous.

  • Example: Microhabitat partitioning (apple race vs. hawthorn race of ancestral plants).

Sexual Selection

  • Females choose mates based on characteristics such as courtship behaviors, songs, or color patterns.

  • Example: African cichlids, where diversifying selection occurs based on color preference.

 

The Pace of Speciation

Gradualism

  • Species evolve continuously over long time spans.

  • Large phenotypic differences arise from the accumulation of many small genetic changes.

Punctuated Equilibrium

  • Species remain in equilibrium for long periods, with sporadic, rapid bursts of changes.

  • Both gradualism and punctuated equilibrium have merit.

  • Larger animals with long generation times evolve more slowly than microbes with shorter generations.

 

Evolutionary Developmental Biology

  • Compares the development of different organisms to understand:

    • Ancestral relationships

    • Developmental mechanisms that drive evolutionary change.

  • Focuses on genes that control development and how their roles vary in different species.

 

Developmental Genes are Key

  • Developmental genes influence:

    1. Cell division

    2. Cell migration

    3. Cell differentiation

    4. Cell death (apoptosis)

  • The interplay of these processes creates an organism's specific body pattern (pattern formation).

  • These genes are crucial in determining individual phenotypes.

 

Chicken vs. Duck Feet

  • Nonwebbed vs. webbed feet: Differences in the expression of two cell-signaling proteins.

    1. BMP4: Causes apoptosis (cell death).

    2. Gremlin: Inhibits BMP4, allowing cells to survive.

  • Mutations in BMP4 and Gremlin expression provide variation.

  • Terrestrial environments: Nonwebbed feet are advantageous.

    • Natural selection favors nonwebbed feet on land.

  • Aquatic environments: Webbed feet are advantageous.

    • Natural selection favors webbed feet in water.

 

Developmental Genes that Affect Growth Rate

  • Genetic variation influences morphology by controlling the relative growth rates of body parts during development.

  • Heterochrony: Evolutionary changes in the rate or timing of developmental events.

 

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