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.