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
Ecological Isolation: Geographic barriers prevent contact.
Temporal Isolation: Species reproduce at different times of day or year.
Behavioral Isolation: Differences in mating signals or sexual selection (e.g., changes in song).
Mechanical Isolation: Incompatible reproductive structures (e.g., size mismatch or shell shape).
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:
Polyploidy
Hybrid speciation
Adaptation to local environments
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:
Cell division
Cell migration
Cell differentiation
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.
BMP4: Causes apoptosis (cell death).
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.