Microevolution and Macroevolution Concepts

Study Notes

Natural selection, genetic drift, mutation, and migration are mechanisms that drive microevolutionary change within a population.
These mechanisms can potentially contribute to macroevolution by leading to the emergence of new species over long periods.

Macroevolutionary diversity refers to the evolution of new species and higher taxonomic groups.
Several species concepts exist:
- Taxonomic (morphological) concept defines species based on distinct physical and anatomical differences.
- Biological concept defines species based on the ability of individuals to interbreed.
Darwin regarded species as groups with similar phenotypes.
Ernst Mayr's concept emphasizes reproductive isolation as a key factor for differentiating species.
Reproductive Isolation: The inability of individuals from different populations to interbreed successfully, leading to the formation of separate species.

Speciation is the process by which new species arise from existing ones through reproductive isolation and divergence.
Allopatric speciation: Occurs when a population is geographically separated, leading to the evolution of unique genetic traits and reproductive barriers.
Sympatric speciation: Occurs when new species arise within the same geographic area, without geographic isolation. This is more challenging due to gene flow, which tends to homogenize the population.

Prezygotic barriers: Prevent mating or fertilization, preventing the formation of a zygote:
- Geographical/Ecological: Separation based on location or resource use.
- Temporal: Reproductive activity occurs at different times between populations.
- Behavioral: Differences in mating behaviors prevent interbreeding.
- Mechanical: Incompatible genitalia prevents mating.
- Cellular: Sperm cannot penetrate the egg.
Postzygotic barriers: Occur after fertilization, resulting in non-viable or infertile offspring:
- Intrinsic postzygotic barriers: Hybrids exhibit reduced viability, fertility, or abnormal development. This can also occur in subsequent generations.
- Extrinsic postzygotic barriers: Hybrids are poorly adapted to the environment, leading to low fitness.

Local Adaptation: Different populations within a species evolve to suit unique environmental conditions, potentially leading to reproductive isolation. This is accelerated by natural selection, as populations diverge faster in response to different ecological pressures.
Ecological Speciation: Speciation driven by adaptations to different ecological environments. Natural selection favors traits that enhance survival and reproduction within a specific environment, leading to reproductive barriers.

Adaptive radiation occurs when a single ancestor diversifies into many species to fill various ecological roles.
Closely related species often exhibit distinct phenotypes linked to their environment (e.g., beak shapes adapted to specific food sources).
Ecological opportunity: Factors that promote adaptive radiation:
- Colonization of new areas with few competitors.
- Extinctions of other species, reducing competition.
- Evolution of novel traits that allow access to new resources.
High propensity for speciation: Some groups have a greater tendency to undergo speciation than others.

Hybridization occurs when two reproductively isolated species interbreed.
Hybrids can be:
- Sterile: Unable to reproduce.
- Fertile: Capable of reproduction, potentially leading to gene flow and merging of species.
Polyploidy: A form of speciation where an organism has more than two complete sets of chromosomes. This is an example of sympatric speciation.
Allopolyploidy: Occurs when two different species hybridize and the offspring possesses a diploid set of chromosomes from both parent species.