Evolution and Speciation Overview

Evolution and Speciation

• Evolution is the descent with modification of species over time, driven by natural selection.
• Genetic variation is the basis of natural selection.
• Natural selection is the process where organisms with advantageous traits are more likely to survive and reproduce, passing those traits to their offspring.
• Variation arises through mutation and sexual recombination.
• Tracking alleles and genotypes across generations is key to understanding inheritance patterns (Mendelian genetics, Hardy-Weinberg Equilibrium).
• Genetic divergence leads to speciation.
• Speciation is the formation of new species through the splitting of existing ones.
• Darwin's On the Origin of Species revolutionized the understanding of evolutionary processes in the past 150 years, and new discoveries and technology continue to refine our understanding.

Species Concepts

• Species are groups of populations whose members have the potential to interbreed in nature to produce fertile offspring (biological species concept, proposed by Ernst Mayr).
• Species can't be defined by outward appearance alone.
• This concept does not apply to asexually reproducing species or extinct organisms.

Reproductive Isolation

• Reproductive isolating mechanisms prevent interbreeding among species.
• These mechanisms can arise from heritable differences in body form, function, or behavior.
  • Prezygotic barriers prevent mating or fertilization.
  • Postzygotic barriers reduce the viability or fertility of hybrid offspring.

Types of Reproductive Isolation

• Prezygotic Isolation Mechanisms
• Temporal isolation: different mating times (e.g., different seasons, or different times of day).
• Habitat isolation: different habitats (e.g., different ecosystems).
• Behavioral isolation: differing courtship rituals (e.g., dances in birds, songs).
• Mechanical isolation: incompatible mating structures (e.g., genital openings unsuitable for mating).
• Gametic isolation: incompatible gametes (e.g., sperm and egg cannot fuse).
• Postzygotic Isolation Mechanisms
• Reduced hybrid viability: reduced development or survival potential of offspring.
• Reduced hybrid fertility: viable but sterile offspring (e.g., mules).
• Hybrid breakdown: first-generation hybrids are viable and fertile, but later generations have reduced fitness.

Mechanisms of Speciation

• Speciation typically begins when gene flow between populations ends, causing distinct evolutionary paths.
• Allopatric speciation: speciation occurring with geographic isolation.
• Sympatric speciation: speciation occurring without geographic isolation; often involving genetic changes or polyploidy.
• Parapatric speciation: speciation occurring without complete geographic isolation, but with a gradation in environmental conditions impacting the population.

Adaptive Radiation

• Adaptive radiation: a burst of speciation when a lineage encounters new ecological niches.
• A key innovation allows a species to exploit a habitat in a new way thus driving evolution and speciation.

Extinction

• Extinction is the irrevocable loss of a species from Earth.
• Mass extinctions are periods of substantial extinctions followed by adaptive radiation events.
• Extinction is a natural process, but environmental pressures can accelerate it considerably.