Speciation and Microevolution

• Speciation and Microevolution are evolutionary processes

Evolution

• Microevolution refers to relatively short term changes in allele frequencies within a population or species
• Macroevolution refers to large scale evolutionary changes over long periods
• Microevolution leads to macroevolution

Biological Species Concept

• A Species is a group of populations with the potential to interbreed in nature to produce viable, fertile offspring
• Species are maintained by gene flow and reproductive isolation

Reproductive Isolation

• Prezygotic barriers block fertilization from happening by preventing different species from attempting to mate
• Prezygotic barriers also prevent an attempted mating from being successful, and they hinder fertilization
• Postzygotic barriers occur after a zygote is formed, by developmental errors, birth defects that cause death, or infertile offspring

Prezygotic Reproductive Isolating Barriers

• Absence of a cross-attraction between individuals of separate species inhibiting any courtship initiation, or individual behavior during copulation does not allow normal fertilization to occur, this is known as Behavioral Isolation
• Habitat isolation is when species occupy diverse habitats due to different biological or genetic tendencies in the same locality, limiting gene flow during breeding season
• Temporal Isolation is when species living in the same area have different breeding seasons
• Mechanical isolation refers to when the reproductive structures are incompatible between species, inhibiting copulation or pollination
• Gametic isolation refers to when the sperm of one species may not be able to fertilize the eggs of another species

Postzygotic Reproductive Isolating Barriers

• Ecological unviability occurs when hybrids cannot find a suitable ecological niche lowering viability, even though normal development occurs
• Hybrid unviability means that hybrids have developmental issues causing complete or incomplete lethality
• Behavioral sterility refers to when hybrids are less fertile and exhibit phenotypes or courtship behaviors that make them less desirable mates, even if normal gametogenesis occurs
• Hybrid Sterility is when hybrids have developmental problems of the reproductive organs or gametes, or can suffer from physiological or neurological issues that prevent effective courtship
• Three postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult:
  • Reduced Hybrid Viability is when genetic incompatibility between two species may stop the development of the hybrid at some embryonic stage, or produce frail offspring
  • Reduced Hybrid Fertility relates to when the hybrid offspring survives, but may be sterile
  • Hybrid breakdown is when some viable, fertile first-generation hybrids mate with another species with either parent species, the offspring of the next generation are sterile or feeble

Speciation

• Development of a new species occurs through a variety of factors
• The rate of speciation depends on generation time, environmental conditions, etc.
• Speciation can be caused by a change in a single gene or a set of genes causing isolation

Types of Speciation

• Allopatric Speciation is when gene flow is interrupted and a population is divided into geographically isolated subpopulations
• Parapatric Speciation occurs when part of a population enters a new habitat bordering the range of the parent species, and some gene flow may occur between populations in the border zone
• Sympatric Speciation occurs in populations that live in the same geographic area, which is less common than allopatric speciation
  • Speciation occurs when gene flow is diminished by polyploidy, habitat differentiation, or sexual selection

How Speciation Occurs

• Gradualism is when one species gradually transforms into another through a series of intermediate forms
  • Evolution occurs in small, incremental changes over many generations
  • Intermediate forms should be able to be seen in the fossil record, but are NOT always present
• Punctuated Equilibrium is when there are relatively brief bursts of rapid evolution interrupting long periods of little change
  • It fits with allopatric speciation
  • Can occur during adaptive radiation, where a population inhabiting a patchy environment gives rise to multiple specialized forms in short time period, common in island groups

Adaptive Radiation

• The evolutionary process in which a single ancestral species rapidly diversifies into multiple new species, each adapted to a different environment or ecological niche

Extinction

• A species goes extinct when all of its members die
• Climate, reproductive barriers, and disease all contribute to extinction
• Extinction occurs when a species fails to adapt
• Small, less genetically diverse populations are more prone to extinction

Types of Extinction

• Background Extinction occurs gradually over time, due to loss of habitat, and small climate change
• Mass Extinction occurs when many number of species disappeared over a relatively short period of time
  • Opens new habitats for adaptive radiation
  • Theories include an impact theory, or movement of earth's crust causing dramatic environmental changes