Speciation

Species Concept

• Biological species concept: species is a group of populations that can breed and produce viable offspring
  • Defined by reproductive isolation and gene flow; basis for understanding macroevo.
• Morphological species concept: distinguishes a species by body shape and other structural features.
• Ecological species concept: defines a species in terms of its ecological niche
  • Species have diff adaptations to environment
• Phylogenetic species concept: set of organisms with unique evolutionary history

Notes

• Speciation: formation of new species
  • Reproductive isolation is main measure for speciation; can involve changes to 1 gene
• In order for speciation to happen, there must be something that prevents interbreeding between closely related population or species → Reproductive Barriers
• If two groups are reproductively isolated, they can accumulate allele differences and eventually diverge
• Reinforcement: Natural selection for traits that prevent interbreeding among populations
  • Selected traits reinforce differences that evolved while the populations were isolated from one another
• Fusion: two species become one
  • Likely to occur when an increasing number of viable, fertile hybrids are produced over the course of generations
  • Reduces species diversity

Methods of Speciation

1. Allopatric Speciation: when a population is divided by a geographic barrier so that interbreeding between two populations is prevented

• Barrier examples: rivers and regions that contain vital resources
• Gene flow is interrupted when a population is geographically isolated → reproductive barriers form and maintain speciation → gene pool/allele frequencies in two populations can diverge bcuz of diff selective pressures
  • If gene pool diverges enough then interbreeding will not occur if barrier removed
• Geographic barriers lead to reproductive isolation & barriers that prevent interbreeding:
  • Ex: Species that were seperated cannot breed when meet each other again

1. Sympatric Speciation: formation of new species without presence of geographic barrier. Appearance of new species in the same area of the parent population. Can occur bcuz of…

1. Sexual Selection:
2. Habitat differentiation: When subpopulation exploits habitat not used by parent → natural selection can act
3. Balanced Polymorphism among subpopulations may lead to speciation

• Ex: a population of insects have polymorphism for color → each color provides camouflage to specific substrate → under these circumstances only insects with same color can associate and mate → similarly colored insects are reproductively isolated

Polyploidy: have more than the normal two sets of chromosome found in diploid (2n) cells

• Occurs as result of nondisjunction of all chromosomes during meiosis                                               
  • Unlike animals, plants more tolerant to changes in chromosome sets
• Ex: Tetraploid plant produces gamete (2n) that fuses with normal gamete from same species (n) → nonviable/infertile triploid plant
  • Bcuz tetraploid & diploid species cannot produce viable offspring = diff species
    • Thus speciation can occur over single generation

1. Hybridization: two different species mate and produce offspring along geographic barrier called hybrid zone

• Sometimes genetic variation of hybrids is greater than either parent so hybrid population can evolve adaptations to environmental conditions in hybrid zone beyond parent range
  • Hybrids become new species when exposed to different selection pressures or can only breed among themselves
• Hybrid Zone: place where two different species meet and mate                                                                 
  • Form when two species do not have complete reproductive barriers

1. Adaptive Radiation: rapid evolution of many species from single ancestor

• Occurs when ancestral species is introduced to an area where diverse geographic conditions are available                 
• Ex 1: 14 species of Darwin’s finches on galapagos islands evolved from single ancestor
• Ex 2: adaptive radiation occurred after mass extinctions → many species go extinct → periods with ecological opportunities for species to colonize → colonization led to competition → promotes speciation

Maintaining Reproductive Isolation

• Reproductive barriers: prevent interbreeding & maintain reproductive isolation when species are not physically separated by geographic barrier
• Often single genes control phenotypic traits that can lead to reproductive isolation → speciation
  • Ex diff species of hummingbird prefer one type of coloured flower and thus only pollinate those

1. Prezygotic Isolating Mechanisms: block fertilization from occurring

• If populations do not attempt to breed, then is not considered prezygotic mechanism

1. Genetic Incompatibility: can’t reproduce bcuz proteins or chromosomes incompatible

• Sometimes when occurs in plants, they can self-pollinate and become new species

1. Habitat isolation: occurs when species do not encounter one another
2. Timing Isolation: occurs when species mate, flower, or are active during different times 

• Ex: nocturnal and diurnal animals

1. Behavioral Isolation: when species do not recognize another species as a mating partner because does not perform courtship rituals, release proper chemicals (scents, pheromones) or have appropriate appearance
2. Mechanical/Anatomical isolation: when male and female genitalia are structurally incompatible or flower structures select diff pollinators
3. Gametic Isolation: when male gametes do not survive females environment or failed recognition
4. Postzygotic Isolating Mechanisms: mechanisms that prevent formation of viable progeny
5. Hybrid Inviability: when zygote fails to develop properly and dies before reaching reproductive maturity
6. Hybrid sterility: when hybrids grow to be adults but are sterile

• Hybrid sterile bcuz chromosomes can’t pair up correctly during meiosis.

1. Hybrid breakdown: when hybrids produce offspring with reduced viability or fertility

Directional, Disruptive, and Stabilizing Selection

• Different ways environment/natural selection can act on a phenotype
• Tip: If FRQ about natural selection involves changing phenotype will probs involve one of these vocab
• Directional selection: when environment favors individuals with one extreme of a phenotypic
  • Ex: Moth case study; an increase number of large seeds over small seeds led to an increase in beak depth
• Disruptive selection: when environment favors individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes
  • Ex: Small-billed birds feed on small, soft seeds, large-billed birds feed on large, hard seeds; intermediate can’t eat either
  • Increases genetic variation
• Stabilizing selection: favors intermediate variants and acts against both extreme phenotypes → reduces variation
  • Ex: human babies at intermediate range tend to be healthier and have higher survival rates

Patterns of Evolution

1. Divergent Evolution: species that originate from a common ancestor become increasingly different over time

• Might happen because of allopatric speciation or sympatric speciation

1. Parallel Evolution: species that originate from a common ancestor have made similar evolutionary changes after divergence

• Species from marsupial mammals and placental mammals have independently evolved similar adaptations

1. Convergent Evolution:
2. Coevolution:

Microevolution vs Macroevolution

1. Microevolution: describes how the population of organisms change from generation to generation (how allele frequencies change)
2. Macroevolution: describes general patterns of change in groups of related species that have occurred over broad geological time; evolution of new species

• Different interpretations of fossil evidence have led to 2 contrasting theories for the pace of macroevolution

Patterns in Fossil Record

1. Phyletic Gradualism: argues that evolution occurs by the gradual accumulation of small changes

• Individual speciation events occur of long periods
• Fossils then reveal only major changes in groups → intermediate stages of evolution not represented and shows incompleteness

1. Punctuated equilibrium: argues that evolutionary history consists of long periods of with little evolution, then interrupted/punctuated by short periods of rapid evolution (new species)

• Most new species accumulate their unique features rapidly as they come into existence, then change little for the rest of their duration as a species.
• Most of species in first static event have become extinct or changed enough to be considered a new species