Lecture 2 – The Origin of Species

Lecture 2 – The Origin of Species Speciation, the origin of new species, is at the focal point of evolutionary theory. • Evolutionary theory must explain how new species originate and how populations evolve Microevolution consists of changes in allele frequency in a population over time. Macroevolution refers to broad patterns of evolutionary change above the species level. The biological species concept states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring. Hybrids are the offspring of crosses between different species. Reproductive isolation can be classified by whether factors act before or after fertilization. Reproductive barrier: Any factor that impeded two species from producing fertile hybrids, thus contributing to reproductive isolation. 2 kinds: 1. Prezygotic barriers: impede mating between species or hinder fertilization of the ova should members of different species attempt to mate. 2. Postzygotic barriers: In the event fertilization does occur, postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult. Prezygotic Barriers 1. Habitat isolation: Two species living in different habitats within the same area may encounter each other rarely if at all, even though they are not technically geographically isolated. 2. Temporal isolation: Two species that breed at different times of the day, seasons, or years cannot mix their gametes. 3. Behavioral isolation: Species-specific signals and elaborate behavior to attract mates are important reproductive barriers among closely related species. 4. Mechanical isolation: Anatomical incompatibility may prevent sperm transfer when closely related species attempt to mate. 5. Gametic isolation: Gametes of different species that meet rarely fuse to form a zygote. Postzygotic barriers 1. Reduced hybrid viability: Genetic incompatibility between the two species may abort development of the hybrid at some embryonic stage. 2. Reduced hybrid fertility: If the two species mate and produce hybrid offspring that are viable, reproductive isolation is intact if the hybrids are sterile because genes cannot flow from one species’ gene poll to the other. 3. Hybrid breakdown: When some species cross-mate, the first generation hybrids are viable and fertile, but when these hybrids mate with one another or with either parent species, offspring of the next generation are feeble or sterile. Reproductive barriers form boundaries around species, and the evolution of these barriers is the key biological event in the origin of species. Limitations of the Biological Species Concept • The biological species concept cannot be applied to asexual organisms (including all prokaryotes) • The biological species concept emphasizes absence of gene flow • However, gene flow can occur between distinct species – For example, grizzly bears and polar bears can mate to produce “grolar bears” Speciation episodes can be classified into two modes based on the geographical relationship of a new species to its ancestral species: 1. Allopatric speciation: Speciation that occurs when the initial block to gene flow is a geographical barrier that physically isolates the population. 2. Sympatric speciation: Formation of new species within the range of parent populations. - Can occur quickly if a genetic change results in a reproductive barrier between the mutants and the parent population. The Time Course of Speciation • Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data Fossil Record • The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear • Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibria to describe periods of apparent stasis punctuated by sudden change • The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence The punctuated pattern in the fossil record and evidence from lab studies suggest that speciation can be rapid. Macroevolution is the cumulative effect of many speciation and extinction events.