The origin of species. AP chapter 22 Miss Kim Jensen

Summary The origin of species or speciation is the basis of the evolution of biological diversity. Anagenesis or phyletic evolution, involves the transformation of a lineage of organisms into forms different enough to be considered new species. In cladogenesis or branching evolution, new species arise from parent species that continue to exist. Cladogenesis is both the more common pattern of evolution and the process that increases biological diversity. Evolutionary theory attempts to determine the mechanism by which new species originate.

The species problem Taxonomists often find that their classification of local species corresponds to the taxonomy of a region morphospecies – anatomical features. Species are most often characterized by their physical form or morphology. Biological species _ Mayr (1942) goes beyond physical differences and considers reproductive and genetic isolation to be the basis for separating species. Have the potential to interbreed and produce fertile offspring. The largest unit in which gene flow is possible.

Reproductive barriers Reproductive Barriers - Reproductive Isolation prevents pop. from interbreeding 1. Pre-zygotic barriers - impedes mating between species or hinders fertilization of ova if mating occurs –a. geographic or habitat isolation - species live in same general area, but habitat preferences Are so different that they rarely mate or cross fertilize each other example: Scarlet oak lives in swamps, poorly drained bottom land, with acidic soils; black oak well drained soils

Prezygotic preproductive barriers. Cont. –b. temporal isolation - breed during different times of the day or different seasons of the year –c. behavioral isolation - most important in animals, courtship patterns different for different species –d. mechanical isolation - differences in structure of genital parts restricts gametic transfer- recently found to be less effective –e. chemical or gametic isolation - due to difference in structure of flower parts - attracts different pollinators

Reproductive Barriers 2. Post-zygotic barriers - prevent zygote from developing into a viable, fertile adult –a. hybrid sterility (reduced hybrid fertility) - sterile hybrid generally occurs during development of reproductive organs/structures or during meiosis when hybrid matures –b. hybrid inviability (reduced hybrid viability) - development of embryo stops - typical stages of susceptibility are at. first cleavage or gastrulation when many genes in the hybrid are suddenly turned on. These events can't be properly coordinated & development stops –c. hybrid breakdown or weakness - weak, abnormal or sterile progeny in 2nd generation, even though first generation is ok

The biogeography of Speciation The evolution of reproductive barriers is the critical event in the origin of new species. When the gene pool of a population becomes separated from other populations, the isolated population can follow its own evolutionary course as a result of selection, genetic drift and mutation. Speciation mechanisms can be grouped by biogeographical factors: Allopatric speciation and Sympatric speciation. Speciation occurs faster in smaller populations

Allopatric speciation occurs when the gene pool of a population becomes segregated geographically from other populations

Sympatric speciation occurs when a subpopulation becomes reproductively isolated in the midst of its parent population.

The mechanisms leading to speciation can be 1. Natural selection (multiple niche polymorphism) 2. Genetic drift (founder effects, bottlenecks) 3. Non-random mating (sexual selection)

Genetic Mechanisms of Speciation Speciation by divergence – This does not exactly favor reproductive barriers; genetic isolation may be a secondary consequence of adaptations to different environments. Speciation by peak shifts – Adaptive peaks occur where a population’s gene pool is at an equilibrium that maximizes fitness.

Punctuated Equilibrium Punctuated equilibrium is an evolutionary theory advocated by Stephen Jay Gould and Niles Eldredge. This theory has been hotly debated for the past 27 years. Evolution occurs in a stepladder fashion, with fits and burst of development (morphological change).Such bursts are followed by long periods of stasis. These periods of stasis may last for millions of years. This contrasts with Darwin’s original theory, which offered that evolution was a gradual process that led to the perfection of species.

Four points of evidence that point towards punctuated equilibrium 1.Individual Cases show that branching occurs, rather that phyletic transformation. An example is that ancestors outlive their descendents, showing that evolution is not a steady process. 2.The majority of evolution cannot be accounted for by gradual development. 90 percent of cases can only be accounted for by punctuated equilibrium. 3.Unstable environments account for rapid genetic change, whereas stable environments allow for the possibility of gradual change. 4.Tests show that punctuated equilibrium is predominant.

Gradualists Some gradualists maintain, however, that fossils show stasis only in external anatomy and that changes in internal anatomy, physiology and behavior go unrecorded. Even reports of long periods of particularly complete set of trilobite fossils, Sheldon found a gradual change in aspects of their morphology, which challenged earlier categorizations of the youngest and oldest fossils in each evolutionary lineage as different species.

Taxonomy: names organisms, assigns hierarchical relationships P opulations: the individuals of a species in a (contiguous) area Cladogenesis: evolution and differentiation of species lineages 8/evolglos.html#gradhttp:// 8/evolglos.html#grad