Evolution Notes Part 2 Natural Selection and Macroevolution Natural Selection shapes a population, making it adapted to its current environment. This happens over a relatively short period of time. Most scientists agree that natural selection, acting over very long periods of time, leads to speciation. (“Adding branches to the tree.”) There are two patterns of evolution described by scientists based on observations of living and extinct organisms. Patterns of Evolution Divergent evolution: the process of two or more related species becoming more and more dissimilar These species will share homologous structures Convergent evolution: the process by which distantly related species become more similar as they adapt to the same kind of environment These species appear similar on the outside, but do not share homologous structures. Their similar structures are considered analogous. Example of Divergent Evolution Each of these mantid species is exquisitely adapted to a very specific environment. The “body plan” of each species is homologous, but adaptation by natural selection has made each look different. Examples of Divergent Evolution What do these 2 examples have in common? Hawaiian Honeycreepers Galapagos Finches Divergent Evolution of Animals Example of Convergent Evolution The placental mouse has more homologous features with the wolf than with the marsupial mouse. The similarities between the placental mouse and marsupial mouse are analogous. Analogous Features Homology vs. Analogy http://www.pbs.org/wgbh/evolution/change/family/ More on Patterns of Evolution Both divergent and convergent evolution involve speciation, the formation of new species. Isolated populations become more different over time as natural selection acts to adapt them to a certain environment, or a unique niche within the environment. If enough time passes and enough genetic change occurs, the isolated populations become different species. Species Definitions Historically, a species has been defined by its structural differences compared to similar species. However, structure is not adequate, by itself, in defining a species. Morphological species concept: defines a species as a group of structurally unique organisms that differ from other described species. Biological species concept: defines a species as a group of organisms able to interbreed to produce viable offspring in nature. http://www.nytimes.com/2010/04/09/science/09fossil.html Isolating Mechanisms are needed for new species to arise A key of isolating mechanisms is that they act as barriers that keep two populations from interbreeding. Geographic isolation: If a population splits into 2 due to a physical barrier (i.e.: mountain range forms, a lake dries up into small ponds, a subpopulation crosses to a different island, etc.), the two populations become different due to natural selection. Over time the populations may become different enough that they can no longer interbreed. Geographic Isolation & Speciation Examples of Geographic Isolation Isolating Mechanisms, continued Reproductive Isolation: type of isolation that prevents interbreeding, even when two populations live in the same geographic region. Prezygotic barriers: these barriers prevent fertilization between gametes of two different species. (ex: males of one population use a different mating call to attract females than a related species.) Postzygotic barriers: these barriers prevent hybrid offspring from either surviving or producing offspring. (ex: an embryo fails to develop and dies or the resulting offspring are sterile like the mule that results from the mating of a donkey and a horse) Reproductive Isolating Mechanisms Natural Selection is not the only mechanism of evolution If a population is changing genetically, then a biologist can investigate if it may be due to one of the following: Natural selection Nonrandom mating (selective breeding) Mutation genetic drift gene flow Closer Look: Nonrandom Mating Farmers and animal breeders have long known that the traits of a population could be changed by nonrandom mating. Example: Oranges with smaller and smaller seeds were bred until “seedless” oranges were created In this case, farmers did not allow “nature to take its course”. They selected a trait they wanted and only bred plants that had that form of the trait. Closer Look: Genetic Drift Genetic drift: the process by which random evolutionary changes occur in small populations due to chance or random events. The evolutionary changes are not necessarily adaptive, unlike evolution by natural selection. Genetic Drift can decrease genetic variation Bottleneck effect: occurs when a population decreases rapidly due to natural disaster, disease, etc. The small remaining population may not have allele frequencies that are representative of the original population. Founder effect: occurs when one or a few individuals leave a large population and colonize a new location. The founders only bring with them a small sample of the alleles of the original population, so the new population is likely to be significantly different genetically.