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Evolution of Populations and Speciation

Variation of Traits Population genetics –The study of evolution from a genetic viewpoint. –Evolution is a gradual change of genetic information over time. In a population individual may vary in observable traits. –The distribution of these traits can be demonstrated using a bell curve.

Variation of Traits What are different causes for genetic variation? Mutation –Flawed copies of individual genes. Recombination –Independent assortment during crossover. Random fusion of gametes –The random chance of which gamete succeeds.

Sexual vs. Asexual Reproduction Sexual Reproduction – Uses the process of meiosis to create gametes. Fertilization results when the embryo receives alleles from both parents. –Genetic variability is a result of independent assortment, recombination of chromosomes, or mutations. Gametes are produced with alleles arranged in new ways.

Genetic changes or variability result in the transcription and translation of new proteins that may improve an organism’s opportunity for survival. These beneficial proteins (traits) will be passed on to the next generation.

Asexual reproduction – Involves only one parent that produces the offspring that are for the most part genetically identical to that parent. –Variation only occurs through mutations passed to the offspring. –The asexual reproduction rate is much faster than the sexual reproduction rate. –Asexual reproduction may have a disadvantage in changing conditions (environment) because all individuals are identical.

Allele Frequencies and the Gene Pool Gene pool –The total genetic information available in a population. Allele frequency –The frequency that a certain allele appears in a population. –May be a percentage or decimal. –Example: Two forms of the A allele (A and a) If there are 20 gametes and 5 are “a” then the allele frequency for “a” is.25.

Allele Frequencies and the Gene Pool Phenotype frequency –The total number of individuals with a particular phenotype divided by the number of individuals in the population. –Example: There are 20 plants in a population. If four are pink then the frequency of pink is.20.

Hardy-Weinberg Genetic Equilibrium Wilhelm Weinberg and Godfrey Hardy independently showed that allele frequencies tend to remain the same from generation to generation unless acted on by outside influences.

Hardy-Weinberg Equilibrium Based on a hypothetical population that is not evolving. 5 Assumptions for evolution not to occur. –No net mutations. Allele frequencies never change from mutation. –Individuals never enter nor leave a population. –The population is large. –Individuals mate randomly. –Selection does not occur. When the equilibrium is disrupted evolution occurs.

Disruption of the Genetic Equilibrium Mutation –Mutations occur spontaneously and constantly at low rates and under normal conditions. –Beneficial mutations are a vital part of evolution.

Disruption of the Genetic Equilibrium Migration –Immigration – the movement of individuals into the population. –Emigration – the movement of individuals out of the population. Migration results in gene flow, the process in which genes move from one population to another.

Genetic Drift –Allele frequencies in a population change as a result of random events. Small populations are more damaged by genetic drift than large populations.

Nonrandom Mating Many species do not mate randomly. Mate selection can be influenced by geographic proximity. –This can result in mating between related individuals. –Amplifies certain traits. Assortative –The selection of a mate based on the similarity of characteristics.

Natural Selection Stabilizing Selection –Individuals with the average form of a trait have the highest fitness. Directional selection –Individuals that display a more extreme form of a trait have greater fitness than individuals with an average form. Disruptive selection –Individuals with either extreme variation of a trait have greater fitness than individuals with the average.

Natural Selection Sexual selection –Individuals tend to mate with individuals displaying certain more attractive traits. –Example: The peacock.

Directional Selection

Microevolution vs Macroevolution Microevolution is evolutionary change within a species. Macroevolution is evolutionary change that transcends species and causes new species to emerge.

Formation of Species Speciation –The process of species formation. Morphological concept of species –Species classification was a result of the appearance and structure of the internal and external of the organism (or morphology). –Convenient b/c morphological characteristics are easy to observe. –Limitations –Species may display different phenotypes within a population.

Formation of Species The Biological Species Concept –A species that can successfully interbreed but cannot breed with another group is a species. –Problems Doesn’t help to determine extinct species. Doesn’t help to determine species that do not reproduce sexually.

Isolating Mechanisms Speciation begins with isolation. –Two formerly interbreeding populations stop interbreeding. Geographic isolation –Populations may be physically separated when their original habitat divides. Example: River changes course.

Formation of Species Reproductive Isolation –Prezygotic isolation Happens before fertilization. Incompatible behavior reduces the chance for a hybrid. Example: Different mating calls. –Postzygotic isolation Happens after fertilization. Offspring of the interbreeding are either underdeveloped or sterile. Example: Mules

Rates of Speciation/Evolution Speciation can require millions of years or it can happen rapidly (several thousand years). Gradual verses punctuated rates. Gradualism –Gradual changes over long periods of time. Punctuated equilibrium –When long periods of stability are followed by instant changes in the species (several thousand years)

Patterns of Evolution Coevolution – The change of 2 or more species in close association with eachother. Examples: Parasites/hosts, predator/prey, plants/pollinators

Cuckoo Bird

Yucca Moth and the Yucca Plant

Patterns of Evolution Convergent Evolution – Organisms that appear to be very similar, such as a shark and a porpoise but are in fact very different developmentally.

Patterns of Evolution Divergent Evolution – Two or more related populations or species become more and more dissimilar.

Divergent Evolution

Patterns of Evolution Adaptive Radiation- Many related species evolve from a single ancestral species.

Patterns of Extinction Gradual Extinction – occurs at a slow rate and may be a result of other organisms, climate change, or natural disasters. Mass Extinction – Usually occurs when a catastrophic event suddenly changes the environment.

Selection Scenarios In a population of lizards, the larger than average individuals may be more easily spotted, captured, and eaten by a predator. On the other hand, lizards that are smaller than average might not be able to run fast enough to escape. What might a population curve look like for a population of lizards? What type of selection is occuring?

Anteaters feed by breaking open termite nests, extending their sticky tongue into the nest and lapping up termites. Suppose that an area was invaded by a new specise of termite that built very deep nests. Anteaters with long tongues could more effectively prey on the termites. What would the population curve look like for the distribution of tongue length in anteaters? What type of selection is occuring?

The shell color of limpets (marine animals) varies from pure white to dark tan. White shelled limpets that are on rocks covered with barnacles that are also white are at an advantage because birds (predators) have difficulty distinguishing lampets from barnacles. On bare, dark colored rocks, dark shelled limpets are at an advantage because birds cannot distinguish them from the rocks either. Those limpets in between in color are at a disadvantage because they don’t blend in with the rocks or the barnacles. What type of selection is occuring? What would the population curve look like?