Transcript Speciation
Speciation Genetic Equilibrium Disruption of Genetic Equilibrium Formation of Species Genetic Equilibrium • Species: a group of individuals that look similar and whose members are capable of producing fertile offspring in the natural environment Morphological Species Concept • Morphological Species Concept: the idea that organisms can be classified by differences in their appearance – Using this concept, scientists can readily communicate about the characteristics, behavior, and relationships of organisms. – The morphological concept of species is limited because it does not account for the reproductive compatibility of morphologically different organisms. Biological Species Concept • Biological Species Concept: the idea that organisms can be classified by their ability to breed with one another • Using both the biological and morphological concepts of species, modern scientists have arrived at the current definition of the term species. Species: a group of individuals that look similar and whose members are capable of producing fertile offspring in the natural environment Variation of Traits in a Population • Population: all the members of a species that live in the same area and make up a breeding group • Within a population, individuals vary in many observable traits. Caribou Population Distribution Variation is the Raw Material of Natural Selection Variation of Traits in a Population What causes variation in traits? • Environmental Factors • Heredity – A difference in genotype usually results in a difference in phenotype. – Variations in genotypes arise through: • Mutation • Genetic Recombination – Independent Assortment – Crossing Over Allele Frequencies and Genetic Equilibrium • Gene Pool: all the genes for all the traits in a population • Allele Frequency: the percentage of an allele in a gene pool • Genetic Equilibrium: the condition in which allele frequencies in a population do not change from generation to generation Genotypic and Allele Frequencies Hardy & Weinberg • Hardy and Weinberg outlined the conditions necessary for genetic equilibrium. Godfrey Hardy Wilhelm Weinberg Hardy-Weinberg Principle • Hardy and Weinberg outlined the conditions necessary for genetic equilibrium. 1. no mutations occur 2. individuals neither enter nor leave the population through migration 3. the population is large 4. individuals mate randomly 5. natural selection does not occur Hardy-Weinberg Principle • If one of these conditions does not hold true, allele frequencies of the population may change. In other words, evolution will occur. Disruption of Genetic Equilibrium Disruption of Genetic Equilibrium Evolution occurs when genetic equilibrium is disrupted. 1. Mutation • Mutations affect genetic equilibrium by producing totally new alleles for a trait. • In addition, mutations can change the frequency of the alleles already present in the gene pool. • The occurrence of mutation in most alleles is too low to cause major changes in the allele frequencies of an entire population. 1. Mutation 2. Migration • Migration: the movement of individual organisms into or out of a population, community, or biome – Immigration: movement of new individuals into a population – Emigration: departure of individuals from a population • Gene Flow: the movement of genes into or out of a population 2. Migration 3. Genetic Drift • Genetic Drift: a shift in allele frequencies in a population due to random events or chance – In small populations, chance can significantly affect allele frequencies from one generation to the next. 3. Genetic Drift 3. Genetic Drift 4. Non-Random Mating • If animals chose their mates then certain traits would be selected for over less desirable ones. 5. Natural Selection • Natural selection is an ongoing process in nature, and is the single most significant factor disrupting genetic equilibrium. 4 Types of Natural Selection 1. 2. 3. 4. Stabilizing Selection Directional Selection Disruptive Selection Sexual Selection Stabilizing Selection • Stabilizing Selection: a type of natural selection in which the average form of a trait causes an organism to have an advantage in survival and reproduction – The extreme forms of the trait confer a disadvantage to the organism. – Stabilizing selection is most effective in a population that has become well adapted to its environment. – Stabilizing selection is the most common type of NS. Directional Selection • Directional Selection: a type of natural selection in which the distribution of a trait is shifted toward one of the extremes Disruptive Selection • Disruptive Selection: natural selection in which individuals with either of the extreme forms of a trait have an advantage in terms of survival and reproduction – The average form of the trait confers a selective disadvantage to the organism. Sexual Selection • Sexual Selection: the preferential choice of a mate based on the presence of a specific trait – Sexual selection may be stabilizing, directional, or disruptive. Formation of Species Speciation • Disruption of genetic equilibrium leads to changes in the gene pool of a population. • Natural selection causes the most significant genetic changes in a population, however, such changes do not necessarily lead to speciation. • Speciation: the formation of a new species Speciation Speciation Isolated Populations • For a new species to arise, interbreeding of the population of fertile offspring must somehow cease among members of a formerly successful breeding population. – For this to occur, populations or segments of populations must somehow become isolated. • Two Forms of Isolation: – Geographic Isolation – Reproductive Isolation Geographic Isolation • Geographic Isolation: the physical separation of populations – Examples: population becomes divided by land or water barrier, colonization of new island, river changes course, highway built across field, etc. Reproductive Isolation • Reproductive Isolation: the inability of formerly interbreeding organisms to produce offspring – Reproductive isolation can occur through disruptive selection. Rates of Speciation • Punctuated Equilibrium: a theory that speciation may occur during brief periods of rapid genetic change • Gradualism: a theory that speciation may occur through a gradual change of adaptations • Extinct: the state of a species that no longer exists – For a species to continue to exist, some members must have traits that allow them to survive and pass their genes on to the next generation.