Transcript Chapter 16
Evolution of Populations Biologists studying evolution often focus on a particular population. Population - a group of individuals of the same species in a given area. Because all members of a population can interbreed, they share a common group of genes, called a gene pool. Gene Pool – the combined genetic information of all the members of a particular population. There are two main sources of genetic variation: Mutations – any change in the sequence of DNA. Gene Shuffling – results from sexual reproduction. Examples: production of gametes crossing over segregation The number of phenotypes produced for a given trait depends on how many genes control the trait. Single-Gene Trait – a trait that is controlled by a gene that has two alleles. Ex. – widows peak Polygenic Trait – traits that are controlled by two or more genes. Ex. – Height in humans Natural Selection on Single Gene Traits can lead to changes in the frequencies of alleles which then lead to evolution. Ex. – peppered moth color Natural Selection on Polygenic Traits - When traits are controlled by more than one gene, the effects of natural selection are more complex. - The effect of selection on varying characteristics in any of three ways: Directional Selection Stabilizing Selection Disruptive Selection Directional Selection - occurs when individuals at one end of the curve have a higher fitness than individuals in the middle or at the other end. Fig. 16-6 Pg. 398 Stabilizing Selection - occurs when individuals near the center of the curve have higher fitness than individuals at either end of the curve. Fig. 16-4 Pg. 396 Disruptive Selection - occurs when individuals at the upper and lower end of the curve have higher fitness than individuals near the middle. Fig. 16-8 Pg. 399 Random Change in allele frequency Bottle neck effect- population size is dramatically reduced Founder Effect- Colonize a new population Sexual reproduction does NOT change allele frequencies Hardy-Weinberg- Conditions for Stability • Random Mating • Large Population Size • No immigration or emigration • No mutations • No Natural Selection Isolating Mechanisms As new species evolve, populations become reproductively isolated from each other. There are two modes of speciation base on how gene flow among populations is interrupted. They are: Allopatric Speciation Sympatric Speciation Allopatric Isolation - occurs when a geographical barrier that physically isolates populations initially blocks gene flow. - Also called: Geographic Isolation Sympatric Isolation - occurs when chromosomal changes (in plants) and nonrandom mating (in animals) alter gene flow. Includes: Behavioral Isolation – occurs when two populations are capable of interbreeding but have different courtship rituals or other types of behavior. Reproductive Isolation The eastern meadowlark (left) and western meadowlark (right) have overlapping ranges. They do not interbreed, however, because they have different mating songs. Temporal Isolation – when two or more species reproduce at different times. • Sympatric populations become genetically isolated even though their ranges overlap. Itext Activity (Page 409) Speciation in the Galápagos finches occurred by the founding of a new population, geographic isolation, changes in the new population’s gene pool, reproductive isolation, and ecological competition. 1. Founders Arrive 2. Separation of Populations 3. Changes in the Gene Pool 4. Reproductive Isolation 5. Ecological Competition 6. Continued Evolution Question: Are Darwin’s finches an example of Allopatric or Sympatric Isolation? Speed of Speciation Gradualism - Species descended from a common ancestor gradually diverge more an more through physical changes as they acquire unique adaptations Punctuated Equilibrium - a new species changes most as it buds from a parent species, and then changes very little for the rest of its existence. WHAT PRECIPITATES AN ADAPTIVE RADIATION? ECOLOGICAL OPPORTUNITY: the invasion of unutilized ecological niches leads to rapid diversification, e.g., colonizing a remote archipelago, surviving a mass extinction. KEY INNOVATION: the acquisition of a novel adaptive trait (behavioral, morphological, or physiological) allows organisms to exploit previously unavailable ecological niches. Seeing this graduation and diversity of structure in one small, intimately related group of birds, one might really fancy that, from an original paucity of birds in this archipelago, one species has been taken and modified for different ends. Darwin, 1842 Character Displacement: Members of one lineage constrain phenotypic evolution in members of other lineages There are two sides to this coin: 1) It may promote divergence between closely related species when there are unexploited ecological niches available. 2) It may constrain divergence when there are no unexploited niches HAWAIIAN HONEYCREEPERS CICHLID FISHES IN AFRICAN RIFT VALLEY LAKES: LAKE TANGANYIKA: 140 SPP. LAKE MALAWI: >500 SPP. LAKE VICTORIA: 250 SPP. Lake Malawi Cichlids Photos by Fredrik Hagblom PARALLEL EVOLUTION IN THE CICHLID RADIATION AUSTRALIAN MARSUPIALS DEMONSTRATE AN ADAPTIVE RADIATION IN THE ABSENSE OF COMPETITIVE INTERACTIONS WITH PLACENTAL MAMMALS THE AMNIOTIC EGG: Perhaps one of the greatest key innovations of all time ANOTHER KEY INNOVATION: WINGS Dana Gardner Trogons of Peru Dana Gardner Fruiteaters of Peru ADAPTIVE RADIATION OF MAMMALS Diversification of the major Mammalian lineages occurred in a relatively short period of time. ADAPTIVE RADIATION OF ANIMALS Warming and retreat of glaciers Oxygenation of ocean Increased availability of phosphorous Niche expansion Cambrian Community Evidence of Predation ADAPTIVE RADIATIONS AND RAPID EVOLUTION Adaptive radiations are often characterized by: Ecological opportunity Acquisition of novel adaptive traits Competitive interactions among closely related taxa Parallel evolution Rapid phenotypic diversification