Chapter 16: How Populations Evolve - Biology PDF

Chapter 16: How Populations Evolve 16.1 Genes, Populations, and Evolution  A population is a group of organisms of a single species living together in the same area.  Diversity exists yet  Population genetics: study of population diversity at the gene level.  These...

Chapter 16: How Populations Evolve 16.1 Genes, Populations, and Evolution  A population is a group of organisms of a single species living together in the same area.  Diversity exists yet  Population genetics: study of population diversity at the gene level.  These variations are described in terms of alleles  Therefore, evolution is described as the change in allele frequencies over time Microevolution  Microevolution pertains to evolutionary change within populations  Various alleles at the gene loci of all individuals in a population make up the gene pool.  Can be described as genotype frequencies or allele frequencies  If you monitor allele frequency in a population overtime and get no change, no microevolution occurred. Allele/Genotype Frequencies  Allele frequency: percentage of each allele in the population's gene pool.  Example: on the board.  Genotype frequency: proportion (or percentage) of the genotypes in the population  Example: on the board. Hardy-Weinberg Equilibrium (HWE)  HWE: a population is said to be in HWE if allele frequencies remain constant. This is assuming:  1. No mutations  2. No migration  3. Large gene pool  4. Random mating  5. No selection  You can use HWE to determine if microevolution has occurred. Breaking down the HWE Assumptions  If microevolution has occurred, we know at least one of the assumptions wasn’t met.  Based on this knowledge, we know the five assumptions are mechanisms for evolutionary change. Breaking down the HWE Assumptions  Mutation  A change to the DNA sequence  Usually made during DNA replication  Genetic mutations are the raw/base materials for evolutionary change  Provides new alleles  Ex. Peppered moths, mice on the lava rocks  Without mutations, there is no variation, which evolution could not occur Breaking down the HWE Assumptions  Migration (immigration, emigration)  Gene flow: movement of alleles between populations  Usually caused by migration (plants and animals)  Continual gene flow could result in populations so similar they resemble a single population  If no gene flow occurs, populations could become so diverse it leads to reproductive isolation  First step in formation of a new species Breaking down the HWE Assumptions  Small population size:  Genetic drift: changes in allele frequencies of a gene pool due to chance events  Environment has no influence  Small populations likely to be affected more so than large populations Genetic Drift  Likely to occur:  Bottleneck: event that prevents majority of the population from passing on genes Natural disasters  Founder Effect: individuals from a population break away to form a new population  Severe inbreeding Breaking down the HWE Assumptions  Nonrandom Mating  Individuals not choosing mates at random  Assortative mating: individuals choosing a mate with a preferred trait  Ex. birds 16.2 Natural Selection  For many traits, there is considerable variation among the phenotypes due to multiple alleles for a single gene  Polygenic traits: traits that are controlled by multiple genes (human height)  When this range of variation is exposed to the environment, natural selection favors the more adaptive variant  This variation usually results in a bell-shaped curve  There are 3 types of selection:  1. Stabilizing  2. Directional  3. Disruptive Stabilizing Selection  An intermediate phenotype is the most adaptive given the environmental conditions.  Extremes are selected against  Ex. Human height, offspring size Directional Selection  An extreme phenotype is favorable, so the curve shifts to one side  Ex. Giraffe necks, NBA Disruptive Selection  Two or more extremes are favored over the intermediate  Curve has two peaks  Ex. Males competing for a mate Sexual Selection  Adaptive changes in males and females that lead to an increased ability to secure a mate  Female Choice: females produce less eggs than males produce sperm, so picking a mate is important  Good genes hypothesis: females chose mates on the basis of traits that improve chances for survival  Runaway hypothesis: females chose mates on the basis of traits that improve male appearance  Sexual dimorphism: male and females differ in size and traits Supports female choice Sexual Selection  Male Choice:  Males can father many offspring due to large sperm count  Males compete for female insemination 16.3 Maintenance of Diversity  Diversity can be maintained in many ways:  Mutation, sexual reproduction, even genetic drift  Natural selection itself causes imperfection adaptations  Benefits have to outweigh the costs Ex. Humans have free hands, but we deal with getting spinal injuries due to the erect posture  Environment includes specific selective agents to help keep diversity  This promotes the rare form survival Ex. Food availability for finches shaped their beaks Heterozygote Advantage  When the heterozygote is favored over the two homozygotes  Heterozygotes “shelter” or “harbor” the recessive allele, keeping it in the population  Ex. Sickle cell anemia  If you are homozygous recessive, you are suffering from sickle cell disease  If you are homozygous dominant, you are more likely to get malaria  If you are heterozygous, you don’t have sickle cell AND you are at a decreased risk for getting malaria  One situation where having one ‘dangerous’ allele is the most favorable.