HARDY WEINBERG CRITERIA & POPULATION EVOLUTION CHAPTER 23 HARDY WEINBERG CRITERIA & POPULATION EVOLUTION (PART 3) Ms.Gaynor AP Biology

Causes of Microevolution A change in the gene pool of a population over a succession of generations We use 5 criteria for non-evolution to determine causes of microevolution The Hardy Weinberg equations are used to determine the degree of microevolution that is occurring for a given allele

Cause #1: Microevolution Genetic drift: changes in the gene pool of a SMALL population due to chance (usually reduces genetic variability) http://highered.mcgraw-hill.com/sites/0072835125/student_view0/animations.html# 2nd to last video

Genetic Drift Example #1 The Bottleneck Effect Results from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population

Ex: NORTHERN ELEPHANT SEAL

Genetic Drift Example #2 Founder Effect: a cause of genetic drift due to the colonization by a limited number of individuals from a parent population http://bcs.whfreeman.com/thelifewire/content/chp24/2402002.html

Founder Effect in Amish Causes dwarfism and polydactyly http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m2/s4/evm2s4_7.htm

Cause #2: Microevolution Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)

Cause #3: Microevolution Mutations: a change in an organism’s DNA original source of genetic variation (raw material for natural selection) Mutations can immediately alter p and q in a population Individual mutations are rare in a population, but there may be cumulative mutations that have an effect  cause evolution

Cause #4: Microevolution Nonrandom mating “picky” mating Unequal chances of each egg getting fertilized Assortative mating - choosing individuals more like self

Sexual Selection Sexual selection is natural selection for mating success It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

Sexual Dimorphism Males are usually more colorful and larger Does NOT help cope with environment DOES lead to reproductive success Can be termed “Female Selection”

Cause #5: Microevolution Natural Selection differential reproductive success in reproduction due to variation only form of microevolution that adapts a population to its environment Phenotypes are selected FOR or AGAINST (not genotypes) In a population at equilibrium, no phenotypes (therefore, alleles) are selected over other alleles http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m2/s1/evm2s1_6.htm

Animations to Review Hardy Weinberg and its Criteria http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/life4e_15-6-OSU.swf

3 Modes of Natural selection Directional selection favors individuals at one end of the phenotypic range Disruptive selection favors individuals at both extremes of the phenotypic range Stabilizing selection favors intermediate variants and acts against extreme phenotypes http://wps.pearsoncustom.com/wps/media/objects/3014/3087289/Web_Tutorials/17_A02.swf

LIGHT/DARK ROCK HABITAT MEDIUM COLOR ROCK HABITAT

Sources of Variations Mutations At first the mutation may not be beneficial to the organism (resistance to antibiotics) Once antibiotics are introduced the mutation is beneficial 2. Sexual recombination increases variation

Population variation Polymorphism coexistence of 2 or more distinct forms of individuals (morphs) within the same population Geographical variation differences in genetic structure between populations (cline) SPRING SUMMER

Evolutionary Fitness “Struggle for existence”/“Survival of the fittest” commonly used but can be misleading Reproductive success is more subtle and depends on many factors Fitness is determined by REPRODUCTIVE AND SURVIVAL success of individual Variations & natural selection affect fitness Sterile individuals even with a relatively fit allele will not be considered fit overall (=0) Fitness is not determined by one trait but by the totality of traits in the organism

A 4th Type of Natural Selection: Neutral Variation Neutral variation is genetic variation that appears to favor NO selective advantage Frequencies can change but no advantage/ disadvantage for population

Asexual vs. Sexual Reproduction Sexual reproduction produces FEWER reproductive offspring than asexual reproduction a so-called “reproductive handicap” Sexual reproduction produces genetic variation

Asexual reproduction Sexual reproduction Female Generation 1 Female