Chapter 23~ Microevolution- small changes in the genetics of populations

Populations The Smallest Unit of Evolution One common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but populations evolve

Genetic variations in populations –Contribute to evolution

Individuals are selected, but populations evolve Population: –a localized group of individuals belonging to the same species Species: –a group of populations whose individuals have the potential to breed and produce fertile offspring Gene pool: –all of the genes in a population at any one time

Population genetics Is the study of how populations change genetically over time Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection Focuses on populations as units of evolution

Traits Polymorphism - different forms of traits –Anatomical/Structural (form) – Physiological (function) –Behavioral (response) Qualitative vs. Quantitative Traits

Alleles- versions of genes that are inherited Genotype is inherited not the phenotype Mutation and sexual recombination produce the variation that makes evolution possible Two processes, mutation (creating alleles) and sexual recombination (shuffling alleles) –Produce the variation in gene pools that contributes to differences among individuals

Stable Gene Pools Mendelian inheritance –Preserves genetic variation in a population In a given population where gametes contribute to the next generation randomly, allele frequencies will NOT change= genetic equilibrium Generation 1 C R genotype C W genotype Plants mate All C R C W (all pink flowers) 50% C R gametes 50% C W gametes Come together at random Generation 2 Generation 3 Generation 4 25% C R C R 50% C R C W 25% C W C W 50% C R gametes 50% C W gametes Come together at random 25% C R C R 50% C R C W 25% C W C W Alleles segregate, and subsequent generations also have three types of flowers in the same proportions

Mutations –Are changes in the nucleotide sequence of DNA –Cause new genes and alleles to arise Mutation types: 1.Lethal 2.Neutral 3.Beneficial

Variation Creators Mutation rates –Tend to be low in animals and plants –Average about one mutation in every 100,000 genes per generation –Are more rapid in microorganisms In sexually reproducing populations, sexual recombination –Is far more important than mutation in producing the genetic differences that make adaptation possible

Hardy-Weinberg Equation p=frequency of one allele (A); q=frequency of the other allele (a); p+q=1 (p=1-q & q=1-p) p 2 =frequency of AA genotype; 2pq=frequency of Aa plus aA genotype; q 2 =frequency of aa genotype; p 2 + 2pq + q 2 = 1

Hardy-Weinberg Theorem Serves as a model for a non-evolving population (equilibrium) 5 conditions: 1- Very large population size; 2- No gene flow; 3- No mutations; 4- Random mating; 5- No natural selection

Natural selection Natural selection is the primary mechanism of adaptive evolution Differential success in reproduction –Results in certain alleles being passed to the next generation in greater proportions Fitness: –contribution an individual makes to the gene pool of the next generation

3 types of Natural Selection: –A. Directional –B. Disruptive –C. Stabilizing

Natural Selection Types 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

A Closer Look at Selection Types From the range of variations available in a population –Natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations

Sexual selection Sexual dimorphism: secondary sex characteristic distinction Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism

Sexual Selection Intrasexual selection –Is a direct competition among individuals of one sex for mates of the opposite sex Intersexual selection –Occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex –May depend on the showiness of the male’s appearance

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

Variation preservation Prevention of natural selection’s reduction of variation Balanced polymorphism Heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia)

Microevolution Gene Flow: –genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations) –Immigration vs. Emigration

Genetic drift changes in the gene pool of a small population due to chance (usually reduces genetic variability) CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW CRCWCRCW CRCRCRCR CRCRCRCR Only 5 of 10 plants leave offspring CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCWCRCW CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW Only 2 of 10 plants leave offspring CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3

Microevolution The Bottleneck Effect: –type of genetic drift resulting from a reduction in population –Caused by natural disasters or human activity –the surviving population is no longer genetically representative of the original population Original population Bottle- necking event Surviving population

Understanding the bottleneck effect –Can increase understanding of how human activity affects other species Bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.

The Evolutionary Enigma of Sexual Reproduction Sexual reproduction –Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap Asexual reproduction Female Sexual reproduction Female Male Generation 1 Generation 2 Generation 3 Generation 4

Why Sex? If sexual reproduction is a handicap, why has it persisted? –It produces genetic variation that may aid in disease resistance

Why Natural Selection Cannot Fashion Perfect Organisms Evolution is limited by historical constraints Adaptations are often compromises Chance and natural selection interact Selection can only edit existing variations