Evolution of Populations

23.1 – Mutation & sexual reproduction produce genetic variation that makes evolution possible 1) Microevolution Change in the allele frequencies of a population over generations Evolution on the smallest scale

2) Mutations The only source of NEW genes & NEW alleles Only mutations in cell lines that produce gametes can be passed on to offspring

Types of Mutations A) Point Mutation B) Chromosomal Mutation Change in one base in a gene Can impact phenotype Sickle cell anemia B) Chromosomal Mutation Delete, disrupt, duplicate, or rearrange many loci at once Most are harmful, but not always

3) Variations due to sexual reproduction Rearranges alleles into new combinations in every generation 3 mechanisms for this shuffling: Next slide

2) Independent assortment 1) Crossing over During Prophase I of meiosis 2) Independent assortment During meiosis (223 different combinations possible) 3) Fertilization 223 x 223 for sperm and egg

Population genetics Population 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving Population genetics Study of how populations change genetically over time Population Group of individuals of the same species that live in the same area Interbreed & produce fertile offspring

Gene pool All of the alleles at all loci in all the members of a population In diploids, each individual has 2 alleles for a gene & the individual can be heterozygous or homozygous If all are homozygous for an allele, the allele is FIXED – only one allele exists at the locus in the population The greater the # of FIXED alleles, the lower the species’ diversity

Hardy-Weinberg Used to describe a population that is NOT evolving Frequencies of alleles & genes in a gene pool will remain constant over generations

5 Conditions for Hardy-Weinberg 1) No mutations 2) Random mating 3) No natural selection 4) The population size must be large (no genetic drift) 5) No gene flow (Emigration, immigration, transfer of pollen, etc.)

If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p2 + 2pq + q2 = 1 where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

Practice Suppose in a plant population that red flowers (R) are dominant to white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?

Mutations can alter gene frequency, but are rare 23.3 – Natural Selection, genetic drift, & gene flow can alter allele frequencies in a population Mutations can alter gene frequency, but are rare 3 major factors alter allelic frequencies 1) Natural selection Alleles are passed to the next generation in proportions different from their frequencies to the present generation Those that are better suited produce more offspring than those that are not

2) Genetic Drift Unpredictable fluctuation in frequencies from one generation to the next The smaller the population, the greater chance Random & nonadaptive A) Founder effect = individuals are isolated and establish a new population – gene pool is not reflective of the source population B) Bottleneck effect = a sudden change in the environment reduces population size – survivors have a gene pool that no longer reflects original

Genetic drift is significant in small populations Genetic drift causes allele frequencies to change at random Genetic drift can lead to a loss of genetic variation within populations Genetic drift can cause harmful alleles to become fixed

3) Gene Flow Populations loses or gains alleles by genetic additions or subtractions Results from movement of fertile individuals or gametes Reduces the genetic differences between populations, makes populations more similar

23.4 Natural Selection is the only mechanism that consistently causes adaptive evolution Relative fitness The contribution an organism makes to the gene pool of the next generation relative to the contributions of the other members Does NOT indicate strength or size Measured by reproductive success

Can alter the frequency distribution of heritable traits in 3 ways: Natural selection acts more directly on the phenotype and indirectly on the genotype Can alter the frequency distribution of heritable traits in 3 ways: 1) Directional selection 2) Disruptive selection 3) Stabilizing selection

1) Directional selection Individuals with one extreme of a phenotypic range are favored, shifting the curve toward this extreme Example: Large black bears survived periods of extreme cold better than small ones, so they became more common during glacial periods

2) Disruptive Selection Occurs when conditions favor individuals on both extremes of a phenotypic range rather than individuals with intermediate phenotypes Example: A population has individuals with either large beaks or small beaks, but few with intermediate – apparently the intermediate beak size is not efficient in cracking either the large or small seeds that are available

3) Stabilizing Selection Acts against both extreme phenotypes and favors intermediate variations Example: Birth weights of most humans lie in a narrow range, as those babies who are very large or very small have higher mortality rates

How is genetic variation preserved in a population? Diploidy Capable of hiding genetic variation (recessive alleles) from selection Heterozygote advantage Individuals that are heterozygous at a certain locus have an advantage for survival Sickle cell anemia – homozygous for normal hemoglobin are more susceptible to malaria, homozygous recessive have sickle-cell, but those that are heterozygotes are protected from malaria and sickle-cell

Why Natural Selection cannot produce perfect organisms: 1) Selection can only edit existing variations 2) Evolution is limited by historical constraints 3) Adaptations are often compromises 4) Chance, natural selection, & the environment interact