PACKET #59 CHAPTER #23 Microevolution 10/31/2015 4:20 PM 1

Microevolution Change in a population’s allele, or genotype, frequencies over successive generations  Occurs when a population does not meet all of the assumptions of the Hardy-Weinberg principle  The small changes are referred to as microevolution. 10/31/2015 4:20 PM 2

Microevolution II 10/31/2015 4:20 PM 3 There are five micro-evolutionary forces: -  Non-random mating  Mutations  Genetic drift  Bottleneck effect  Founder effect  Gene flow  Natural selection  Stabilizing selection  Directional selection  Disruptive selection

NON-RANDOM MATING Microevolution 10/31/2015 4:20 PM 4

Non-random Mating 10/31/2015 4:20 PM 5 Assortative Mating  Individuals select mates on the basis of phenotype— indirectly selecting a corresponding genotype.  This may lead to interbreeding Leads to an increased homozygous allele composition  May lead to interbreeding depression and lowered “fitness” in the population Commonly seen in plants Fitness Ability to pass on genes to the next generation

MUTATIONS Microevolution 10/31/2015 4:20 PM 6

Mutations Unpredictable changes in DNA resulting in the production of new alleles  Introduces variation  Cause small deviations from Hardy-Weinberg equilibrium  Mutations in somatic cells are not heritable  Mutations in alleles found in sex cells allow those changes to be passed to the offspring 10/31/2015 4:20 PM 7

GENETIC DRIFT Microevolution 10/31/2015 4:20 PM 8

Genetic Drift Random events that change allele frequencies in small populations  Small populations are more prone to lose alleles present in low frequencies 10/31/2015 4:20 PM 9

Bottleneck Effect Rapid and severe declines in population size due to an adverse environmental factor Results in an increase in different allele frequencies 10/31/2015 4:20 PM 10

Bottleneck Effect Example Examples  Northern elephant seals  Cheetah  American bison  Wollemi Pine 10/31/2015 4:20 PM 11

Founder Effect Occurs when a small population colonizes a new area. Common in island populations  Finnish population exhibits much less allelic variation than the general European population  Amish population in Pennsylvania has a significant number of individuals with the allele composition for a form of dwarfism. 10/31/2015 4:20 PM 12

GENE FLOW Microevolution 10/31/2015 4:20 PM 13

Gene Flow The movement of alleles caused by a migration of individuals between populations  Migration of breeding individuals introduces new allelic frequencies to a population  Tends to counteract natural selection and genetic drift  Causes populations to become more genetically similar  Humans have experienced an increase in gene flow in the last few hundred years. 10/31/2015 4:20 PM 14

NATURAL SELECTION Microevolution 10/31/2015 4:20 PM 15

Natural Selection Natural selection results in changes in allele composition that lead to adaptation and is based on differential reproduction Natural selection does not act directly on an organism’s genotype but acts on the phenotype.  The phenotype represents an interaction between the environment and all the alleles in the organism’s genotype.  Natural selection weeds out those individuals whose phenotypes are less adapted to environmental changes  Allowing the better adapted organisms to survive and pass their alleles to future generations Natural selection acts indirectly on the genotype. 10/31/2015 4:20 PM 16

Natural Selection 10/31/2015 4:20 PM 17 There are three types of natural selection: -  Stabilizing selection  Directional selection  Disruptive selection

Natural Selection Stabilizing Selection Favors intermediate phenotypes Results in reduced variation in a population  Classic example is human birth weight 10/31/2015 4:20 PM 18

Natural Selection Directional Selection Favors one phenotype over another Favors one extreme of the normal distribution over the other phenotypes. 10/31/2015 4:20 PM 19

Natural Selection Disruptive Selection Favors phenotypic extremes Selects for two or more different phenotypes  May result in splitting of a population into 2 or more separate species. 10/31/2015 4:20 PM 20

Necessities for Natural Selection Populations contain genetic variation, a necessity for natural selection, introduced because of  Mutations  Crossing Over  Independent Assortment  Random Fertilization In order to investigate genetic variation scientists examine genetic polymorphism  The presence of two or more alleles at a given locus.  Locus is the location of a gene on the chromosome.  Scientists compares different forms of a protein using gel electrophoresis  Resulting from slight changes in the gene’s DNA nucleotide sequence 10/31/2015 4:20 PM 21