Evolution

Evolutionary mechanisms Forces that change the genetic structure of a population Mutation Gene flow Genetic drift Nonrandom mating Natural selection

Evolutionary mechanisms Mutations generate genetic variation Origin of genetic variation; Any change in the DNA Rate is typically one in a million locus Can create considerable genetic variation because: Large number of genes may mutate Chromosomal rearrangements may change genes Populations contain large numbers of individuals Each human gamete contains 3 x 109; average of 3 point mutations. So, each zygote has ~6 mutations. Current population is 6.5 billion people; would expect ~40 billion mutations that were not in the previous generation.

Evolutionary mechanisms Gene flow may change allele frequencies Migration of individuals; movements of gametes between populations (gene flow) are common

Evolutionary mechanisms Genetic drift can cause large changes in small populations Random events that change the gene pool Population bottlenecks Founder effects The loss of genetic variation when a new colony is formed by a very small number of individuals from a larger population.

Bottleneck Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

Founder Effect Polydactyl -- extra fingers or sometimes toes -- is one symptom of Ellis-van Creveld syndrome. Commonly found among the Old Order Amish of Pennsylvania, a population that experiences the "founder effect." Genetically inherited diseases like Ellis-van Creveld are more concentrated among the Amish because they marry within their own community Which prevents new genetic variation from entering the population. Children are therefore more likely to inherit two copies of the particular recessive genes that lead to genetic disease.

Evolutionary mechanisms Nonrandom mating changes genotype frequencies Individuals of choose particular genotypes as mates If they mate with the same genotypes then homozygous genotypes will be overrepresented Self-fertilization (type of nonrandom mating) Sexual selection Changes allele frequencies

Evolutionary mechanisms resulting in adaptation Allele frequencies in the population change in a way that adapts individuals to the environment that influenced such reproductive success Fitness capability of an individual of certain genotype to reproduce manifested through its phenotype Determined by the average rates of survival & reproduction

Natural selection ~Produces variable results Stabilizing selection Preserves the average characteristics of a population (favors the average individual) Reduces variation in populations; does not change the mean Example: In humans the incidence of infant mortality is higher for very heavy as well as for very light babies

Natural Selection Directional Selection Favors individuals that vary in one direction from the mean of the population Evolutionary trend toward that extreme For example, breeding of the greyhound dog Early breeders were interested in dog with the greatest speed. They carefully selected from a group of hounds those who ran the fastest. From their offspring, the greyhound breeders again selected those dogs who ran the fastest. By continuing this selection for those dogs who ran faster than most of the hound dog population, they gradually produced a dog who could run up to 40mph.

Natural Selection Directional Selection

Natural Selection Disruptive Selection Favors individuals that vary in opposite directions from the mean Individuals from both extremes have high fitness Example Smaller-billed birds feed on soft seeds Large-billed birds crack hard seeds Birds with intermediate sizes cannot use either kind of seed efficiently & survive poorly

Natural Selection Sexual selection Natural selection that acts on characteristics that determine reproductive success. Intrasexual selection – improve ability to compete for access to mates Intersexual selection – more attractive to members of the opposite sex