Mechanisms (Features) of Evolution What is needed for Natural Selection to occur?

Evolution of Populations Population: A particular species in one area By focusing on evolution of populations, we define evolution on the smallest scale (microevolution) Because: Individuals do not evolve Populations are isolated from others of their own species; keeps gene pool from being manipulated by outside traits Allele frequencies can be monitored over generations Allele: traits passed on from parents (You have two alleles for each trait, one from each parent) Changing allele frequencies point to an evolving population

Genetic Variation The most important factor needed for NS to occur Random Mutations in DNA Mutation cannot be silent or non-beneficial to reproductive success of individual Mutation must occur in cell line that produces gametes Less limiting in plants and fungi as many cell lines produce gametes Mutations produced in somatic cells die with individual

Genetic Variation The most important factor needed for NS to occur Random Mutations in DNA This is likely how vertebrates evolved Invertebrates have a gene called the Hox complex that direct the major development of body parts during fetal (embryological) development 520 million years ago a duplication of the Hox complex may have provided genetic material associated with the first vertebrates A second duplication (leading to 4 Hox complexes in current vertebrates) points to a linear evolution of these genes

Hox Gene Evolution

Genetic Variation The most important factor needed for NS to occur Sexual Reproduction (only in organisms that reproduce in this manner) Shuffles genetic material in individuals to contribute variability to population Crossing over during Meiosis – happens during synapsis in which homologous chromosomes touch, exchanging chromatids and ensures that specific variation of traits do not have to be inherited together Independent Assortment – homologous chromosomes from each parent do not have to all line up on one side of metaphase plate; ensures that parental DNA is shuffled in offspring creating genetic variability Random Fertilization – each gamete produced has a unique combination of parental DNA and fertilized at random; ensures variability of offspring with each combination of gametes

Genetic Variation The most important factor needed for NS to occur Asexual Reproduction – creates offspring that are genetically identical to parents Advantageous only when environment is stable

Genetic Variation How can asexually reproducing organisms be genetically variable? Random Mutations in DNA Mutation cannot be silent or non-beneficial to reproductive success of individual Mutation must occur in cell line that produces gametes Horizontal Gene Transfer: transfer of genes between organisms in a manner other than traditional reproduction Transformation Transduction Conjugation

Genetic Variation How can asexually reproducing organisms be genetically variable? Bacterial Transformation: Direct uptake of DNA from environment by bacteria through cell wall Can occur naturally in some bacteria but has to be induced in others E.coli Lab

Genetic Variation How can asexually reproducing organisms be genetically variable? Bacterial Transduction: Uptake of DNA through viruses known as bacteriophages Bacteriophage: virus that infects bacteria Happens naturally; once virus takes over bacterial cell, new viruses have bacterial DNA incorporated into them; those viruses then go onto infect other bacteria and are more readily accepted by the bacteria because of the recombinant DNA Now commonly used in gene therapy to insert therapeutic DNA or siRNA into cells

Bacterial Transduction

Genetic Variation How can asexually reproducing organisms be genetically variable? Bacterial Conjugation: transfer of genetic material (DNA or RNA) between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells

Bacterial Conjugation

Genetic Variation Genetic Drift: Chance events that alter the allele frequencies of a population with no regard to how trait is related survival Not considered NS Example: You stepping on a large population of green bugs; trait has nothing to do with survival, just an oops Extreme drift can lead to: Founders Effect Bottleneck Effect

Genetic Variation Founders Effect: A few individuals become isolated from original population; allele frequency may differ from original population Not necessarily a negative effect unless frequency reflects less variation

Genetic Variation Bottleneck Effect: A dramatic reduction in population size that may not be representative of allele frequencies original population Generally reduces genetic variation Can lead to endangerment of species Example: Cheetahs Two Bottlenecking Events: Ice age (100,000 years ago) and extreme hunting and poaching (100 years ago) So genetically similar that tissue grafts are never rejected between unrelated cats (less than 1% genetic variation)

Genetic Variation Gene Flow: Transfer of alleles into or out of a population due to movement of fertile individuals or gametes Not considered NS Introduced alleles would change frequencies of next generation Transferred alleles can be both beneficial and damaging to the survival of population Reduces variation between populations of different geographic locations Example: HUMANS! Much more mobile than previous generations; much less variation based on geographic location

Relative Fitness - Competition Variation doesn’t matter unless organism that is passing on traits is in some way improving its chances of reproductive success Competition for Limited Resources Food Space Mates Survive long enough to have offspring Interactions with environment such as coloration or symbiotic relationships with other organisms

Relative Fitness - Competition Sexual Selection: Form of natural selection in which certain individuals are more likely to obtain mates Intrasexual selection: individuals compete with member of the same sex for mate More often occurs in males, but can occur in females Intersexual selection: mate choice; individuals are choosy in selecting mates Often it is females choosing males based on behavior or appearance Results in sexual dimorphism

Overall Requirements for NS to Occur Reproduction Heredity (ability to pass on heritable traits) Variation of traits in the population Relative Fitness of members of the population