Biology ECO SC.912.L Sophiaa Lopez, P2.

As powerful as natural selection is, unfortunately, it is sometimes misunderstood. A persistent misconception is that natural selection occurs mainly through differences between organisms in death rates, or differential mortality. Selection normally proceeds in a much more subtle and inconspicuous way. Whenever one organism leaves more successful offspring than another, in time its genes will come to dominate the population gene pool. Eventually, the genotype leaving fewer offspring must become extinct in a stable population, unless concomitant changes confer an advantage on it as it becomes scarcer. Ultimately, natural selection operates only by differential reproductive success. Differential mortality can be selective but only to the degree that it creates differences between individuals in the number of reproductive progeny they produce. Hence, phrases such as “the struggle for existence” and “survival of the fittest” have had an unfortunate consequence.

Simpson Evolution Example

Peppered Moth Lab Individuals with favorable traits are more likely to leave more offspring better suited for their environment Example: English peppered moth moth (Biston betularia )Biston betularia Natural Selection

Peppered Moth Natural Selection Simulation

Normal phenotype MUTATION A mutation can create a new allele in an individual. When this happens, the population experiences a change in its allele frequencies and, consequently, experiences evolution. MECHANISMS OF EVOLUTION EVOLUTIONARY CHANGE: MUTATION Despite mutation’s vital role in the generation of variation, mutations almost always cause early death or lower the reproductive success of an organism.  DNA Mutagen Mutated protein Mutated phenotype Mutated base- pair sequence Normal base- pair sequence Normal protein #1

Mutations Are rare because you have self correcting enzymes Natural Process that produces genetic diversity Not all mutations are bad –Some won’t affect the body at all Blood types/ear lobes –Some are advantageous (thumb) wrist bone five digit s

POPULATION BEFORE GENETIC DRIFT Allele frequencies: GENETIC DRIFT MECHANISMS OF EVOLUTION GENETIC DRIFT A population can experience random changes in allele frequency that do not influence reproductive success and, consequently, the population experiences evolution. FIXATION Genetic drift leads to fixation when an allele’s frequency becomes 100% in a population. If this occurs, there is no longer genetic variation for the gene. POPULATION AFTER GENETIC DRIFT There are now more recessive alleles in the population than before. REPRODUCTION In this example, a heterozygous couple (Cc) could have two children that are homozygous recessive (cc), causing an increase in the proportion of recessive alleles in the population. cleft chin (dominant) smooth chin (recessive) Neither allele is related to reproductive success. Inheritance is based solely on chance.

MECHANISMS OF EVOLUTION FOUNDER EFFECT The founding members of a new population can have different allele frequencies than the original source population and, consequently, the new population experiences evolution. GENETIC DRIFT: FOUNDER EFFECT 5 digits per hand (recessive) NEWLY FOUNDED POPULATION The new population will be dominated by the genetic features present in the founding members. SOURCE POPULATION Allele frequencies: A group of individuals may leave a population and become the founding members of a new, isolated population. >5 digits per hand (dominant) AMISH

MECHANISMS OF EVOLUTION MIGRATION After a group of individuals migrates from one population to another, both populations can experience a change in their allele frequencies and, consequently, experience evolution. MIGRATION (GENE FLOW) 1 BEFORE MIGRATION Two populations of the same species exist in separate locations. In this example, they are separated by a mountain range. Population 1Population 2

2 MIGRATION A group of individuals from Population 1 migrates over the mountain range. MECHANISMS OF EVOLUTION MIGRATION After a group of individuals migrates from one population to another, both populations can experience a change in their allele frequencies and, consequently, experience evolution. MIGRATION (GENE FLOW) Population 1Population 2

3 AFTER MIGRATION The migrating individuals are able to survive and reproduce in the new population and they may experience evolutionary changes from population 1. MECHANISMS OF EVOLUTION MIGRATION After a group of individuals migrates from one population to another, both populations can experience a change in their allele frequencies and, consequently, experience evolution. MIGRATION (GENE FLOW) Population 1Population 2

3 Conditions that must occur for Natural Selection 1. VARIATION OF A TRAIT IN A POPULATION 2. The trait must be inheritable The tiniest dog in a litter has reduced differential reproductive success. Its more robust siblings prevent access to the food it needs to grow and thrive. 3. One version of the trait must be in greater abundance than a different version of the trait. Mechanism of evolution Natural Selection

MECHANISMS OF EVOLUTION NATURAL SELECTION When these three conditions are satisfied, the population’s allele frequencies change and, consequently, evolution by natural selection occurs. 1 VARIATION FOR A TRAIT Different traits are present in individuals of the same species 3 DIFFERENTIAL REPRODUCTIVE SUCCESS In a population, individuals with traits most suited to reproduction in their environment generally leave more offspring than individuals with other traits. 2 HERITABILITY Traits are passed on from parents to their children. EVOLUTION BY NATURAL SELECTION: A SUMMARY