Biology ECOSC.912.L.15.13

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 Individuals with favorable traits are more likely to leave traits are more likely to leave more offspring better suited more offspring better suited for their environmentfor their environment

Example:Example:English pEnglish peeppppeerreedd moth moth (Biston betularia)

Natural Selection

Peppered Moth Natural Selection Simulation Peppered Moth Natural Selection Simulation

Normalphenotype

MUTATIONA mutation can createa new allele in an individual. When this happens, the population experiences a change in its allele frequencies and, consequently, experiences evolution.

MECHANISMSOF

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

Mutatedphenotype

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 digits

POPULATION BEFOREGENETIC DRIFTAllele frequencies:

GENETIC DRIFT

MECHANISMSOF EVOLUTION

GENETIC DRIFTA population can experience

random changes in allele frequency that do not influence

reproductive success and, consequently, the population

experiences evolution.

FIXATIONGenetic 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 DRIFTThere are now more recessive alleles in the population than before.

REPRODUCTIONIn 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.

MECHANISMSOF EVOLUTION

FOUNDER EFFECTThe 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 POPULATIONThe new population will be dominated by the genetic features present in the founding members.

SOURCE POPULATIONAllele 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

MECHANISMSOF EVOLUTION

MIGRATIONAfter 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 MIGRATIONTwo populations of the same species exist in separate locations. In this example, they are separated by a mountain range.

Population 1 Population 2

2 MIGRATIONA group of individuals from Population 1 migrates over the mountain range.

MECHANISMSOF EVOLUTION

MIGRATIONAfter 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 1 Population 2

3 AFTER MIGRATIONThe migrating individuals are able to survive and reproduce in the new population and they may experience evolutionary changes from population 1.

MECHANISMSOF EVOLUTION

MIGRATIONAfter 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 1 Population 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

MECHANISMSOF EVOLUTION

NATURAL SELECTIONWhen these three conditions are satisfied, the population’s allele frequencies change and,

consequently, evolution by natural selection occurs.

1 VARIATIONFOR A TRAITDifferent traits are present in individuals of the same species

3 DIFFERENTIAL REPRODUCTIVE SUCCESSIn a population, individuals with traits most suited to reproduction in their environment generally leave more offspring than individuals with other traits.

2 HERITABILITYTraits are passed on from parents to their children.

EVOLUTION BY NATURAL SELECTION: A SUMMARY