Maintaining VariationMaintaining Variation

Sexual selection

Balanced polymorphism

Sexual SelectionSexual Selection Most species have distinctively male and

female phenotypes: ____________________

One of the sexes is often larger, more colorful, more aggressive

Sexual selection is based on any trait that gives the individual a competitive edge in: _______________ ___________________________________________

Females are usually the agents of selection when they pick their mates

Courtship varies partially due to the varying parental involvement of males and females

Sexual SelectionSexual Selection

Figure 17.12, page 274

Balanced PolymorphismBalanced Polymorphism With _______________________________ ,

two or more alleles of a gene persist at high frequencies in a population.

Balancing selection occurs when conditions favor heterozygotes

Hb gene: codes for hemoglobin (oxygen-transporting protein in the blood) HbA : _______________ HbS : ________________

Causes: ___________________________

Balanced PolymorphismBalanced Polymorphism Mosquitoes transmit Plasmodium,

parasite which causes malaria

Humans that are homozygous for sickle-cell anemia develop the disease and die at an early age

However, individuals with alleles for both normal (HbA) and sickle-cell (HbS) have the greatest chances of surviving malaria

a) Distribution of malaria cases in 1920’sb) Distribution of people who carry the sickle-cell alleleFigure 17.13, page 275

Balanced PolymorphismBalanced Polymorphism

Genetic Drift: Chance ChangesGenetic Drift: Chance Changes Genetic drift: the random fluctuation in

allele frequencies over time, due to chance occurrences alone

More significant in small populations ___________________ helps explain the difference

Increases the chance of any given allele becoming more or less prevalent when the number of individuals in a population is small

_______________ means that one kind of allele remains at a specified locus in a population

Occurs when all are homozygous for one allele

Genetic DriftGenetic Drift

Effect of population size on genetic drift

Figure 17.14, page 276

Bottlenecks and the Founder Bottlenecks and the Founder EffectEffect

Genetic drift is most pronounced in small or inbred populations

_________________ : some stressful situation greatly reduces the size of a population, leaving a few (typical or atypical) individuals to reestablish the population

_________________________: a few individuals (carrying genes that may or may not be typical of the whole population) leave the original population to establish a new one

Genes of those individuals are the only ones that get passed on

Founder EffectFounder Effect

Figure 17.15, page 277

Inbred PopulationsInbred Populations

Inbreeding: nonrandom mating among closely related individuals

Tends to increase the homozygous condition, thus leading to lower fitness and survival rates

Human populations that remain isolated and inbred may concentrate disease-causing alleles

Old Order Amish in Lancaster County, PA

Gene FlowGene Flow Genes move with individuals when they

move out or into a population

______________: individuals leave

______________: individuals move in

Gene flow: movement of alleles among populations

Helps keep populations of same species _________

Physical flow and resultant shuffling tends to minimize genetic variation between populations

Species and SpeciationSpecies and Speciation How can we define a species?

Morphological species concept:

Species: Latin word for ________ – its simplest interpretation

Attempting to determine whether similar yet different animals are the same species by appearance (phenotype) is not reliable due to the subtle variations that are displayed

Biological Species ConceptBiological Species Concept The biological species concept uses

______________ to define relatedness of species

Developed by biologist Ernst Myer

“Species are groups of interbreeding natural populations that are reproductively isolated from other such groups”

As good as it is, this definition does not work so well for organisms that are non-sexually reproducing and those only known from fossils

Only applies locally

SpeciationSpeciation

New species arise by the evolutionary process of ______________, which begins as gene flow ends between populations

Difficult to classify a new species – how can you define the exact moment which made it significantly different than predecessors?

Reproductive IsolationReproductive Isolation Individuals of a sexually reproducing species

can produce fertile offspring, but are _________________ ________________

Reproductive isolation occurs when gene flow between populations stops

Reproductive isolating ________________ begin as two populations diverge from one another:

Heritable features of body form, function, or behavior that prevent interbreeding between genetically divergent populations

Divergences may lead to new species

Reproductive IsolationReproductive Isolation

Prezygotic: prevent successful pollination or mating (______________ the zygote)

Temporal

Mechanical

Behavioral

Ecological

Gamete incompatibility

Postzygotic: result in offspring which are weak or infertile hybrids (___________the zygote)

Prezygotic MechanismsPrezygotic Mechanisms

Prezygotic isolating mechanisms take effect before or during fertilization

______________ isolation: different groups may not be reproductively mature at the same season, month, or year (for example, periodical cicadas)

_______________ isolation: Two populations are mechanically isolated when differences in reproductive organs prevent successful interbreeding

Example: floral arrangements in sage plants discriminate between different bee pollination

Prezygotic MechanismsPrezygotic Mechanisms _____________ isolation: patterns of courtship may be

altered to the extent that sexual union is not achieved

Ex: albatross courtship rituals

______________ isolation: potential mates may be in the same general area, but not in the same habitat where they are likely to meet

Ex: different species of manzanita shrubs live at different altitudes and habitats

__________________: incompatibilities between egg and sperm prevent fertilization

Ex: signals to pollen grains to begin growing toward egg

Postzygotic MechanismsPostzygotic Mechanisms

______________ or ______________

Sometimes fertilization does occur between different species, but the hybrid embryo is weak and dies

In some instances, the hybrids are vigorous but sterile

Example: a mule, produced by a male donkey and a female horse

Reproductive Reproductive Isolating Isolating MechanismsMechanisms

Figure 17.17, pg. 278

Allopatric SpeciationAllopatric Speciation

Genetic changes leading to a new species usually begin with physical separation

_____________ speciation: refers to the “different lands” occupied by two species

In this model, a _________________separates two populations, ending the gene flow between them

Most common way new species form

Reproductive isolating mechanisms evolve in the genetically diverging populations and will result in complete speciation when the two species can no longer interbreed

Allopatric SpeciationAllopatric Speciation

The pace of geographic isolation can vary depending upon the organism’s means of travel

Isolation may be sudden, such as an earthquake that results in the separation of species

Glaciers may produce conditions that yield separate species over vast periods of time

ArchipelagosArchipelagos

An _________________ is an island chain some distance away from a continent

Example: the islands of ________ , which formed less than 1 million years ago

The islands’ isolation is enough to favor divergence, but not enough to stop the occasional colonizers

While the islands of Hawaii make up just 0.01 percent of the world’s land mass, 40 percent of the 1,450 species of Drosophila species arose there