Maintaining VariationMaintaining Variation Sexual selection Balanced polymorphism.
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Transcript of Maintaining VariationMaintaining Variation Sexual selection Balanced polymorphism.
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