Download Ch. 16 The Evolution of Populations and Speciation

Ch. 16 The Evolution of
Populations and Speciation
Background Information
• Genetics- the study of heredity; how traits are
passed from parents to offspring
• Allele- an alternative form of a gene;
– Ex: alleles for tallness in pea plants
• T - tall;
• t - short;
• Genotype- the genetic makeup of an organism for a
trait;
– Ex: Tt
• Phenotype- the physical make-up of an organism,
what the organism physically looks like;
– Ex: Tall
Genetic Equilibrium
• Population Genetics- the study of evolution from a
genetic point of view.
– Bell curve- useful tool used to show variation in traits or
characteristics w/in a population of a species.
• some variations are caused by the environment; heredity; both
usually play a role population variations.
– Variations arise from:
» 1. Mutation- flawed copies of genes.
» 2. Recombination- reassociation of genes.
» 3. Random fusion of gametes- the game of chance of gamete fusion
– Gene pool- used to describe the total genetic information
available in a population.
• allows allele frequencies to be predicted if present alleles w/in
population are known; predict genotypes for future generations.
Allele Frequency- divide the # of a certain allele by the total # of
ALL alleles in the population.
-Ex: 12/16= .75 (‘R’ frequency)
4/16= .25 (‘r’ frequency)
Phenotype Frequency- the # of indiv. w/ a particular phenotype
divided by the total # of indiv. in the population.
-Ex: 1/8= 0.125 (white four o’clock flowers)
2/8= 0.25 (pink four o’clock)
5/8= 0.625 (red four o’clock)
• Wilhelm Weinberg (German physician) and Godfrey
Hardy (British mathematician)
– wanted to answer what happened to allele frequencies
over generations.
– independently they showed that allele frequencies will
remain the same w/in a population over multiple
generations as long as there are no outside influences on
the population.
• Their idea is based upon an ideal hypothetical population that is
not evolving.
Hardy-Weinberg Genetic Equilibrium
1.
2.
3.
4.
5.
NO net mutations occur.
Individuals neither enter nor leave the population.
The population is large (ideally, infinitely large).
Individuals mate randomly.
Selection does not occur.
True genetic equilibrium is a theoretical state.
“Hardy-Weinberg genetic equilibrium allows us to consider
what forces disrupt equilibrium.”
(Modern Biology © 2009)
*any change in the 5 conditions can result in evolution*
Disruption of Genetic Equilibrium
1.) Mutation
2.) Migration
Immigration
Migration
3.) Genetic Drift
4.) Nonrandom Mating
5.) Natural Selection
Stabilizing Selection
Directional Selection
Disruptive Selection
Sexual Selection
1. Mutation- will cause frequency changes by creating
new alleles to be introduced into the population.
2. Migration- will cause frequency changes because
the population will not remain constant.
• Immigration- individuals move into a population.
• Emigration- individuals move out of a population.
– can cause gene flow- the process of genes moving from 1 population to
another
3. Genetic Drift- allele frequencies are controlled by
random events or chance.
• can cause extinction of a species because a lack of variation of
alleles for specific traits; becomes a serious issue for small
populations.
– the failure of even 1 organism to reproduce or 1 to reproduce above
norm can extensively disrupt allele frequency.
4. Nonrandom Mating- species select mates according
to similarities in physical traits.
• called assortative mating.
• usually influenced by geographical proximity.
– Random mating would indicate mating w/out regard to genetic make-up
» random mating can affect the combination of alleles in a population
but will not affect allele frequencies.
5. Natural Selection- individuals selecting mates
according to favorable traits
» single most disruptive factor to genetic equilibrium.
A. Stabilizing Selection- individuals w/ the average form of
a trait have the highest fitness.
– the average represents the optimum for most traits, the extreme forms of
most traits relinquish lower fitness for those individuals that have them.
» Selection against the extremes reduces # of organisms w/ the trait.
– Most common type of selection.
B. Directional Selection- individuals w/ an extreme form of
the trait have greater fitness than those individuals w/ the
average form of the trait.
– Usually will be dictated by an environmental change; coevolution
C. Disruptive Selection- individuals w/ either extreme
variation of a trait are more fit than individuals w/ the
average form of the trait.
D. Sexual Selection- females choosing male mates based
upon certain traits.
– Genes of successful reproducers not of those that merely survive are
amplified through natural selection.
Formation of Species
• Speciation- the formation of a new species
• Morphology- the study of the internal and external structure
and form of an organism.
• was used as the chief criterion for species classification, but had some
limitations; it means there can be NO phenotypic differences w/in a
population.
• The biological species concept arose when biologists
recognized that organisms which looked like different
species interbred and produced fertile offspring.
• Proposed by American Biologist Ernest Mayr it says a species is a
population that can successfully interbreed but not w/ other groups.
• reasonable idea for sexually reproducing modern organisms; cannot be
tested on extinct species nor is it useful in organisms that reproduce
asexually.
Isolating Mechanisms
Speciation begins w/ isolation because 2 parts of a
formerly interbreeding population quit interbreeding
• Geographic Isolation- physical separation of
members of a population.
» Habitat could become divided
• once subpopulations become isolated then gene flow ceases
between them.
• Reproductive Isolation- genetic isolation even w/out
geographic isolation.
» May arise from disruptive selection.
– Prezygotic isolation- occurs before fertilization
– Postzygotic isolation- occurs after fertilization
• Prezygotic Isolation
• These are mechanisms that prevent mating of organisms.
» Incompatible behavior- different mating calls
» Different mating times- spring/summer
• Postzygotic Isolation
• These are occurrences that happen after fertilization.
» Offspring may die or not develop completely
» Offspring may not be fertile therefore adults wasted their gametes
• Rates of Speciation
– Speciation may occur gradually overtime or quickly.
• Gradual- species evolve gradually overtime.
• Punctuated equilibrium- species arise abruptly; a sudden shift in
form.