gene pool- total aggregate of genes in a population
an allele is fixed if all members of a population are homozygous
for that allele
usually there are two or more alleles for a gene in a population
Hardy-Weinberg theorem- frequencies of alleles and genotypes in a
population's gene pool remain constant from generation to generation
generalized Hardy-Weinberg equation- use p to represent the
frequency of one allele and q to represent the frequency of the second
allele
p+q=1 (100%)
p2+2pq+q2=1
p2 represents the frequency of homozygotes (of the first
allele)
2pq represents the frequency of heterozygotes
q2 represents the frequency of homozygotes (of the second
allele)
Changes in gene pool- microevolution
Causes of microevolution
genetic drift- changes in the gene pool of a small population due
to chance
examples of situations that can lead to genetic drift-
bottleneck effect- occurs when a population is reduced in size , usually
reducing overall genetic variability in a population
founder effect- can occur when a few individuals colonize a new habitat
gene flow- genetic exchange due to migration of individuals
between populations
mutations- changes in DNA, mutation that is transmitted in gametes
can immediately change the gene pool by substituting one allele for
another
natural selection- differential success in reproduction
Sources of genetic variation
variation within populations- can be in quantitative or discrete
characters
morphs- contrasting forms (with respect to a discrete
character) in a population
variation between populations- example: cline= graded change in
some trait along a geographic axis
genetic variation due to mutation
sexual recombination- trading of information between homologous
chromosomes, random segregation of chromosomes in meiosis
Preservation of genetic variation
diploidy- hides some genetic variation from selection
balanced polymorphism- ability of natural selection to maintain
diversity in a population
neutral variation
Modes of natural selection
stabilizing selection- acts against extreme phenotypes
directional selection- shifts frequency curve for variation in a
phenotypic character in one direction
diversifying selection- favors variants of opposite extremes over
intermediate individuals
Speciation
biological species concept- defines a species as a population or
group of populations whose members have the potential to interbreed with
one another in nature to produce viable, fertile offspring, but who cannot
produce viable, fertile offspring with members of another species
Barriers to reproduction between species
prezygotic- before formation of zygote (fertilized egg)
habitat isolation- two species that live in different
habitats within the same area may not encounter each other
behavioral isolation- signals and behaviors that attract
mates are unique to a species
temporal isolation- species differ in time of day or
season in which they breed
mechanical isolation- anatomic incompatibility
gametic isolation- gametes meet but don't fuse to form a
zygote
sympatric- intrinsic factors alter gene flow, sympatric
populations become genetically isolated, e.g. due to polyploidy
(autopolyploid or allopolyploid)