Genetic Diversity
Biology/Env S 204
Spring 2009
Genetic diversity
• Heritable variation within and between
populations of organisms
• Encoded in the sequence of 4 basepairs that make up DNA
• Arises by mutations in genes
and chromosomes
Genetic Diversity
• Very small fraction of genetic
diversity is outwardly expressed
• Estimated 109 different genes across
the Earth’s biota
• Represents a largely untapped genetic
library
Genetic Diversity
• Genetic diversity is the foundation for
all higher levels of biodiversity
• Genetic diversity provides the recipe
for populations and species, which in
turn form communities and ecosystems
• Genetic variation enables evolutionary
change and artificial selection
Genetic Diversity
• Genetic diversity may have direct economic
value (genes for disease resistance,
biologically active compounds)
• But effective conservation for whatever
purpose depends upon accurate, thoughtful
assessment of genetic diversity
• Preservation of genetic diversity is usually
a high priority in conservation programs
Nature of Genetic Diversity
• Information for all of life
stored in the structure
of DNA
• Genetic code or the units
(bases, nucleotides) that
make up DNA are
essentially universal
Nature of Genetic Diversity
• A length of DNA (often with extra
proteins) is a chromosome
• A section of DNA along the
chromosome that contains the
information to make a protein (or
RNA) is a gene
Chromosome structure
gene
gene
gene
gene
chromosome
DNA Structure
A
T
T
G
C
T
G
G
A
C
A
T
T
A
A
C
G
A
C
C
T
G
T
A
Bases:
A = adenine
T = thymine
C = cytosine
G = guanine
Nature of Genetic Diversity
• A gene may be several hundred to up
to about two thousand units (bases)
long
• A gene contains the information to
make a protein or RNA
• A gene is a discrete unit of
hereditary information
Nature of Genetic Diversity
gene
gene
protein (enzymes, membranes, etc.)
RNA (essential for production
of proteins)
Nature of Genetic Diversity
A given gene may have more than one form;
the different forms of a single gene are
called alleles.
flower color
gene with
two alleles
Nature of Genetic Diversity
flower color
Nature of Genetic Diversity
or
flower color
gene
Homozygous
(both alleles in an
individual are the same)
Heterozygous
(two different alleles
present in an individual
for one gene)
Nature of Genetic Diversity
Prokaryotes vs. Eukaryotes
bacteria,
archaebacteria
protists, fungi,
plants, animals
Prokaryotes
• One-celled, no compartments (no
nucleus)
• Genetic material in a single, circular
chromosome
• Therefore only 1 copy of each gene
per bacterium
• A typical bacterium has 1,000-2,000
genes
Eukaryotes
• One-celled or many-celled, with
compartments (e.g., a nucleus is present)
• Genetic material in two to many linear,
separate chromosomes in the nucleus
• Normally two copies of each gene present
in an individual in part of the life cycle
• A eukaryote has about 50,000 genes on
average
Origin of Genetic Diversity
• Mutation = change in the sequence of
bases of DNA along a chromosome
• Change in a base or chunks of DNA
can be rearranged
• Mutations can occur anywhere along a
chromosome
• This is the ultimate source of all
genetic variation
Measuring Genetic Diversity
• Chromosome = a collection of genes
plus “extra” DNA in between that
doesn’t code for anything
• Genes are used in measuring genetic
diversity but
• The “extra” DNA is free to change
and is also useful in assessing genetic
diversity
Measuring Genetic Diversity
• Different parts of the DNA evolve at
different rates—”extra” DNA changes
faster than DNA in the genes
• Some genes evolve slowly and help in
the study of deep branches of life
(ancient lineages)
• “Extra” DNA can change so rapidly
that every individual is distinct
(except for clones)
Measuring Genetic Diversity
• Within an individual: %
heterozygosity (alleles same or
different in a given set of genes)
• Among individuals in a population:
allele frequencies for given genes
• Between populations: %
heterozygosity, allele frequencies,
unique molecular markers
Measuring Genetic Diversity
Measuring diversity between
evolutionary lineages usually
involves comparing sequences of
DNA and looking for changes in
bases or major rearrangments.
Measuring Genetic Diversity
Loss of genetic diversity = loss of
useful genetic diversity in the
short term and reduction of
evolutionary options in the long
term.
Evolutionary Processes
1) Natural Selection
2) Gene Flow
3) Genetic Drift
Evolutionary Processes—
1) Natural Selection
• A major mechanism of evolution as proposed
by Darwin
• A filter for genetic variation: the best
adapted individuals survive and reproduce in
greater numbers over time
• Not a directed process!
• Changes in direction and intensity depend
on conditions and time span and available
genetic diversity
Evolutionary Processes—
1) Natural Selection
SURVIVOR
http://science.discovery.com/interactives/literacy/darwin/darwin.html
Evolutionary Processes—
2) Gene Flow
• The exchange of genetic material
within a population, between
populations of a species, and even
between species
• Gene flow among populations of a
species maintains the integrity of the
species
• Lack of gene flow can lead to
speciation
Evolutionary Processes—
2) Gene Flow
Population A
Species A
A
gene
flow
barrier
arises
Population B
B
time
reproductive
isolation
Species B
Evolutionary Processes—
2) Gene Flow
Species A
allopatric speciation =
gene
flow
Species B
geographic isolation
+
reproductive isolation
Evolutionary processes—
2) Gene Flow
Species A
(AA)
X
Species B
(BB)
Hybrid AB
(infertile,
cannot cross
with either
parent either)
Evolutionary processes—
2) Gene Flow
Hybrid AB
Chromosome
doubling
AABB (now sex
cells can be
produced!)
AA
X
AABB
sex cell A
sex cell AB
AAB (infertile)
Evolutionary Processes—
2) Gene Flow
sympatric speciation =
reproductive isolation of
parent species from
hybrid derivatives through
hybridization and
chromosome doubling
without geographic isolation
Evolutionary Processes—
3) Genetic Drift
• Changes in the gene pool of a small
population due to chance events
• Founder effect = one or two
individuals disperse and start a new
population with limited genetic
diversity
• Bottleneck = extreme reduction in
population size and therefore genetic
diversity
Conservation Genetics
• Involves the use of genetic data
and principles to guide
conservation activities
• Genetics should be prominent in
the practice of conservation
Conservation Genetics
1) Rate of evolutionary change in a
population is proportional to the
amount of genetic diversity available
2) Higher genetic diversity is usually
positively related to fitness
3) Global pool of genetic diversity
represents all of the information for
all biological processes (= genetic
library)
Conservation Genetics
Small populations tend to lose
genetic diversity over time!!!
Conservation Genetics
• Habitat fragmentation and destruction
now produce and will continue to
produce small, isolated populations
• Understanding the genetic status of
species and populations and the
consequences of small population sizes
is vital to conservation, management,
and recovery efforts.
Conservation Genetics
A major goal is to preserve
natural patterns of genetic
diversity to the extent possible
to preserve options for future
evolutionary change.
greater prairie chicken example
Conservation Genetics—
Example: Prairie Chickens
• 35-year study of a remnant population of
prairie chickens in Illinois
• In 1962, about 2,000 individuals present;
in 1994, fewer than 50
• Fertility and hatching rates declined
significantly, as did genetic diversity
• Translocation program established in 1992
to bring in birds from MN, KS and NE
Conservation Genetics—
Example: Prairie Chicken
• By 1994, increased survival of young
prairie chickens was verified
• By 1997, there were significant
increases in mean rates of fertility
and hatching
• Once the main population in Illinois
became isolated, it began to lose
viability and without intervention, it
most likely would have disappeared