BIOL 4120: Principles of Ecology
Lecture 2: Adaptation and
Evolution
Dafeng Hui
Office: Harned Hall 320
Phone: 963-5777
Email: dhui@tnstate.edu
Evolutionary Ecology
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2.1 Nature selection as a force of evolution
2.2 Heritability is the feature of Natural
Selection
2.3 Genes are the units of inheritance
2.4 Genetic variation is the ingredient of NS
2.5 Evolution is the outcome of NS resulted
from gene frequency change
2.6 Speciation and mechanisms
Introduction to evolutionary ecology
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Major question in Ecology: What determines
distribution & abundance of species?
Two classes of answers
• Contemporary, local factors (domain of
traditional Ecology, biomes and PPT and
T)
• Historical factors (= evolutionary ones)
 Why different species live in different
environments? (Adaptation)
 E.g., long necked giraffe in savannas of
Africa (widely dispersed, umbrellashaped trees); white coated polar bear
in Arctic (invisible to prey)
2.1 Nature selection as a force
of evolution
What is Darwin’s natural selection?
The differential success (survival
and reproduction) of individuals
within the population that results
from their interaction with their
environment.
“Survival of fitness, elimination of
‘inferior’ individual”
Two conditions (assumptions):
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1. There is variation in populations. Variation is
heritable.
2. In every generation some organisms are more
successful at surviving and reproducing than others.
Survival and reproduction are not random, but are
related to variation among individuals. Organisms
with best characteristics are ‘naturally selected.’
If 2 conditions are met then the population will
change from one generation to the next. Evolution
will occur.
Evidence of natural selection
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Evolution of beak shape in Finches.
Peter and Rosemary Grant’s (and
colleagues) work on Medium Ground
Finches Geospiza fortis.
Natural
selection
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Darwin’s
Finches
Genetic
studies show
all arise from
a single
ancestral
species.
40-ha Daphne
Major island
Rosemary & Peter Grant
Is there a phenotypic variation in
beak size?
Is variation in beak size correlated with
variation in fitness?
In 1978, there was
a severe drought,
small seeds
declined more than
large seeds.
Small beak birds
have difficult to find
seeds, and suffered
heavy mortality,
especially females.
post-drought
pre-drought
Beak size evolves
Conclusion: Nature selection indeed caused evolution in
beak size
Types of natural selection
black bellied seed cracker
beak size
2.2 Heritability is an essential
feature of NS
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Heritability: individual’s characteristics
are passed from one generation to the
next. Measured as proportion of the
variation in a trait in a population that
is due to variation in genes.
Mendel’s genetics and inheritance
Mendle’s Pea experiment
Mendle’s conclusions:
1. Flower color is a heritable trait
2. For each inherited char, an organism has two units, one from each parent
(one each from egg and sperm). The unit may be the same or different.
3. When the two units are different, one is fully expressed, another one has no
noticeable effect of the organism’s outward appearance.
Dominant: the unit is expressed (purple)
Recessive: not expressed (white).
2.3 Genes are the units of
inheritance
What are genes?
Genes: discrete subunit of chromosome,
carry genetic information
Chromosomes: the threadlike structures
where DNA is contained
DNA: Deoxyribonucleric acid. All DNA is
composed of the same 4 nucleotide (ATGC),
differ in sequence.
Alleles: alternate forms of a gene (A, a).
Locus: the position of an allele occupies on a
chromosome
Homozygous (AA, aa) vs heterozygous (Aa)
Dominance (A vs a), incomplete dominance
(Aa shows different trait to AA or aa)
How are genes transmitted?
Genotype: the sum of
genes carried by the
individual.
Gene pool: total
collection of genes
across all individual in
the population at any
one time
Phenotype: the
observed expression
of genotype (color etc)
Phenotypic plasticity
Phenotypic plasticity: the ability of a genotype to give a range of
phenotypic expressions under different environmental conditions.
Norm of reaction
The set of phenotype across a range of environmental conditions.
2.4 Genetic variation is the
ingredient for Natural Selection
Genetic variation within a population is
absolutely necessary for natural selection to
occur
If all individuals are identical within a
population then their fitness will all be the
same
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Sources of genetic variation
• Mutation: inheritable changes in a gene or a chromosome
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Gene mutation: (point mutation)
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Chromosome mutation
• deletion, duplication, inversion, translocation
• Genetic recombination
Sexual reproduction
two individuals produce haploid gametes (egg or sperm) – that
combine to form a diploid cell or zygote.
• Reassortment of genes provided by two parents in the
offspring
• Increases dramatically the variation within a population by
creating new combinations of existing genes.
Asexual reproduction: less variation (only mutation)
What do we mean by
genetic variation?
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Range (variance) of phenotypes, as in
Darwin’s Finch example
Different chromosomal arrangements
(cytogenetics)
DNA sequence differences among
individuals
Electrophoresis--> electromorphs =
allozymes
Molecular marks: RFLP, RAPD, etc.
2.5 Evolution is a change in gene
frequency
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Evolution is a change of gene
frequencies within a population (or
species) over time
• Individuals do not evolve, populations
evolve
• Focus on gene pool, collective
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But why do we see populations are still
the ‘same’ over many generations?
The Hardy-Weinberg Principle
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The Hardy-Weinberg Principle:
Gene frequencies will remain the same in successive
generations of a sexually reproducing population if
the following five conditions hold:
• Random mating
• There is no mutation
• The population is very large
• There is no selection
• There is no migrations (isolated from other
populations)
The Hardy-Weinberg Principle
Five Causes of evolution
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Mutations
Gene flow - Emigration and immigration of
individuals (Flow of alleles)
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Genetic Drift – Changes in the gene pool of
a small population due to chance
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Nonrandom mating: (AA mates AA, Aa)
Natural selection
One Example: Genetic drift in a small population
2.6 Speciation and mechanisms
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Speciation: splitting of one species into 2
different species.
Concept of species:
• Biological Species: a species is a group of
organisms whose individual have the potential
to interbreed and produce fertile offspring.
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Reproductively isolated: don’t produce fertile
hybrids
Natural conditions: artificial breeding doesn’t
count. For example, artificial insemination,
keeping 2 species locked up together.
• Morphological species: members of the same
species look similar to each other. Many
examples of organisms that look similar but
can’t produce fertile offspring.
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• No one species concept applies to all
organisms
Flickers
Genetic isolation mechanisms
(reproductive barriers)
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Premating mechanisms
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Habitat selection; temporal isolation; behavior or
mechanical or structure incompatibility
Postmating mechanisms
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Genetic barrier such as hybrids are sterile; seed
abortion; hybrid inviability.
Mechanisms of speciation
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Several are well documented:
• Allopatric speciation (probably most
vertebrates)
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allopatric species occupy areas separated by
time or space.
• Sympatric speciation (especially plants and
insects)
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Sympatric species occupy the same place at the
same time
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Allopatric Speciation
• Geographic isolation, Easy to understand.
Allopatric
speciation:
Ring species
salamander
San Joaquin
Valley, CA
Speciation via geographic isolation and divergence
Sympatric speciation
(e.g: Speciation by Polyploidy)
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About half of all flowering plants are polyploid:
more than 2 copies of each gene.
Polyploids are the result of failure of cell division
(mitosis or meiosis) to separate the chromosomes
into 2 cells.
2.4 Adaptations reflect trade-offs
and constraints
Adaptation: Individuals of a species
have certain characteristics that
enable an organism to thrive in a
given environment.
Adaptations maintain or increase
fitness of an organism in a given
environmental conditions.
Tradeoff
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Evolution is reflected in changes in
gene frequencies and phenotypes
However, changes in gene
frequencies involve tradeoffs
Giraffes do not graze well on grass
due to their long necks
Snow leopards can hide in snowy
mountains, ordinary leopards can
hide well in green trees.
Constraint
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There are many other
factors that can influence
species adaptation.
For example, response of
an organism to an
environmental gradient
such as temperature.
All these factors influence
species adaptation.
The End
Brief history of integration of
Genetics into Ecological studies
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Natural Selection—Darwin (1859) The Origin of Species
Particulate genetics & inheritance—Gregor Mendel (1856-1864)
Mutations & chromosomes—Hugo Devries & others (1901)--sources of variation
in populations; rediscovery of Mendel’s work
“The Modern Synthesis” (Dobzhansky, Wright, Fisher, Haldane, Mayr, Simpson-1930s & 1940s)
• Integration Natural Selection & mutation; genetic drift; migration
• Appreciation of genetic variation within populations in nature
DNA structure/importance elucidated by Watson & Crick (1953) double helix
structure of DNA
Molecular variation in natural populations (Harris; Lewontin & Hubby 1966)-using starch gel electrophoresis  molecular biology.
Synthesis of Ecology with Genetics --> Evolutionary Ecology (starting in 1970s)!
Evolution by Natural Selection
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The major factors affecting the organisms’
survival in the system is Fitness
Darwinian fitness: ability of an organism to
survive and reproduce in its environment.
Adaptation is a characteristic or trait of an
organism that increases its fitness relative to
individuals that do not possess it. It is an
inherited characteristic that increases the ability
of an organism to survive and reproduce.
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The traits selected for by natural selection need to be inheritable
so that the next generation after selection retains the change
Three types of selection
• Directional
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See finches
• Stabilizing
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Long necks in giraffes
• Disruptive
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Sexual dimorphism
Fig. 5-6 An example of
microevolution – directional
selection
Summary
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What is natural selection?
Different types of NS
Gene and genetic variation
Evolution
Hardy-Weinberg principle
Concept of species.
Speciation and mechanisms
Adaptation and trade-offs
What is a species
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Ecology is based on the idea that we can identify
different groups of organisms within an ecosystem
These groups are generally called species
There are three specific means of defining a species
• Morphological species concept
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A species is defined as a morphologically consistent group of
organisms than can be distinguished from all other species
• Can fail. So called cryptic species
• Biological species concept
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A group of populations whose individuals can interbreed and
produce fertile offspring and cannot interbreed with other species
Reproductive isolation
• Still fails. If you cannot tell the individuals apart morphologically, how
can you tell if they are interbreeding or not
• Also, some species can interbreed and produce viable offspring
 Bontebok and Blesbok in South Africa
• Genetic species concept
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A group of populations whose individuals have a distinct genetic
makeup and who do not interbreed with others groups of
populations for some reason
• Bontebok and Blesbok are genetically distinct as well as being
morphologically different. Do not naturally overlap in range
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Geographical isolates
Salamanders
Adaptations of Honeycreepers on the island of Hawaii
Recap
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What’s Nature selection? 2
assumptions?
Three types of natural selection
Concept in genetics: gene,
chromosome, DNA, allele, locus,
genotype, phenotype, dominance,
incomplete dominance etc