Unit 3- Populations
3.1-Testing for Plasticity and Adaptation
Chapter 4 pages 90–93, 98–100
Phenotype: is a product of the interaction between an organism’s genes; organism's
observable characteristics or traits: such as its morphology or development
Genotype: genetic makeup of a cell, an organism, or an individual
common garden study: representatives from different populations that had distinctive
growth forms, which he called ecotypes, were grown under the same environmental
conditions in a common location. If plasticity was the only cause of the variation in
morphology in a species, then you would expect that plants grown in a common garden
would look the same
ecotypes: genetically distinct geographic variety, population or race within species (or
among closely related), which is adapted to specific environmental conditions.
genetic drift: change in gene frequencies in a population due to chance or random events
local adaptation: a population survives best in its home environment. If the genetic
differences are the result of adaptation, then the growth of each plant in its home
environment should be better than in the other environments.
reciprocal transplant experiment: used to determine if genetically differentiated
populations are adapted to their home environments.
Heritability: the proportion of total phenotypic variation in a trait attributable to genetic
variation; determines the potential for evolutionary change in a trait
homozygous: having identical alleles at a given locus
heterozygotes: having different alleles at a given locus
Phenotypic Variation in a Desert Lizard
- Ted Case (1976) explored variation in body size among Sauromalus (lizard
known at Chuckwalla which lives in hot, dry places) populations at twelve sites
distributed across its geographic range, and found that average summer
temperatures at his desert study sites ranged from 23.8 C to 35 C, while average
annual rainfall varied from approx. 35 to 194 mm. Because the environments in
which Sauromalus lives vary greatly across its range, we might expect that
selection have favored different characteristic in different parts of the species’
range. Prefers to eat herbaceous plants and the amount of winter rainfall largely
determines the amount of plant growth in these desert environments therefore
higher rainfall means more available food.
- Variation in rainfall translates into variations in food availability. Lizards at lower
elevations on average have access to less food and the amount available on any given year is unpredictable.
- This case found that lizards from the food-rich higher elevations are
approximately 25% longer than those from lower elevations therefore different
body mass. Therefore the best predictor of body length with these lizards is
average winter rainfall.
- Christopher Tracy (1999) collected 12 to 15 juvenile Chuckwallas from 6
populations in Arizona, California, and Nevada, living at elevations ranging from
200 to 890m. He then raised these lizards under identical environmental
conditions in a laboratory to determine the contributions of environmental versus
genetic factors to size differences among this population. He concluded that
lizards from higher elevations grew to a larger size, approximating in a lab
common garden for lizards the pattern of variation in body size found in the field.
Genetic Variation and Heritability
- i2 an equation variability can be defined as:
h =V GV Pwhich is genetic variance and phenotypic variance
- sub dividing V gPves an equation of:
h =V G(V +G ) E
- Peter Boag and Peter Grant (1978) the latter formerly a professor at Mcgill,
estimated bill width in the Galapagos finch (geospiza fortis) to have a heritability
of 0.95. By comparison they estimated that bill length in the species has a
heritability of 0.62.
Calculating Gene Frequencies
- Chia-Chen Tan (1946), Chia-Chen Tan and Ju-Chi Li (1934), and Theodosius
Dobzhansky (1937) determined that the variation in colour patterns shown by
Harmonia (Asian lady beetles) is due to the effects of more than a dozen
alternative alleles for colour pattern
- Hardy-Weinberg Principle states that in a population mating at random in the
absence of evolutionary forces, allele frequencies will remain constant. To
maintain constant allele frequencies in a population:
• Random mating
• No mutations
• Large population size
• No immigration
• All genotypes have equal fitness
- A form of natural selection that favours twos or more extreme phenotypes over
the average phenotype in a population. Acclimation/plasticity or adaptation/evolution?
• Organisms can optimize their performance three key ways: (1) acclimation or
(2) dispersal or range shifts, and
(3) evolution (adaptation) or extinction.
• The first and third ways involve expressing a new phenotype to suit a new
• Two things to keep in mind are that acclimation and plasticity represent the
capacity of an individual to adjust to environmental change within their own
lifetime, and it occurs very quickly, whereas adaptation is an evolutionary
response of a population that occurs from generation to generation and therefore
takes more time.
Testing for developmental plasticity
1) Phenotype = Genotype × Environment
• One genotype (or clone) can be grown in different environments. If phenotypic
differences are observed between environments then plasticity can be implicated
as the cause of that phenotypic variation.
2) P = G × E ↑ Held constant
• French botanist, Gaston Bonnier, observed in his early studies on variability in
plant populations in Europe (Bonnier 1890). On his travels in various mountain
ranges he observed that many species of plants grew over a wide range of
elevations, and differed dramatically in their form in different locations. These
observations prompted him to conduct transplant experiments on the effects of
climate on plant morphology. • Bonnier was able to produce genetically identical clones of a single plant, and
transplant one into a lowland site and the other into an alpine site. He then made
measurements and drawings comparing each pair of plants. These results show
that the phenotypic differences between these plants were caused by
developmental plasticity; a single genotype was able to produce morphologically
different phenotypes in response to the environment.
Testing for genetic variability among populations
• Gote Turesson (1925) extended this type of research farther and looked for
evidence for genetic variability among populations. He used a common garden
study in which representatives from different populations that had distinctive
growth forms, which he called ecotypes, were grown under the same
environmental conditions in a common location. If the morphological differences
persisted when grown in the same environment, this would be evidence for
genetic differences among populations.
1) P = G × E ↑ Held constant
• It is important to realize that change in gene frequencies in a population can be
due to chance or random events (i.e., genetic drift) and mutation, or it can be the
product of natural selection and therefore adaptive.
Testing for adaptation
• To demonstrate adaptation, you would have to show that:
(1) phenotypic differences among populations are based on genetic differences,
using a common garden study.
(2) each population survives best in its home environment (local adaptation)
when compared to others derived from other environments. A reciprocal
transplant experiment can be used to determine if genetically differentiated
populations are adapted to their home environments.
(1) Common garden comparison: If plasticity was the only cause of the variation in
morphology in a species, then you would expect that plants grown in a common garden
would look the same (2) Local adaptation (reciprocal transplant) comparison: If the genetic differences are the
result of adaptation, then the growth of each plant in its home environment should be
better than in the other environments.
“Unintentional” experiments to test for adaptation
• An unintentional experiment occurred when three new species of plants in the
Sapindaceae family were brought to the United States in the 1950s and were
planted farther north than the existing species’ range. Soapberry bugs (Jadera
haemotaloma) feed exclusively on plants in this family. They have piercing
mouthparts (beaks) that they insert into the fruits to feed, and the length of the
beak must be a certain length to reach the seeds within the fruit.
• Scott Carroll and Christin Boyd (1992) seized the opportunity to ask whether the
soapberry bugs were able to adapt to these new food sources and expand their
range. They first collected eggs from each population of soapberry bug and grew
them in a common garden. Differences in beak length among populations were
maintained in the common garden, suggesting that beak length is genetically
• The researchers also found a positive correlation between beak length and the
radius of fruits each population used, suggesting that each soapberry bug
population had adapted to its plant host. But this was only observational
• Scott Carroll, Stephen Klassen, and Hugh Dingle (1998) continued the study.
They asked whether soapberry bug populations were ecotypes, and answered this
question by doing a reciprocal transplant study and measuring juvenile
survivorship of each population on different plant hosts.
Chapter 4 pages 103–107
Inbreeding: mating between close relatives; more likely in small populations
microsatellite DNA: short repeating units of DNA that can be used identity relatedness
Biological species concept: a group of actually or potentially interbreeding populations,
which are reproductively isolated from other such groups
Isolating Mechanisms: some process that prevents the production of a viable offspring
between two individuals. Isolating mechanisms are critical to the species integrity
Allpatric Speciation: speciation that occurs when isolating mechanisms evolve among
geographically separated populations
Parapatric speciation: speciation that occurs when a population expands into a new habitat-type within the pre-existing range of the parent species
Sympatric speciation: speciation that occurs when isolation mechanisms evolve among
populations with overlapping geographic ranges
Assorative mating: mating among phenotypically similar (positive assortative mating),
or dissimilar (negative assortative mating), individuals
Parallel Evolution: the independent evolution of similar traits in geographically
Genetic Diversity and Butterfly Extinctions
- Richard Frankham and Katherine Ralls (1998) point out that one of the
contributors to higher extinction rats in small populations may be inbreeding.
Combining already low genetic variations in small populations with a high rate of
inbreeding has several negative impacts on populations, including reduced
fecundity, lower juvenile survival, shortened life span and it further accelerates
the loss of genetic diversity especially in Plantago lanceolata and Veronica spicata
that act as hosts for the Glanville fritillary butterfly, Malitaea cinxia.
- Ilik Saccheri and colleagues (1998) reported one of the first studies giving direct
evidence that inbreeding contributes to extinctions in wild populations. Studied
dry meadows. Documented an average of 200 extinctions and 114 colonization’s.
Conducted genetic studies on populations of Melitaea in 42 meadows estimating
heterozygosity and indicator of genetic variability with respect to seven enzyme
systems and one locus of nuclear microsatellite DNA. Results of the study
indicated that influence of inbreeding on the probability of extinction was very
significant. High inbreeding=probability of extinction.
Speciation: what is a species?
-Ernst Mayr (1942) defined species as “groups of actually or potentially interbreeding
populations, which are reproductively isolated from other such groups.” Based upon a
real ecological concept: reproductive isolation
Speciation: what is reproductive isolation?
- isolating mechanisms can be categorized into two groups, pre and postzygotic.
Prezygotic isolating mechanisms are processes which prevent two individuals
from forming a zygote. Postzygotic isolating mechanisms are equally efficient at
maintaining species integrity, but they occur after a zygote has been formed.
Speciation: what causes speciation?
- in both allopatric and parapatric speciation, the evolution of reproductive isolation
could occur through drift or natural selection. Since genetic drift is random,
fluctuations in allele freguencies in subpopulations will be independent, and thus
loss of genetic diversity through drift could lead to reproductive isolation Reproductive Isolation and Ecological Divergence
- Dan Funk and colleagues (2006) decided to conduct a broad test of this prediction
that increased ecological divergence will be associated with increased
reproductive isolation. Using both parametric and nonparametric tests for
relations they found that the more different the habitats were the more
reproductively isolated the pairs of related species were.
- Mckinnon and colleagues (2004) wanted to know to what extent ecological
difergence plays in the early stages of speciation. What they did was collected fish
from marine and stream populations and placed them in an experiemental
aquarium to see which individuals would and would not mate. Concluded that
reproductive isolation occurred based upon ecological differentiation and not
primarily geographic isolation.
3.3 Distribution and Abundance of Populations
Chapter 10 pages 255–256, 259–267, 269–274
- G. Caughley and his colleagues (1987) found a close relationship between climate
and the distribution of the three largest kangaroos in Australia. One in the eastern
third of the continent of Australia that includes several biomes (temperate forest
grows in the southeast and tropical forests in the north, and mountains with their
varied climates. Another in the southern and western regions of Australia
(temperate woodland and shrubland biome). Third in the savanna and desert.
These limited distributions may not be determined by climate directly but suggest
that climate often influences species distributions through factors such as food
production, water supply and habitat.
- Also studied was the tiger beetle (Cicindela longilabris) that lives at higher
latitudes and high elevations that just any other species of tiger beetle in north
America. Thomas Schultz, Michael Quinlan, and Neil Hadley (1992) set out to
study the environmental physiology of widely separated populations of the
species. Found that metabolic rates are higher and its preferred temperatures
lower than those of most other tiger beetle species that have been studied. None of their measurements differed significantly among populations.
-There are three distributions of patterns:
1. Random Distribution: an individual has an equal probability of occurring
anywhere in an area. Neutral interactions between individuals and local
2. Regular Distribution: individuals are uniformly spaced through the environment.
Antagonistic interactions between individuals or local depletion of resources
3. Clumped Distribution: individuals live in areas of high local abundance which are
separated by areas of low abundance
Organism Size and Popul