Ecology & evolution
Chapter 2
“Nothing makes sense except in the light of evolution”
Theodosius Dobzhansky
The evolutionary play on the ecological theater
 Developed by Hutchinson in 1965
 Recognizes that evolution occurs within ecosystems
Natural selection
 Two conditions necessary:
 Variation in a heritable character
 Variation results in differences in survival and reproduction
 From Darwin:
 “Among those individuals that do reproduce, some will leave more offspring than
others. These individuals are considered more fit that the others because they
contribute the most to the next generation. Organisms that leave few or no
offspring contribute little or nothing to the succeeding generations are so are
considered less fit.” - Fitness
Natural selection in action
 Variation in bill size
During dry years….
Population shift
Types of natural selection
 Directional selection
 Stabilizing selection
 Disruptive selection
Heritability
 Mendel provided the information on how characters were passed from parent to
offspring
 Genes are the unit of inheritance
Genetics review
Individuals have 2 alleles for each gene
A population has a frequency of alelles
Hardy Weinberg Equation
 Original proportions of genotypes in a population will remain constant from
generation to generation
Evolution = Change in allele frequencies over time
Species concept
 What is a species?
The biological species concept (BSC) emphasizes reproductive isolation
 Ernst Mayr 1942
 A biological species is defined as a population or group of populations whose
members have the potential to interbreed and produce viable, fertile offspring but
cannot do so with members of other species
 The species is the largest unit of population in which gene flow is possible
 It is defined by reproductive isolation from other species in natural environments
(hybrids may be possible in the lab or in zoos)
Other species concepts
Gene pools of biological species are isolated by pre- and post-zygotic barriers
 Any factor that impedes two species from producing viable, fertile offspring
contributes to reproductive isolation
 Various barriers classified by whether they function before or after zygote formation:
 Pre-zygotic barriers
 Post-zygotic barriers
Mechanisms of speciation
 Allopatric speciation
 Founder or quantum speciation
 Sympatric speciation
 Parapatric speciation
Speciation
 Very difficult to see speciation in action due to long timescale
 Geographic variation in species give clues to speciation processes
 Cline
 Ecotype
 Geographic isolates
Cline
 Measurable, gradual change in a character over a geographic region
 Usually associated with an environmental gradient
Ecotype
 Changes with abrupt changes or step clines
Geographic isolates
 Can be classified as subspecies
Ring species
 Populations encircle unsuitable habitat
Tradeoffs
 Are very important to consider in ecology and evolution
 Successful traits in one environment limit success in another environment
Stickleback evolution & speciation
 Sympatric pairs of species of three-spine stickleback inhabit small low-elevation lakes
in coastal British Columbia
 In lakes with 2 species: large-bodied benthic species & small bodied limnetic species
are present.
 In lakes with 1 species, form is intermediate
 Slight differences in morphology when sampling limnetic versus benthic forms
Within species variation
Phenotypic differences are heritable
Feeding trials
 Foraging success related to morphological features
 Tradeoff
Speciation
 How do you think differences in morphology, diet and habitat of the two phenotypic
forms within the population of fish might lead to reproductive isolation and eventual
speciation?