LECTURE 6: EVOLUTION AND ECOLOGY
Trophy Hunting and Inadvertent Evolution: A Case Study
Bighorn sheep populations have been reduced by 90% by hunting, habitat loss, and
introduction of cattle. Hunting is now restricted; permits for a large “trophy ram” cost over
Trophy hunting removes the largest and strongest males—the ones that would sire many
In one population, 10% of males were removed by hunting each year, the average size of males
and their horns decreased over 30 years of study.
This is also being observed in other species:
o African elephants are poached for ivory; the proportion of the population that have tusks
o Rock shrimp are all born male, and become females when they are large enough to carry
eggs. Commercial harvesting takes the largest individuals, which are all females.
Genes for switching sex at a smaller size became more common, resulting in more females, but
smaller females lay fewer eggs.
What is Evolution?
Evolution can be viewed as genetic change over time or as a process of descent with
Biological evolution is change in organisms over time
Evolution can be defined more broadly as descent with modification.
As a population accumulates differences over time and a new species forms, it is different from
But the new species has many of the same characteristics as its ancestors, and resembles them
Populations change over time through natural selection:
o Individuals with certain heritable traits survive and reproduce more successfully than
Mechanisms of Evolution
Natural selection, genetic drift, and gene flow can cause allele frequencies in a population to
change over time
Phenotype: Observable characteristics that are determined by the genotype.
Individuals differ from one another in part because they have different alleles for genes
Different alleles arise by mutation: change in DNA
Mutations can result from copying errors during cell division, mechanical damage, exposure to
chemicals (mutagens) or high-energy radiation
Formation of new alleles is critical to evolution
o If mutation did not produce new alleles, all members of a population would have
identical genotypes and evolution could not occur
Mutations are actually very rare
o In a generation, one mutation would occur in every 10,000 to 1,000,000 copies of a gene
o In one generation, mutation acting alone causes virtually no change in allele frequencies
of a population
o Mutations are not a strong source of genetic variation – more like a seed that introduces
it, and other factors such as natural selection or gene flow, act upon it and increase
Types of Natural Selection
Directional selection: Individuals at one phenotypic extreme (e.g., large size) are favored
o Example: Drought favored large beak size in medium ground finches Stabilizing selection: Individuals with an intermediate phenotype are favored.
o Example: Parasitic wasps select for small gall size of Eurosta flies; while birds select for
large gall size
Disruptive selection: Individuals at both phenotypic extremes are favored.
o Example: African seedcrackers (birds) have two food sources—hard seeds that large
beaks are needed to crack, and smaller, softer seeds that smaller beaks are more suited
Genetic drift occurs when chance events determine which alleles are passed to the next
o Only significant for small populations
Genetic drift has four effects on small populations:
1. It acts by chance alone, thus causing allele frequencies to fluctuate at random. Some may
disappear, other may reach 100% frequency (fixation)
2. Because some alleles are lost, genetic variation of the population is reduced
3. Frequency of harmful alleles can increase, if the alleles have only mildly deleterious
4. Differences between populations can increase
Number 2 and 3 can have dire consequences
o Loss of genetic variation reduces the ability of the population to respond to changing
o Increase of harmful alleles can reduce survival and reproduction
o These effects are important for species that are near extinction
Greater prairie chicken populations in Illinois have been reduced by loss of habitat to farmland.
o In 1993, population was less than 50. DNA from this population compared with museum
specimens from the 1930s showed a decrease in genetic variation.
o 50% of eggs failed to hatch, suggesting fixation of harmful alleles
Gene flow: Alleles move between populations via movement of individuals or gametes.
Gene flow has two effects:
1. Populations become more similar – alleles are shared, becomes more uniform
2. New alleles can be introduced into a
In the 1960s, new alleles that provide resistance
to insecticides arose by mutation in mosquitoes
in Africa or Asia
o Mosquitos with the new alleles were
blown by winds or transported by
humans to new locations
o The allele frequency increases rapidly in
populations exposed to insecticides
o Resistance to insecticide – became a very
high selection pressure
Natural selection is the only evolutionary
mechanism that consistently causes adaptive
Adaptations are features of organisms that improve their ability to survive and reproduce
Natural selection is not a random process By consistently favoring individuals with certain alleles, natural selection causes adaptive
evolution – traits that confer advantages tend to increase in frequency over time
Example: Soapberry bugs feed on fruits by piercing them with a needle-like beak.
o Feeding is most efficient if beak size matches fruit size
o In populations with different food sources, Carroll and Boyd (1992) predicted that beak
size would evolve to adapt to fruits of introduced tree species
o Beak length is a heritable trait, so the observ