Ecology Lecture No. 6: Evolution & Ecology
Thursday September 27 , 2012
Case Study #1 (Trophy Hunting & Inadvertent Evolution):
-Trophy hunting removes the largest and strongest males—the ones that would sire many healthy
offspring. 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. Among other species, African elephants are
poached for ivory; the proportion of the population that have tusks is decreasing.
-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.
Introduction To Evolution:
-Evolution can be viewed as genetic change over time or as a process of descent with modification.
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 its ancestors. But the new species has many of the same characteristics as its ancestors,
and resembles them.
Mechanisms Of Evolution:
-Natural selection, genetic drift, and gene flow can cause allele frequencies in a population to change
-A phenotype is an observable characteristic that is determined by the genotype. Individuals differ from
one another in part because they have different alleles for genes.
-Different alleles arise by mutation or a change in DNA. Mutations can result from copying errors during
cell division, mechanical damage, exposure to chemicals (mutagens) or high-energy radiation. The
formation of new alleles is critical to evolution. 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. In a generation, one mutation would occur in every 10,000 to
1,000,000 copies of a gene. In one generation, mutation acting alone causes virtually no change in allele
frequencies of a population. Recombination:
-Recombination also produces different genotypes within a population. Offspring have combinations of
alleles that differ from those of their parents.
Types Of Natural Selection:
-There are three types of natural selection:
-Directional selection: Individuals at one phenotypic extreme are favored.
-Stabilizing selection: Individuals with an intermediate phenotype are favored.
-Disruptive selection: Individuals at both phenotypic extremes are favored.
-Genetic drift occurs when chance events determine which alleles are passed to the next generation. It
is significant only for small populations.
The Effects & Consequences Of Genetic Drift:
-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. Loss of genetic variation
reduces the ability of the population to respond to changing environmental conditions.
3. Frequency of harmful alleles can increase, if the alleles have only mildly deleterious effects. Increase
of harmful alleles can reduce survival and reproduction.
4. Differences between populations can increase. These effects are important for species that are near
Gene Flow & Its Effects:
-Gene flow describes when alleles move between populations via movement of individuals or gametes.
Gene flow has two effects:
1. Populations become more similar.
2. New alleles can be introduced into a population. Natural Selection & Adaptation: