Ecology Notes – Sept. 27/12
- 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 and
permits for a large trophy ram cost over $100,000, but trophy hunting still has negative effects.
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, and the average size
of males and their horns decreased over 30 years of study. This is also being observed in other
species. African elephants are poached for ivory and 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 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 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. When evolution occurs, both descent (shared
ancestry, resulting in shared characteristics) and modification (the accumulation of differences) can
be observed. This can be seen in stickleback fish fossils. The lakebed they’re in is layered, allowing
the age of the fossils to be determined. The more modern fish are mostly the same as the ancestors,
but with some modification. The pelvic bone gradually became smaller over a period of 5000 years.
This is an evolutionary process. Given time, ancestral structures can be changed.
- Charles Darwin used the phrase descent with modification. He proposed that populations change
over time through natural selection, the process by which individuals with certain heritable traits
survive and reproduce more successfully than other individuals.
- 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. Phenotype is determined by genes
and the environment. Different alleles arise by mutation, which is change in the DNA of a gene.
Mutations can results from copying errors during cell division, mechanical damage, exposure to
chemicals (mutagens), or high-energy radiation. 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 each 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. Mutation is a weak agent of allele
frequency change, but it is still very important for evolution because it provides the raw material
(new alleles) on which natural selection and other mechanisms can act. Recombination also produces
different genotypes within a population. Offspring have combinations of alleles that differ from those
of their parents.
- There are three types of natural selection: 1. Directional selection – Individuals at one phenotypic
extreme (ex. large size) are favoured over other individuals (small and medium size). An example of
this was when drought favoured large beak size in medium ground finches. 2. Stabilizing selection –
Individuals with an intermediate phenotype are favoured. An example of this is parasitic wasps select
against small gall size of goldenrod plants where Eurosta fly larvae are located inside and birds select
against large gall size. Fly larvae inside intermediate-sized galls survived at the highest rates. 3. Disruptive selection – Individuals at both phenotypic extremes are favoured. An example of this is
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 to.
- Genetic drift occurs when chance events determine which alleles are passed to the next generation. It
is significant only for small populations. It is random. 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, others 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 effects. 4. Genetic differences between populations can increase (an allele
might reach fixation in one population, but be lost from another). Effects 2 and 3 can have dire
consequences. Loss of genetic variation reduces the ability of the population to respond to changing
environmental conditions. An increase of harmful alleles can reduce survival and reproduction.
These effects are important for species that are near extinction.
- The greater prairie chicken populations in Illinois have been reduced by loss of habitat to farmland.
In 1993, the population was less than 50. DNA from this population compared with museum
specimens from the 1930s showed a decrease in genetic variation. 50% of eggs failed to hatch,
suggesting fixation of harmful alleles. This example shows the negative effects of genetic drift on
small populations. When greater prairie chickens from other populations were brought to Illinois,
new alleles entered the Illinois population and egg hatching rates increased.
- Gene flow: Alleles move between populations via movement of individuals or gametes (pollen). Gene
flow has two effects – 1. Populations become more similar. 2. New alleles can be introduced into a
population. In the 1960s, new alleles that provide resistance to insecticides arose by mutation in
mosquitoes in Africa or Asia. Mosquitos with the new alleles were blown by winds or transported by
humans to new locations. The allele frequency increases rapidly in populations exposed to
- Natural selection is the only evolutionary mechanism that consistently causes adaptive evolution.
Adaptations are features of organisms that improve their ability to survive and reproduce in their
environment. Natural selection is not a random process. By consistently favouring individuals with
certain alleles, natural selection causes adaptive evolution, which is a process of change in which
traits that confer survival or reproductive advantages tend to increase in frequency over time. Gene
flow and genetic drift can improve the effectiveness of an adaptation (by increasing the frequency of
an advantageous allele), but they can also do the reverse (by increasing the frequency of a
disadvantageous allele). Thus, natural selection is the only evolutionary mechanisms that
consistently results in adaptive evolution. Example of adaptive evolution – Soapberry bugs feed on
fruits by piercing them with a needle-like beak. Feeding is most efficient if beak size matches fruit
size. In populations with different food sources, Carroll and Boyd (1992) predicted that beak size
would evolve to adapt to fruits of introduced tree species. Bugs in Florida that still feed on the native
host have beak lengths similar to the historical average. Beak length is a heritable trait, so the
observed changes must have been due to changes in allele frequencies. The goldenrain tree was
introduced to Florida about 35 years ago, so natural selection caused adaptive evolution in a
relatively short time.
- There are many example of rapid adaptive evolution: antibiotic resistance in bacteria, insecticide
resistance in insects, drab coloration in