Chapter 6 Evolution and Ecology 4/9/2013 11:24:00 PM
Trophy Hunting and Inadvertent Evolution: A Case Study
Trophy hunting removes the largest and strongest individuals from a population, preventing
populations from recovering.
Harvesting of species may also affect sex determination in some species.
What Is Evolution?
Biological evolution is the change in organisms over time.
Evolution includes the relatively small fluctuations that occur continually within populations, as
when the genetic makeup of a population changes from one year to the next.
Evolution is allele frequency change
Genes are composed of DNA, and they specify how to encode proteins.
A given gene can have two or more forms (known as alleles) that result in the production of different
versions of the protein that the gene encodes.
The genotype of an individual can be designated by the letters that represent the individual’s two
copies of each gene.
Evolution can be determined as change over time in the frequencies of different alleles in a
In scientific studies, researchers often use an approach based on the Hardy-Weinberg equation to test
where a population is evolving at one or more genes.
Evolution is descent with modification
Populations accumulate differences over time, and hence, when a new species forms, it differs from
its ancestors in a relatively small number of ways.
A new species not only differs from its ancestors, but also resembles its ancestors, because it
descended from then and continues to share many characteristics with them.
Darwin proposed that populations accumulate differences over time primarily by natural selection,
the process by which individuals with certain heritable characteristics survive and reproduce more
successfully than other individuals because of those characteristics.
Darwin argued that if two populations experience different environmental conditions, individuals
with one set of characteristics may be favored by natural selection in one population, while
individuals with a different set of characteristics may be favored in the other population.
By favoring individuals with different heritable characteristics in different populations, natural
selection can cause populations to diverge genetically from one another over time; thus
accumulating more and more genetic differences.
Populations evolve, individuals do not
Natural selection acts as a sorting process, favoring individuals with some heritable traits over
Natural selection ensures that allele frequencies will change over time, causing the population to
Individuals do not evolve, they either r have traits favored by selection or they don’t.
Mechanisms of Evolution
Mutation generates the raw material for evolution
Individuals in populations differ from one another in their observable characteristics, or phenotype.
Many aspects of an organisms phenotype are influenced by its genotype.
Different alleles arise from mutations, caused by events that damage DNA.
Individuals in a population also differ genetically because of recombination, the production of
offspring that have combinations of alleles that differ from those in either of the parents.
Mutations occur too rarely to be a direct cause of significant allele frequency change over short
periods of time.
Natural selection increases the frequencies of advantageous alleles
Natural selection occurs when individuals with particular heritable traits consistently leave more
offspring than do individuals with other heritable traits.
Natural selection can be categorized into three types:
o Directional selection occurs when individuals with one extreme of a heritable phenotypic
trait are favored over other individuals.
o In stabilizing selection, individuals with an intermediate phenotype are favored.
o In disruptive selection, individuals with a phenotype at either extreme are favored.
Some individuals will always have heritable phenotypes that give then an advantage in survival or
reproduction, causing them to leave more offspring than other individuals.
Genetic drift results from chance events
When chance events determine which alleles are passed from one generation to the next, genetic
drift is said to occur.
Genetic drift has four related effects on small populations:
o Because it acts by chance alone, genetic drift can cause allele frequencies to fluctuate
randomly in small populations over time. When this occurs, some alleles eventually
disappear from the population, while other reach fixation.
o By causing alleles to be lost from a population, genetic drift reduces the genetic variation of
the population, making the individuals within the population more genetically similar to one
o Genetic drift can increase the frequency of a harmful allele. If a population size is very small,
and an allele has only slightly deleterious effects, genetic drift can overrule the effects of
natural selection, causing the harmful allele to increase or decrease in frequency by chance
o Genetic drift can increase genetic differences between populations because chance events
may cause an allele to reach fixation in one population, yet be lost from another population.
A loss of genetic variation in a population can reduce the capacity of a population to evolve in
response to changing environmental conditions, potentially placing it at risk of extinction, as well as
an increase in the frequency of harmful alleles of any particular gene from one generation to the
Gene flow is the transfer of alleles between populations
Gene flow occurs when alleles are transferred from one population to another via the movement of
individuals or gametes.
Gene flow has two important effects:
o Gene flow tends to make populations more similar to one another genetically.
o Gene flow can introduce new alleles into a population.
Evolutionary change that results in a closer match between the traits of organisms and the conditions
of their environment is an example of adaptive radiation.
Adaptations result from natural selection
Natural selection causes adaptive evolution, which is a process of change in which traits that confer
survival or reproductive advantage tend to increase in frequency over time.
Although gene flow and genetic drift can improve the effectiveness of an adaptation, they can also
do the reverse.
Natural selection is the only evolutionary mechanism that consistently results in adaptive evolution.
Adaptive evolution can occur rapidly
Hundreds of species have altered the timing of key events in their lives in ways that may be a
response to global warming.
Gene flow can limit local adaptation
Gene flow is one of the factors that can limit the extent to which a population is adapted to its local
Adaptations are not perfect
Trophy hunting and inadvertent evolution: a case study. Trophy hunting removes the largest and strongest individuals from a population, preventing populations from recovering. Harvesting of species may also affect sex determination in some species. Biological evolution is the change in organisms over time. Evolution includes the relatively small fluctuations that occur continually within populations, as when the genetic makeup of a population changes from one year to the next. Genes are composed of dna, and they specify how to encode proteins. A given gene can have two or more forms (known as alleles) that result in the production of different versions of the protein that the gene encodes. The genotype of an individual can be designated by the letters that represent the individual"s two copies of each gene. Evolution can be determined as change over time in the frequencies of different alleles in a population.