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WEB TUTORIAL 17.1
Agents of Change
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Section 17.3 Five Agents of Microevolution, p. 274
Introduction
Microevolution can be defined as a change of allele frequencies in a population
over a relatively short period of time. Natural selection is but one of the agents of
microevolution. In this tutorial we’ll explore natural selection as well as four other
factors that can change the allelic frequencies of a population: mutation, gene flow,
genetic drift, and nonrandom mating.
Learning Objectives
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Know how different factors can change allelic frequencies in a population.
Understand why natural selection is generally considered to be the most
powerful force in evolution.
Narration
Mutations
A mutation is a permanent change in a cell's DNA. Such mutations in the sex cells
are heritable, meaning they can be passed on to future generations. Mutations are
fairly rare. Still, mutation is critical to evolution because it provides the only means
for creating new genetic information.
A point mutation is a nucleotide sequence error often resulting from incorrect
insertion of a single base during DNA replication.
Mutations can also be more dramatic. For example, a section of a chromosome
may be duplicated, resulting in a longer chromosome containing additional DNA.
Likewise, a section of a chromosome may be deleted, resulting in a loss of DNA
and an incomplete chromosome.
Most mutations that arise do nothing or are harmful to an organism. However, a
few mutations occur that are adaptive, and contribute new genetic information to a
population.
Gene Flow
Gene flow is the movement of alleles between populations. This movement occurs
when individuals from one population migrate into the territory of another population with a different gene pool.
Plants called California tarweeds are found throughout California. Although divided into many local groups, the species forms a single population. Gene flow occurs
between the groups, so no group has a gene pool separate from the rest. At some
time in the past, one or a few of these tarweeds somehow got to the Hawaiian
Islands, over 3,900 kilometers away, possibly as a seed stuck to a bird's foot or
feathers. Once in Hawaii, the plant reproduced and formed a new population.
The great distance separating Hawaii and North America prevented most plants or
seeds from traveling from one to the other, making gene flow between the populations nonexistent. Therefore, the gene pools of the two groups were reproductively isolated, and evolution of the mainland tarweeds and the Hawaiian tarweeds proceeded separately. As new mutations arose among the Hawaiian tarweeds, those
alleles that helped them to adapt to and survive in their new home were selected
for. Eventually, new species of plants found only in Hawaii, including the
Hawaiian silversword, arose from the ancestral population derived from the
California tarweeds.
Genetic Drift
Genetic drift is a change in allele frequencies that can occur by chance in a small
population. Consider a hypothetical population of 10,000 penguins, in which 1
individual in 10 carries a given allele. A natural disaster strikes and the population
loses half its members, including 550 individuals who carried the allele. The frequency of the allele in the population thus drops from 10% to 9%, but this is a
small effect on allele frequency—and no alleles are lost.
Now consider a population of 10 with the same allele frequency of 1-in-10. There
is now but a single carrier of the allele. If this population likewise loses half its
members, and the one member of the population who carried the allele is not a survivor, the frequency of the allele in the population drops from 10% to zero. This
allele can now only be replaced by mutation (unlikely), or by migration from
another population.
This is an example of genetic drift: the chance alteration of allele frequencies in a
population, with such alterations having the greatest impact on small populations.
Sexual Selection
Sexual selection occurs when some members of a population mate more often than
other members.
In practice, this is mating based on phenotype, which is any observable trait in an
organism, including differences in appearance and behavior. It is the behavior of
strutting and chest-swelling in a male sage grouse that causes a succession of
female grouse to mate with it rather than nearby competitors. It's easy to see that if
one male mates four times as much as the average male of his generation, his alleles stand to increase proportionately in the next generation.
Differential mating success among members of one sex in a species often is based
on choices made by members of the opposite sex in that species.
Natural Selection
Some individuals may have traits that give them a better "fit" to their environment,
and consequently greater success at survival, than other members of their population. The individuals more likely to survive long enough to reproduce are more
likely to pass on their traits. Thus the traits that aid in survival are the ones most
likely to be passed on to the next generation. This phenomenon is known as natural selection.
Natural selection is the only agent of microevolution that consistently acts to adapt
populations of organisms to their environment. Because of this fact, natural selection is generally regarded as the most powerful force in evolution.
Sometimes evolution through natural selection can be observed over just a few
years. Such an example was provided when a drought occurred in 1977 on Daphne
Major, one of the Galápagos Islands. A large part of the island's population of the
finch Geospiza fortis died during this drought.
In 1978 it was observed that the average beak depth of the finches that survived the
drought was greater than the average beak depth that had been measured in a survey conducted before the drought in 1976. Individuals with the larger beaks were
better able to crack open the large, tough seeds that were available during the
drought. Finches with smaller beaks starved during the drought from lack of an
accessible food source. The offspring of the survivors also were observed to have
a larger average beak size than the population before the drought. Thus, the population had evolved through natural selection over the course of just a few short
years.
You should now be able to…
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List and define the five agents of microevolution.
Explain why mutation is unique among all the agents of microevolution.
Discuss in evolutionary terms why the California tarweeds are considered
to be the same species but are considered distinct from the Hawaiian tarweeds.
Use allele frequencies to demonstrate the effects of genetic drift in small
and large populations.