Download Natural selection works directly on the expression or appearance of

7. The frequency of an allele in a gene pool of a population depends on many factors and may be stable or
unstable over time. As a basis for understanding this concept:
a. Students know why natural selection acts on the phenotype rather than the genotype of an organism.
Natural selection works directly on the expression or appearance of an inherited trait, the phenotype, rather than
on the gene combination that produces that trait, the genotype. The influence of a dominant allele for a trait over
a recessive one in the genotype determines the resulting phenotype on which natural selection acts.
b. Students know why alleles that are lethal in a homozygous individual may be carried in a heterozygote and
thus maintained in a gene pool. Two types of allele pairings can occur in the genotype: homozygous (pairing
two of the same alleles, whether dominant, codominant, or recessive) and heterozygous (pairing of two different
alleles). Recessive lethal alleles (e.g., Tay-Sachs disease) will, by definition, cause the death of only the
homozygous recessive individual. Healthy heterozygous individuals will also contribute the masked recessive
gene to the population’s gene pool, allowing the gene to persist.
c. Students know new mutations are constantly being generated in a gene pool.
Mutation is an important source of genetic variation within a gene pool. These random changes take the form of
additions, deletions, and substitutions of nucleotides and of rearrangements of chromosomes. The effect of
many mutations is minor and neutral, being neither favorable nor unfavorable to survival and reproduction.
Other mutations may be beneficial or harmful. The important principle is that culling, or selective breeding,
cannot eliminate genetic diseases or unwanted traits from a population. The trait constantly reappears in the
population in the form of new, spontaneous mutations.
d. Students know variation within a species increases the likelihood that at least some members of a species
will survive under changed environmental conditions.
As environmental factors change, natural selection of adaptive traits must also be realigned. Variation within a
species stemming either from mutation or from genetic recombination broadens the opportunity for that species
to adapt to change, increasing the probability that at least some members of the species will be suitably adapted
to the new conditions. Genetic diversity promotes survival of a species should the environment change
significantly, and sameness can mean vulnerability that could lead to extinction.
Notes:
"Nothing in biology makes sense except in the light of evolution." Theodosius Dobzhansky
The History of Evolution
By the 1800's a number of scientists came to the realization that species could change, and that this change had
occurred throughout earth's history. But the fossil record did not indicate how or why one species could evolve into
one or a number of other species (adaptive radiation).
Jean Baptiste de Lamarck proposed that changes acquired by an organism during its lifetime could be passed on
to its offspring. This hypothesis is called the inheritance of acquired characteristics.
Charles Darwin and Alfred Russel Wallace. Both these naturalists came up with an acceptable mechanism for
evolution. Both men had traveled around the world collecting data and samples which convinced them of the
veracity of evolution. And both presented papers to the Royal Society in London outlining similar mechanisms for
evolution. Since it was known that Darwin had spent 20 years working on his theory, he was given the majority of
the credit for the theory now known as Natural Selection. Darwin's book On the Origin of Species By Means of
Natural Selection .was published in 1859 and overnight changed the course of Biological studies forever.
Natural selection, like all credible scientific hypotheses can be tested, and has been tested successfully many times.
Natural selection is actually a group of postulates (or hypotheses) which are summarized below.
1.
2.
3.
4.
Variations (controlled by genes) exist within every population
Some variations are more advantageous for survival and reproduction than others.
Resources (food, shelter, etc.) can not grow as quickly as the reproduction rate of most organisms.
Over time, offspring of organisms best fit to the local environment will make up a larger proportion of the
population.
Each of these proposals have been tested and refined. Evolution is not in question. Defined as the change in the
frequency of genes within a population over time, evolution is merely an observation which most scientists accept
as fact.
Natural selection, which is the theory Darwin proposed to explain evolution, is not a simple fact and so can never
be proven like a mathematical theorem can. Even today some of the basic premises forming the theory of natural
selection have been refined or modified to reflect new data and observations.
Evolution is the most important concept in biology if not the most controversial in some circles. Biologists today
have abundant evidence for evolution. Evolution is accepted as part of science because its concepts are useful.
Evolutionary theory generates testable predictions about the biology of organisms.
Genetic Equilibrium
"Natural Selection acts only upon the phenotype of an organism."
Genes that are not expressed -- recessive genes in the heterozygous condition -- are hidden from the effects of
Natural Selection. Many harmful traits -- controlled by recessive alleles can remain indefinitely in the gene pool of a
large population.
Since Natural Selection acts only on the phenotype variability is maintained in the population. When Natural
Selection is operating in a population it actually tends to reduce variation by removing individuals (and their genes)
which exhibit extreme phenotypes (very tall, very small etc.).
Natural Selection can cause evolution only when something upsets the dynamic equilibrium in a population and
genes which once were not favored help individuals within a population adapt to the new conditions.
In order to understand now this can come about we need to define 3 terms: gene pool, allele frequency, and genetic
equilibrium.
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A gene pool is all the genes in all the individuals of a population.
Allele frequency is the proportion of each allele in the gene pool
Genetic equilibrium occurs when allele frequencies in a population remain the same from one generation
to the next.
Changes in Genetic Equilibrium (Microevolution)
What are the mechanisms that cause the genetic equilibrium to change? The mechanisms include:
Mutation -- provides the variations that can be acted upon by natural selection. Even though mutations are:
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relatively rare
rarely beneficial
often recessive and hidden
Mutations are the only source of new genetic material added to a gene pool.
In a species all other mechanisms of evolution merely shuffle the genetic material that is already present. These
include:
Genetic recombination during cross over events in meiosis and the shuffling of genes caused by the random
pairing of gametes during fertilization can bring about a change in gene frequencies.
Genetic drift results in changes in allele frequencies because of random (chance) fluctuations possible in small
populations. Flip a coin 4 times and it's possible that it could come up heads every time. It is much less possible
(actually impossible) that a coin tossed 1000 times would always come up heads. The larger the sample size the
more likely the expected ratio (500 heads: 500 tails)
Gene flow occurs when individuals from different populations of the same species migrate from one group to
another. Since genes are carried along with the organisms the gene pool must increase or decrease in size.
Gene flow between different species although once thought to be impossible, then rare, is now acknowledged as a
major source of change in gene frequency, especially in prokaryotes
Variation in Natural Selection
When the frequencies of genes change by whatever mechanism Natural Selection can operate on the individuals to
establish a new genetic equilibrium in one of the following ways:
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Stabilizing Selection
Directional Selection
Disruptive Selection
Stabilizing Selection favors the average individual in a population. Variation at the extremes is selected against by
opposing biotic or abiotic factors in the environment. An example might be if an organism is too small to compete
for a mate or too large to hide from a predator. Only those organisms of average size are least vulnerable.
Directional selection. If one of the opposing forces in the environment dominates selection occurs favoring genes
which help the population overcome the new pressure until genetic equilibrium is reestablished. An example is a
pesticide (such as DDT) which kills 99% of a mosquito population. The population of the one percent that survives
quickly increases along with the gene or genes which helped them overcome the toxic effects of the pesticide.
Disruptive selection. In some cases the average condition becomes the target of environmental pressure.
Individuals at either extreme are better able to cope with the change and their genes increase in frequency.