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CP Biology Chapter 11
11.1 Genetic variation within populations
Genetic variation in a population increases the chance that some individuals
will survive
A population of organisms may have a wide range of phenotypes. Imagine a large
population of frogs. They might all look the same at first, but look closer and you
will see that they differ in leg length, tongue shape, skin color, body size, and many
other traits.
The more variation in phenotypes, the more likely that some individuals can survive
in a changing environment. For example, imagine that during most summers, greenskinned frogs are easily hidden in the green plants around a pond. But if there is a
dry summer, brown-skinned frogs might have an advantage and be better hidden in
the dry grasses.
A population with a lot of genetic variation likely has a wide range of phenotypes.
Genetic variation is stored in a population’s gene pool. A gene pool is made up of all
of the alleles of all of the individuals in a population. Each allele in a gene pool exists
at a certain rate, or frequency. An allele frequency is a measure of how common an
allele is in a population.
Genetic variation comes from several sources
Genetic variation comes from two main sources: mutation and recombination
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Mutation A mutation is a random change in DNA. This change can result in
a new allele. If the mutation happens in a cell that makes gametes – sperm or
eggs – then the mutation can be passed on to offspring. Mutations increase
the genetic variation in a gene pool.
Recombination New combinations of alleles form during meiosis – the type
of cell division that forms gametes. When gametes are made, each parent’s
alleles are arrange in a new way. This can result in many different genetic
combinations.
Some scientists are also researching other possible sources of genetic variation. One
of these other possible sources is called hybridization. This occurs when organisms
breed with organisms of another closely-related species, resulting in new
combinations of alleles.
11.3 Other mechanisms of evolution
Gene flow is the movement of alleles between populations
In addition to natural selection, other factors can lead to the evolution of
populations. For example, when an organism joins a new population, and
reproduces, its alleles become part of that population’s gene pool. The movement of
alleles from one population to another is called gene flow.
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For many animals, gene flow occurs when individuals move between populations.
As shown in the map, a young bald eagle might mate and reproduce 2500 kilometers
from where it hatched. Gene flow also occurs in plant and fungi populations when
seeds or spores are spread to new areas.
Genetic drift is a change in allele frequencies due to chance
Imagine you have a huge bag full of hundreds of candies – exactly half are mints and
half are lemon drops. If you randomly grab 50 candies, it is likely that they will be
about half mints and half lemon drops. If you reach into the bag and just pick out a
few candies, you might get exactly half mints and half lemon drops. But, just by
chance, you might also get all lemon drops, or all mints, or another combination –
different from half and half.
With the larger “population” of 50 candies, it is likely that both types of candy are
equally represented. But with the small sample size of just a few candies, it is more
likely that they few candies will not represent the proportions of the whole bag.
Allele frequencies in small populations can also be affected by chance. Changes in
allele frequencies that are due to chance are called genetic drift. Two processes
commonly cause populations to become small enough for genetic drift to occur: the
bottleneck effect and the founder effect.
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The bottleneck effect is genetic drift that occurs after a population has been
greatly reduced in size. A natural disaster, for example, can leave only a few
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survivors of a population. The few survivors do not represent the genetic
diversity of the original population
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The founder effect is genetic drift that occurs after a small number of
individuals begin to live in a new area.
Genetic drift can cause problems for populations because it results in a loss of
genetic variation. Small populations have only a small amount of the genetic
variation from the original larger population. With little genetic variation, a
population is less likely to have some individuals that are able to survive a changing
environment.
Sexual selection occurs when certain traits increase mating success
Some traits help an organism find a mate. Sexual selection occurs when a trait
increases mating success, and gets passed on to the next generation. These traits
can be very showy – like bright coloring or large antlers. Research suggests that
some showy traits may be linked with genes for good health and fertility.
11.4 Hardy-Weinberg equilibrium
Hardy-Weinberg equilibrium describes populations that are not evolving
Biologists often use models to learn more bout complex situations. Hardy-Weinberg
equilibrium is a model that can be used to study how a population is changing. It
was developed in 1908 and is named after the two people who developed it. Here,
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equilibrium refers to genotype frequencies that stay the same from generation to
generation. If genotype frequencies of a population stay in equilibrium then the
population is not evolving. Populations that meet five conditions listed below are
said to be Hardy-Weinberg equilibrium
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Very large population No genetic drift can occur
No emigration or immigration no gene flow can occur
No mutations no new alleles can be added to the gene pool
Random mating no sexual selection can occur
No natural selection all traits must equally help in survival
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Real populations rarely meet all five conditions. But the model is a useful tool for
understanding changes in populations. Real data can be compared with data
predicted by the model.
11.5 speciation through isolation
The isolation of populations can lead to speciation
The more gene flow there is between two populations, the more similar the
populations will be because they will have similar alleles in their gene pools. But if
there is very little gene flow between two populations, these populations are likely
to become genetically different as changes in the gene pools add up over time.
If gene flow between two population stops, the populations are said to be isolated.
Over time, isolated populations become more and more genetically different.
Members of the two populations may begin to look and behave differently from one
another.
These changes can result in reproductive isolation, which is when members of
different populations can no longer mate successfully. Members of the two
populations might not be physically able to make with each other, or they might not
produce offspring that can survive and reproduce. Reproductive isolation is the last
step in speciation, the process of one species becoming two or more separate
species.
Populations can become isolated in several ways
Several things can prevent mating between populations, leading to reproductive
isolation. Things that prevent mating are called barriers, because they block
mating. Three types of barriers to mating are behavioral, geographic, and temporal.
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Behavioral barriers result in behavioral isolation, isolation caused by
differences in courtship and mating behaviors. Over 2000 species of fireflies
are isolated in this way. Each species produces a different pattern of flashes
that attracts mates of their own species.
Geographic, or physical, barriers result in geographic isolation, such as wen
a river or mountain divides a population into two or more groups. For
example, populations of snapping shrimp have become isolated by a strip of
land, called the Isthmus of Panama, that formed between the Atlantic and
Pacific Oceans. If shrimp from different sides are placed together, they will
not mate.
Temporal barriers result in temporal isolation, when timing prevents
reproduction between populations. Reproduction periods may change to a
different time of year or day. For example, two pine tree species in California
are separated in this way. One species sheds its pollen in February, while the
other sheds its pollen in April.
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