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Transcript
Chapter 8: Evolution
Lesson 8.3: Microevolution and the Genetics of Populations
Microevolution refers to varieties within a given type. Change happens within a group, but the
descendant is clearly of the same type as the ancestor. This might better be called variation, or
adaptation, but the changes are "horizontal" in effect, not "vertical." Such changes might be
accomplished by "natural selection," in which a trait within the present variety is selected as the best for
a given set of conditions, or accomplished by "artificial selection," such as when dog breeders produce a
new breed of dog.
Lesson Objectives
• Distinguish what is microevolution and how it effects changes in populations.
• Define gene pool, and explain how to calculate allele frequencies.
• State the Hardy-Weinberg theorem
• Identify the five forces of evolution.
Vocabulary
• adaptive radiation
• allele frequency
• directional selection
• disruptive selection
• gene flow
• gene pool
• genetic drift
• Hardy-Weinberg theorem
• macroevolution
• microevolution
• migration
• population genetics
• stabilizing selection
Introduction
Darwin knew that heritable variations are needed for evolution to occur. However, he knew
nothing about Mendel’s laws of genetics. Mendel’s laws were rediscovered in the early 1900s. Only then
could scientists fully understand the process of evolution.
Microevolution is how individual traits within a population change over time. In order for a
population to change, some things must be assumed to be true. In other words, there must be some
sort of process happening that causes microevolution. The five ways alleles within a population change
over time are natural selection, migration (gene flow), mating, mutations, or genetic drift.
262
The Scale of Evolution
We now know that variations of traits are heritable. These variations are determined by
different alleles. We also know that evolution is due to a change in alleles over time. How long a time?
That depends on the scale of evolution.
 Microevolution occurs over a relatively short period of time within a population or species. The
Grants observed this level of evolution in Darwin’s finches which will be discussed later in this
lesson.
 Macroevolution occurs over geologic time above the level of the species. The fossil record reflects
this level of evolution. It results from microevolution taking place over many generations and will be
discussed in the next lesson of this chapter.
Genes in Populations
One common misconception about evolution is the idea that individuals can evolve. Individuals
do not evolve. Their genes do not change over time. Individuals can only accumulate adaptations that
help them survive in the environment. Evolution takes a long time, spanning several generations, to
happen. While it is possible for individuals to mutate and have changes made to their DNA, this does
not mean the individual has evolved. In other words, mutations or adaptations do not equal
evolution. There are no species alive today that have individuals that live long enough to see all of
evolution happen to its species. A new species may diverge from an existing species’ lineage, but this
was a buildup of new traits over a long period of time and did not happen spontaneously in an instant.
So if individuals cannot evolve, then how does evolution happen? Populations can evolve. The
unit of evolution is the population. A population consists of organisms of the same species that live in
the same area and can interbreed. In terms of evolution, the population is assumed to be a relatively
closed group. This means that most mating takes place within the population. Populations of individuals
in the same species have a collective gene pool in which all future offspring will draw their genes
from. This allows natural selection to work on the population and determine which individuals are more
“fit” for their environments. The aim is to increase those favorable traits in the gene pool while weeding
out the ones that not favorable. Natural selection cannot work on a single individual because there are
not competing traits in the individual to choose between. Therefore, only populations can evolve using
the mechanism of natural selection. The science that focuses on evolution within populations is
population genetics. It is a combination of evolutionary theory and Mendelian genetics.
Gene Pool
The genetic makeup of an individual is the individual’s genotype. A population consists of many
genotypes. Altogether, they make up the population’s gene pool. The gene pool consists of all the
available genes of all the members of the population that are able to be passed down from parents to
offspring. For each gene, the gene pool includes all the different alleles for the gene that exist in the
population. The more diversity there is in a population of a species, the larger the gene pool. The gene
pool determines which phenotypes are visible in the population at any given time. For a given gene, the
population is characterized by the frequency of the different alleles in the gene pool.
The gene pool can change in an area due to migration of individuals into or out of a population.
If individuals that have certain traits are the only ones in the population and they emigrate to a different
population, the gene pool shrinks and those traits are no longer available to be passed down to
offspring. If individuals that have different traits immigrates into a population, they increase the gene
pool and a new type of diversity can be seen within the population in that area as they interbreed with
the others who already life there.
The size of the gene pool directly affects the evolutionary trajectory of that population. The
Theory of Evolution states that natural selection acts on a population to favor the desirable traits for
that environment while simultaneously weeding out the unfavorable characteristics. As natural selection
works on a population, the gene pool changes. The favorable adaptations become more plentiful and
the less desirable traits become fewer or even disappear from the gene pool completely.
263
Populations with larger gene pools are more likely to survive as the environment changes than
those with smaller gene pools. Larger populations with more diversity will tend to have at least some
desirable characteristics within the population as the environment changes and requires
new adaptations. A smaller and more homogeneous gene pool puts the population at risk for extinction
if there are few or no individuals have the genes for the traits necessary to survive the change. For
example, in bacteria populations, individuals that are antibiotic resistant are more likely to survive any
sort of medical intervention and will live long enough to reproduce. Therefore, the gene pool has now
changed to include only bacteria that are antibiotic resistant.
Allele Frequencies
Allele frequency or genetic variation is how often an allele occurs in a gene pool relative to the
other alleles for that gene. In genetic variation, the genes of organisms within a population change.
Gene allele frequencies determine genetic variation and the distinct traits that can be passed on from
parents to offspring. Gene variation is important to the process of natural selection. The genetic
variations that arise in a population happen by chance, but the process of natural selection does not.
Natural selection is the result of the interactions between genetic variations in a population and the
environment. The environment determines which variations are more favorable. More favorable traits
are thereby passed on to the population as a whole.
Genetic variation occurs mainly through DNA mutation, gene flow (movement of genes from
one population to another) and sexual reproduction. Due to the fact that environments are unstable,
populations that are genetically variable will be able to adapt to changing situations better than those
that do not contain genetic variation.
Look at the example in Table 8.3. The population in the table has 100 members. In a sexually
reproducing species, each member of the population has two copies of each gene. Therefore, the total
number of copies of each gene in the gene pool is 200. The gene in the example exists in the gene pool
in two forms, alleles A and a. Knowing the genotypes of each population member; we can count the
number of alleles of each type in the gene pool. The table shows how this is done.
Table 8.3: Number of Alleles in a Gene Pool (for one gene with two Alleles, A and a)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Genotype
Number of Individuals
in the Population
with that Genotype
Number of Allele A
Contributed to the
Gene Pool by that
Genotype
Number of Allele a
Contributed to the
Gene Pool by that
Genotype
--------------------------------------------------------------------------------------------------------------------------------------------------------
AA
Aa
aa
Totals
50
40
10
100
50 × 2 = 100
40 × 1 = 40
10 × 0 = 0
140
50 × 0 = 0
40 × 1 = 40
10 × 2 = 20
60
--------------------------------------------------------------------------------------------------------------------------------------------------------
Let the letter p stand for the frequency of allele A. Let the letter q stand for the frequency of allele a. We
can calculate p and q as follows:
• p = number of A alleles/total number of alleles = 140/200 = 0.7
• q = number of a alleles/total number of alleles = 60/200 = 0.3
• Notice that p + q = 1.
Evolution occurs in a population when allele frequencies change over time. What causes allele
frequencies to change? That question was answered by Godfrey Hardy and Wilhelm Weinberg in 1908.
264
Hardy and Weinberg and Microevolution
Charles Darwin’s Theory of Evolution touched briefly on favorable characteristics being passed
down from parents to offspring, but the actual mechanism for that was flawed. Gregor Mendel did not
publish his work until after Darwin's death. Both Hardy and Weinberg understood that natural selection
occurred because of small changes within the genes of the species.
The focus of Hardy's and Weinberg's work was on very small changes at a gene level either due
to chance or other circumstances that changed the gene pool of the population. The frequency at which
certain alleles appeared changed over generations. This change in frequency of the alleles was the
driving force behind evolution at a molecular level, or microevolution.
Since Hardy was a very gifted mathematician, he wanted to find an equation that would predict
allele frequency in populations so he could find the probability of evolution occurring over a number of
generations. Weinberg also independently worked toward the same solution. The Hardy Weinberg
Equilibrium Equation used the frequency of alleles to predict genotypes and track them over
generations.
The Hardy Weinberg Equilibrium Equation
In order for this equation to work, it is assumed that all of the following conditions are not met
at the same time:
1. Mutation at a DNA level is not occurring. Therefore, no new alleles are being created.
2. Natural Selection is not occurring. Thus, all members of the population have an equal chance of
reproducing and passing their genes to the next generation.
3. The population is infinitely large.
4. All members of the population are able to breed and do breed.
5. All mating is totally random. This means that individuals do not choose mates based on
genotype.
6. All individuals produce the same number of offspring.
7. There is no emigration or immigration occurring. In other words, no one is moving into or out of
the population.
The list above describes causes of evolution. If all of these conditions are met at the same time, then
there is no evolution occurring in a population. Since the Hardy Weinberg Equilibrium Equation is used
to predict evolution, a mechanism for evolution must be happening.
However, when all these conditions are met, allele frequencies stay the same. Genotype
frequencies also remain constant. In addition, genotype frequencies can be expressed in terms of allele
frequencies, as Table 8.4 shows.
Table 8.4: Genotype Frequencies in a Hardy-Weinberg Equilibrium Population (for one gene with two
alleles, A and a)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Genotype
Genotype Frequency
--------------------------------------------------------------------------------------------------------------------------------------------------------
AA
Aa
aa
p2
2pq
q2
--------------------------------------------------------------------------------------------------------------------------------------------------------
(p = frequency of A, q =frequency of a): p2 + 2pq + q2 = 1.
Remember in Table 8.3 we saw that p + q = 1
In Table 8.4, if p = 0.4, what is the frequency of the AA genotype using the Hardy-Weinberg Equilibrium
Equation: p2 + 2pq + q2 = 1? See Table 8.5 on the next page for the solution.
265
Table 8.5: Genotype Frequencies of the AA genotype when p = 0.4 using the Hardy-Weinberg
Equilibrium Equation (for one gene with two alleles, A and a)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Genotype
Genotype Frequency
--------------------------------------------------------------------------------------------------------------------------------------------------------
AA
Aa
aa
p2= 0.16 or 16%
2pq
q2
--------------------------------------------------------------------------------------------------------------------------------------------------------
(p = frequency of A, q =frequency of a): p2 + 2pq + q2 = 1.
Remember in Table 8.3 we saw that p + q = 1
CHECK YOUR WORK:
Since p + q = 1 and p= 0.4; q = 0.6
p2 + 2pq + q2 = 1
(0.4)2 + 2(0.4 x 0.6) + (0.6)2 = 1
Summary of what the Hardy-Weinberg Equilibrium Equation shows:
1. p = the frequency or percentage of the dominant allele in decimal format
2. q = the frequency or percentage of the recessive allele in decimal format
3. Since p is the frequency of all dominant alleles (A), it counts all of the homozygous dominant
individuals (AA) and half of the heterozygous individuals (Aa)
4. Likewise, since q is the frequency of all recessive alleles (a), it counts all of the homozygous
recessive individuals (aa) and half of the heterozygous individuals (Aa)
5. Therefore, p2 stands for all homozygous dominant individuals
6. q2 stands for all homozygous recessive individuals
7. 2pq is all heterozygous individuals in a population
8. Everything is set equal to 1 because all individuals in a population equals 100%
For a further explanation of this theorem, see http://khanexercises.appspot.com/video?v=4Kbruik_LOo.
The Hardy-Weinberg Theorem
Today the Hardy-Weinberg Equilibrium Equation is commonly referred to as the HardyWeinberg Theorem and is regarded as the founding principle of population genetics. Its mathematical
equation shows that allele frequencies do not change in a population if certain conditions are met and
the population remains in genetic equilibrium, and under these conditions evolution cannot occur. Such
a population is said to be in Hardy-Weinberg equilibrium. However, if the conditions are not met then
evolution can occur.
Forces of Evolution
The conditions for Hardy-Weinberg equilibrium are unlikely to be met in real populations. The
Hardy-Weinberg theorem also describes populations in which allele frequencies are not changing. By
definition, such populations are not evolving. How does the theorem help us understand evolution in
the real world?
From the theorem, we can infer factors that cause allele frequencies to change. These factors
are the forces of evolution. There are five such forces: natural selection, migration (gene flow), mating,
mutations, and genetic drift.
-Natural Selection
Charles Darwin was not the first scientist to explain evolution, or that species change over time.
However, he gets most of the credit simply because he was the first to publish a mechanism for how
evolution happened. This mechanism is what he called Natural Selection. As time passed, more and
266
more information about natural selection and its different types was discovered. With the discovery of
Genetics by Gregor Mendel, the mechanism of natural selection became even clearer than when Darwin
first proposed it. It is now accepted as fact within the scientific community.
Natural selection is the main mechanism for microevolution. Alleles that produce favorable
adaptations are more likely to be passed down to future generations because they allow those
individuals to live long enough to reproduce. As a result, unfavorable adaptations are eventually bred
out of the population and those alleles are removed from the gene pool. Over time, changes in allele
frequency become more apparent when compared to previous generations.
Below is more information about the types of selection known today.
1. Artificial Selection
Artificial selection is not a type of natural selection, obviously, but it did help Charles Darwin
obtain data for his theory of natural selection. Artificial selection mimics natural selection in that certain
traits are chosen to be passed down to the next generation. However, instead of nature or the
environment in which the species lives being the deciding factor for which traits are favorable and which
are not, it is humans that do the selecting of traits during artificial selection.
Darwin was able to use artificial selection on his birds to show that desirable traits can be
chosen through breeding. This helped back up the data he collected from his trip on the HMS
Beagle through the Galapagos Islands and South America. There, Charles Darwin studied
native finches and noticed those on the Galapagos Islands were very similar to the ones in South
America, but they had unique beak shapes. He performed artificial selection on birds back in England to
show how the traits changed over time.
2. Directional Selection
The first type of natural selection is called directional selection. It derives its name from the
shape of the approximate bell curve that is produced when all individuals' traits are plotted. Instead of
the bell curve falling directly in the middle of the axes on which they are plotted, it skews either to the
left or the right by varying degrees. Hence, it has moved one direction or the other. In other words one
of two extreme phenotypes is selected for or favored over another. This phenomena is usually seen in
environments that have changed over time. Changes in weather, climate, or food availability lead to
directional selection.
Directional selection curves are most often seen when one coloring is favored over another for a
species. This could be to help them blend into an environment, camouflage themselves from predators,
or to mimic another species to trick predators. Other factors that may contribute to one extreme being
selected for over the other include the amount and type of food available.
Human interaction can speed up directional selection. Hunters most often kill the bigger
individuals of the population for their meat or other large ornamental or useful parts. Therefore, the
population tends to skew toward the smaller individuals. This causes the directional selection bell curve
to shift to the left. Similarly, many slower individuals in a prey population are killed and eaten, meaning
directional selection in this case would skew toward the faster individuals.
The population bell curve shifts either farther left or farther right due to directional selection.
However, the height of the bell curve does not change. There are far fewer "average" individuals in this
population.
267
--Example of Directional Selection:
Charles Darwin studied what was to become known as directional selection while he was in the
Galapagos Islands. The beak length of the Galapagos finches changed over time due to available food
sources. When there was a lack of insects to eat, finches with larger and deeper beaks survived because
they could crack seeds. Over time, as insects became more plentiful, directional selection
favored finches with smaller and longer beaks.
All of the finches probably descended from one bird that arrived on the islands from South
America. Until the first bird arrived, there had never been birds on the islands. The first bird was a seed
eater. It evolved into many finch species. Each species was adapted for a different type of food. This is
an example of adaptive radiation. This is the process by which a single species evolves into many new
species to fill available niches.
Figure 8.25: Galápagos Finches
3. Disruptive Selection
Disruptive selection is also named for the way the bell curve skews when individuals are plotted
on a graph. To disrupt means to break apart and that is what happens to the bell curve of disruptive
selection. Instead of the bell curve having one peak in the middle, disruptive selection's graph has two
peaks with a valley in the middle of them.
The shape comes from the fact that both extremes are selected for during disruptive selection,
in other words the phenotypes in the middle of the range are selected against. The median is not the
favorable trait in this case. Instead, it is desirable to have one extreme or the other, with no preference
over which extreme is better for survival. This is the rarest of the types of natural selection.
The normal bell curve is changed greatly in disruptive selection. In fact, it looks almost like two
separate bell curves. There are peaks at both extremes, and a very deep valley in the middle. Disruptive
268
selection can lead to speciation (the appearance of new species), and form two or more different
species in areas of drastic environmental changes.
Like directional selection, disruptive selection can be influenced by human interaction.
Environmental pollution can drive disruptive selection to choose different colorings in animals for
survival.
--Example of Disruptive Selection:
One of the most studied examples of disruptive selection is the case of London’s peppered
moths. In rural areas, the peppered moths were almost all a very light color. However, these same
moths were very dark in color in industrial areas. Very few medium colored moths were seen in either
location. It seems that the darker colored moths survived predators in the industrial areas by blending in
to the polluted surroundings. The lighter moths were seen easily by predators in industrial areas and
were eaten. The opposite happened in the rural areas. The medium colored moths were easily seen in
both locations and were therefore very few of them left after disruptive selection.
Figure 8.26: London’s Peppered Moths
4. Stabilizing Selection
The most common of the types of natural selection is stabilizing selection. In stabilizing
selection, the median phenotype is the one selected for during natural selection. This does not skew the
bell curve in any way. Instead, it makes the peak of the bell curve even higher than what would be
considered normal.
Stabilizing selection is the type of natural selection that human skin color follows. Most humans
are not extremely light skinned or extremely dark skinned. The majority of the species fall somewhere in
the middle of those two extremes. This creates a very large peak right in the middle of the bell curve.
This is usually caused by a blending of traits through incomplete or codominance of the alleles.
Diversity in a population is decreased due to stabilizing selection. However, this does not mean
that all individuals are exactly the same. Often, mutation rates in DNA within a stabilized population are
actually a bit statistically higher than those in other types of populations. This and other kinds
of microevolution keep the population from becoming too homogeneous.
269
--Example of Stabilizing Selection:
Many human characteristics are a result of stabilizing selection. Human birth weight is not only a
polygenic trait, but it is also controlled by environmental factors. Infants with average birth weight are
more likely to survive than a baby that is too small or too large. The bell curve peaks at a birth weight
that has the minimum death rate.
-Migration (Gene Flow)
Migration is the movement of individuals into or out of a population. Just like the migration of
birds from the north to the south in the winter, organisms will sometimes change their locations and
therefore change the gene pool of that population. Gene flow occurs when individuals move into or out
of a population. If the rate of migration is high, this can have a significant effect on allele frequencies.
Both the population they leave and the population they enter may change. If a new individual moves
into a population, that is called immigration. If an individual moves out of a population, that is called
emigration. Immigration causes different alleles to be introduced into the population and those are now
available to be passed down via breeding with other members of the population. Emigration results in
the loss of alleles from a population, which in turn would decrease the available genes in the gene pool.
During the Vietnam War in the 1960s and 1970s, many American servicemen had children with
Vietnamese women. Most of the servicemen returned to the United States after the war. However, they
left copies of their genes behind in their offspring. In this way, they changed the allele frequencies in the
Vietnamese gene pool. Was the gene pool of the American population also affected? Why or why not?
- Mating
Many species are not selective when it comes to mating. Asexual reproduction essentially clones
the parent by copying its alleles without any sort of mating between individuals. Some species that use
sexual reproduction will choose any available individual that is available as a partner with no regard for
which characteristics they show. This keeps the alleles that are being passed down from generation to
generation random.
However, many animal species are selective when finding a mate. These individuals look for
particular traits in a mate that will translate to an advantage for their offspring. Since this mating is no
longer random, many undesirable alleles are bred out of the population over several generations. This
makes the gene pool shrink and fewer traits available for the next generation, causing microevolution.
-Mutations
Mutations change alleles by changing the actual DNA of the organism, thus creating genetic
variation in a gene pool. It is how all new alleles first arise. There are several types of mutations that can
occur and have varying degrees of change that accompany them. Alleles may not necessarily change if
the change in DNA is small, like a point mutation, but could be lethal to organisms if it has a profound
change, like a frame shift mutation. This change in DNA, if it occurs in the gametes can be passed down
to the next generation. This either creates new alleles or removes existing traits from the population.
Cells are equipped with a system of checkpoints to make sure mutations do not occur and are corrected
if they do. Mutations within populations that actually change the gene pool are rare. However,
mutations provide the genetic variation needed for other forces of evolution to act. Mutations result in
genotypic variations within organisms and these variations in genotypes sometimes lead to variations in
the phenotypes on which evolutionary forces act.
-Genetic Drift
Unlike natural selection, genetic drift is a random, chance event that depends solely on
statistical chance instead of desirable traits being passed down to offspring. Genetic drift changes allele
frequencies in small populations with small gene pools. The smaller a population, the more likely it is to
see significant microevolution related differences over generations. When a small number of parents
270
produce just a few offspring, allele frequencies in the offspring may differ, by chance, from allele
frequencies in the parents. This is like tossing a coin. If you toss a coin just a few times, you may, by
chance, get more or less than the expected 50 percent heads or tails. In a small population, you may
also, by chance, get different allele frequencies than expected in the next generation or an allele may
disappear completely from a gene pool, even if it is a desirable trait. This random change in the gene
pool can affect the speed of evolution of a species. Instead of taking several generations to see a change
in allele frequency, genetic drift can cause the same affect within a single generation or two. The smaller
the population size, the greater the chance of genetic drift occurring. These changes are caused by the
environment and other factors in everyday life. The survival and reproduction of individuals could be
affected and in turn change the frequency of some alleles seen in future generations of the population.
Genetic drift is not the same as mutations. While some environmental factors can mutate the DNA,
genetic drift has more to do with behaviors, such as breeding times or geographic obstacles for smaller
organisms. Larger populations tend to function through natural selection much more than genetic drift
due to the sheer number of alleles that are available for natural selection to work on as compared to
smaller populations. The Hardy-Weinberg Equilibrium Equation cannot be used on small populations
where genetic drift is the main contributor to diversity of alleles.
There are two special conditions under which genetic drift occurs. They are called bottleneck
effect and founder effect.
--Bottleneck Effect
One specific cause of genetic drift is the bottleneck effect, or population bottleneck. The
bottleneck effect occurs when a larger population shrinks significantly in size in a short amount of time.
Usually, this decrease in population size is generally due to a random environmental affect like a natural
disaster or spread of disease. This rapid loss of alleles makes the gene pool much smaller and some
alleles are completely eliminated from the population.
Out of necessity, populations that have experienced population bottleneck increase the
instances of inbreeding to build the numbers back up to an acceptable level. However, inbreeding does
not increase diversity or numbers of possible alleles and instead just increases the numbers of the same
types of alleles. Inbreeding can also increase the chances of random mutations within DNA. While this
may increase the number of alleles available to be passed down to offspring, many times these
mutations express undesirable traits such as disease or reduced mental capacity.
--Founders Effect
Another cause of genetic drift is called founders effect. The root cause of Founders Effect is also
due to an unusually small population. However, instead of a chance environmental effect reducing the
numbers of available breeding individuals, the founders effect is seen in populations who have chosen
to stay small and do not allow breeding outside of that population.
Often, these populations are specific religious sects or offshoots of a particular religion. The
mate choice is significantly reduced and is mandated to be someone within the same population.
Without immigration or gene flow, the number of alleles is limited to only that population and often the
undesirable traits become the most frequently passed down alleles.
---Example of Founders Effect:
An example of Founders Effect happened in a certain population of Amish people in
Pennsylvania. Since two of the founding members were carriers for Ellis van Creveld Syndrome, the
disease was seen much more often in that colony of Amish people than the general population of the
United States. After several generations of isolation and inbreeding within the Amish colony, the
majority of the population became either carriers or suffered from Ellis van Creveld Syndrome. An
explanation of who the Amish are and how the Founders Effect imbedded into their gene pool is
explained in Figure 8.27 on the next page.
271
Figure 8.27: Founder Effect in the Amish Population. The Amish population in the U.S. and Canada had a small
number of founders. How has this affected the Amish gene pool?
Eyewitness to Evolution
In the 1970s, biologists Peter and Rosemary Grant went to the Galápagos Islands. They wanted
to re-study Darwin’s finches. They spent more than 30 years on the project. Their efforts paid off. They
were able to observe evolution by natural selection actually taking place. While the Grants were on the
Galápagos, a drought occurred. As a result, fewer seeds were available for finches to eat. Birds with
smaller beaks could crack open and eat only the smaller seeds. Birds with bigger beaks could crack and
eat seeds of all sizes. As a result, many of the small-beaked birds died in the drought. Birds with bigger
beaks survived and reproduced (see Figure 8.28). Within 2 years, the average beak size in the finch
population increased. Evolution by natural selection had occurred.
Figure 8.28: Evolution of Beak Size in Galápagos Finches
272
Why were the Grants able to observe such a dramatic evolutionary change?
Oceanic islands are frequently inhabited by clusters of closely related species that fill a variety of
ecological niches, often niches that are filled by very different species on continents. Such clusters, like
the Finches of the Galapagos, Hawaiian honeycreepers, members of the sunflower family on the Juan
Fernandez Archipelago, and wood weevils on St. Helena are called adaptive radiations because they are
best explained by a single species colonizing an island (or group of islands) and then diversifying to fill
available ecological niches. Such radiations can be spectacular; 800 species of the fruit fly family
Drosophila, nearly half the world's total, are endemic to the Hawaiian islands. Another illustrative
example from Hawaii is the Silversword alliance, which is a group of thirty species found only on those
islands. Members range from the Silverswords that flower spectacularly on high volcanic slopes to trees,
shrubs, vines and mats that occur at various elevations from mountain top to sea level, and in Hawaiian
habitats that vary from deserts to rainforests. Their closest relatives outside Hawaii, based on molecular
studies, are tarweeds found on the west coast of North America. These tarweeds have sticky seeds that
facilitate distribution by migrant birds. Additionally, nearly all of the species on the island can be crossed
and the hybrids are often fertile, and they have been hybridized experimentally with two of the west
coast tarweed species as well. Continental islands have less distinct biota, but those that have been long
separated from any continent also have endemic species and adaptive radiations, such as the 75 lemur
species of Madagascar, and the eleven extinct moa species of New Zealand.
Can Microevolution Lead to Macroevolution?
No matter how controversial the Theory of Evolution is in some circles, it is rarely argued that
microevolution happens in all species. There is pretty significant amounts of evidence that DNA changes
and in turn can cause small changes in the species, including thousands of years of artificial selection via
breeding. However, the opposition comes when scientists propose that microevolution over very long
periods of time can lead to macroevolution.
After all, thousands of years of breeding different species has not led to completely new species
being formed. Doesn't that prove that microevolution does not lead to macroevolution? Proponents for
the idea that microevolution leads to macroevolution point out that not enough time has gone by in the
scheme of the history of life on Earth to show if microevolution does lead to macroevolution.
The bottom line is that this is one controversy that has not been solved. Both sides have
legitimate arguments for their causes. It may not be solved within our lifetimes. It is important to
understand both sides and make an informed decision based on the evidence that fits in with your
beliefs. Keeping an open mind while remaining skeptical is often the hardest thing for people to do, but
it is necessary when considering scientific evidence.
- Basics of Microevolution
Microevolution is the changes in species at a molecular, or DNA, level. All species on Earth have
very similar DNA sequences that code for all of their characteristics. Small changes can happen through
mutations or other random environmental factors. Over time, these can affect the available traits that
can be passed down through natural selection to the next generation. Microevolution is rarely argued
and can be seen through breeding experiments or studying population biology in various areas.
-Changes in Species
Species do change over time. Sometimes these are very small changes caused by
microevolution, or they may be larger morphological changes describe by Charles Darwin and now
known as macroevolution. There are different ways species change based on geography, reproductive
patterns, or other environmental influences. Both proponents and opponents of the microevolution
leading to macroevolution controversy use the idea of speciation to support their arguments. Therefore,
it does not really settle any of the controversy.
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-Basics of Macroevolution
Macroevolution was the type of evolution Darwin described in his time. Genetics and
microevolution were not discovered until after Darwin died and Gregor Mendel published his pea plant
experiments. Darwin proposed that species changed over time in morphology and anatomy. His
extensive study of the Galapagos finches helped shape his Theory of Evolution through Natural
Selection, which is now most often associated with macroevolution.
Lesson Summary
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Microevolution occurs over a short period of time in a population or species. Macroevolution occurs
over geologic time above the level of the species.
The population is the unit of evolution. A population’s gene pool consists of all the genes of all the
members of the population. For a given gene, the population is characterized by the frequency of
different alleles in the gene pool.
The Hardy-Weinberg theorem states that, if a population meets certain conditions, it will be in
equilibrium. In an equilibrium population, allele and genotype frequencies do not change over time.
The conditions that must be met are no mutation, no migration, very large population size, random
mating, and no natural selection.
There are five forces of evolution: natural selection, migration (gene flow),mating, mutation, and
genetic drift. Natural selection for a polygenic trait changes the distribution of phenotypes. It may
have a stabilizing, directional, or disruptive effect on the phenotype distribution.
References/ Multimedia Resources
Bailey, Regina. "The Importance of Genetic Variation." About.com Biology. Web. 16 June 2014.
Scoville, Heather. "Can Microevolution Lead to Macroevolution?" About.com Evolution. Web. 16 June
2014.
Scoville, Heather. "Gene Pool." About.com Evolution. Web. 16 June 2014.
Scoville, Heather. "Genetic Drift." About.com Evolution. Web. 16 June 2014.
Scoville, Heather. "Processes of Microevolution." About.com Evolution. . Web. 16 June 2014.
Scoville, Heather. "3 Types of Natural Selection (and 1 Type of Not-So-Natural Selection)." About.com
Evolution. Web. 16 June 2014.
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