Download Chapter 24 - Evolution and Population Genetics

Chapter 18 - Evolution and Population Genetics
Review
Haploid, Diploid
Diploid cells (2N) have two complete sets of chromosomes. The body
cells of animals are diploid.
Haploid cells have one complete set of chromosomes. Some organisms
are haploid. Animals are diploid but their gametes (sperm and eggs) are
haploid.
Mitosis
Mitosis is a type of cell division that results in daughter cells that are
identical (genetically) to the parent cell. If the parent cell is diploid, the
two daughter cells will be diploid. Similarly, a haploid cell that divides by
mitosis will produce two haploid daughter cells.
Meiosis
Meiosis is a type of cell division in which the daughter cells have 1/2 the
number of chromosomes as the parent cell. If the parent cell is diploid, the
daughter cells will each be haploid.
Meiosis has two separate divisions resulting in four daughter cells.
Evolution and Population Genetics
Evolution Occurs in Populations
Species
There is not a good definition of species; perhaps the concept of species
is artificial but it is useful because it allows people to classify organisms.
Most biologists would agree that members of a sexually-reproducing
species are able to interbreed and have a shared gene pool.
Different species do not exchange genes with each other; they do not
interbreed.
This definition of species is based on sexual reproduction and therefore
does not work with prokaryotes or other asexual species.
Population
A population is an interbreeding group of organisms (the same species)
that occupies a particular area.
The size of the area is somewhat arbitrary. There could be a population of
fish in an aquarium and a population of fish in a lake.
Gene Frequency and Evolution
Gene frequency refers to the proportion of alleles that are of a particular
type. For example, if 60% of the alleles in a population are "a" and 40%
are "A", then the gene frequency of "a" is 0.6 and the gene frequency of
"A" is 0.4.
On a small scale, evolution involves changes in gene frequencies.
Population Model
A population is a group of interbreeding organisms that occupies a
particular area.
Initial Population
Circles are used to represent genes in this diagram of a population.
Individuals are diploid, so two circles are used to represent an individual.
Gene Frequencies in the Model Population
In the population above, 33% of the genes for eye color in a population are
"A" and 67% are "a". The frequency of "A" is therefore 0.33 and the
frequency of "a" is 0.67.
Gametes
During meiosis, "AA" individuals will produce all "A" gametes. Similarly,
1/2 of the gametes produced by an "Aa" individuals will be "A" and the
other half will be "a"; "aa" individuals will produce all "a" gametes.
Individual
AA
Aa
aa
Gametes
all A
1/2 A, 1/2 a
all a
The proportion of A and a in the gametes will be the same as in the
population. In the example population we have been using, suppose that
each individual produces four gametes.
In reality, males produce many millions of gametes and females produce
relatively few. This is not a concern for our model because in either case,
the gene frequency of the gametes will be the same as that of the
population that produced them.
The gene frequency of "A" and "a" in the gamete pool will remain 0.33
and 0.67.
Gene frequency: The next generation
Because the gene frequency in the gamete pool did not change, the gene
frequency in the population the next generation remains the same.
The Hardy-Weinberg law states that under certain conditions (discussed
below), the gene frequency of a population does not change from
generation to generation.
Should There Be Fewer Recessive Alleles?
The population model described above predicts that gene frequencies will
not change from one generation to the next even if there are more
recessive alleles.
There is sometimes a misconception among students beginning to study
genetics that dominant traits are more common than recessive traits. It isn't
true. For example, blood type O is recessive and is the most common type
of blood. Huntington's (a disease of the nervous system) is caused by a
dominant gene and the normal gene is recessive. Fortunately, most people
are recessive; the dominant is uncommon.
The misconception comes from the observation that in a cross of Aa X Aa,
3/4 of the offspring will show the dominant characteristic. However, the
3:1 ratio comes only if the parents are both Aa. If there are many recessive
genes in a population, then most matings are likely to be aa X aa and most
offspring will be aa.
Forces that Change Gene Frequencies
Migration can change the gene frequency of a population if the migrants
have a different gene frequency than that of the population they are
leaving or entering.
The founder effect occurs when the gene frequency of a newly established
population is somewhat different from the parental population. This may
be due to the small sample of founding individuals.
The sample-size phenomenon can be illustrated by flipping a coin. The
expected number of "heads" from flipping a coin is 50% but if a coin is
flipped only 4 times, you may get all "heads" or all "tails". If the coin is
flipped 1000 times, the actual number of "heads" and "tails" will probably
not deviate much from 50%. Thus, the larger the sample size of emigrants,
the more likely it is to reflect the population from which it is leaving.
Below: The population on the right was formed from a few individuals
emigrating from the population on the left.
During a bottleneck, a large population undergoes a decrease in size so
that relatively few individuals remain. Because there are few individuals,
the gene frequency is more likely to drift.
Below: The gene frequency of the initial population (left) changes because
many of the individuals have died. The population on the right is the same
population after the bottleneck has occurred.
Genetic drift refers to random fluctuations in the gene frequency of a
population. This is more likely to occur in a small population. As with
bottlenecks and the founder effect, it is a sample-size phenomenon. The
smaller the population, the more likely that gene frequencies are likely to
fluctuate from generation to generation.
Mutation changes gene frequencies when genes of one type ("A" for
example) mutate to another type ("a" for example).
Natural selection changes gene frequencies when genes or gene
combinations are more likely to result in greater reproductive success of
the individual that possesses them.
Conditions Necessary for Hardy-Weinberg Equilibrium
Notice that the gene frequency the next generation is the same as that of
the initial population. The Hardy-Weinberg principle states that if the
following conditions are met, the gene frequency of a population will not
change from generation to generation:
No migration
Large population size
No mutation
Random mating
No selection
Natural Selection
Adaptation
Adaptations are structures or behaviors that allow efficient use of the
environment. For example, the webbed foot of a duck enables it to swim
better than a foot that is not webbed.
Adaptations are due to genes, that is, they are inherited.
Natural Selection
Natural selection operates to produce individuals that are better adapted to
their environment. It is important to keep in mind as you read below that
natural selection does not act on individuals; it acts on populations.
Individual organisms cannot become better-adapted to their environment
because they cannot change their genes.
Natural selection produces changes in the genetic composition
of a population from one generation to the next. As a result,
organisms become better adapted to their environment.
Natural selection occurs because
1.
2.
3.
4.
Individuals within a population vary; they are not all identical.
Some variants are “better” than others.
The traits that vary are heritable.
The “better” individuals will have more success reproducing; they
will have more offspring.
In successive generations, more offspring will have the better trait.
These items are discussed below.
Variation
Sexual reproduction promotes genetic variation.
For many traits that occur in a population,
individuals are often not all identical. For
example, if running speed were measured, some
individuals would likely be able to run faster
than others but most individuals would probably
be intermediate.
If number of individuals is plotted against the
trait in question (running speed for example), a
graph like the one shown is often produced.
We would get a similar bell-shaped curve if we
plotted height, weight, performance on exams,
or almost any other characteristic.
Some Variants are Better
Some individuals are bound to be better than others. Perhaps their body
structure allows them to escape predators better or to find food faster or to
better provide for their young. For example, suppose that the fasterrunning animals diagrammed below are better able to escape predators
than the slower ones. You would expect that more of the faster ones would
survive and reproduce than the slower ones.
The slower rabbits will not reproduce as much because predators kill them
more than they kill the faster rabbits.
Traits Are Heritable
Those individuals that survive better or reproduce more will pass their
superior genes to the next generation. Individuals that do not survive well
or that reproduce less as a result of "poorer genes" will not pass those
genes to the next generation in high numbers. As a result, the population
will change from one generation to the next. The frequency of individuals
with better genes will increase. This process is called natural selection.
Natural Selection Produces Evolutionary Change
If the conditions discussed above are met, the genetic composition of the
population will change from one generation to the next. This process is
called natural selection.
The word "evolution" refers to a change in the genetic composition of a
population. Natural selection produces evolutionary change because it
changes the genetic composition of populations.
A variety of other mechanisms can also produce evolutionary change. For
example, suppose that 65% of the eye-color genes in a population were for
individuals with blue eyes and 35% of the genes were for brown eyes. If
most of the immigrants entering the population carried the blue gene, the
overall composition might change from 65% blue to 70% blue.
Natural selection acts on populations; a single individual cannot evolve.
Natural selection does not act on an individual to make it better adapted to
its environment.
Example of Natural Selection: Industrial Melanism
Kettlewell studied the peppered moth (Biston betularia) from insect
collections in England. He observed that in polluted areas, most of the
peppered moths were the dark form. In clean areas, most were the pale
form.
During the early 1800's, the dark form comprised less than 2% of the
population and the pale form made up more than 98%. During the 1800’s
the dark form increased in frequency in urban areas.
Kettlewell suggested that dark moths survived better in polluted areas
because they were more difficult for avian (bird) predators to see on the
darkened tree trunks. Similarly, he suggested that light-colored moths
were more difficult to see in unpolluted areas because the tree trunks were
light-colored.
To test this, he released moths of each type (light and dark) in both
polluted and unpolluted areas. In the unpolluted area, he recaptured
13.7% of the light moths and 4.7% of the dark moths. In the polluted area,
he recaptured 13% of the light and 27.5% of the dark moths.
Sexual Reproduction and Evolutionary Change
Variation
Individuals with in a population usually are not all identical and much of
this variation is due to genetic differences among individuals.
Sexual reproduction acts to increase variation in populations by shuffling
genes. Offspring have some genes from each of two different parents and
therefore are not identical clones of their parents. The increased variation
due to sexual reproduction allows natural selection (and thus evolution) to
produce changes in populations as described above.
Ultimately, all variation in a population comes from changes in the DNA.
These changes are called mutations.
Recombination during sexual reproduction promotes variation. Sperm and
eggs (gametes) are produced by a type of cell division called meiosis.
During meiosis, crossing-over and independent assortment act to
shuffle the genes before gametes are produced.
Fluctuating environments
Evolutionary change due to natural selection would not be necessary if the
environment never changed and the organisms within the environment
were optimally adapted to the environment. For example, imagine a plant
that is adapted to an environment that has an average annual rainfall of
100 cm. If the climate were to change so that the amount of rainfall
decreased, individuals that could tolerate less rain would survive and
reproduce better, thus establishing their drought-tolerant genes in
subsequent generations. If there was no variation in the plant population,
there would not be any drought-tolerant individuals and the species would
likely go extinct in areas of decreased rainfall.
Sexual reproduction therefore, enables species to survive in fluctuating
or changing environments because it promotes variation, which in turn
allows natural selection.
Model Chromosomes
The drawings of chromosomes below will be cut out and used in class for
reviewing mitosis and meiosis in the "Review" section at the beginning of
this page.
Be sure that you can do the following using these models of
chromosomes:
Create a haploid cell.
Create a diploid cell.
Simulate mitosis in a diploid cell.
Simulate mitosis in a haploid
Simulate meiosis in a diploid cell.
Use the models to create two gametes: an egg and a sperm.
Simulate the fusion of the two gametes to create a fertilized
egg (called a zygote).