Download Mechanisms of Evolution: Microevolution

Lesson: Measuring Microevolution
Recall that a GENE is a unit of inheritance.
Different forms of the same gene are called ALLELES (uh-LEELZ’)
Alleles arise from an original gene via the process of MUTATION.
Mutations can happen in any cell as the DNA replicates before the cell divides.
Usually, the mistake is repaired. Sometimes it is not.
Mutations that happen in a SOMATIC CELL (body cell) can’t be passed on.
Mutations that happen in a GERMLINE CELL can be passed on to offspring.
• In females, germline cells undergo meiosis to produce eggs (ova)
• In males, germline cells undergo meiosis to produce sperm.
Like traits, a mutation can be
• beneficial – helps survival and reproduction
• harmful (= deleterious) – interferes survival and reproducrtion
• neutral – does not affect survival and reproduction
•
Probability
The probability of an event is the precise chance that it will happen.
For example, if you roll a die (singular of dice), there is a one in six probability (chance) that you
will roll a one because a die has six sides, and only one side bears a one. There’s also a one in six
probability that you will roll a six or any other number.
You also can calculate the probability that two events will happen together. This is called combined
probability.
If two events are independent of one another, (For example, if you roll a die twice, then the result the
second roll does not depend on the result of the first roll.), then the probability of both of them
happening is the product of their independent probabilities. This is called The Product Rule.
For example, if you roll a die twice, then the probability that you will roll a one and a six is the product
of each of those results’ probability.
• The chance of rolling a one is one in six (1/6)
• The chance of rolling a six is one in six (1/6)
• The chance of rolling a one and a six in two rolls is equal to:
1/6 x 1/6 = 1/36
That means that, statistically, you should have to roll a die 36 times to roll a one and a six, back-toback. You might get a one and a six on the first try or the hundredth. But the most probable result is
36 rolls. Statistics is about predictions. Once something happens, it no longer has a probability. It’s
already happened.
What does this have to do with genetics? Everything!
Because every time an organism sends its eggs or sperm on the road to create a member of the next
generation, the likelihood that any given allele of a gene will end up in that egg or sperm is like
a roll of a die.
If a gene has two alleles, and an individual has one of each (Xx), then the probability that X will end
up in the sperm is 1/2 (50%, or 0.5). The probability that x will end up in the gamete is also 1/2 (50%,
or 05.) If there are two events possible, then each event has a 50:50 chance of happening.
Probability and Inheritance
Every diploid organism has two copies of every gene (one from mom, one from dad).
Let’s consider a gene called A. It codes for an important enzyme, enzyme A.
• A dominant allele (A) codes for normal, functional enzyme A.
• A recessive allele (a) codes for non-functional enzyme.
Genotypes and Phenotypes
An organism with genotype AA has phenotype of all normal, wild type enzyme A.
An organism with genotype Aa has phenotype of half the amount of normal, wild type enzyme X.
An organism with genotype aa has phenotype of no enzyme a, and it is not viable.
In this case, half the amount is enough to function normally. (This makes the A allele dominant.)
An organism with genotype AA can make sperm or eggs carrying ONLY the A allele.
An organism with genotype aa can make sperm or eggs carrying ONLY the a allele.
An organism with genotype Aa can make sperm or eggs carrying either the A or a allele.
You can predict the expected genotypes of offspring of any given mating with a Punnett Square.
Let’s say you are breeding two flies, one with genotype AA(female), and one with genotype Aa
(male). What will be the genotypes and phenotypes of their babies?
1. The homozygous female produces eggs with only A alleles. Put an A beside each square.
2. The heterozygous male produces sperm with either A or a. Put one of those above each square.
A
a
A
A
The inside of each box represents the possible genotype of 1/4 of the offspring possible from this
mating. Fill the squares by entering the letter (A or a) that borders each of its sides. Like so:
A
a
A
AA
Aa
A
AA
Aa
In this mating, 50% of the offspring will be genotype AA, and 50% will be genotype Aa. All are viable.
What if two of the Aa flies from the offspring above were to mate together? Fill in the square!
A
a
A
a
If you did this correctly, 25% will be AA, 50% will be Aa (carriers) and 25% will be aa.
You can do a Punnett Square for any possible genotype combination and predict the expected
ratio of offspring genotypes (and phenotypes).
To Share Genes, or Not.
If a recessive allele is harmful (as in the case of the mutant enzyme a), there can be problems.
Inbreeding (mating between close relatives) increases the probability that two copies of a particular
allele will be identical because they were inherited from a common ancestor. If that allele happens to
be harmful (= deleterious), then it is clear that inbreeding increases the chance of harmful recessive
traits being expressed.
This is why a hybrid organism (one whose parents are not closely related) tends to be healthier and
more vigorous than an inbred one.
Heterozygosity at many gene loci means that even if an organism carries a harmful recessive allele,
it will not express (show) it, because it is masked by the dominant allele. Organisms heterozygous at
many gene loci are usually healthier and more robust than those who are homozygous at many gene
loci. They exhibit HYBRID VIGOR.
Outbreeding (matings between unrelated individuals) increases heterozygosity at many gene loci,
and thus increases hybrid vigor.
Send in the Clones
Some organisms can reproduce exact copies of themselves without mixing their genes with those of
a mate. Their offspring are all genetically identical to the parent and to each other. A population of
genetically identical organisms is called a CLONE.
In a stable environment, the clone may persist. (If it ain’t broke, don’t fix it.)
But most environments are not stable. They change. And if the clone individuals are not well-suited
to a change, they may ALL die.
Sex ensures a variety of genetically different offspring, some of which might
survive in a changing environment. Sex is life’s way of not putting all of its eggs
in one basket.
So sex is good in a hostile, changing world.
~>~>~>~>~>~>~>~>~>~>~>~>~>~>~>
Changing Environments, Changing Genotypes
Remember that mutation is the raw material of evolution. Without multiple forms of genes (alleles),
there can be no evolution.
MICROEVOLUTION is a change in a population's genes without speciation (reproductive isolation).
MACROEVOLUTION is the origin of two new species from an ancestral one.
Also remember that there are five different processes that can make a population evolve.
1.
2.
3.
4.
MUTATION: A gene in the population changes
SMALL POPULATION SIZE: small gene pool leads to quicker evolution.
MIGRATION: Individuals immigrate into or emigrate out of the population
ASSORTATIVE MATING: Individuals pair off according to phenotype.
a. positive assortative mating – similar individuals prefer each other as mates
b. negative assortative mating – dissimilar individuals prefer each other as mates
5. NATURAL SELECTION: Individuals of a particular genotype have a better chance
of reproducing than individuals of other genotypes.
Measuring Changes in Gene Frequencies
Most natural populations are not clones. Genetic diversity (heterozygosity) is a good
“insurance policy” that helps prevent a population from going extinct.
What is “frequency”?
The frequency of anything is how often it occurs compared to all the chances it has to occur.
For example, if a population of 1000 beetles has 800 red members and 200 black members, then you
can calculate their frequencies by dividing the number of each color of beetle by the total number of
beetles:
Red beetles: 800/1000 = 0.8 (80%)
Black beetles: 200/1000 = 0.2 (20%)
We can do the same thing with genes in a population. REMEMBER THAT EVERY MEMBER OF
THE POPULATION HAS TWO COPIES OF EVERY GENE. SO THE NUMBER OF GENES (and
alleles) WILL ALWAYS BE DOUBLE THE NUMBER OF INDIVIDUALS IN THE POPULATION.
In our beetles, a single gene codes the enzymes responsible for the color of the wing covers (elytra).
** Dominant allele (R) codes for red elytra (RR or Rr beetles will be red)
** Recessive allele (r) codes for black elytra (rr beetles will be black)
How do we determine the frequency of R and r in our population of 1000 beetles?
In population genetics “shorthand”…
• the frequency of the dominant allele is represented as p
• the frequency of the recessive allele is represented as q
• because there are only two alleles at this locus, p + q = 100% of the alleles; p + q = 1.0
Back in 1908, two scientists, George Hardy (a British mathematician) and Wilhelm Weinberg (a
German physician) independently reported that the relative frequencies of genotypes (at ONE gene
locus with TWO alleles) could be represented by this equation:
p2 + 2pq + q2 = 1.0
in which…
p2
= the proportion (frequency) of RR homozygotes
2pq = the proportion (frequency) of Rr heterozygotes
q2
= the proportion (frequency) of rr homozygotes
If you know the values of p and q, you can plug them into the Hardy-Weinberg equation and predict
how many of the individuals in the next generation should be RR, Rr, and rr IF THE
POPULATION IS NOT EVOLVING. (A population that is not evolving is said to be in HardyWeinberg equilibrium.)
In our example, p = 0.8 and q = 0.2. Plug in!
(0.8)2 + 2(0.8)(0.2) + (0.2)2
=
In plain English, this means that in the next generation…
• 64% of the beetles should be RR
• 32% of the beetles should be Rr
• 4% of the beetles should be rr
0.64 + 0.32 + 0.04
If you don’t know the values of p and q, you can estimate them by counting the number of individuals
expressing the recessive trait, because you KNOW that all those individuals are rr.
Let’s say in a neighboring population of 1000 beetles there are 500 red beetles and 500 black
beetles. Unpack the problem:
1.
2.
3.
4.
5.
6.
7.
The black beetles (rr) make up 50% (0.5) of the population.
The frequency of rr individuals is equal to q2.
To find the value of q, take the square root of q2.
The square root of 0.5 = 0.7, which means that q = 0.7
You can now solve for p.
If p + q = 1.0, then 1.0 – q = p
1.0 – 0.7 = 0.3, which means that p = 0.3
You can now plug the values into the Hardy-Weinberg equation:
(0.3)2 + 2(0.3)(0.7) + (0.7)2
=
0.09 + 0.42 + 0.5
…which means that the next generation of beetles in this population should be 9%
(0.9) RR, 42% (0.42) Rr, and 50% ().5) rr if the population is NOT EVOLVING (is in
Hardy-Weinberg equilibrium).
For the population to not evolve, the following criteria must be met:
1. NO MUTATION: The gene in question does not change.
2. NO MIGRATION: Individuals do not immigrate into or emigrate out of
the population
3. LARGE POPULATION: The population consists of very great number
(theoretically infinite!) of individuals.
4. NO ASSORTATIVE MATING: Every individual in the population has
an equal chance to mate
5. NO NATURAL SELECTION: Neither allele confers an advantage over
the other, resulting in differential reproduction
IF YOUR POPULATION IS IN HARDY-WEINBERG EQUILIBRIUM, THEN THE
GENOTYPE PROPORTIONS AND ALLELE FREQUENCIES SHOULD NOT
CHANGE FROM GENERATION TO GENERATION.