note: mutations may be beneficial, harmful/deleterious or neutral to the organism in which they occur.
Rarely, wild populations consist of 100% genetically identical organisms, produced without sex. They make up a clone.
Without sex, there is no genetic recombination, and all the individuals born to a population like this have exactly the same genes at all loci.
If the clone's environment is stable, it might be advantageous to keep reproducing without sex. After all, the object of breeding is to get as many of your genes into the next generation as possible. So getting 100% of your genes into the next generation (via cloning) would be ideal, would it not?
There's only one problem with this. Environments are not always stable.
Because it lacks any genetic diversity, a clone population is highly susceptible to changes in the environment. If a change occurs that harms one, then it is quite likely that ALL will die, since they are identical! A clone lacks the genetic "safety net" of variation that will allow at least some members of the population to survive and become the parents of the next generation.
So sexual reproduction is good in a cruel and changing world. Most natural populations are NOT clones. They consist of individuals (of the same species), each of which is genetically (and phenotypically) unique. Their DNA is different because they inherited small changes/mistakes in the DNA from their parents. Such changes are called MUTATIONS.
MICROEVOLUTION is genetic change in a population over generations that does not result in new species.
MACROEVOLUTION is genetic change in a population over generations that does result in new species being generated from the original species.
(Recall the definition of a species.)
Not everyone expresses every trait to the same degree. When a population has a particular trait that is variable, that population is said to be polymorphic for that trait. Let's think of a few examples...
As each new generation is born, it carries genes that are slightly different from the genes of the generation that gave birth to it. But unless that new generation is a new species (which is not likely!), then these changes are said to represent microevolution.
Fortunately, even a harmful mutation can be masked if it is recessive and the organism has a dominant version of the gene to mask the harmful recessive one. But what happens if an organism gets two copies of a harmful allele--one from each parent?
Let's use the example of a mystery gene called "X". The normal, dominant allele (X) codes for an important enzyme that allows the animal to function. A mutation of the gene that tells the cell how to build enzyme X causes the cell to instead build a mutant form of the enzyme, x, which is non-functional. The organism with the recessive x allele dies without a normal, functioning X enzyme.
Fortunately, every organism has two copies of every gene (one from mom, one from dad)
If this bug mates with a female bug who does not carry this mutation, you can predict the genotypes of the offspring the pair will produce with a Punnett Square:
The mutant male produces sperm with either X or x. The normal female produces sperm with only X alleles, since she doesn't have the mutation. The insides of each box represents the possible genotype of 1/4 (25%) of the offspring possible from this mating. If you go through and fill the matrix, you'll see that 50% will be XX and 50% will be Xx.
All of these offspring will survive, since they can produce protein X--but half of them are carrying a harmful allele.
mated with
Do the Punnett Square:
As you can see, only 25% will be fully homozygous dominant, 50% will carry the harmful allele without expressing it in the phenotype and 25% will be homozygous recessive and DIE.
INBREEDING increases the chances that two harmful (= deleterious) copies of a gene will be inherited by the offspring of the inbreeding individuals. If this happens, the homozygous recessive individual will express that harmful condition.
A hybrid organism is one that is produced by parents that have dissimilar genotypes. The most distantly related the parents are, the more likely it is that the offspring will be heterozygous for most of its gene loci. Heterozygosity (the state of being heterozygous) at many gene loci means that even if you carry a harmful allele, you will not express it in your phenotype.
When heterozygosity makes an organism less likely to express harmful genetic conditions because it has many wild type, dominant alleles that function normally, we say that the individual shows hybrid vigor. OUTBREEDING (matings between individuals that are not closely related) increases heterozygosity at many gene loci, and thus increases hybrid vigor.
ALWAYS REMEMBER:
species: similar organisms that can interbreed in nature to produce fertile, viable offspring.
population: all individuals of the same species living in a defined geographic area. (anything from your eyebrows to the Himalayas).
gene pool: all the genes at all gene loci in every member of an interbreeding population.
evolution: CHANGE over time
organic evolution: change in living populations over time.
Does evolution have a goal? Is it due to random events?
A Poll: What is the "most highly evolved species?"
Ever thought about...
To measure evolution, the population geneticist measures the frequency of occurrence of various alleles in a population, and how those frequencies change over time.
In 1908, Godfrey H. Hardy, a British mathematician, and Wilhelm Weinberg, a German physician.
independently reported a mathematical rule that describes allele
frequencies in a population at any given moment in time.
They noted:
For a population having two alleles of a particular gene in which
the total number of all alleles, A + a = 1.0 (100% of all the alleles in the population)
The total number (frequency) of either allele is equal to:
Hence, if A + a = 1.0 (100%), then p + q = 1.0 (100%)
Hardy and Weinberg independently noted that at any given point in time, the allele frequencies in a population that is not evolving would be equal to
...in which
How can we calculate the expected percentages of each genotype for a population? Let's do an example.
Let's try it with tongue rolling in our group of Research in Ecology participants
How many people can roll their tongue? (dominant trait, R)
How many people cannot roll their tongue (recessive trait, r)
If you're a non-roller, your genotype is easy. You can be ONLY rr.
But what if you're a roller? You could be either RR or Rr. How can we tell? Not by looking. But the HW equation gives us a way to estimate the proportion of rollers who are RR and who are Rr.
What should we expect in our own little population?
HARDY AND WEINBERG KNEW.
Let's figure out what the percentages of RR, Rr and rr should be expected in our little population, if we are not evolving (with respect only to the tongue-rolling gene).
# of non-rollers = ?
Remember that the number of non-rollers in the HW equation is equal to q2 (homozygous recessive individuals, rr).
We can't tell which of the rollers are RR or Rr, because rolling is dominant, and masks the non-rolling version of the gene. But we can figure out what SHOULD be the number by using the Hardy Weinberg equation.
If you know the value of q2, how do you calculate q?
Okay, now that you know the value of q, how can you calculate p?
Hint: Remember that
Now that you have both p and q, just plug it into the Hardy-Weinberg equation:
This means that of our total number of participants:
But are we not evolving? Remember, this model assumes...
Similarly, if members of a population leave, they take their genes with them. This will have a similar effect.
If any of these five conditions are not met in a population, then allele and genotype frequencies will NOT remain constant over generations. That means the population is EVOLVING at that gene locus. And sometimes, small (microevolutionary) changes can add up and foster large (macroevolutionary) change, resulting in new species.