Recall that in the last lecture, I defined evolution as "genetic changes in populations through time."
Several important points logically follow from this definition:
1. In order for evolution to occur, populations must have genetic variation -- e.g. genes will 2 or more alleles -- have to be able to change from something to something else -- genetic variation is the "raw material of evolution."
2. It is clear from this definition that in order to study evolution, scientists must be able to describe the genetic characteristics of populations in order to see if genetic changes are occurring through time.
Describing the genetic characteristics of populations is the goal of the field of biology called population genetics.
A population is a group of organisms of the same species at a particular place -- a population does not have just a single phenotype or genotype, but it is a collection of many phenotypes and genotypes.
Gene pool -- all the genes and alleles in a population.
Two primary statistics used by population geneticists to characterize gene pools:
1. genotype frequencies -- relative proportions of genotypes in a population.
2. allele frequencies -- relative proportions of alleles in a population.
Let's introduce these statistics by way of an example:
In humans, there is a blood group system called the MN system -- controlled by a single gene with alleles M and N -- blood is collected from a population of Native Americans in the Southwest U.S. -- genotypes of each individual are determined for MN blood group by a simple blood test.
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Note: genotype frequencies ALWAYS sum to 1.00.
Allele frequencies: relative frequencies of alleles M and N -- freq. of M = p and freq. of N = q.
Question: what is the total number of alleles in the sample? Answer: 416 since their are 208 diploid individuals and each has two alleles for this gene.
Question: what proportion (p) of these alleles are allele M and what proportion (q) are allele N?
Answer: p = ((119 x 2) + 76)/ 416 = 0.75
q = ((13 x 2) + 76)/ 416 = 0.25
Note: p and q must add to 1.00; Note: if p is known, q = 1 - p (p + q = 1)
Now, we have described this gene pool in very specific, mathematical terms with respect to this particular genetic trait (MN blood group) -- could do this for other traits as well.
NOTE: If genotype and/or allele frequencies change over time, then evolution is occurring!
In the early part of this century, an English mathematician (Hardy) and a German physician (Weinberg) independently discovered an important population genetics principle.
Hardy-Weinberg Law: Under certain conditions, allele and genotype frequencies will remain the same from generation to generation.
Populations in which allele and genotype frequencies remain constant through time are said to be in Hardy-Weinberg equilibrium -- notice that populations that are in H-W equilibrium for a trait are not evolving for that trait according to our definition of evolution.
Populations will remain in H-W equilibrium unless they are acted upon by evolutionary forces.
Evolutionary forces: mechanisms that cause genetic changes in populations -- forces will change p, q and/or genotype frequencies.
There are 4 important evolutionary forces:
1. Mutation.
2. Gene flow.
3. Genetic drift.
4. Natural selection.
Mutation: random changes in the DNA --results in new alleles being introduced into the populations -- important as a source of genetic variation.
Gene flow: nonrandom movement of genes into or out of a population by migration -- e.g. suppose our population of 208 Native Americans received 20 new immigrants and all 20 were MM --clearly this will change p, q and genotype frequencies -- gene flow is also a way that new genetic variation can be introduced into populations.
Gene flow between populations has a homogenizing effect -- tends to make allele frequencies similar in populations exchanging genes -- e.g. consider a population with p = 1.0 (q = 0.0) exchanging genes over time with another population with p = 0.0 (p = 1.0) -- will eventually reach a point where p = q = 0.5 in both populations -- notice that gene flow is responsible for introducing genetic variation into these populations.
Genetic drift: random changes in frequencies due to sampling error in gamete production and fertilization -- most important in small populations.
Sampling error: the fewer times a chance event occurs, the greater the variance from the expected outcome of that event.
Example: flipping a coin N times -- expected outcome is 1/2 heads to 1/2 tails. Suppose you flipped a coin 10 times -- expected outcome is 5 H to 5 tails, but the observed outcome is 3 heads and 7 tails (20% deviation). Suppose you repeated this experiment, but the coin is flipped 1,000 times -- would 300 heads to 700 tails be a "reasonable outcome?" As number of flips gets larger, the % deviation gets smaller!
In reproduction, the production of gametes and fertilization are probabilistic events -- e.g. in theory, an individual that is Aa will produce gametes such that 1/2 have allele A and 1/2 have allele a -- we expect the offspring of a Aa X Aa cross to be 1AA:2Aa:1aa -- but these are expectations and sampling error can result in deviations -- e.g. Aa X Aa cross produces 10 offspring such that 4 are AA, 5 are Aa and 1 is aa -- this is due to sampling error.
When sampling error result in changes in allele and genotype frequencies, we call this effect genetic drift.
Figure 16.20 in the text illustrates how genetic drift can change population genetic characteristics and how population size influences genetic drift -- 18 populations of stoneflies -- each start with p = q = 0.5 -- 9 populations have N = 25 and the other 9 have N = 500 -- population size kept constant (25 or 500) over time
Several things to notice from these figures:
1. Since genetic drift is a random process, the direction and magnitude of changes cannot be predicted for any given population.
2. Magnitude of change is greater in small populations (N = 25).
3. Genetic drift causes alleles to disappear from populations thereby reducing genetic variation.
Natural selection: differential reproduction of genotypes -- if different genotypes leave different numbers of offspring, then the frequencies of those genotypes will change over time.
Genotypes that adapt individuals to their environment will survive and leave more offspring having the same genotype than genotypes that are less well adapted to their environments.
Natural selection was proposed as a mechanism of evolutionary change independently by Charles Darwin and Alfred Russell Wallace.
Several important points about natural selection:
1. the relative contribution of a genotype to future generations is called the fitness of the genotype -- genotypes with high fitness are favored by natural selection (increase in frequency), while genotypes with low fitness are selected against (decrease in frequency).
2. Genetic traits that aid the organism in surviving and reproducing in its environment are called adaptations -- adaptations, by definition, have high fitness and are favored by natural selection.
3. natural selection operates through agents in the environment -- ecological factors such as disease, predation, competition, etc. are agents of selection -- e.g. resistance to disease is often genetically determined -- nonresistent genotypes will be selected against by the environment (disease) -- genotypes that make an organism more difficult for predators to detect will be favored by selection.
4. fitness is environment-specific -- a genotype with high fitness in one environment may have lower fitness in another environment -- fitness of a genotype may change seasonally, may change with changes in the density of the population.
5. selection operates to increase the average fitness of the population -- operates to make population better adapted to its environment.
NOTE: of the evolutionary factors that we've discussed, natural selection is the only one that consistently increases a population’s adaptedness to its environment -- changes due to mutation, drift, etc. may be nonadaptive.
Let's look at how natural selection can change the genetic characteristics of populations.
Biologists recognize 3 types of natural selection:
1. Directional selection.
2. Stabilizing selection.
3. Disruptive selection.
We are going to use wing patterns in populations of butterflies to study how selection operates --
1. assume each population has genetic variation for wing pattern -- e.g. there are there patterns (phenotypes) in the population and these phenotypes are genetically determined.
2. phenotypes can be represented as a frequency distribution -- e.g. Figure 16.11.
Directional selection -- natural selection operates to shift underlying allele frequencies in a consistent direction.
Figure 16.11 again:
Orange arrows indicate phenotypes that are disadvantageous -- don't survive as well and/or don't leave many descendants -- can expect this distribution to shift to the right.
Directional selection occurs when their is a directional change in the environment or a new mutation produces a new phenotype that is more adaptive than existing phenotypes.
There are many examples of directional selection -- resistance of bacteria and other organisms to chemicals designed to control them (e.g. antibiotics, DDT).
Stabilizing selection -- selection operates against extreme phenotypes and favors intermediate ones.
Figure 16.13.
Notice the orange arrows indicating phenotypes being selected against.
Here's what happens over time -- notice how variation is reduced.
Stabilizing selection often results when environmental conditions have been constant.
Example: Human birth weight -- newborns with birth weights much more or much less than the average (ca. 7 lbs.) have increased mortality.
Disruptive selection -- selection against intermediate phenotypes in favor of extreme phenotypes.
Figure 16.15.
Here's what happens over time:
Disruptive selection can result in the formation of genetically distinct "subpopulations/"
Next time: Speciation.