Evolution: The Unifying Theme of Biology
I. Defining Evolution
A. Evolution is the change in the frequency of an allele in a gene pool.
B. Extension of this definition: Biologists argue that changes in the gene pool
can cumulatively account for the evolution of new species over time.
C. Darwin and Evolution:
Alfred Russel Wallace described natural selection at the same time.
The Theory of Natural Selection
1. Differences: All individuals within a species display unique, genetic differences. Some of these differences help them survive in their world and/or attract mates.
2. Competition: All species, given enough time, will overpopulate. The result will be ultimate competition for resources and mates.
3. Changing Gene Pool: In part, those that survive and pass on their genes will do so because they are well adapted to their environment. ("Survival of the fittest," a term never used by Darwin, is often used to sum this point. I don’t mind the term, but care should be exercised to remember that ‘fit’ means adapted to one environment, not strong, rich, white, male or any other definition that has creeped into the lingo of Social Darwinism over the years.)
II. Details of Evolution
A. Individuals don’t evolve, only populations can evolve.
We are born with our genetic code and this code doesn’t change (ignoring mutations in body cells). Evolution occurs because of genetic change, and this occurs during gamete formation (the goal of meiosis is genetic shuffling). Each new offspring represents a new genetic combination, and offspring are the result of populations.
B. Genes are the functional units of evolution.
Only those traits that are influenced by genes are affected by evolution. Acquired traits (nongenetic) don’t influence the course of evolution as such traits can’t be passed on to offspring. Mutations in somatic (body) cells don’t influence evolution as they can’t be passed on to offspring.
C. If genes are the functional units of evolution, mutations are the authors of the changes.
This is important: evolution doesn’t start from scratch! The process must work with the existing material, thus explaining homologous structures, vestigial structures and, amongst other things, the human eye!
D.In the evolutionary game, success in measured in the number of healthy offspring.
Desired traits result in the individual surviving, finding a mate, and passing on healthy offspring. Health may be a good thing to an individual, but it provides no evolutionary benefit if it doesn’t result in increased reproductive success.
E. There is no direction or goal in evolution.
Too often we draw an evolutionary ladder that lead to humanity, placing our species at the top of the evolutionary chain. It is true that humans are remarkably successful at this particular time, but if an asteroid strikes Earth, one would be better off as a beetle, bacterium or rat. There are no favored species in the process of evolution, just temporarily well-suited species for a given environment.
III. Darwin’s Insight
Darwin/Wallace
Lamark
IV. Microevolution
A. Variation, can exist in two forms:
1) Polymorphism, which means there are two or more specific alleles (or forms of a given
gene), or
2) Continuous variation, such as height (you don’t inherit a gene for being exactly 5’6",
rather it is a combination of genes, diet, hormones, etc.) in which numerous genes
and environmental conditions lead to many, many variations.
B. Measuring Evolution:
If evolution is the change in the frequency of an allele within a gene pool,
then evolution can be measured.
V. Hardy-Weinberg Equation
Hardy-Weinberg Equation:
An equation to measure the frequency of alleles in a balanced population.
1. There are no mutations,
2. The population is infinitely large,
3. There is no migration or immigration, meaning no interbreeding with another
population,
4. Mating is random (the presence or absence of the allele doesn’t give one an advantage
in breeding), and
5. All individuals have an equal chance of surviving and
breeding.
B. If the assumptions are true for a population, then the allele frequency shouldn’t change. A change in frequency indicates an evolutionary force is acting on the population.
p2 + 2pq + q2 = 1
p + q = 1
In this equation, p2 = the frequency of AA, 2pq = the frequency of Aa, and q2 = the frequency of aa.
For a trait with two forms (alleles A or a), the sum of the frequencies of AA,Aa, and aa should equal 1, or 100%.
Sample Problem
30% of a population is made up of homozygous recessive individuals.
(= p)
(=q)
p2 + 2pq + q2 = 1
p + q = 1
30% = q2
.54 = square root of q2
q = .54
p = .46
Sample Problem
50% of a population is made up of homozygous recessive individuals.
a) What is the relative frequency of the dominant allele?
b) Recessive allele?
c) What % of the population is heterozygous?
p2 + 2pq + q2 = 1
p + q = 1
a)
b)
c)
Sample Problem
9% of a population is made up of homozygous recessive individuals.
a. What is the relative frequency of the dominant allele?
b. Recessive allele?
c) What % of the population is heterozygous?
p2 + 2pq + q2 = 1
p + q = 1
a)
b)
c)
Sample Problem
In a population, 1 in 1000 individuals display a recessive disease.
a. What is the relative frequency of the dominant allele?
b. Recessive allele?
c) What % of the population is heterozygous?
p2 + 2pq + q2 = 1
p + q = 1
a)
b)
c)
Sample Problem
In a population, the relative frequency of the dominant allele is .60.
a) What is the relative frequency of the homozygous genotype?
b) Dominant gentoype?
c) What % of the population is heterozygous?
p2 + 2pq + q2 = 1
p + q = 1
a)
b)
c)
V. Causes of Evolution
A.One true cause: Environmental Conditions.
A "fit" organism is able to survive and reproduce in a given environment.
B. Example: Peppered Moths
1. Peppered moths exist in two morphs:a dark and a light phase.
2. Prior to the industrial revolution, many trees in England had lighter bark and
therefore the light morph tended to blend in with the trees and therefore
escape predation .
3. Industrial Revolution darkened trees, allowing dark
morphs to blend in a avoid predation.
4. Result = increase in dark morph allele, which is
evolution.
C. Selective pressure – Environmental condition that favors one phenotype over another.
1. Disruptive selection – When selective pressures result in two or more distinct morphs in a population.
2. Stablizing pressure – Intermediate forms of a trait are favored.
3. Directional Pressure – Selecting for a specific trait.
E. Example 2: Antibiotic Paradox.
At one point, antibiotics (drugs with special properties that disrupt the life cycle of prokaryotes but not eukaryotes) were occasionally used for the treatment of specific diseases. However, in recent years, the use of antibiotics has exploded. They can now be found in soaps, cleaning products, dishrags, cutting boards, animal feeds and are commonly over prescribed (used when the disease would fade without the use of antibiotics or used for nonbacterial diseases). The result? We are creating an environment with low levels of antibiotics, presenting an evolutionary pressure/selective pressure on bacteria to grow stronger and resistant to antibiotics
VI. Extreme Phenotypes
A. Stabilizing pressures –
Pressures that tend to discourage extreme phenotypes.
B. Balanced Polymorphism –
When selective pressures favor two or more different morphs within a population.
C. Sexual Dimorphism –
Special case of balanced polymorphism. Why would there exist females lacking ornamentation while males seem to be in a race to be more ‘flashy’? When such sexual dimorphism exists, you should immediately think of reproductive advantage. Male with bright feathers or large antlers must enjoy a reproductive advantage over those with dull
colors or small antlers.
Case Study: Sickle-cell Anemia
Homozygous Dominant: Healthy red blood cells.
Heterozygous: Some misshaped red blood cells: increase likelyhood of surviving malaria, increased likelyhood to suffer from mild to severe effects.
Homozygous recessive: Fatal.
VII. Gene Flow
A. Genetic drift is random change in gene frequency. Genetic drift often occurs when a population gets small.
B.The ultimate genes expressed by a population will be a reflection of the founder community, or those individuals that serve as the foundation of the new community.
C. Bottleneck: When a once large population is drastically reduced, this is known as a bottleneck.
D. Inbreeding: Lack of genetic diversity. Often, bottlenecks are damaging as
inbreeding occurs