Modern Theories of Evolution

The Hardy-Weinberg Equilibrium Model

According to the modern definition, evolution is a change in frequencies of the alleles in the gene pool of a population.  Remember the bunny lab?  It used to be thought that dominant alleles would swamp out recessive alleles.  But Hardy and Weinberg came up with an algebraic equation that describes the frequency of alleles in a population ( p² + 2pq + q² = 1) and demonstrates that the frequency of these alleles will remain constant from one generation to the next, provided that certain conditions are met.

1. Mutation is not occurring
2. Natural selection is not occurring
3. Mating must be entirely random
4. The population must be sufficiently large to be statistically relevant
5. No new variants can be introduced into the gene pool or leave
6. All individuals are equally successful at surviving and reproducing

 These conditions can never all be met, so evolution will inevitably occur.  The equation made it possible to predict the allele frequencies in entire populations at a point in time, not just individuals.  (Punnett Squares).  It also allows scientists to see how populations are evolving by comparing one generation to the next algebraically.

The Mechanisms that Can Cause Evolution to Occur (4 Main Processes, 2 Ancillary)

• MUTATION

Mutations are chance alterations of genetic material that produce new variation.  They occur during DNA duplication in cell division and fall into four categories.

1.	DNA base substitution, insertion, and deletion
2.	unequal crossing-over and related structural modifications of chromosomes
3.	partial or complete gene inversion and duplication
4.	irregular numbers of chromosomes

In order for a mutation to be subject to natural selection, it must be expressed in the phenotype.  Natural selection favors adaptively advantageous expressions and even when mutations are recessive, they can add to the genetic load of a population as a hidden variability that may show up in later generations.  Mistakes have made evolution possible.

• NATURAL SELECTION

Darwin had trouble with the concept of ever growing populations.  He knew that organisms produced more offspring than were needed to keep the population steady, but he couldn't at first understand why if that were the case, why we weren't up to our armpits in flies.  Do you? He decided that natural selection was the answer... that the environment selected the best adapted specimens.  We now know that the environment selects the individuals with the best suited genotypes to survive.  (Remember though, that selection operates on an individual's phenotype).  For evolution to happen, natural selection must occur for or against one or more of an individual's allele pairs.

Examples:

Selection against one homozygote results in a lower frequency of that homozygote and a higher frequency of the other resulting homozygote.  Remember the bunny lab and that the recessive allele never disappeared totally.  Another example of selection against a recessive allele is juvenile diabetes, which can now be treated, so the frequency of the allele is not decreasing.  The most striking example of this selection is the homozygous inheritence of the recessive gene CCR5- delta 32 which in its recessive form, gives the individual immunity to the AIDS virus.  See movie clip.

An example of selection against both homozygotes is the case of sickle cell anemia and heterozygous resistance to malaria.  See movie clip.

There are other examples, but what do you think happens when natural selection selects against all genotypes?

Here are some ethical considerations.  Modern Humans have been able to alter the natural pressures for or against genes with medical intervention and counseling.  They have also altered the environment with mutagenic chemicals and radiation.

• Diabetes used to be fatal
• Retinoblastoma (childhood cancer of the eye) was also a dominant trait that caused early childhood death 100% of the time.  Now it's treatable.

What do you think of this turn of events?

We are increasing our genetic load of harmful genes in our populations.

• SMALL POPULATION SIZE EFFECTS

Genetic Drift

In small, reproductively isolated populations, special circumstances can be at work aside from mutation and natural selection.  These evolutionary changes can be due to chance.  For example, when we flip a coin 10 times, the chance that it will be exactly 5 heads and 5 tails is small, but as the number of tosses increases, the chance that the heads and tails will be 50%/50% increases.

With a heterozygous cross, the expected probability is 25% AA, 50% Aa, and 25% aa. (Make a punnett)  But if there's even a chance deviation from the expected, for example, if none of the offspring have a homozygous recessive, the odds change to 33.3 AA and 66.7% Aa.  This chance deviation is the norm... no one gets the expected probable offspring, but with a large enough population, things even out.  But if the group of humans is a closed breeding group, like the Amish, isolated from other gene pools, then genetic anomalies can take precedence.

Founder Principle

This anomaly occurs when a small amount of people have many descendents surviving after many generations.  There are then high frequencies of specific genetic traits inherited from a few common ancestors.

• ex.  There was a woman in the remote Lake Maracaibo region of Venezuela named Maria Conception Soto who had a rare degenerative nerve disorder called Huntington's disease in the early 1800's.  Because the disease does not show up until after the child-bearing years, she was able to have many children before she died.  Because of her many descendents, this region in Venezuela has a very high rate of Huntington's disease, much higher than the rest of the world.
• ex.  The Old Order Amish have a high rate of microcephaly because of a single Amish couple who migrated to America nine generations ago.  In the last 40 years, 61 babies with microcephaly were born to 23 Amish families.

Bottleneck Effect
Dramatic natural selection in catastrophic periods where most individuals died, but then the few that were left were reproductively successful but created a population that is very reduced in genetic diversity.  The bottleneck is usually caused by a natural catastrophe such as an epidemic, storm, earthquake or volcanic eruption..  Sometimes there can be a combination bottleneck and founder effect if in the wake of the natural disaster, a surviving individual is carrying a genetically inherited trait or disorder. 

• GENE FLOW

1.  This can occur with migration.  For example, if all redheads left Scotland, the next generation would have dramatically reduced frequency of red hair.

2.  Can occur without migration.  For example, during the Vietnam War, many men had children with Vietnamese women and significantly changed the gene pool in that country

• RECOMBINATION

During meiosis, when the chromosomes break apart to form the sex cells, there is sometimes a crossing over at the ends of homologous before recombination in the zygote.  If this occurs, it creates a new sequence of alleles to be selected for or against.  It in itself is not evolution, but provides greater diversity.

• NON-RANDOM MATING

In all human populations, mating is not random.  Nor is it in the animal population, or for that matter in the farming population.  Humans and animals look for specific traits in a mate, for humans these selections are based on social mores and rules... ie. culture.  In other words, mate selection is a powerful force of evolution, not unlike natural selection.

Tale of the Peacock PBS Evolution Library video

When farmers use breeding practices

Sources :  O'Neal, Dennis.  (2011).  Physical Anthropology Tutorials.  Retrieved from http://anthro.palomar.edu/tutorials/biological.htm.

Please see the Anthropology Tutorials to clear up any questions or to take the practice quiz.