The Modern Synthesis

Goals

1. To understand the concepts of the gene pool and allele frequency, and how they relate to the study of evolution.

2. To understand why evolution won't occur in Hardy-Weinberg equilibrium, and what will happen if some of the rules are broken.

3. To understand the implications of natural selection in the modern synthesis, and how genetic diversity can be maintained in the face of selection.

Outline

The Modern Synthesis (Neodarwinism)

In the 30's a reconciliation between evolution and genetics was worked out, so that by the 40's the Modern Synthesis emerged: a refinement of Darwinism that incorporates genetics. It includes more mechanisms of evolution than just selection.

Population genetics considers how genes behave in populations. A gene pool is the population considered as, well, a pool of genes.

Evolution is redefined as a change in allele frequencies over time.

Allele frequency is the proportion of the alleles at a locus that are a particular allele

p = the frequency of the dominant allele (e.g., A)

q = the frequency of the recessive allele (e.g., a)

Hardy-Weinberg equilibrium

The Hardy-Weinberg Principle states that a population will not evolve if:

1. The population is large
2. There is no immigration or emigration (no gene flow)
3. There is no net mutation
4. Mating is random (no mate choice)
5. All genotypes have equal reproductive success (no selection)

(also, in equilibrium, genotype frequencies will be described by the equation p² + 2pq + p² = 1.)

This is a big null hypothesis -- what's interesting is how evolution will occur if the Hardy-Weinberg conditions are not met....

So let's break a few rules:

1. No net mutation

In real life mutations are rare events, and cannot effect a significant change in allelic frequencies. However, mutations are important in the Modern Synthesis as the source of genetic variation in a population.

2. No migration

Allelic frequencies of a population's gene pool can be altered if there is an influx of individuals or gametes from another population that has different allele frequencies. This is called gene flow, and is a common thing, especially in humans.

3. Large population size

Small populations are liable to genetic drift, a change in allele frequency caused by sampling error. This can result in alleles being fixed, and a loss of genetic variation in a population.

There are two situations where genetic drift commonly occurs:

1. Founder effect
2. Genetic bottleneck

In the above examples evolution was non-adaptive: changes in allele frequencies did not result in populations that were better adapted (more "fit"). Natural selection is the only means of adaptive evolution.

Adaptive vs. non-adaptive evolution

There are several mechanisms for evolution:

1. Genetic Drift
   1. Founder effect
   2. Genetic bottleneck
2. Gene flow
3. Mutation (less a mechanism for evolution than a source of new alleles)
4. Natural Selection

The only one of these that can result in adaptive evolution is natural selection.

The outcome of selection is greater adaptation, which can be measured in terms of fitness.

The way natural selection functions depends on how the traits being selected for or against are inherited. We will look at single-gene traits (as opposed to polygenic inheritance) where the trait selected against is dominant and recessive.

Kinds of selection

• stablizing
• directional
• disruptive

Selection against the dominant phenotype

Dominant lethal traits can be eliminated from the gene pool in a single generation. Even traits that are only deleterious are eliminated rather quickly

Selection against the recessive phenotype

Selection against recessive alleles always proceeds very slowly, even with lethal recessive alleles' The lower q is, the greater the proportion of the recessive allele that is present in the heterozygotes (i.e., not expressed). This is what underlies the occurrence of many genetic diseases in humans. The alleles may have become so (relatively) common because of genetic drift in the past.

Natural selection & variation

Q: How can natural selection allow the maintainance of genetic variation in a population?

A: assuming variation isn't neutral (i.e., that there is selection):

1. Diploidy -- recessive genes may "hide" from selection (selection is weak)
2. Balanced polymorphism -- selection favors genetic diversity
   1. Heterozygote advantage: If selection favors the heterozygote, both alleles may be preserved (neither will be eliminated through selection. e.g., sickle-cell anemia
   2. Patchy environment
   3. Frequency dependent selection (the effect of selection decreases selection pressure)

 

Vocabulary

click here to go to modern synthesis vocabulary