Anthropology Mini Essays
kevcnypngh7315
lecture_6_the_modern_synthesis.pdf
Lecture 6—The Modern Synthesis
Today’s Questions
• What is the “modern synthesis” or the “synthetic theory of evolution”?
• What are the factors that produce and distribute variation in populations?
• Why do some deleterious conditions persist in populations?
The Modern Synthesis
• Melding of Darwin and Mendel in the early to mid 20th century – Expansion of evolutionary theory
• More precise definition of evolution – A change in the frequency of alleles in a
population over time
• Evolution as a two stage process: – The production and redistribution of variation
– Natural selection acting on that variation
The Modern Synthesis
• Natural Selection acts on variation
• Factors that produce and redistribute variation:
– Mutation
– Gene Flow
– Genetic Drift
Mutation
• Mutation—an alteration of genetic material such that a new variation is produced – Point mutation or substitution—a change in a single
nucleotide base – Non-disjunction—failure of chromosomes to separate
correctly; can result in cells with extra or missing chromosomes
– Insertion or deletion—insertion of extra or deletion of DNA
• Only mutations that occur during meiosis will have consequences for natural selection.
Mutation
• Point mutation
Mutation
• Nondisjunction
Mutation
Down Syndrome is a result of non- disjunction
Mutation
• Insertion
• Deletion
Gene Flow
• Changes in the frequency of alleles in a population as a result of interbreeding between populations.
• Gene flow decreases genetic variation between populations but increases variation within populations
Gene Flow
• Gene flow — also called migration — is any movement of individuals, and/or the genetic material they carry, from one population to another. Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries. If gene versions are carried to a population where those gene versions previously did not exist, gene flow can be a very important source of genetic variation. In the graphic below, the gene version for brown coloration moves from one population to another.
• http://evolution.berkeley.edu/evolibrary/article/evo_21
Gene Flow
http://evolution.berkeley.edu/evolibrary/images/evo/geneflow_beetles.gif
Genetic Drift
• Random fluctuation in gene frequencies due to a variety of forces. – Small population size
– Bottlenecks
– Non-selective forces • Environmental catastrophes or change
• Random chance
• The chance loss of rare genes or fixing of certain genes reduces genetic diversity.
Genetic Drift
• Genetic drift — along with natural selection, mutation, and migration — is one of the basic mechanisms of evolution.
• In each generation, some individuals may, just by chance, leave behind a few more descendents (and genes, of course!) than other individuals. The genes of the next generation will be the genes of the "lucky" individuals, not necessarily the healthier or "better" individuals. That, in a nutshell, is genetic drift. It happens to ALL populations — there's no avoiding the vagaries of chance.
• http://evolution.berkeley.edu/evolibrary/article/evo_24
Genetic Drift
• Founder Effect—a small portion of a larger population migrates
– reproductive isolation
– any rare allele will become more common over time
• Genetic Drift decreases genetic variation within populations but increases variation between populations
• Combined with natural selection and mutation, genetic drift can lead to speciation
Genetic Drift
http://evolution.berkeley.edu/evolibrary/images/evo/beetles_mech3.gif
Founder Effect
The Genus Pan
• The Last Common Ancestor of Pan paniscus (bonobos) and Pan troglodytes (chimpanzees) lived throughout equatorial Africa
– ~2-3mya Course of the Congo River changed, cutting off gene flow between a larger population to the north and a smaller population to the south
– Mutations + Natural Selection + Genetic Drift Speciation
• Speciation—the evolution of new species
Genetic Drift + Natural Selection = Speciation
Photo by Me
Photo by Kyleb Wild
Adapted from de Waal and Lanting 1997
Why do some deleterious conditions persist in populations?
• Evolutionary theory predicts that fatal genetic conditions will be selected out of populations and, therefore, occur at a relatively low rate and decline over time. However, some fatal genetic conditions persist at a higher rate in some populations than others.
• Why?
– Evolutionary theory can explain this, too.
Sickle-Cell Anemia
• Hemoglobin Alleles
– HbA = Normal Hemoglobin
– HbS = Hemoglobin S (sickle-cell)
• Genotypes and Phenotypes
– HbAHbA = Normal Hemoglobin
– HbAHbS = Sickle-Cell Trait
– HbSHbS = Sickle-Cell Anemia
Malaria and Sickle-Cell Anemia
• Sickle-cell TraitResistance to Malaria S
– Selection for individuals with SCT means more recessive alleles in the population
– Increases the likelihood that two heterozygotes will reproduce, which increases the likelihood of homozygous recessive offspring
– Sickle-cell anemia is more common in malarial environments
Malaria Around the World
Sickle-cell anemia is most common in the dark red areas
Natural Selection
• Natural Selection is the primary mechanism of evolution
– Selection acts on variation introduced by mutation and gene flow
– Selection acts from conception to reproduction in the life of an organism
Natural Selection
• The cost or benefit of any particular genotype (and phenotype) is dependent upon the environment in which the organism lives. – Those that are well adapted are more likely to
reproduce and to produce more offspring that reach reproductive age.
• By selecting for advantageous traits, or against disadvantageous traits, natural selection alters allele frequencies by differential survival and reproduction.
Putting It All Together—Tay- Sachs and Evolution
• Evolutionary theory predicts that fatal genetic conditions will be selected out of populations and, therefore, occur at a relatively low rate and decline over time. However, some fatal genetic conditions persist at a higher rate in some populations than others.
• Why?
– Evolutionary theory can explain this, too.
Putting It All Together
• The Diamond article pulls everything we have done so far together:
– Mendelian Genetics
– Protein Synthesis
– Natural Selection
– Modern Synthesis (he talks about mutation, gene flow, and genetic drift)
Autosomal Recessive
• Tay-Sachs is an autosomal recessive condition. Individuals must inherit 2 copies of the recessive allele to be affected by Tay- Sachs.
http://019221f.netsolhost.com/gpx/twoCarrierFamily.gif
Tay-Sachs
The gene associated with Tay-Sachs is found on the 15th chromosome.
http://1.bp.blogspot.com/-KZWFlcs0JYE/TV7LxuoeKZI/AAAAAAAACe8/OZz9Yoh_e7g/s1600/Tay%2Bsachs%2Bdisease.png
http://hattix19.wikispaces.com/Tay-Sachs+Disease
Go to this page to learn more…
• http://ghr.nlm.nih.gov/condition/tay-sachs- disease
• It reviews the basics of the disease (Diamond does this, too).
• Be sure you understand the relationship between hexosaminidase A and GM2 ganglioside. THIS IS IMPORTANT.
Tay-Sachs In the Ashkenazim
• The Ashkenazim are Eastern European Jews
• Tay-Sachs occurs at a higher rate among the Ashkenazim than other populations. As people interested in evolution, we wonder WHY?
– What can explain the high rate of TS in Ashkenazi Jews?
Diamond’s Counterarguments
• Diamond explicitly engages 3 counterarguments or alternative hypothesis for the high rate of TS among the Ashkenazim 1. Higher rate of mutation than other populations
2. Acquired it from the Khazar Turks
3. Genetic drift and founder effect
• Diamond refutes each of the possibilities. How?
Diamond’s Argument
• Diamond ultimately argues that there is a fourth possible explanation and this is his thesis: “something about [the Ashkenazim] favored accumulation of GM2 ganglioside and related fats” (196).
• What was this?
• Check out the Diamond Reading Guide
