Introduction to Evolution

Evolution is Descent with Modification

• Evolution: descent with modification

• Decent: traits from one generation are passed along to future generations

• Modification: ultimate source of genetic modification is mutations-- provides new versions of genes (novel alleles)

• Evolution is a property of groups of organisms

• Descent without modification: all organisms on earth would look exactly like the earliest organism

• Identical twins look the same but don’t have the exact same genes due to mutation

• Two living things tend to resemble each other more if they share a recent common ancestor

Historical Relationships Among Genes

• You are a genealogical descendant of a large group of people, but a genetic descendant of a relatively small sample of them

• Chance you share genes with an ancestor is very very small

Charles Darwin

• Created a tree of life diagram genealogy: an explanation of the historical relationship among organisms

A Brief History of Evolutionary Thought

• Scientists agreed that populations evolved even before Darwin, they just didn’t know how

• Lacked a mechanism for evolution

Early Theories of Evolution: Erasmus Darwin & Jean-Baptiste Lamarck

• Erasmus Darwin: Darwin’s grandfather, proposed that species change, but no evidence and no theory for how the change occurs, lacked a mechanism of evolution

• Jean-Baptiste Lamarck: French naturalist attempted to explain mechanisms of evolution before Darwin was even born

  • Looked at fossils and noted organisms change over time

  • Discovered unimportant traits atrophy (reduced or lost)

  • Concluded that organisms inherit traits that their parents acquired, allowing them to become better adapted to their environment over time

Charles Darwin and the Theory of Natural Selection

• Darwin visited the Falkland Islands in 1833, found distinct versions of Antarctic Foxes in the West and East

• Visited the Galapagos Islands and found 13 varieties of finches

• He was influenced by Charles Lyell’s Principles of Geology: Earth’s current geological features were caused by events that happened in the past, such as wind, rain, erosion, earthquakes

• And Thomas Malthus’ An Essay on the Principle of Population: populations may grow exponentially/geometrically at first, but growth will decrease due to limited resources, food grows arithmetically

  • Malthus was wrong, Industrial Revolution and Green Revolution brought changes to allow for more food growth

Evolution Through Natural Selection

• Natural Selection: differential reproductive success of certain individuals with more or less desirable traits

• Evolution: change in biological populations over time

• Natural Selection is one mechanism by which evolution occurs

• Alfred Wallace independently developed a similar theory of natural selection

• Darwin and Wallace are credited with the idea of evolution by natural selection

• Real breakthrough: Darwin’s book On the Origin of Species

The Forces of Evolution:

• Evolution occurs when there is a change in allele frequency in a population over time

• 6 forces of evolution, 4 cause a change in allele frequencies in a population, 2 which cause a change in genotype frequencies or distribution of alleles in the genome

Four Forces that cause Allele Frequencies to Change

• Mutations:

  • Mutation: heritable change in DNA, ultimate source of genetic variation

  • Genetic mutation is ultimately the origin of all genetic variation

  • Mutations occur randomly in respect to fitness, but not in the context of organism, genomic, age, or environment

  • Can be beneficial, neutral, or harmful

  • Per-nucleotide base substitution mutation rate is very low due to DNA polymerase accuracy

  • Human genome experiences about 50 mutations per generation

  • E. coli only 1/1000 per generation

• Random Genetic Drift:

  • Genetic drift: change in allele frequency due to:

  • Random sampling of gametes: gametes containing one allele may be randomly chosen more than another

    • Genetic drift due to random sampling has a more significant evolutionary impact if population size is small

    • Variance: measure of dispersion, can be calculated as the sum of the squared deviations from the mean, divided by number of observations -1

      • Σ(x1-x)^2/(n-1)

    • Variance in allele frequency determines strength of genetic drift

• Random Death and Survival of Individuals:

  • External event, natural disaster, alleles no longer in the gene pool

  • You and your friend are walking, you fall into a manhole and die, genes no longer in the gene pool, nothing to do with fitness

• Random Variation in # of Offspring:

  • Some organisms randomly have more offspring, has nothing to do with fitness

  • Plant makes 2 seeds, one lands on dung and one on a rock, random and has nothing to do with fitness

• Genetic drift is random, natural selection is not

• Migration (Gene Flow):

  • Migration refers to gene flow, or movement of individuals (and their alleles) from one population to another

  • Gene flow and genetic drift:

    • Gene flow is migration, genetic drift is unpredictable, random fluctuations in allele frequencies due to a population’s small size

    • Gene flow increases genetic diversity, genetic drift can reduce genetic diversity

    • Both have larger impact when initial population is small

• Natural Selection:

  • refers to the differential reproductive success of certain individuals with more or less desirable traits

Two Forces that Cause Genotype Frequencies (not allele frequencies) to Change

• Recombination:

  • Rearrangement of genetic material, particularly during crossing over, during meiosis 1

  • Changes distribution of alleles in genome

• Nonrandom Mating:

  • Situation in which gametes are not equally likely to fertilize any other gametes, nonrandom mating in human populations

  • Changes genotypic frequencies, not allele frequencies

Hardy-Weinberg Equilibrium:

• Frequency of alleles is same from one generation to the next

• p^2 + 2pq + q^2 = 1

• p^2 = AA freq of dominants

• 2pq = Aa freq of heterozygotes

• q^2 = aa freq of recessives

• Allele frequencies should be the same between one generation to the next, if not, we can conclude that evolution is occurring

• If a population is not at Hardy-Weinberg equilibrium, a single generation of truly random mating will restore that population to equilibrium

Modes of Selection:

• Selection: general term that refers to the differential reproductive success of individuals who are more likely than others to survive and reproduce

• Artificial selection:

  • when humans intentionally produce crops, livestock, or pets that have desirable traits to humans

• Natural selection:

  • individuals who are better adapted to their environments tend to survive and produce more offspring

  • Adaptation: a beneficial mutation that is favored by natural selection

  • Individuals with adaptations are more likely to survive and reproduce, have higher fitness

• Qualitative traits: discrete quantities and are influenced by alleles at a single locus or just a few loci

  • discrete: “black” “white”

• Quantitative traits: continuous variation and influenced by a larger group of genes

  • body size, height, skin color

• Directional selection: extreme phenotype is the fittest, trait moves in direction of the extreme phenotype over time

• Stabilizing selection: occurs when genetic variation decreases as the population stabilizes around one intermediate trait

  • favors average individuals

  • reduces genetic variation by reducing variance, but does not change the mean value

• Disruptive selection: occurs when an intermediate trait is selected against and tends towards both extreme phenotypes

  • taken to its extreme could cause emergence of 2 separate species

  • prevalent in environments where more than one strategy works well for organisms

• An average phenotype is most fit = stabilizing

• One extreme phenotype is most fit = directional

• Both extreme phenotypes are most fit = disruptive

How Genetic Polymorphisms are Maintained

• Polymorphisms: multiple forms/alleles in most populations

• Heterozygote advantage: occurs when heterozygous individuals are more fit than homozygous individuals

  • hemoglobin in humans

• Geographic Variation in Natural Selection: natural selection acts in different ways on populations that live in geographic areas

  • selective pressures are different in different environments

• Frequency-dependent selection: occurs when fitness depends on the relative frequency of a genotype or phenotype in the population

  • if common genotypes have lower fitness than rare types, then rare types will be selected for

• Sexual Selection: special case of natural selection in which certain characteristics of one sex are favored, either bc the other sex prefers them, or bc those characteristics allow the individual to outcompete other members of the same sex for mates

More on Natural Selection:

Darwin noted the following:

1. More organisms are born than can survive to reproduce

2. The limitations to population growth create a struggle for existence, in which some organisms will survive and others will not

3. Populations exhibit significant variability

4. Some variation must be heritable

5. Any variation that confers an advantage will tend to increase in frequency in the population over time. Any variation that confirms a disadvantage will tend to decrease in frequency in the population over time

6. Therefore, populations will change over time-- evolve

More on Fitness

• Fitness: an individual’s measure of its contribution to the next generation

• Absolute Fitness (W): refers to a lifetime productive success, which is approximately the probability that an individual will survive multiplied by number of offspring the individual has

• Relative Fitness (w): is an individual’s fitness relative to some reference type

  • w = W/Wmax

What Maintains Genetic Variation

• Temporal Variation in Selection: selection could vary over time

• Fitness tradeoffs

  • Pleiotropy: situation in which a single gene impacts multiple traits

  • Antagonistic pleiotropy: occurs when a gene’s impact on one trait conveys a fitness advantage, while its impact on another trait conveys a disadvantage

• Mutation Selection Balance: MSB equilibrium that occurs when purifying selection removes deleterious variation at the saem rate that mutation introduces new variation

More on Adaptations:

• Adaptation (noun): refers to a trait that increases in frequency in the population

• Adaptation (verb): process by which the favored trait arises

• Neutral evolution: a change in populations solely due to the effects of mutation and random genetic drift

• Indirect Selection on Correlated Traits: Legs to become longer (directional selection) and indirect effect on arm length will be shorter

Group Adaptation:

• Group selection: differential selection of groups based on heritable group variation

• Does not contradict Darwin’s theory of evolution

Inbreeding:

• Inbreeding: occurs when closely related individuals breed one another

• Illegal/stigmatized: inbreeding increases the frequency of deleterious phenotypes in populations because it reduces the frequency of heterozygotes in each generation

Sex and Recombination:

• Lateral gene transfer: how prokaryotes recombine

The Costs and Benefits of Sex:

Costs:

• Ecological costs: time, energy, resources to find a mate, could have sexually transmitted diseases

• Genetic costs: loss of adaptive combinations of alleles, two-fold cost (asexually reproducing populations grow twice as fast as sexually), the cost of meiosis (only 50% chance that each particular gene copy will be included in any particular gamete)

• Sexual evolved after asexual, must offer advantages that outweigh disadvantages listed above

Advantages:

• Bringing together adaptive combinations of alleles, populations adapt much faster over time

• Removal of deleterious mutations in infinite populations: asexual reproduction has no way to eliminate deleterious mutations

• Red Queen hypothesis: selective changes over time in such a way that alleles that used to be deleterious are now advantageous, and vice versa

  • worm findings confirmed the Red Queen hypothesis and highlighted importance of sexual recombination