Evolution and Natural Selection

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Evolution (what is it and what are the two types)

• Evolution is the process of biological change that makes descendants differ from their ancestors (happens over generations)

  • There are two types:

    • 1. Microevolution: Evolutionary change affecting a single population

      • Ex: peppered moths during the industrial revolution

    • 2. Macroevolution: Evolutionary change affecting species across populations

      • Happens worldwide over millions of years

        • Ex: Whales evolving from terrestrial beings

Darwin’s Observations

• Through Darwin’s observations, he discovered 3 major patterns

  • 1. Similar species inhabit similar ecosystems

  • 2. Different but related species inhabit different habitats within a region

  • 3. Fossil records show there are extinct species that are similar to modern species

Mechanisms for Evolution

• While Darwin discovered natural selection, that is not the only was evolution happens

  • 1. natural selection

  • 2. mutations

  • 3. genetic drift

  • 4. gene flow

Evolutionary mechanism: natural selection

• Individuals with traits better suited for their environment are more likely to survive. reproduce, and pass those traits onto their offspring in the next generation

• “Survival of the fittest”

Fitness (viability & fertility, absolute vs. relative)

• Fitness is a measure of an organisms reproductive success

  • Ability to survive to the age of reproduction, find a mate, a produce viable offspring

    • Viability: Ability to live to the age of reproduction

    • Fertility: The expected number of offspring an organism produces at maturity

• Absolute fitness (W): The average amount of viable offspring produced by individuals with a specific phenotype or genotype

• Relative fitness (W): The reproductive success of a genotype compared to the most successful genotype of the population

  • Natural selection is dried by the differences in relative fitness

Absolute Fitness (W)

• Absolute fitness (W): the total reproductive success of a genotype (how many offspring it produces that survive to reproduce)

  • W = the average number of viable offspring per genotype

  • AA = 10 viable offspring => W = 10

  • Aa = 7 viable offspring => W = 7

  • aa = 3 viable offspring => W = 3

Relative fitness (w)

• Relative fitness (w): the reproductive success of a genotype compared to the most successful genotype of the population

  • The highest relative fitness you can have is 1

• Ex: w(AA) = 10

  • 10/10 = 1 (highest rf)

• w(Aa) = 7

  • 7/10 = 0.7

• w(aa) = 3

  • 3/10 = 0.3

How does natural selection work?

• The environment (not humans) influence fitness

• Populations continue to change as they become better adapted or as their environment changes

• Natural selection does not make an organism “better,” it makes it more likely to survive and reproduce

Selection Pressures

• Selective Pressure: an environmental factor that influences which traits are advantageous and which as disadvantageous

  • Selective pressures “select” which individuals are more likely to survive and reproduce

• Examples: predators, climate, disease, competition

Change in allele frequencies

• Allele Frequency: how much an allele (not phenotype) shows up in a population

  • 10 organisms => 20 alleles (every organism has 2 alleles)

• Natural selection changes what alleles are more common over time

  • Cause and effect chain of event

    • Genetic variation exists

    • The environment creates selective pressures

    • Some traits have higher survival/reproduction rates

    • The alleles that those individuals have are passed down more often => alleles from that individual have increased in frequency

Evolution and Allele frequencies

• Evolution => change in allele frequencies in a population over time

  • Individuals do not evolve, populations do

  • Evolution takes place over many generations, not within a lifespan

  • Natural selection acts on phenotypes but changes genotype (allele) frequencies

  • If allele “E” increases, that means that allele “e” decreases

• Natural selection is a mechanism by which evolution works, allele frequencies are evidence for that evolution

Evolutionary mechanism: mutations

• Mutations: random differences in genetic code that create more variation

• Happens through:

  • Crossing over during meiosis

  • Errors in DNA replication, exposure to radiation, or chemical agents

• This random appearance of new genotypes and phenotypes allow more traits to be acted on by natural selection

• Biggest mechanism of genetic diversity

Evolutionary mechanism: genetic drift

• Genetic drift: random change in allele frequencies within a population, over time, during to chance events

• 1. Founder effect

  • A small group of individuals start a new localized population. The population’s genetics in the later generations will reflect the genetic makeup of those who started the population

• 2. Bottleneck effect

  • a. Large scale death of a population due to a natural disaster. The populations genetics in the later generations will reflect the genetic makeup of the survivors

  • b. The survivors did not survive due to their genetic traits, but by random chance

Evolutionary mechanism: Gene flow

• Gene flow: the movement of genes from one population to another through the migration of individuals

  • Prevents populations from diverging into different species

Molecular clock

• Evolution involves changes in DNA sequences (genomes)

  • Analyzing DNA sequences can tell scientists how closely species are related

  • Molecular clock: using the mutations rates in DNA to estimate the time that two species have been evolving independently will tell how long ago they have diverged from a common ancestor

    • Many mutations have no effect on phenotype so they will get passed down without any ill effect on the species

    • There is a predictable pace of mutation, allowing for estimations from scientists

      • The more differences = the more time between shared ancestors

Speciation

• Speciation: the formation of a new species by evolution from pre-existing species

  • Gene pool gradually becomes different and are no longer able to reproduce

  • Some kind of isolation must occur

• There are 3 types of isolation

  • Behavioral

  • Geographic

  • Temporal

Behavioral isolation

• Populations are capable of interbreeding but do not because of their behaviors (not attracting to one another)

Geographic isolation

• Populations are physically separated from each other leading to genetic drift

  • Ex: sea separating them

Temporal isolation

• Species begin reproducing at different times

  • Ex: nocturnal and non-nocturnal animals, flowers that bloom at different times

Population genetics

• The study of how allele frequencies in a population change over time

• Evolution = change in allele frequency

• Population: a group of interbreeding organisms

• Gene pool: all the alleles within a population

Measuring allele frequencies

• Frequency = portion of an allele in the gene pool

• p = frequency of the dominant allele (A)

• q = frequency of the recessive allele (a)

• p + q = 1

  • Ex: p = .7 q = .3 => .7 + .3 = 1

Hardy-Weinberg Equilibrium

• HWE describes a population that is NOT EVOLVING, it is a null model

• In order for a population to be at HWE, populations must have

  • No mutation

  • Random mating

  • No natural selection

  • Extremely large population

  • No gene flow

• Real populations almost always violate these conditions

HWE

• Scientists use HWE calculation to create a baseline, or null hypothesis, to compare real populations. This allows scientists to analyze evolutionary forces

• HWE equation: p² + 2pq + q² = 1

  • p² = frequency of homozygous dominant genotype (AA)

  • 2pq = frequency of heterozygous genotype (Aa)

  • q² = frequency of homozygous recessive genotype (aa)

• Equation allows us to find what genotypic frequencies would be without any selective pressures