17-1 Genes and Variation

• Darwin based his evolutionary theory on heritable variation, but he had no idea where the variation came from or how heredity worked

• Mendel’s work was published during Darwin’s lifetime, however, no one realized how it related to Darwin’s work

• Biologists connected the work of Mendel to Darwin in the 1930s

20th century geneticists discovered that heritable traits are controlled by genes and carried on chromosomes - changes in genes and chromosomes generate variation

• individual organisms don’t evolve → populations evolve from generation to generation

population - a group of individuals of the same species that mate and produce offspring

plants and animals have two sets of genes in their nuclei, one from each parent

• alleles — specific forms of genes — may vary from individual to individual

natural selection works directly on phenotype, not genotype

• it acts on an organism’s characteristics, not directly on its alleles

  • it “selects” an entire organism, not a single gene or group of genes, to survive and reproduce or die without reproducing

some individuals have phenotypes that are better suited to their environment than phenotypes of other individuals

• better suited individuals have higher fitness, produce more offspring, and pass more copies of their genes to the next generation that poorly suited individuals do

  • inherited variation can lead to natural selection because it can result in differential reproductive success for individuals with different phenotypes

members of a population interbreed with one another — they share a common group of genes called a gene pool

gene pool - a gene pool consists of all genes, including all alleles for each gene, that are present in a population

• gene pools are studied by examining the number of different alleles they contain

• allele frequency - the number of times an allele occurs in a gene pool

• allele frequency has nothing to do with whether it is dominant or recessive, just whether it occurs more frequently

in genetic terms, evolution is any change in the frequency of alleles in a population over time

two main sources of genetic variation:

1. Mutation: any change in a sequence of DNA

   1. caused by a mistake in replication or radiation and chemicals in the environment

   2. some mutations are harmless, others affect an organism’s fitness

   fitness - the ability to survive and reproduce

2. Gene shuffling from sexual reproduction:

   1. MEIOSIS

   2. Review

      1. Segregation - separation of alleles during meiosis

      2. Independent Assortment - genes for different traits assort independently

      3. Crossing over - homologous chromosomes exchange segments, increases variation

   3. sexual reproduction is a major source of genetic variation

The number of phenotypes produced for a given trait depends on how many genes control the trait

• single-gene trait: any trait controlled by one gene that has two alleles

  • ex. widow’s peak

  • only 2 possible phenotypes

• polygenic traits: traits controlled by two or more genes

  • ex. skin color, height

  • traits have more than 2 alleles

  • many possible phenotypes — not just 2

17-2 Evolution as Genetic Changes

Natural selection never acts on a single gene — it acts on an entire organism because the whole organism either survives to reproduce or dies before reproducing — populations evolve over time, not single individuals

• natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution

  ex. brown lizards reproduce, but because of mutations, black and red babies are born

  • the black is more advantageous because it absorbs more sunlight — warm up faster, more energy, run faster

  • the red is very visible to predators and doesn’t survive to reproduce like the black lizard does

The effects of natural selection are more complex on polygenic traits because there are many possible phenotypes and genotypes

• often display a range of phenotypes that form a curve — fitness of individuals may vary from one end of a vurve to the other

Natural selection can affect the distributions of phenotypes in any of 3 ways:

1. Directional Selection:

   • when individuals at one end of the curve have a higher fitness than individuals in the middle or at the other end

   • curve moves in one direction

2. Stabilizing Selection:

   • when individuals near the center of the curve have higher fitness than individuals at the two other ends

   • curve gets narrower

3. Disruptive Selection:

   • when individuals at the upper and lower end of the curve have a higher fitness

   • curve dips in the middle

Genetic Drift: In small populations, chance alone can cause a change in the allele frequency in a population

Ex.

founder effect - when allele frequencies change because of the migration of a small number of individuals in a population

Evolution vs Genetic Equilibrium:

• Hardy-Weinberg Principle: allele frequencies in a population will remain constant unless one ore more factors cause those frequencies to change

  • genetic equilibrium - the situation in which allele frequencies remain constant

5 conditions disrupt genetic equilibrium & cause evolution to occur

1. Non-random mating - sexual selection - individuals select mates based on size, strength, or coloration

   • genes for traits selected for or against are not in equilibrium

2. Small population size - genetic drink affects small populations

3. Gene flow from immigration or emigration - changes in allele frequency can be produced by gene flow

   • this causes evolution to occur

4. Mutations - can introduce new alleles into the gene pool, changing allele frequencies and causing evolution to occur

5. Natural selection - if different genotypes have different fitness, natural selection will disrupt genetic equilibrium and evolution will occur

17-3 The Process of Speciation

species - a population or a group of populations whose members interbreed and produce fertile offspring

interbreeding permits any genetic change that occurs to spread throughout populations of a species

• if some members of population stop breeding with other members, the species gene pool can split

• once members of two populations stop interbreeding, changes in one gene pool cannot spread to the other — reproductive isolation

speciation - when populations become reproductively isolated, they may evolve into two separate species

3 ways for reproductive isolation to occur:

1. Behavior Isolation: two populations have differences in courtship rituals or other reproductive strategies that involve behavior

   ex. Eastern and Western Meadowlarks use different songs to attract males

2. Geographic Isolation: two populations are separated by geographic barriers such as rivers and mountains

3. Temporal Isolation: reproduction occurs at different times of the year

   ex. 3 different orchids live in the rain forest. Each produces a flower that only blooms one day and must be pollinated on that day to produce seeds. Because the three species flowers bloom on different days they cannot pollinate each other

   • 

Testing Natural Selection in Nature:

• Darwin’s Finches - natural selection shaped the bird beaks on each island depending on the food source

Speciation in Darwin’s Finches:

• Speciation in the Galapagos finches occurred by founding of a new population, geographic isolation, changes in the new population’s gene pool, reproductive isolation, and ecological competition

Studying Evolution Since Darwin:

• Scientific evidence since Darwin’s time supports the theory that living species descended with modification from common ancestors that lived in the ancient past

• research is limited to what we can observe or find in fossils

• many questions are still unanswered