Darwin & Microevolution Chapter 19-20

Charles Darwin ( ) Former divinity and medical student Secured an unpaid position as ship's naturalist on the H.M.S. Beagle Voyage provided Darwin a unique opportunity to study plants, animals, and their environment Gathered a great deal of evidence he would later incorporate into a theory of evolution

EQUATOR Galapagos Islands Voyage of the Beagle 5 yr. mission to chart South America

Darwin’s Theory of Natural Selection Individuals in a population have variable levels of success in reproducing Left unchecked, populations tend to expand exponentially, leading to a scarcity of resources In the struggle for existence, some individuals are more successful (fit) than others, allowing them to survive and reproduce Those organisms best able to survive and reproduce will leave more offspring

Darwin’s Theory of Natural Selection Over time there will be heritable changes in phenotype (genotype) of a species These changes may result in a transformation of the original species into a new species similar to, but distinct from, its parent species Common Descent, due to these changes similar species have common ancestors. This means that nearly all of life is linked

What is Evolution? Evolution is a process that results in heritable changes in a population spread over many generations Evolution can be precisely defined as “any change in the frequency of alleles within a gene pool from one generation to the next” Populations, not individuals evolve Traits within a population vary among individuals

Variation Within a population most phenotypic traits are polymorphic, they have two or more forms Those traits that have many forms show continuous variation Individual inherit different combinations of alleles leading to different phenotypes All these genes & their alleles within a population is known as the gene pool. This variation is the raw material for evolution This variation is also what allows for natural selection

What Determines Alleles in an Individual? Mutations, many are lethal, but some can be neutral & some may confer an advantage Crossing over at meiosis I, shuffles alleles Independent assortment, genes that may work together, but are on different chromosomes will not be inherited together Fertilization, sexual reproduction Change in chromosome number or structure, often deleterious, but can be advantageous

Genetic Equilibrium Point when a population is not evolving The opposite of evolution Allelic frequencies are not shifting Calculated using the Hardy-Weinberg equations p + q = 1 p 2 + 2pq + q 2 = 1 Where p = frequency of Dominant allele (A) and q = frequency of recessive allele (a)

Genetic Equilibrium Five conditions need to be met: No mutations Random mating Gene does not affect survival or reproduction Large population No migration

Hardy-Weinberg Equilibrium AA(p 2 ) Aa(pq) aa(q 2 ) A p a q A p a q p 2 = frequency of AA (Homozygous dominant) 2pq = frequency of Aa (heterozygous) q 2 = Frequency of aa (homozygous recessive)

490 AA butterflies Dark-blue wings 420 Aa butterflies Medium-blue wings 90 aa butterflies White wings Hardy Weinberg Equilibrium: Example Starting population:

Frequencies in Gametes: A A Aaaa 0.49 AA0.42 Aa0.09 aa A0.3a F 1 genotypes: Gametes: Hardy Weinberg Equilibrium: Example

490 AA butterflies THE NEXT GENERATION 420 Aa butterflies 90 aa butterflies THE NEXT GENERATION 490 AA butterflies 420 Aa butterflies 90 aa butterflies NO CHANGE STARTING POPULATION 490 AA butterflies Dark-blue wings 420 Aa butterflies Medium-blue wings 90 aa butterflies White wings Hardy Weinberg Equilibrium: Example

Mechanisms of Evolution Evolution of a population over time may occur as a result of  New mutations  Natural selection  Nonrandom mating (Sexual selection)  Genetic drift because of small population  Gene flow – immigration and emigration (Opposite of Genetic Equilibrium)

Mutations Mutations that alter protein structure enough to impact its function  more likely to be harmful but may be beneficial  our genome is product of thousands of generations of selection Fuel for evolution Mutant allele may enable an organism to fit its environment better & increase reproductive success especially likely if environment is changing

Natural Selection Difference in the survival & reproductive success of different genotypes and/or phenotypes Over time, the alleles that produce the most successful phenotypes will increase in the population Less successful alleles will become less common Change leads to increased fitness Selection is not a “force” it is merely the favoring of some genetic changes over others

Types of Natural Selection Directional Selection Stabilizing Selection Disruptive Selection

Number of individuals in the population Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3 Number of individuals in the population Number of individuals in the population Directional Selection Shift in the variation in a consistent direction within the phenotypic range Examples:  Pesticide resistance in insects  Antibiotic resistance in bacteria

Number of individuals in the population Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3 Stabilizing selection Loss of extreme forms with stabilization of an intermediate form Example: Infants greater or less than 7.5 lbs have increased mortality

Number of individuals in the population Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3 Number of individuals in the population Number of individuals in the population Disruptive Selection favors individuals at the extremes with a reduction of intermediate forms Example: African Finches selection favors small or large beak size

Sexual Selection A type of natural selection Selection that is driven by the competition for mates

Gene Flow Movement of alleles into a population Tends to keep the gene pools of populations similar Counters changes due to mutation, natural selection, and genetic drift

Genetic Drift Random change in allele frequencies brought about by chance Effect is most pronounced in small populations This can cause similar, but isolated populations to become dissimilar due to the loss or fixation of alleles

Bottleneck Genetic drift is most pronounced when small populations grow into larger ones, usually after a catastrophe A bottleneck as only a few alleles survive and are now disproportionally expressed in the population A founder effect results in rare or even disadvantageous alleles being found in a population at a level higher than normally expected

Macroevolution: Evidence for evolution Biogeography Fossil Record Comparative anatomy Comparative embryology Molecular Biology

Evolution evidence: Biogeography Geographical distribution of species Indicates that populations that are isolated from one another geographically evolve separately

Evolution evidence: The Fossil Record Fossils are created when organisms become buried in sediment, bone and other hard tissue is converted to rock The fossils contained in sedimentary rock layers reveal a history of life on earth

Evolution evidence: Comparative Anatomy Comparing body forms and structures of major lineages By studying homology or how similarly derived body parts have evolved, we can put together an evolutionary tree and find common ancestors even if the body parts no longer serve the same function

Evolution evidence: Comparative anatomy Homologous structures  Similar anatomy, different functions  Indicate divergent evolution

Evolution evidence: Comparative anatomy Analogous structures  Similar function, different anatomy  Indicated Convergent Evolution  Example: bird’s wing vs. fly’s wing

Evolution evidence: Comparative anatomy Vestigial Organs  Remnants of evolution  Organs that were required in ancestor that are not needed in present-day organism  Examples: appendix, tailbone

Evolution evidence: Comparative Embryology Development of early embryos of resemble each other due to a common developmental plan During later embryonic development this plan becomes modified to create the different body types we see

Evolution evidence: Molecular Biology Tracking mutations in sequences of DNA and/or proteins can trace the evolutionary history of organisms The more nucleotides/ amino acids in common, the more closely related the species