EVOLUTION OF POPULATIONS Chapter 16

Genes and Variation 16-1  Darwin did not know how genetics worked in the mid 1800’s  Despite Mendel’s work on genetics in the mid 1800’s, most scientists did not make a connection between the two until the 1930’s  Current evolutionary theory deals entirely with genetics and heredity

How common is genetic variation?  Many genes have at least two forms (alleles)  Many traits are made of a combination of genes  Skin color, height, horses coat color  In addition, all organisms have additional “invisible” genetic variation  Small differences in biochemical processes  Animals are only heterozygous for 4% to 15% of their traits  Remember, humans have 35,000 genes and even more traits that they code for

Variation and Gene Pools  Genetic variation is studied in populations  Groups of individuals of the same species that interbreed  Each population shares a common group of genes called a gene pool  All of the available genes and the specific alleles for them in that particular group  The relative frequency of an allele is determined by how often it pops up in the gene pool  Described as a %  In genetic terms, evolution is ANY change in the RELATIVE FREQUENCY of ALLELES in a population

Where does the variation come from? 1. Mutations  Chromosomal or gene  Few are beneficial, most are benign or harmful  May occur naturally or artificially with mutagens  May or may not effect the fitness of an organism 2. Gene Shuffling  Crossing over during meiosis provides new gene combinations  Sexual reproduction combines genes from two different organisms  DOES NOT change the relative frequency of alleles  Think of a deck of cards….shuffling it over and over won’t change the number of Kings

Single Gene vs. Polygenic Trait  The number of phenotypes produced for a given trait depends on how many genes control the trait  Single-gene trait  Has two alleles (dominant or recessive)  Has two phenotypes  In real populations, phenotype ratio is determined by relative frequency AND dominance vs. recessive  Allele frequencies might NOT match Mendelian ratios  Polygenic trait  Has more than two alleles therefore many possible phenotypes  Ex: Height, skin color Single-gene trait: Free vs. Attached Earlobes Polygenic trait: Eye Color

Evolution as Genetic Change 16-2  Natural selection on single-gene traits can lead to changes in allele frequencies over time and thus to evolution  Recall the two moths sitting on the tree  Organisms of one color, for example, may produce fewer offspring than organisms of other colors  This is due to not reaching reproductive age

Natural Selection on Polygenic Traits  When more than one gene is considered, natural selection can lead to evolution in three ways: 1. Directional selection 2. Stabilizing selection 3. Disruptive selection

Directional Selection  When individuals at one end of the curve have a higher fitness than those in the middle or other end, the whole population will shift towards the individuals with higher fitness  Evolution towards one end  Ex: human pinky fingers are getting smaller, the average global height of people is getting taller

Stabilizing selection  When individuals in the middle of the curve have a higher fitness than either extreme, the population remains stable  Selection occurs against the extremes to produce more individuals in the middle of the curve  No evolution  May lead to a population crash  Ex: Birth weight in human babies

Disruptive Selection  When individuals at either end of the curve have higher fitness than those in the middle, the population moves towards the extremes.  Evolution to either side  Can create two new species  Ex: suppose birds lives in an area where medium-size seeds decrease in number, but small and large seeds increase  Birds with unusually large or tiny beaks would have higher fitness  May result in development of two species, one who eat small seeds and one who eat large

Genetic Drift  The concept that evolution can happen in small portions of large populations due entirely to chance  In a small population, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of these chance occurrences can cause an allele to become more common in a population

Hardy-Weinberg Principle  Allele frequencies will remain constant (Genetic Equilibrium) unless one or more factors causes those frequencies to change  For Genetic Equilibrium to occur, the following must happen:  Random Mating  Very large population  No movement in or out of the population  No mutations  No natural selection  Based on this principle, evolution MUST occur