Chapter 11 Evolution of Populations

Genes and Variation Darwin’s theory had 2 big gaps: 1. He had no idea how heritable traits passed from generation to generation 2. He had no idea how variation appeared It wasn’t until the 1930’s that Darwin’s theories were connected to Gregor Mendel’s genetics principles - focus then turned to genes as mechanism for passing heritable traits - Watson and Crick’s contribution of DNA also helped explain the origins of mutation and variation

How Common is Variation? Every gene has two alleles Each gene that is heterozygous for a trait has a chance of showing variation -Certain insects may have 15% of genome with heterozygous genes -Most mammals are between 4 & 8%

Variation and Gene Pools Population is a group of individuals of the same species that can successfully interbreed - these members share a common group of genes - Gene pool – accumulation of all the different genes that are present in a population - Relative frequency – refers to the number of times an allele occurs in the gene pool - has nothing to do with dominant or recessive - Higher % means it occurs more often vs. lower % being less often Evolution is any change in relative frequency of alleles in a population

Sources of Genetic Variation Where does variation come from? 2 Main sources of genetic variation: 1. Mutation – any change in the sequence of DNA either internally or externally ( radiation or chemicals in environment) 2. Gene shuffling from sexual reproduction – crossing over during meiosis is major source of variation among eukaryotes - 8.4 million combinations possible in humans - even though many combinations are possible, does not alter the relative frequency of genes

Single vs. Polygenic The number of phenotypes produced for a given trait depends on how many genes control it Single –gene trait – has only two alleles that control the single gene Polygenic traits – two or more genes control the expressing of a trait – more than 2 alleles Ex: height of students in class

Evolution as Genetic Change Evolutionary fitness is measured as an individuals success at passing genes to the next generation Natural selection acts only on the organism not the genes Organisms that produce many offspring keep their alleles in the gene pool and may increase the frequency of those alleles Populations are the smallest entity, not the individuals that can evolve over time

Natural Selection on Traits Natural selection on single gene traits can lead to changes in allele frequencies, or evolution Ex: red or brown vs. black lizards

Natural Selection on Traits Polygenic traits can be affected 3 ways by natural selection: 1. Directional selection – those at one end of curve have higher fitness vs. middle or other end 2. Stabilizing selection – favors those at the center of curve in terms of fitness 3. Disruptive Selection – those at the extreme ends have higher fitness than the middle group

Genetic Drift Genetic drift occurs when allele frequencies change unpredictably or by chance - can make an allele more common in the population - separation to a new habitat can be a source of change – founder effect *population will be genetically different from parent population

Gene Flow Gene flow – movement of alleles from one population to another - can increase genetic variation of the receiving population - lack of gene flow from neighboring populations can increase the separation of species Ex: animals migrate between populations and reproduce for plants and fungi wind blown dispersal of spores and seeds

Are there conditions where evolution will not occur? Hardy- Weinberg principle – states that allele frequencies in a population will remain constant unless one or more factors cause a change -creates genetic equilibrium -established in 1908 as the counter principle to evolutionary change

Hardy- Weinberg Principle 5 conditions must be met to maintain genetic equilibrium: 1. Random mating must occur 2. Population must be very large 3. No migration 4. No mutations 5. No natural selection Rarely are all conditions met in nature – thus change is imminent or evolution will occur

Speciation Speciation – origin of a species Species – group that can breed and produce fertile offspring - common genes in the gene pool - any changes that increase fitness will as change allele frequency as it passes from generation to generation

Isolating Mechanisms Reproductive isolation – members of two populations cannot interbreed and produce fertile offspring - leads to establishment of new species Two types of reproductive isolation: 1. Behavioral – two populations in close proximity will not interbreed because of different courtship rituals, or reproductive strategies 2. Temporal – species reproduce at different times (either day, season, or year)

Isolating Mechanisms Geographic (allopatric) isolation – populations are separated by a geographic barrier Ex: rivers, mountains, or larger bodies of water - isolation depends on organisms mobility - can create new species with genetic changes not present in other group - does not always guarantee formation of new species; if two population can still interbreed then still one species

Natural Selection in Nature

Speciation of Darwin’s Finches Speciation of Galapagos finches occurred by: - Founding a new population - Geographic isolation - changes in new gene pool - reproductive isolation - ecological competition Collectively known as Adaptive radiation

Adaptive Radiation evolution of many diversely adapted species from a common ancestor after being introduced into new environmental opportunities and challenges Explains differences between each islands inhabitants and differences from having organisms in close proximity