CP Biology Ms. Morrison

Genes and Variation  Gene pool = combined genetic information of all members of a particular population  Relative frequency = number of times allele occurs in a gene pool (usually %)  2 main sources of genetic variation:  Mutations = change in DNA sequence, some change phenotype, some do not, can affect fitness or not  Gene shuffling = chromosomes moving independently from each other during meiosis and crossing-over, does not change relative frequency of alleles

Genes and Variation, pg 2  Number of phenotypes produced for a trait depends on how many genes control trait  Single gene – usually only 2 alleles so 2 phenotypes  Polygenic – several alleles, many genotypes and even more possible phenotypes, bell curve distributions common (normal distribution)

Evolution = Genetic Change  Natural selection acts on phenotypes  Over time changes in relative frequencies of alleles  Leads to evolution

Natural Selection in Polygenic Traits  Directional selection: individuals at one end of normal curve have higher fitness (shifts left or right)  Stabilizing selection: individuals in center of normal curve have higher fitness (curve narrows in middle)  Disruptive selection: individuals at each end have higher fitness than those in middle, can cause curve to split into two

Genetic Changes  Genetic drift = particular allele changes frequency by chance, over time it can become common in the population  Founder effect = changes in allele frequencies when small subgroup of a population migrates to a new habitat  Hardy-Weinberg principle = allele frequencies remain constant unless one or more factors causes them to change

Genetic Equilibrium  Alleles frequencies remain constant  5 conditions required to maintain equilibrium  Random mating (equal opportunities to pass on alleles)  Large population (less effect from genetic drift)  No movement in or out of population (could gain or lose alleles otherwise)  No mutations (would result in new alleles)  No natural selection (no phenotype has selective advantage over another)

Process of Speciation  Speciation = formation of a new species  New species evolve when populations become reproductively isolated from each other, 3 ways can happen:  Behavioral isolation – two populations have different courtship rituals or other behaviors  Geographic isolation – two populations separated by geographic barriers  Temporal isolation – two or more populations reproduce at different times

Steps of Speciation  Founders arrive  Separation of population (do not go back to previous environment)  Changes in gene pool (result from adapting to new environment)  Reproductive isolation  Ecological competition (become more specialized for particular food source)  Continued evolution