Mechanisms of Evolution PDF

Mechanisms of Evolution AND THEIR EFFECT ON POPULATIONS  Remember… ► Genetic variation of individuals within a population make evolution possible ► Species that reproduce sexually, each individual inherits a new combo of alleles from parents RESULT? ► Each new generation m...

Mechanisms of Evolution AND THEIR EFFECT ON POPULATIONS  Remember… ► Genetic variation of individuals within a population make evolution possible ► Species that reproduce sexually, each individual inherits a new combo of alleles from parents RESULT? ► Each new generation made up of genetically unique individuals ALSO: ► Mutations occur randomly David Gallo: Underwater Astonishments  What is Natural Selection? (Review) ► Acts on the heritable variations of individuals and results in a population that is better adapted to its current environment ► E.g. N.S. favours weeds with genes for herbicide resistance; these individuals able to reproduce and pass on genes for herbicide resistance  Key Ideas ► Individuals do NOT evolve, populations do! ► N.S. acts on individuals; their characteristics affect their chances of survival and their reproduction with a local environment ► However, N.S.’s impact on Evolution is only apparent when tracking how a population changes over time  Factors that Change Allele Frequencies in Populations ► Allele frequencies: #of copies of an allele compared to total # of alleles in the pop’n ► When frequency of an allele in a pop’n changes, microevolution has occurred ► Microevolution: small events that lead to evolution in a population  Gene Pool ► Consists of all alleles (alternative forms of genes which are specific nucleotide sequence in DNA) in all individuals in a population ► In studying evolution at the population level, geneticists focus on the gene pool HOW CAN THE GENE POOL CHANGE? 1) Mutations 2) Gene Flow 3) Non-random Mating 4) Genetic Drift 5) Natural Selection  1) Mutations Change in DNA structure which changes the genotypes and phenotypes of a population ►Mutations are the original source of variation that lead to changes in traits 2) Gene Flow (Migration) ► Net movement of alleles of a population due to migration of individuals  3) Non-Random Mating ► Mating based on mate selection for particular phenotype or due to inbreeding ► Sexual Selection- natural selection for mating based on competition between males and choices made by females ► Effect on Population: -leads to males & females of a species evolving appearances & behaviours that are very different from one another = sexual dimorphism Sexual Selection Examples Video clips ► Sounds from the Lyre bird ► Mating dance – birds of paradise ► Two male red deer stags fighting Natural Selection and Sexual Selection  4) Genetic Drift ► Change in allele frequencies due to chance events ► Changes are more pronounced in a small population ► E.g. Flipping a coin 1000x 🡪 would not expect 700 heads, 300 tails; BUT what about 10 coin flips? ► 2 Types: Bottleneck Effect & Founder Effect 1 3 Genetic Drift: The Founder Effect ► New pop’ns formed by only FEW individuals aka founders ► Founders will carry SOME (not all) of the alleles from original pop’n gene pool ► SO: diversity in new gene pool is LIMITED ► WHERE DOES IT OCCUR? ► Islands ► Isolated human pop’ns Genetic Drift: The Founder Effect Example ► Pennsylvania Amish Community ► founded in 1720 by only 30 individuals ► Current pop’n: ► Unusually high frequency of polydactylism (presence of sixth finger or toe) Genetic Drift: The Bottleneck Effect ► A severe event (e.g. natural disaster, starvation, disease etc…) quickly reduces size of large pop’n ► Only a small sample of alleles remain in the population from survivors ► By chance, certain genes (traits) may then be represented more frequently than others among the survivors Genetic Drift: Bottleneck Effect  Genetic Bottleneck – A Historical Case Note: A genetic bottleneck creates random genetic changes without regard to adaptation. A severe genetic bottleneck occurred in northern elephant seals-from fur hunting  The Effect of Genetic Drift is Inversely Related to Population Size Large populations = small effects Small populations = large effects 5) Natural Selection ► Environment selects for individuals in a pop’n with certain traits that make them better suited to survive and reproduce than others in the pop’n ► EFFECT? ► Over many generations, allele frequencies of many different genes may change resulting in significant changes in the characteristics of pop’n Forms of Natural Selection ► Natural selection has three modes of action: 1. Stabilizing selection 2. Directional selection 3. Disruptive selection Number of Individuals Trait  1.Stabilizing Selection ► N.S. favours intermediate phenotypes and acts against extreme variants of phenotype Number of Individuals Trait  2.Directional Selection ► N.S. favours the phenotypes at one extreme Number of Individuals Trait  3.Disruptive (Diversifying) Selection ► N.S. favours both extremes of range of phenotypes (can result in elimination of intermediate phenotypes) Number of Individuals Trait  Changing Allele Frequencies in Populations What’s Special about N.S.? ► Of all causes of microevolution, only natural selection usually leads to adaptation Summary of Microevolution https://ed.ted.com/lessons/five-fingers-of-evolution# watch Five Fingers of Evolution  Practice questions: 1. How does evolution favour behavioural traits in males? 2. How do mutations affect genetic variation? 3. What does “allele frequency” mean? 4. Why does gene flow increase diversity in one population? 5. What is the difference between random and non-random mating? 6. Why does microevolution apply to populations and not individuals? 7. What are the effects or outcomes of non-random mating?