Mechanisms of Evolution

1. Mechanisms of Evolution 8.1 – Microevolution • sickle-cell anemia is a genetic disease caused by a recessive allele resulting in abnormal RBC [diploid organisms have two copies of every gene; versions (alleles) of genes can be dominant or recessive; if alleles are the same it is termed homozygous, and heterozygous if different] • the sickle-cell allele is rare in Canada (1 in 3800) but common in Africa (1 in 25) • individuals homozygous for the allele suffer severe symptoms, but those heterozygous for the allele are resistant to malaria • the allele is most prevalent in areas with a high population of Plasmodium parasites, so natural selection has maintained a higher frequency of a harmful allele than would be expected if the allele had only negative effects • if Darwin had known about Gregor Mendel’s work on heredity, he likely would have been able to apply genetics to natural selection

2. Gene Pools and Variation • a gene pool consists of all the alleles of all the individuals in a population, and is where genetic variation is stored • the genotype (allele combination) of an individual is reflected in their phenotype (expressed traits) • a populations exhibit a range of phenotypes, with some appearing more frequently than others

3. Sources of Variation • one source of genetic variation is mutation resulting from an alteration of DNA • most variation in sexually reproducing organisms comes from the recombination of existing alleles during meiosis (random assortment and crossing over)

4. Changes to Gene Pools • Hardy-Weinberg equilibrium is a condition that exists when gene pools are not changing, and is expected when: • there is random mating • the population is large • there is no migration • there are no mutations • there is no natural selection • if the above conditions are not met, genetic equilibrium is disrupted and gene pool frequencies may change, and is termed microevolution, evolution on the smallest scale

5. Mechanisms of Microevolution • by providing the set of conditions under which genetic change would not occur, the Hardy-Weinberg Principle helps identify key factors that can cause evolution (i.e. a change to the gene pool of a population or species) Natural Selection • increases the frequency of alleles that provide a reproductive advantage to individuals and leads to the evolution of adaptations • stabilizing selection occurs when individuals near the centre of the phenotype range are selected over those at either extreme

6. directional selection occurs when individuals at one end of the phenotype range are selected for, and the range of phenotypes shifts in one direction • disruptive selection occurs when individuals at both extremes of the phenotype range have higher fitness than individuals near the middle, and may lead to two distinctivephenotypes in the population

7. Pesticide Resistance: Directional Selection in Action • survivors of initial applications have alleles that enable them to resist attack, and they pass that resistance on to their offspring • the proportion of resistant individuals increases in successive generations, and the population undergoes directional selection

8. Sexual Selection • a common violation of the Hardy-Weinberg equilibrium is the fact that mating if often non-random • in sexual selection, other individuals of the same species select the traits, while the environment selects for traits in large populations • Sexual Selection in Widow Birds • (see P. 218) • in an experiment, the tail lengths of male birds were altered so that some were shortened, some lengthened, with control groups of unaltered tails and one with tails cut and glued back on • results showed that males with lengthened tails were more successful at mating than other groups

9. Artificial Selection • breeders select for desirable traits in a small population instead of the environment • mating is non-random, evolution occurs rapidly, but genetic variability is reduced, with unintended negative results • inbreeding in quarter horses has resulted in thousands of descendents of one horse, Impressive, chosen for his speed • many inherited a disorder called hyperkalemic periodic paralysis (HYPP)

10. Genetic Drift • low allele frequency is more at risk being lost in a small population (i.e. if 2 of 100 individuals carry an allele in a population of 100 000 and ½ of the population was wiped out, 1000 individuals still carry the allele, but if the population is only 100 . . ?

11. The Bottleneck Effect • genetic drift in which a natural disaster reduces a population size and therefore the depth of its gene pool • some alleles may be better represented than others by chance, while others may be eliminated The Founder Effect • genetic drift resulting from a few individuals colonizing another, isolated, habitat • the change in allele frequencies will reflect the founding population • isolated Amish populations in NA exhibit traits that are rare in the larger population (i.e. female beards)

12. Gene Flow • a random process in which fertile individuals or their gametes migrate between neighbouring populations • two populations of flowers with different allele frequencies for colour may receive windblown pollen from each other, changing the allele frequency in both populations • gene flow disrupts genetic equilibrium and tends to reduce the genetic differences between populations • if gene flow is extensive, it may result in a single population with a common gene pool