Modes of Natural Selection

Modes of Natural Selection Directional Selection Favors individuals at one end of the phenotypic range Most common during times of environmental change or when moving to new habitats Disruptive selection Favors extreme over intermediate phenotypes Occurs when environmental change favors an extreme phenotype

Directional Selection

Disruptive Selection

Modes of Natural Selection Stabilizing Selection Favors intermediate over extreme phenotypes Reduces variation and maintains the current average Example: Human birth weight

Variations in Populations

Geographic Variations Variation in a species due to climate or another geographical condition Populations live in different locations Example: Finches of Galapagos Islands & South America

Heterozygote Advantage Favors heterozygotes (Aa) Maintains both alleles (A,a) instead of removing less successful alleles from a population Sickle cell anemia > Homozygotes exhibit severe anemia, have abnormal blood cell shape, and usually die before reproductive age. > Heterozygotes are less susceptible to malaria

Sickle Cell and Malaria

Other Sources of Variation Mutations In stable environments, mutations often result in little or no benefit to an organism, or are often harmful Mutations are more beneficial (rare) in changing environments  (Example:  HIV resistance to antiviral drugs) Genetic Recombination source of most genetic differences between individuals in a population Co-evolution -Often occurs between parasite & host and flowers & their pollinators

Coevolution

Hardy-Weinberg Principle

The Hardy-Weinberg Principle Used to describe a non-evolving population. Shuffling of alleles by meiosis and random fertilization have no effect on the overall gene pool.   Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium.

The Hardy-Weinberg Principle Deviation from Hardy-Weinberg equilibrium usually results in evolution Understanding a non-evolving population, helps us to understand how evolution occurs                         .

5 Assumptions of the H-W Principle Large population size - small populations have fluctuations in allele frequencies (e.g., fire, storm). No migration - immigrants can change the frequency of an allele by bringing in new alleles to a population. No net mutations - if alleles change from one to another, this will change the frequency of those alleles

5 Assumptions of the H-W Principle Random mating - if certain traits are more desirable, then individuals with those traits will be selected and this will not allow for random mixing of alleles. No natural selection - if some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation.

Traits Selected for Random Mating

The Hardy-Weinberg Principle The gene pool of a NON-EVOLVING population remains CONSTANT over multiple generations (allele frequency doesn’t change)   The Hardy-Weinberg Equation:                 1.0 = p2 + 2pq + q2  Where: p2 = frequency of AA genotype 2pq = frequency of Aa q2 = frequency of aa genotype

The Hardy-Weinberg Principle Determining the Allele Frequency using Hardy-Weinberg:                 1.0 = p + q  Where: p = frequency of A allele q = frequency of a allele

Allele Frequencies Define Gene Pools 500 flowering plants 480 red flowers 20 white flowers 320 RR 160 Rr 20 rr As there are 1000 copies of the genes for color, the allele frequencies are (in both males and females): 320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R 160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r