Part VI and Chapter 20 Biology Sixth Edition Raven/Johnson (c) The McGraw-Hill Companies, Inc.

Practically every plant, insect, and vertebrate gene exhibits some level of variation. This variation ultimately lies at the core of evolution.

Inheritance of acquired characteristics – behavior changes acquired during lifetime were passed onto the next generation (Lamarck). Variation is the result of preexisting genetic differences among individuals – not experience (Darwin)

A natural population can contain a great deal of genetic variation. Polymorphism – a locus with more variation than can be explained by mutation alone. Heterozygosity – the probability that a randomly selected gene will be heterozygous for a randomly selected individual

The Hardy-Weinberg Principle: (Know the assumptions listed on page 424) (p + q) 2 = p 2 + 2pq + q 2 B = 0.6; b = 0.4 Why do allele frequencies change? Assumption violations – that’s why.

Five agents of evolutionary change: **

Genetic Drift – when frequencies of particular alleles change drastically by chance alone. Founder effect – a few individuals disperse and become the founders of a new population. Bottleneck effect – Drastic reduction in population size.

Artificial selection – the breeder selects for desired characteristics. Natural Selection – Environmental conditions determine which individuals in a population produce the most offspring. Conditions for natural selection: 1)Variation must exist among individuals in a population 2)Variation among individuals results in differences in number of offspring surviving in the next generation 3)Variation must be genetically inherited

Examples of natural selection

Maintaining Polymorphism: Adaptive Selection Theory – Heterogenous environments generate a condition in which many alleles exist. The Neutral Theory – alleles are ‘neutral’ to selection. Promoted by high mutation rate and low population size (genetic drift). Population size: DNA sequence in humans vs. fruit fly. The Nearly Neutral model: assumes that many of the variants are slightly deleterious, not neutral.

Genetic drift can continue to bring disadvantageous alleles to a population. But, can also bring advantageous alleles to a population.

Heterozygote Advantage

Over 80% of thoroughbred gene pool originated from just 31 individuals from late 18 th century. Intense selection has removed variation. Same genes affect both eyes – differences are due to developmental processes.

The End.