Part VI and Chapter 20 Biology Sixth Edition Raven/Johnson (c) The McGraw-Hill Companies, Inc.
Dec 30, 2015
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 = p2 + 2pq + q2
B = 0.6; b = 0.4
Why do allele frequencies change? Assumption violations – that’s why.
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
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.
Over 80% of thoroughbred gene pool originated from just 31 individuals from late 18th century. Intense selection has removed variation.
Same genes affect both eyes – differences are due to developmental processes.