Part VI and Chapter 20

BiologySixth 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 = p2 + 2pq + q2

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 18th century. Intense selection has removed variation.

Same genes affect both eyes – differences are due to developmental processes.

The End.