Where does Physiological variation come from?

January 19, 2005

Why do we all look different?

Why do we all look different?

• Yali’s question: diversity.• Humans come in many forms:

– Body shape– Colors– Lifestyles

How to explain diversity?

• Avoid flawed labels like “civilized,” “barbaric,” “savage,” “primitive.”

• Diamond: History followed different courses in different places because of environment, not biology, genetics or intelligence.

Evolution

• Simplest definition: change over time.

• Nature and Culture

• What does it mean to be human biologically?

Evolution

• What does it mean to be a theory?

• Other important theories:– Gravity– Earth revolves around Sun

• Do you believe in evolution?

Evolution part I: Natural Selection

• Evolution: Change over time

• Microevolution: change in gene (allele) frequencies in a population over time

• Macroevolution: change of species over time (speciation)

Natural Selection

• Change in gene (allele) frequency as a result of:– Differential fertility– Differential mortality

Example No. 1: Guppies

• Continuous variation:– Color– Spots

Example No. 1: Guppies

• Colorful fish:

• Enjoy differential fertility• Suffer differential mortality

Example No. 2: Peppered Moth

• Pre-industrial revolution:– Most moths are light colored.

Example No. 2: Peppered Moth

• Post-industrial revolution:– Majority of moths are very dark.

Example No. 2: Peppered Moth

• Explanation:– Change in environment (soot

discolored tree trunks) caused a change in mortality among moths which changed the gene (allele) frequency affecting coloration.

Darwin’s Three Postulates:

• Variation exists.• Variation is heritable.• There is a struggle for existence.

• Conclusion: “Descent with modification” aka Natural Selection

Survival of the Fittest?

• “Struggle for existence DOES NOT MEAN survival of the fittest!

What Darwin Didn’t Know:

• How traits are inherited and why.

• Where new variation comes from.

Genetics

• Gregor Mendel – 1822-1884

Inside the Nucleus of a Cell:

• Chromosomes– 23 pairs (46 total) make up 1 gene

• Body Cells – Diploid (contain 2 copies of each

chromosome)• Sex Cells

– Haploid (only 1 copy of each)

DNA

• Introns– No function– Vast majority of DNA

• Exons– Code for functions– Minority of DNA

Inheritance

• Parent Generation: homozygous• F1 Generation: heterozygous• Genotype: allele combination

Phenotype

• Observable characteristic controlled by:

• Genotype • Environment

Why do we all look different?

Back to Yahi’s Question

• Over many thousands of years, human bodies have evolved into different shapes, sizes and colors to adapt to different environments.

BUT . . .

Environment does not determine phenotype!

• Environment cannot cause a feature to appear that is not in the genotype.

• Culture is also necessary for people to adapt to any environment.

Key Terms:• Allele-half of a gene pair, each half comes

from one parent.• Genotype-genetic code for any particular

trait, contains 2 alleles.• Phenotype-expressed trait based on

genotype and environment.• Adaptation-change that is advantageous

for survival in a given environment.• Heritability-ability of traits to be passed

from parents to subsequent generations.