Why is Evolution Important?
❖ Pre-Darwinian Views
o Plato and Aristotle
■ Single, graded continuum of animal s and plats going from less perfection (plants) to more perfection (humans)
o Homer
■ Golden Chain (incorporates Creator)
❖ The Study of Variations: Ordering Nature
o Carolus Linnaeus (1707-1778)
■ Developed the modern system of taxonomy
■ Taxonomy = the science of biological classification
■ Taxon
o Binomial nomenclature
■ Ex. Homo sapiens
o Systema Natura (1735)
❖ Evidence for Change?
o Early research in geology provided evidence that the natural world had changed over time
■ Robert Hooke (1635-1703) and others
■ James Hutton (1726-1797) and others
■ Geoges Cuvier (1769-1832)
■ Charles Lyell (1797-1875)
❖ Exploring Change: Early Models
o Uniformitarianism:
■ Idea that all landforms were developed by processes (erosion, sedimentation, volcanism that can be seen today
■ Thus processes are uniform over time
■ Model was incompatible with tradition as a great age for the earth is required
o Catastrophism:
■ Idea that periodic catastrophes caused:
• Changed in landforms
• Replacement of fossil communities
❖ Evolving Organisms
o In the same period, many suggested models to explain variation and changes in organisms:
■ Comte de Buffon (1707-1778)
• Observed plants and animals from many habitats
• Noted patterns in variation
• Proposed mechanism
■ Jean-Baptiste Lamarck (1744-1829)
• Inheritance of acquired characteristics
• Each environment has different demands
• Animals change over their lifetimes to adapt to these demands
• Changed are passed directly to offspring
• No a process of selection, but introduced idea of adaptation
■ Charles Darwin (1809-1882)
• Natural selection was developed independently
■ Alfred Wallace (1823-1913)
• Both Darwin and Wallace familiar with debates in geology, debates in evolution, theories of Thomas Malthus (political economist)
❖ Evolving Organisms: Darwin
o Charles Darwin (1809-1882)
■ Wealthy family
■ Medicine, theology
■ Interest in the natural sciences
■ HMS Beagle - Galapagos
■ Finches - he noticed they were different on every island
o The Origin of Species by Natural Selection (1859)
■ Change in organisms over time - this change being neither positive or negative o The Descendant of Man (1871)
■ Monkeys, apes and humans are descendants from a common ancestor, long ago
■ Physical (andgenetic) divergence of that common ancestor into monkeys and humans caused by the process of natural selection
❖ Evolving Organisms: Wallace
o Alfred Russel Wallace (1823-1913)
■ Born in a small village in Manmouthshire, Wales
■ Was the 8th of 9 children
■ No formal education
■ The Mischief - Amazon
■ "On the Tendency of Varieties to Deport Indefinitely from the Original Type”
What is Natural Selection?
❖ Evolution by Natural Selection
o Darwin's principles of evolution
1. Producing offspring at a faster than food supplies increase
2. Biological variation exists within all species
3. There is competition among individuals
4. Favourable traits lead to fitness
5. Environment matters
6. Traits are inherited, leading to reproductive success
7. Time = more variation (different species)
8. Geographical isolation (new species)
❖ Natural Selection
o Main process that increases the frequency of adaptive traits through time
■ Adaptive = results in greater reproductive success in a particular environment o Operates through three main principles
1. Variation
2. Heritability
3. Differential reproductive success
o Variability between individuals within a population gives reproductive edge to some in a changing environment, leading to gradual change over time in the population as a whole
■ Process become encapsulated in "survival of the fittest" (Herbert Spencer)
What are the Basics of Contemporary Genetics?
❖ Heredity: Genetic Revolution
o Darwin cont.
■ Variations transmitted through heredity but what was the mode of inheritance?
■ Genetics = The study of heredity and the variation of inherited characteristics
❖ Genetic Principles
o Gregor Mendel (1822-1884)
■ Was a botanist
■ Discovered the principles of inheritance even before chromosomes were discovered
■ Grew more than 28000 pea plants
■ "Discrete" physical unit is inherited
■ Challenged the model of blended inheritance
■ Three main principles or postulates
❖ Mendel's Postulates
1. Principles of Segregation
a. That parents contribute
2. Principles of Dominance and Recessiveness
a. Recessive
b. Dominant
c. Heterozygous
d. Homozygous Genotype (genetic) / Phenotype (physical)
3. Principle of Independent Assortment
a. The distribution of one pair of alleles into gametes does not influence the distribution of another pair
b. The genes controlling different traits
❖ Genetics and Variation
Crossing Over
o Makes possible the independent assortment of linked genes
o Crossing over during meiosis leads to allele combinations in sex cells that are not present in the parent chromosomes
Human Biological Variation
❖ Studying Human Variation
o Key questions
■ How do Humans vary? How did this variation arise?
❖ Methods: Genotype/Phenotype
We can study human variation in:
o Genotype
■ DNA sequences carried by an individual
■ Nuclear DNA, mDNA
■ May be used to study:
o Inheritance of a given trait
o Population ancestry and relationships
o Phenotype
■ Observable physical traits
o Hair, skin, eye colour
o Blood type
o Body dimensions
o Enzyme production
■ Results of genes and environment
■ Impact of genes vs. environment varies and is not always known
❖ DNA and Population Ancestry
o To trace population ancestry:
■ Look for shared gene sequences
■ These should be passed on unchanged from parents to offspring
■ Key to study DNA sequences that are not recombined
• mtDNA - traces maternal ancestry
• Y - chromosome traces paternal ancestry
o Result is a “tree” of population ancestry and relationships
■ Reflects descent of populations from common ancestral group
■ Splitting of populations may be given a time estimate based on estimated rate on mutation in DNA
❖ Breeding Populations
o Humans are divided into breeding populations by factors such as:
■ Geographical barriers
■ Customs and attitudes
o This separation is incomplete die to gene flow:
■ Exchange of mates with other breeding populations
■ Can lead to exchange of traits
■ Maintains genetic unity of our species
❖ Environmental Stress
o Much environmental stress is dealt with through cultural adaptation
■ Ex. Inuit
o Removes much selective pressure on human populations
o If culture does not remove stress, it may have a selective effect
❖ Environmental Stress
o The body also deals with stresses through acclimatization
o This is a response on the individual level, often reversible
■ Tanning
■ Increase in red blood cell count at high altitudes
o Can cause phenotypic differences between genetically similar groups
❖ Environment and Body Build
o Bergmann's Rule:
■ General relationship between body size and temperature
• Warmer = Slender
• Colder = Robust
o Allen's Rule:
■ General relationship between limb proportions and temperature
• Warmer = Longer Limbs
• Colder = Shorter Limbs
❖ Genetic Drift (Wright Effect)
o A trait may become common or rare in a population due to chance
o Such effects are referred to as genetic drift
o Genetic drift contributes to differences between populations
o Is most significant in small populations
❖ Genetic Drift: Special Cases
o Founder Effect
■ Part of a breeding population branches off as a new population (ex. colony)
■ Rare traits may be absent from or overrepresented in new population
■ Result may be group of related but different populations in a region o Bottleneck Effect
■ Part of a breeding population dies in a catastrophe
■ By chance, most individuals with a trait may die or survive
■ Population now has a different proportion of that trait than before the catastrophe
❖ Sexual Selection
o A form of natural selection
o Individuals with “most attractive” traits mate more often
o Traits may be practical
o Traits may be impractical
■ Beauty is culturally defined
o Impact on humans is uncertain
❖ Selection: An Example
o Sickle-cell anemia
■ Relatively common in populations in malarial regions
■ Result of selective pressure of malaria
• Single copy of the allele helps to protect against malaria, this individual is more likely to survive and reproduce
• Balancing selection
■ Researchers have studied the gene sequence associated with this trait to understand its origin and spread through populations
❖ Patterns of Variation
o Recent studied of human variation indicate that:
■ Most variation in traits is clinical
• Cline: gradual differences across an area
• Few sharp boarders in traits between human populations
• Most groups similar to nearby groups
• Reflects the effects of gene flow
■ Most traits vary independently
• Reflects independent inheritance, different selective pressures
• Populations could be groups in different ways using different traits
o mtDNA and Y-DNA variation an be use to group relationships
■ All human groups appear to share a relatively recent common ancestor
■ Smaller and more closed related groups may share some typical traits
■ Reflects recent common ancestry
■ They still show much more internal variation
o Most genetic variation is seen within racial groups
■ Variation between racial groups accounts for relatively small % of total human variation
■ Reflects broad nature of classic racial groups
■ Reflects fact that racial groups are defined using few selected traits
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