Download evolution: natural selection

CHARLES DARWIN
• Proposed Theory of Natural
Selection; “Father of Evolution”
• Naturalist who set out on a 5 year
journey around the world on the
ship HMS Beagle
• Made many observations in South
America and the Galapagos Islands
• Wrote book “Origin of Species”
Theory of Natural Selection
•Organisms that are best
adapted to an environment are
able to survive and reproduce
more than others
SURVIVAL OF THE FITTEST
• No agent acting on natural selection
• Process of elimination
• Individuals whose current traits are best adapted for the current
environment are the ones that survive and pass on their genes to the
next generation
Includes some fundamental ideas:
• Species change over time and space – representatives of a species
today are not the same as the ones from recent past
• Share common ancestor – as organisms divide and evolve they split
from common ancestors which helps explain similarities between
them; gives rise to new species
• Evolutionary change is slow – supported by fossil record and lack of
sudden appearance of new species
ADAPTIVE RADIATION
• Organisms diversify rapidly from ancestral species into many new
forms in response to environmental changes (good or bad)
Darwin’s Theory has four componets:
•Variation
•Inheritance
•High Population Growth
•Competition
VARIATION
• Each organism has a unique combination of inherited traits
• These traits are expressed in color variation as well as behavior
making no two individuals exactly alike
• Higher species variation means higher chance of survival
VARIATION
INHERITANCE
• Favorable traits are consistently passed down from
parent to offspring
• Best traits and adaptations will allow organisms to
live/reproduce
• Natural selection acts on the phenotype (physical
appearance) not genotype (genetic makeup) of
organism
HIGH POPULATION GROWTH
• Organisms will overproduce offspring
• Each generation of offspring experiences high death rates
COMPETITION
• Organisms compete for limited resources such as food, shelter, mates
• Those with favorable traits out-compete others and are able to
reproduce
SELECTION PRESSURES ON A POPULATION
• Selection Pressure – agent that tends to cause a
genetic change in a population:
• Competition for food
• Competition for shelter
• Competition for mates
• Changes in the environment
• Predation
• Parasitism
MAIN TYPES OF SELECTIONAL PRESSURES
• Stabilizing Selection – genetic diversity decreases and the population
stabilizes on a particular trait value (average phenotype is favored)
MAIN TYPES OF SELECTIONAL PRESSURES
• Directional Selection – an extreme phenotype (physical characteristic)
is favored, causing trait frequency to shift over time in the direction
favoring that phenotype
MAIN TYPES OF SELECTIONAL PRESSURES
• Disruptive Selection – change in population genetics that favors
extreme values for a trait causing the population to divide into two
groups.