Natural SelectionAnd other mechanisms for
Evolution
Darwin’s Observations and InferencesGenetic Variation•Individuals in a species vary from one to another. These variations can be passed from parent to offspring. Over Production of Offspring•Individuals in populations tend to produce more offspring than can survive.Struggle for existence•Over population creates competition.•Environmental resources are limited.•Those that are more efficient will have an advantage for survival.
Differential survival and reproduction
Darwin’s Observations and InferencesInference 1: Production of more individuals than can be supported by the environment leads to a struggle for existence among individuals, with only a fraction of offspring surviving in each generation.
Inference 2: Survival in the struggle for existence is not random• depends in part on the
heritable characteristics • Those who inherit
characteristics most fit for their environment are likely to leave more offspring
Darwin’s Observations and Inferences
Inference 3:•The unequal ability of individuals to survive and reproduce leads to a gradual change in a population, with favorable characteristics accumulating over generations (natural selection).
This is Evolution by Natural Selection
Darwin’s Ideas Updated
• A major problem in Darwin’s theory was the lack of a mechanism to explain natural selection– How could favorable variations be
transmitted to later generations
• With the rediscovery of Mendel’s work and its vast extension in the first half of the 20th century, the missing link in evolutionary theory was forged.
• Darwinian theory supported by genetics is known as the modern synthesis.
Darwin in his early years.
Genetics of Evolutionary Theory
Darwin did not understand how traits were passed on to each generation. Today we define
fitness, adaptation, species, and process of evolutionary change in genetic terms.
Terms to Know
• FITNESS– Ability of an individual to survive and
reproduce• ADAPTATION
– Fitness is the result of adaptations– Adaptation is any individual characteristic that
increases an organism’s chance of survival
What is the purpose of the adaptation in this animal?
Support for Darwin’s Theory
1. Inherited variation exists within the genes of every population or species
– Law of segregation
– Law of independent assortment
Support for Darwin’s Theory
2. In a particular environment, some individuals of a population or species are better suited to survive and have more offspring
Support for Darwin’s Theory
3. Overtime, the traits that make certain individuals of a population able to survive and reproduce tend to spread in that populationDivergence –
Accumulation of differences between groups
Support for Darwin’s Theory
4. There is overwhelming evidence that living species evolve from organisms that are extinct.
Kaibab squirrel of north rim of Grand Canyon
Albert squirrel of south rim of Grand Canyon
Mechanisms for Evolution• Geographic isolation leads to reproductive
isolation– Two populations are separated by geographic
barrier•Example; river, mountain, ocean
– Two gene pools formed
Mechanisms for Evolution• Temporal isolation leads to reproductive
isolation– Two or more species reproduce at different times– Example: brown trout and rainbow trout live in
same stream but cannot interbreed because they breed in different months of the year
Brown Trout
Rainbow Trout
Patterns for Natural Selection• Divergent evolution – pattern in evolution in
which species that were once similar to an ancestral species diverge and become increasingly distinct.
• divergent evolution eventually results in a new species
Polar Bear Brown Bear
Patterns for Natural Selection• Adaptive radiation – when an ancestral species
evolves into an array of species to fit the number of diverse habitats.– Shown when new species evolve rather quickly– Happens when a colony moves to a new environment
that lacks any competing organisms– adaptive radiation is a form of divergent evolution.
•Examples: Hawaiian honeycreepers and finches of Galapagos.
Patterns for Natural Selection
A) organ pipe cactus (N. & S. America)
B) Euphorpia (African deserts)
Convergent EvolutionCan lead to development of analogous structures
PATTERNS OF EVOLUTION • Parallel
Evolution– Two related
species making similar evolutionary changes after their divergence.
– Marsupial and Placental mammals
Patterns for Natural Selection• Convergent Evolution – when two
species look alike but do not have a common ancestor
• Similar in appearance• Similar in function• Different in development, origin, &
anatomy
Opossum
Aye-Aye
DOLPHIN
SHARK
Patterns for Natural SelectionCoevolution – when two species evolve in response to changes in one another
•Flowers & Insects
•Flowers & Birds
•Flowers & Mammals
Evidence for Evolution – Structural Adaptation
• STRUCTURAL ADAPTATIONS– Mimicry– Camouflage
http://science.howstuffworks.com/animal-camouflage2.htm
Natural Selection in Action
Camouflage
Figure 22.11
(a) A flower mantidin Malaysia
(b) A stick mantidin Africa
When the environment changes over time
–Natural selection may result in adaptation to these new conditions
Natural Selection in Action Counter Shading
Natural Selection in Action
Eye spots
Natural Selection in Action
Disruptive Coloration
Natural Selection in Action
Mimicry Monarch or Viceroy Butterfly
Summary• Evolution is change in
species over time• Heritable variations
exist within a population
• These variations can result in differential reproductive success
• Over generations this can result in changes in the genetic composition of the population.