Transcript Evolution for Beginners

Evolution
What is evolution?
A basic definition of evolution…
“…evolution can be precisely defined as any
change in the frequency of alleles within a
gene pool from one generation to the next."
- Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974
So what does the definition mean?
• Evolution is a change in the number of
times specific genes that code for specific
characteristics occur within an interbreeding
population
• Individuals don’t evolve, populations do
• There is no implied “improvement” in
evolution
Genetic Variation and
Evolution
• Evolution: changes through time
1. Species accumulate difference
2. Descendants differ from their ancestors
3. New species arise from existing ones
A brief history of evolution
• Jean-Baptist Lamack
• First person to come up with a theory of
how evolution occurred.
• Inheritance of acquired characteristics:
changes that an organism aquired during it’s
life were passed on. (no genetic link).
• WRONG
A brief history of evolution
Charles Darwin was born on
February 12, 1809 in Shrewsbury,
England.
From 1831 to 1836 Darwin served as
naturalist aboard the H.M.S. Beagle on
a British science expedition around the
world.
He observed much variation in related
or similar species of plants and animals
that were geographically isolated from
each other.
These observations were the basis for
his ideas.
Evidence of Evolution
• The Fossil Record
• Geographic Distribution of Living Things
• Homologous Body Structures
• Similarities in Early Development
Evidence for Evolution
• The Fossil RecordLayer show change
• Geographic
Distribution of Living
Things
• Homologous Body
Structures
• Similarities in Early
Development
Evidence of Evolution
• The Fossil Record
• Geographic
Distribution of
Living Things-similar
environments have
similar types of
organisms
• Homologous Body
Structures
• Similarities in Early
Development
Homologous Structures
• Homologous Structures-structures that
have different mature forms in different
organisms, but develop from the same
embryonic tissue
Evidence for Evolution
• Vestigial organs-organs that serve
no useful function in an organism
• i.e.) appendix, miniature legs, arms
Similarities in Early
Development
Conclusion
• After making all these observations Darwin
came up with a theory as to how
populations evolve.
• NATURAL SELECTION!
Natural selection: mechanism of
evolutionary change
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Natural selection: proposed by Darwin as the
mechanism of evolution
individuals have specific inherited characteristics
they produce more surviving offspring
the population includes more individuals with
these specific characteristics
the population evolves and is better adapted to its
present environment
Natural Selection
Darwin knew nothing of
genes, but what he did have
were two observations and a
little inference that provided
the motive force for
evolution.
•Darwin: Evolution is descent with modification
Natural Selection
Observation 1: Organisms generally
have more offspring than can survive to
adulthood.
Observation 2: Offspring are not
identical. There is variation in their
appearance, size, and other
characteristics.
Natural Selection
Inference: Those organisms that
are better adapted to their
environment have a greater
likelihood of surviving to adulthood
and passing these characteristics
on to their offspring.
Survival of the “fittest.”
Survival of the “fittest.”
Darwin’s
theory for
how long
necks
evolved in
giraffes
Evolution of species
• Based on 3 mechanisms
– 1.
Sources of variation
– 2.
Method of selection for those
characteristics that would be passed on
– 3.
A mechanism for retaining changes
1. Sources of variation
• Genetic diversity thru mutations that are not
lethal
– Physical or behavioral traits
• Sexual reproduction between genetically
different individuals
1. Sources of variation
Gene Flow
• Many species are made up of local populations whose members tend to
breed within the group.
• Each local population can develop a gene pool distinct from that of
other local populations.
• Members of one population may breed with occasional immigrants
from an adjacent population of the same species.
-introduce new genes or alter existing gene frequencies in the
residents.
This is called hybridization.
2. Method of selection
• Natural selection: environmental
conditions determine which individuals in a
population produce the most offspring
• Reproductive fitness is the method of
selection.
– Competition, escaping from and eluding
predators
– Finding a good mate
Fitness and Its Measurement
• Fitness: A phenotype better fitted to it’s
environment usually increases in
frequency.
• Fitness is a combination of:
– Survival: how long does an organism live
– Mating success: how often it mates
– Number of offspring per mating that survive
3. Maintenance of Variation
• Fittest organisms reproduce.
• The beneficial alleles are passed down to
the offspring.
• This maintains the ability for change.
Selection in 3 Directions
• There are three ways a population can
change.
• Stabilizing Selection
• Disruptive Selection
• Directional Selection
Stabilizing Selection
• Organisms with the average traits have the
highest fitness.
Disruptive Selection
• Individuals that have either extremes have
the highest fitness.
Directional Selection
• Individuals with extreme characteristics in
one direction have the highest fitness.
Selection
• Artificial selection: a breeder selects for
desired characteristics
Hardy–Weinberg principle states that the genotype frequencies in a
population remain constant or are in equilibrium from generation to
generation unless specific disturbing influences are introduced.
Those disturbing influences include
Genetic equilibrium is
a basic principle of
population genetics.
Genetic Drift
• If a population is small, Hardy-Weinberg principal may be violated.
• Chance alone may eliminate certain members out of the population. In
such cases, the frequency of an allele may begin to drift toward a
certain trait or set of traits.
• Ultimately, the allele may represent 100% of the gene pool or, just as
likely, disappear from it.