Download Evolution Notes Prt II

Evolution
part II
“Explanations for Evolution, Natural
Selection, Evidence for Evolution, and
mechanisms of Evolution”
Explanations for Evolution

Catastrophism
– states that natural disasters like floods
and earthquakes have happened during
Earth’s long history and that they have
caused species extinction in some cases

Gradualism
– Canyons carved by rivers are examples
of gradual changes over time altering
the landscape
Explanations for Evolution

Uniformitarianism
– Geologic processes that shape the earth
are uniform through time and can be
seen by look at rock strata like an
ancient “yearbook”
Explanations for Evolution

Fossils !
– Traces of organisms that lived in
the past
– After deposition into the Earth
some specimens are “preserved”
and their bones, over a long
period of time, become
mineralized into a rock-like
substance we call fossils
Evidence for Evolution

Charles Darwin saw evidence for evolution in
his travels that convinced him that life must
have evolved over time
– Fossils – Darwin found fossils that closely
resembled living counterparts
– Geography – as Darwin traveled from island to
island he noticed similar species with different
adaptations depending on food sources
 Biogeography- is the study of different organisms and
their relatedness around the world
– Embryology – comparison of embryos between
similar species shows closely similar traits!
Evidence for Evolution

Anatomy – some of darwin’s best evidence
came from the comparison of the body parts
of different species
– Homologous structures – are features that are
similar in structure but appear in different
organisms and often have different functions
– Analogous structures – structures that perform a
similar function but are not similar in origin
– Vestigial structures – remnants of organs or
structures that had a function in an early
ancestor
Evidence for Evolution
Natural Selection

One of the accepted definitions for natural
selection is:
– “the mechanism by which individuals that have
inherited, beneficial adaptations produce more
offspring (on average) than do other individuals”

Darwin was comparing the rates of
reproduction in nature to that of human
reproduction according to Malthus and
concluded:
– “variation within a population must be the answer
to success in population growth”
 Four Main Principles of Natural Selection:
– Variation – heritable differences exist in all
populations and natural selection “acts on” those
traits present in the environment
– Overproduction – having many offspring results
in competition for resources and promotes natural
selection
– Adaptation – the better adapted individuals in any
population live longer and pass their successful
genes to their offspring
– Descent with modification – over time, natural
selection will result in species with adaptations that
make them well suited for survival and reproduction
in that particular environment
Natural Selection

MAIN IDEA
– Natural selection “acts on” phenotypes rather than
on the genetic material itself. New alleles are not
“made” by natural selection – they occur by natural
selection. Natural selection can act only on traits
that exist in a population!

Fitness –
– The measure of the ability to survive long enough
to produce “successful” offspring
 The key is to have your offspring survive long enough to
have their own offspring
Mechanisms of Evolution

Natural selection is not the only mechanism
by which populations evolve
– Gene flow – the movement of alleles between
populations
– Genetic drift – a change in allele frequencies due
to chance
– Bottleneck effect – genetic drift that occurs after
an even greatly reduces the size of a population
– Founder effect – genetic drift that occurs after a
small number of individuals colonize a new area
Mechanisms of Evolution

Bottleneck effect occurs
after an event greatly
reduces the size of a
population such as over
hunting or a meteor
from outer space!
Mechanisms of Evolution
Random mutations occur in populations that may
provide a new allele that is advantageous or not.
Changes in allele frequency are due to genetic drift

Factors in Evolution
Genetic drift – allele
frequencies change due
to chance
 Gene flow – movement
of alleles from one
population to another
 Mutation – new alleles
form through mutation
 Sexual selection –
certain traits improve
mating success and
increase in frequency
