Darwin and Natural Selection

Was from England.
 Beagle Expedition: found
unique species that looked
similar to species from
other parts of the world.
 Found that many of the
plants and animals on
Galapagos resembled
those in nearby South
America
Voyage of the HMS Beagle
The Origin of the Species


On The Origin of Species
by Means of Natural
Selection Nov. 1859
Charles Darwin
Darwin suggested that
humans are related to
apes
Other scientist
 Lamarck
believed that over the lifetime of an
individual, physical features increase in size
because of use and decrease from nonuse
 Malthus suggested that human pops. do not
grow unchecked because death caused by
disease, war, and famine slows growth
Natural Selection
 Natural
selection: organisms with
favorable traits for a particular
environment survive, reproduce, and pass
these traits on to the next generation
(those with less favorable traits are less
likely to survive).
 Adaptation- inherited trait that has become
common in a population because the trait
provides a selective advantage
Evolution of Humans

The Descent of Man,
and Selection in
Relation to Sex
Charles Darwin 1871
 Darwin outlined
sexual selection to
explain the evolution
of humans
4 Tenets of Natural
Selection
 All
populations have genetic variation.
 The environment presents challenges to
successful reproduction.
 Individuals tend to produce more offspring
than the environment can support.
 Individuals that are better fit their
environment tend to live longer, reproduce
more, and spread their genes throughout the
population.
Darwin’s 4 Facts
 Exponential
Growth of Populations
 Factors will prevent Exponential
Growth(Steady State)
 Variability among species
 Traits are inheritable
Darwin’s 2 Inferences
 Competition
for resources
 Individuals most fit for their environment
will survive.
Sources of Genetic Variation
 Mutation/
Translocation- Mutation slowly
introduces new alleles which natural selection
helps distribute (can be harmful, beneficial or
have no significance)
 Crossing Over- exchange of homologous
chromosome segments
 Independent Assortment- assortment of
chromosomes
 Sexual Reproduction- combination of allelles of
two individuals
 Immigration- incoming allelles to a gene pool
Mechanisms for Evolutionary
Change
 Natural
Selection
Population Genetics and
Evolution

Populations evolve when natural selection causes changes
in their allele distribution.

Natural selection causes the greatest changes to
populations’ gene pools, but is not the only cause
of change

Gene Pool- all the alleles of a population of a species
 In a population’s gene pool, each allele has a certain
frequency which governs the frequency of phenotypes
 If a population’s allele frequencies do not change, it is not
evolving and is said to be in genetic equilibrium
Individuals don’t evolve, populations of species do!

Other Influences on the Gene
Pool
 In
small populations, change events
(crossing over, translation errors) can
cause shifts in a gene pool = genetic drift
 Individuals may move into or out of a
population, bringing or taking alleles with
them = migration
Population Evolution
 Gene
flow-the movement of genes into or
out of a population due to interbreeding.


Immigration- the flow of alleles into a
population.
Emigration-the flow of alleles out of a
population.
 Genetic
drift- the random change of allele
frequency in a population, not due to natural
selection.
Population Evolution

Artificial selection- a process in which humans
consciously select for or against particular features
in organisms.
 Genetic bottleneck-an event in which the
populations’ size is greatly reduced; reduces
genetic variability
 Founder effect-changes in gene frequency from
starting a new population from a small number of
individuals; reduces genetic variability.
Adaptations


Structural:
 Can change the size and/or shape of a body part
 For example: thorns, wings, mimicry (copy the appearance
of another species), camouflage (blend in with environment)
 Change within a population over time (anywhere from 100
years to millions of years) depending on type of adaptation,
rate of reproduction and environmental factors
Physiological
 Help populations overcome chemicals they encounter
(antibiotics, pesticides, herbicides, etc)
 Develop rapidly (example: bacterial resistance to penicillin)
Evidence for Evolution
 Fossils
 Anatomical
relationships
 Embryological development
 Genetic comparisons
Fossils
 Fossils
can help provide a record for life
 However, the fossil record is incomplete there are large gaps - therefore each and
every step of evolution can not be followed
 Scientists need to find intermediate
species - and once they do, they can
provide us with a step by step sequence of
evolution
Anatomical Comparisons
 Homologous


Same form, different use
Indication of common evolutionary ancestor
 Vestigial

structures
structures
Evolutionary remnants of structures that were
important in a past ancestor
Embryological Development
 There
are noted similarities between the
embryological development of all
vertebrates (fish, chicken, rabbit, human,
etc)

Presence of gills and tails in all supports
evidence that gill-breathing vertebrates
preceded air-breathing, terrestrial species
Genetic Comparisons
 All
living things use DNA.
 All living things use the same genetic code.
 Can show relationships between species
(human and chimpanzee DNA are 99%
identical and are closer than chimpanzees
are to apes)
 Can help reveal when species diverged
from their ancestral types
Types of Natural Selection
 Stabilizing
Selection - favors average traits
(average sized spiders fair better than
both large and small)
 Directional Selection - favors one extreme
of a trait (the longer the beak the better)
 Disruptive Selection - favors both
extremes of a trait (intermediate at
disadvantage - often eliminated)
Evolution of a Species =
Speciation
 Species
= group of organisms that look
alike and have the ability to interbreed and
produce fertile offspring in nature
 Speciation occurs when


Interbreeding is prevented
Production of fertile offspring is prevented
Causes of Speciation

Isolation of populations - Two populations of the
same species do not mate because of some
form of separation
If populations can’t interbreed due to isolation, their
allele frequencies can become more and more
different from each other
 Each population will adapt to its environment,
eventually preventing breeding with members of
other populations (reproductive isolation)
 Changes in chromosome number causes
instantaneous speciation
 Most speciation happens gradually

Types of Isolation
 Geographic-
geographic barriers form that
separate two populations.
 Ecological- different environmental factors
may cause isolation (wet vs. dry, soil
conditions, etc.)
 Reproductive- mating structures and
behaviors may isolate two populations
 Temporal- two populations may reproduce at
different times
Types of Evolution
 Divergence

(ex: adaptive radiation);
Single populations splits to adapt to different
environments
 Convergence


Unrelated species become similar as they
adapt to a similar environment.
Analogous structures
• Same use, different form
• Not evidence of a common evolutionary
ancestor
Rate of Speciation
 Fossil


record suggests the rate is variable
Gradualism: slow, steady process of
change
Punctuated equilibrium: long periods of
genetic stability punctuated by rapid
changes to new environmental
conditions
Pace of Evolution
 Gradualism-
the pace of natural selection
is gradual change.
 Punctuated Equilibrium- model of
evolutions that states there are differing
periods of rapid change and periods of
little to no change.
 Why? What could cause this?