Transcript Darwin and Natural Selection

Evolution
Unit 13
p. 275-292, 327-332
Darwin and Natural Selection

Beagle Expedition: found
unique species that looked
similar to species from
other parts of the world.
 Noticed changes in form
among members of the
same and closely related
species.
Voyage of the HMS Beagle
Darwin’s Voyage
Development of Darwin’s
Theory
 In
1844, Darwin finally wrote down his ideas
about evolution and natural selection in an early
outline that he showed to only a few scientists
he knew and trusted.
 Darwin decided to publish after he received a
letter and essay in June 1858 from the young
English naturalist Alfred Russel Wallace (1823–
1913), who was in Malaysia at the time.
Wallace’s essay described a hypothesis of
evolution by natural selection.
Darwin’s Theory
Darwin’s theory of evolution by natural selection
is supported by four major points:
1.Variation exists within the genes of every
population or species.
2.In a particular environment, some individuals of
a population or species are better suited to
survive and have more offspring.
3.Over time, the traits that make certain
individuals of a population able to survive and
reproduce tend to spread in that population
4.There is overwhelming evidence from fossils
and many other sources that living species
evolved from organisms that are extinct.
The Origin of the Species




On The Origin of Species
by Means of Natural
Selection Nov. 1859
Charles Darwin
He avoided the term
"evolution”
“light will be thrown on
the origin of man and his
history".
"endless forms most
beautiful and most
wonderful have been,
and are being, evolved."
Natural Selection
 Natural
selection: mechanism for change
in populations that occurs when organisms
with favorable variations for a particular
environment survive, reproduce, and pass
these variations on to the next generation
(those with less favorable traits are less
likely to survive).
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
Darwin’s 4 Facts
 Exponential
Growth of Populations
 Variability among species
 Factors will prevent Exponential
Growth(Steady State)
 Traits are inheritable
Darwin’s 2 Inferences
 Competition
for resources
 Individuals most fit for their environment
will survive.
4 Tenets of Natural
Selection
 Individuals
tend to produce more offspring
than the environment can support.
 All populations have genetic variation.
 The environment presents challenges to
successful reproduction.
 Individuals that are better fit their
environment tend to live longer, reproduce
more, and spread their genes throughout the
population.
Natural Selection
Sources of Genetic Variation
 Mutation-
changes in DNA can slowly
introduces new alleles which natural selection
helps distribute (can be harmful, beneficial or
have no significance)
 Meiosis


Random Alignment of chromosomes
Crossing Over- exchange of homologous
chromosome segments
 Sexual
Reproduction- combination of alleles of
two individuals
 Immigration- incoming alleles to a gene pool
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!
Mechanisms for Evolutionary
Change
 Natural
Selection
 Artificial Selection
 Gene Flow
 Genetic Drift
 Founder Effect
 Genetic Bottleneck
Population Evolution
 Artificial
selection- a process in which
humans consciously select for or against
particular features in organisms.
 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.
Population Evolution

Genetic drift- the random change of allele
frequency in a population, not due to natural
selection.
 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)
Darwin’s Finches
Evidence for Evolution
 Fossil
Record
 Anatomical relationships
 Embryological development
 Genetic comparisons
Fossil Record
 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
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Same form, different use
Indication of common evolutionary ancestor
 Vestigial

structures
structures
Evolutionary remnants of structures that were
important in a past ancestor
Chapter 13
Forelimbs of Vertebrates
Evidence of Whale Evolution
Evolutionary Relationship
Between Whales and Hoofed
Mammals
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
Comparing Vertebrate
Embryo Development
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
Chapter 13
Hemoglobin
Comparison
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)
Types of Natural Selection
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
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Unrelated species become similar as they
adapt to a similar environment.
Analogous structures
• Same use, different form
• Not evidence of a common evolutionary
ancestor
Pace of Evolution
 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