Download Theory of evolution by natural selection

Bio - Evolution
• Various theories of evolution and life exist:
– 300 BC Greeks theorized that an organism’s form is related to its function.
– 1769 Charles Bonnet observed that fossils do not resemble modern organisms,
and theorized that they are from a previous era. He is the first to use the term
‘evolution’ to describe the development of diverse life-forms.
– 1809 Jean-Baptiste Lamarck reasoned that the fossils of extinct animals were the
ancestors of those living today.
• Organisms constantly strive to improve themselves
• The Principle of Use and Disuse – body structures develop or waste away
depending on their use or disuse
• Principle of Inheritance of Acquired Characteristics – once a structure is
modified, that organism will pass it on to its offspring.
– Weismann would disprove Lamarck’s theory by showing that although an
organism may adapt to its surroundings during its lifetime, these
adaptations are not passed on to offspring.
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Bio - Darwinian Evolution
• During Darwin’s voyage on the HMS Beagle, he found evidence that challenged the
accepted idea that all critters had been created exactly as they are now seen, and
were unchanging.
• Charles Lyell had written in Principles of Geology that the surface of the Earth had
changed slowly over many centuries. Darwin’s observations of different islands and
shorelines presented questions that could be explained only thru a similar process of
slow change over time.
– Ex: Fossils of armadillos that closely resembled, but were not exactly like the living ones
in the same area.
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Bio - Darwinian Evolution
• In the Galapagos Islands, Darwin noted that many of the species of
plants and animals were very similar to those on the coast of South
America, but were not identical. He would propose that a common
ancestor species had migrated to the islands, and its offspring
developed differently over time from the offspring of the same species
on the mainland.
• Darwin named this idea of change over time as descent with
modification – evolution.
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Change Over Time……
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Bio - Darwinian Evolution
• Darwin looked at the practice of artificial selection and
hypothesized that there was a force in nature that acted the
same way.
– Artificial selection is the purposeful breeding of animals with
specific traits, in the hopes that those traits would be passed on to
offspring. ….. i.e. dog breeding, etc.
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Bio - Darwinian Evolution
• Thomas Malthus in 1798 proposed that human populations
are capable of increasing faster than their food supply.
– We do not outdistance our food supply because our population
growth is slowed by death due to disease, famine, war.
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Bio - Darwinian Evolution
• Darwin applied Malthus’ idea to all species populations – the idea that
one organism has the potential to produce many offspring in its
lifetime, BUT only a limited number of those offspring are able to
survive due to environmental limiting factors.
– Only the individuals that survive will be able to mate and reproduce,
so……”Individuals that have physical or behavioral traits that better suit their
environment are more likely to survive and will reproduce more successfully
than those that do not have such traits.” ----- natural selection
– In time, the individuals that carry these favorable characteristics increase in a
population and the individuals without decrease, so the nature of the population
changes. ----- evolution (change over time)
– Since different habitats present different challenges, organisms of a species will
differ because it has changed in response to its specific environment ----adaptation.
• Adaptation – feature that has become common in a population due to the selective
advantage it provides.
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Bio - Darwinian Evolution
• Darwin wrote a paper on his theory of evolution in 1844, but would
not publish his ideas until 1858, when Alfred Russel Wallace, a
naturalist, asked him to help get his essay published – Wallace’s essay
also proposed evolution by natural selection, so Darwin presented his
manuscript with Wallace’s essay at a public scientific meeting.
• Though there was very strong resistance to the ideas, the evidence and
arguments made people think and slowly the idea gained acceptance
by other biologists.
• Later discoveries, especially in the field of genetics, have given more
information on how natural selection can bring about evolution of a
species, resulting in some modernization and tweaking of the original
hypothesis, but NOT fundamentally changing it.
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Bio - Darwinian Evolution
•
Darwin’s Theory of evolution by natural selection is
supported by four main points:
1. Variation (differences among individuals in a species) exists
within the genes of every population or species as a result of
random mutations and translation errors.
2. Because of this variation, in a particular environment, some
individuals of a population or species are better suited to survive
and have more offspring (natural selection).
3. Over time, the traits that make certain individuals of a population
able to survive and reproduce, tend to spread thru the population.
4. There is overwhelming evidence from fossils and many other
sources that living species have evolved from organisms that are
now extinct.
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•
The evolutionary history of a species is called phylogeny.
Bio – Darwinian Evolution - Updated
1. Now that we know genes are responsible for inherited
traits, we can see that natural selection results in certain
forms of a trait becoming more common in a
population…..Natural selection causes the frequency of
certain alleles to increase or decrease over time.
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New variations caused by mutations and recombinations of
alleles during sexual reproduction provide endless sources of new
variation for natural selection to act on.
The Fossil Record
 Fossils provide a record of Earth’s
past life-forms, and present
evidence that species have changed
over time….fossilized species
resemble but are different from
living individuals.
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Paleontologists (scientists who study
fossils) use radiometric dating to
determine the age of fossils, and are
fairly accurate.
Once the fossils and/or sediment
surrounding them are dated, then
fossils of organisms can be arranged
from oldest to youngest, and a pattern
of evolution can be seen.
Darwin predicted that intermediate
forms would be found that would
bridge the gap between organisms that
are extinct and living organisms…..and
many have been.
• The fossil record is the only evidence we
have of many organisms that are now
extinct. However, the fossil record is
not, and will probably never be
complete.
– Some critters live in environments that
do not form fossils, as fossils are
generally formed when the organism or
its traces are quickly buried by
sediments carried by water, wind or
volcanic eruptions
• Environments that are conducive to the
formation of fossils include wet
lowlands, slow-moving streams, lakes,
shallow seas, and areas near volcanoes
that spew volcanic ash.
• Upland forests, mountains, grasslands
and deserts generally do not produce a
large amount of fossils.
– Fossils only form if the animal remains
have not been scattered, eaten or decayed
before being buried.
• Animals with hard exoskeletons like
arthropods, are more likely to be
fossilized than a soft-bodied organism.
• Even incomplete, the fossil record is
strong evidence for evolution.
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Bio – Evidence for Evolution
• Evidence of new species evolving, is found by comparing
several different aspects of the organisms – living and
fossils.
– Homologous structures – traits that are similar in different
species because they share a common ancestor.
• Ex: Forelimbs of vertebrates are similar – arm/hand in humans, front feet of
dogs, wings in birds
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Bio – Evidence for Evolution
– Vestigial organs – structures or organs that seem to serve no
useful function.
• Ex: appendix in humans, hind legs on whales
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Bio – Evidence for Evolution
– Analogous structures – traits that are similar in function, but are
not inherited from a common ancestor.
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Bio – Evidence for Evolution
– Biochemistry – Species that share a recent common ancestor,
should have fewer differences in the proteins and nucleotide
sequences…..a long ago common ancestor would result in more
differences in related species’ proteins and nucleotide
sequences.
• Ex: Amino acid differences from human Hemoglobin Protein:
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Gorilla – 1
Rhesus monkey – 8
Mouse – 27
Frog – 67
Lamprey - 125
Bio – Evidence for Evolution
– Embryology – embryos of different species may have almost identical patterns
of development, suggesting that they share common genes for embryonic
development, and therefore a common ancestor.
• At some point during embryonic development, all vertebrates have a tail, buds that
become limbs, and pharyngeal pouches (develop into structures in the throat in
humans).
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Bio – Natural Selection
• There are four driving points of natural selection for all real
populations…..
– All populations have genetic variation
– The environment presents challenges to successful reproduction
– Individuals tend to produce more offspring than the environment
can support, so that individuals of a population must compete to
survive.
– Individuals that are better suited to the challenges survive and
reproduce more than individuals who are poorly suited to the
challenges.
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Bio – Natural Selection
• The process of forming new species
(members of a species can only
breed successfully within their own
species) is called speciation.
– Speciation results from the divergence
of a species.
• Divergence refers to an accumulation
of differences between groups of the
same species they adapts to different
environments.
• Subspecies are the first step of
speciation – the organisms of the
species have developed differing
adaptations, but are still able to
interbreed successfully.
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An experiment demonstrating allopatric
speciation in the fruit fly (Drosophila
pseudoobscura) conducted by Diane Dodd. A
single population of flies was divided into two,
with one of the populations fed with starchbased food and the other with maltose-based
food. After the populations had diverged over
many generations, the groups were again
mixed; it was observed that the flies would
mate only with others from their adapted
population.
Mechanisms for Evolution
• Individuals do not evolve –
populations do.
• In any population, a specific gene will occur in a percent of
the individuals – this is called allele frequency.
– If the frequency of that gene remains fairly constant, then the
population is said to be in genetic equilibrium.
– These population are not evolving – little to no change is occurring
because it is not needed.
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Mechanisms for Evolution
• Many things can alter the percentage of genes in a given
gene pool, which will result in evolution.
• Natural Selection is the predominant mechanism of
evolution for larger populations and established gene pools.
**Remember, natural selection acts as a result of genetic
variation.
– Stabilizing Selection favors average individuals in a population –
variation outside the norm makes the individual less likely to
survive.
– Directional Selection favors an extreme of a trait, eliminating the
average and opposite extremes.
– Disruptive selection favors individual either extreme, but
eliminates the average.
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Mechanisms for Evolution
• In small populations, other factors can act to spur an
evolutionary change in that population.
• Mutations - disruptions in an individual’s genetic code
– Many are lethal and will thus be quickly eliminated
– Useful mutations may result in evolution through natural selection
• Ex: The recessive gene that causes sickle cell anemia also provided
immunity to malaria
• Genetic Flow occurs when
individuals move in or out of a
population, resulting in new genes
being introduced to one population
and becoming unavailable to
another.
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Mechanisms for Evolution
• Genetic Drift – alteration of allelic frequencies by chance events.
– Small populations becoming isolated thru belief or religion, or from
environmental factors
• Amish in Pennsylvania – percentage of a recessive allele that results in short arms
and legs, and extra fingers and toes is 1/14 as opposed to 1/1000 in the rest of the US
• Blue People of the Troublesome Creek, Kentucky
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Mechanisms for Evolution
– Geographic isolation may separate similar populations from one
another, so that they evolve separate from one another, possibly
resulting in speciation over time.
• Lava flows…..islands breaking from the mainland….
• Eventually the two different populations that have been separated from one
another, may become so different, that they can no longer interbreed.
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Mechanisms for Evolution
– Reproductive isolation occurs when formerly
interbreeding populations can no longer mate and
produce fertile offspring.
• Genetic material is so different that fertilization cannot occur
• May also result as a difference in behaviors…..
– One population mates in the fall, the other in spring…..or one is
nocturnal and the other is diurnal.
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Mechanisms for Evolution
– Polyploidism (individual or
species with a multiple of the
normal set of chromosomes) can
also cause speciation…..
• Remember, polyploidism is
caused by total nondisjunction
during meiosis – none of the
chromosomes separate, so one egg
or sperm gets twice as many as it
is supposed to have.
• The polyploid individuals will not be able to mate and produce fertile offspring with
normal individuals in its population.
• But, they may be able to interbreed with other same polyploids, and have fertile
offspring, resulting in a totally new species (since members of a species should have
the same number of chromosomes, with small exceptions due to genetic mutations,
i.e. Trisomy 21, etc)
• Common in plants, not so much in animals
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Bio – Darwinian Evolution - Updated
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There are two tempos of evolution….both have evidential
support and neither has been shown to be the only correct
possibility……
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Gradualism in which change resulting in a new species occurs
gradually over a long period of time.
Punctuated equilibrium, where a species may remain
unchanged for very long periods of time, only to undergo rapid
spurts of change as a result of major environmental changes.
Patterns of Evolution
• Divergent radiation – species that were once very similar,
gradually become less and less alike.
– Adaptive radiation: One ancestral species evolves into an array
of species, to fit differing habitats…common on islands.
• Ex: Darwin’s finches
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Patterns of Evolution
• Convergent evolution – distantly related organisms evolve
similar traits.
– Share a similar environment, so make some of the same
adaptations over time.
– They are not related, and do not interbreed, they just become more
and more alike in
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mantispid
preying mantis