Chapter 15 - Mechanisms of evolution.
Use of the word theory:
Non scientific use: a theory is, at best, an educated guess. It implies that something is unproven or speculative.
Scientific use: A scientific theory summarizes a group of hypotheses that have been supported with repeated testing and it becomes accepted as a valid explanation of a phenomenon.
Charles Darwin
In 1831, Charles Darwin, a 22-year-old naturalist, accepted a position aboard the ship HMS Beagle that began a voyage around the world; it provided Darwin with many observations.
Prior to Charles Darwin
· Pre-Darwinian world-view was determined by theological beliefs.
o The earth is young.
o Each species was specially created and did not change
o Variations are imperfections
o Observations are to substantiate the prevailing worldview.
o Aristotle and Judeo-Christian viewpoints – “scala natuae”
o Scientists were also usually theologians
Linneaus (1707-1778)
o Swedish taxonomist (taxonomy is the science of naming and classifying organisms.
o He developed a binomial system of nomenclature (two-part names for each species [e.g., Homo sapiens]).
o Like other taxonomists of his time, Linnaeus believed in the ideas of
a)special creation—each species had an“ideal”structure and function; and
b)fixity of species—each species had a place in thescala naturae, a sequential ladder of life.
Lamarck
• first to state that descent with modification occurs
• Proposed inheritance of acquired characteristics
• he believed that organisms become adapted to their environment.
• Lamarck mistakenly saw “a desire for perfection” as inherent in all living things
Should be recognized for:
claim of evolution, theorizing about great age of earth, emphasis on adaptation
Charles Lyell
Lyell was a geologist that popularized the idea that earth had been shaped by slow-acting forces that are still at work today. 2 ideas that Darwin took from Lyell’s work:
The earth must be very old (much older than 6000 years)
Many gradual changes over time will yield substantial changes.
Charles Darwin 1809 - 1882
HMS Beagle Sailed in 1831
Darwin was ship’s naturalist
Sailed for 5 years and surveyed coast of South America
Voyage of the HMS Beagle
Plants and animals
Uniquely South American
More closely related to each other than to organisms on other continents
Fossils – uniquely SA
Galapagos Islands
Organisms lived nowhere else on earth
Closely resembled organisms from the mainland
Three major propositions by Darwin
1. species are not immutable –they change over time
2. Divergent species share a common ancestor
3. The mechanism that produces change is natural selection
Natural Selection
Darwin decided that adaptations develop over time; he sought a mechanism by which adaptations might arise.
Natural selection was proposed by both Alfred Russel Wallace and Darwin as a driving mechanism of evolution caused by environmental selection of organisms most fit to reproduce, resulting in adaptation
What is Natural Selection?
1. Differential success in reproduction
2. Occurs through interaction between environment and variability in individuals
3. Product of natural selection is the adaptation of populations of organism to their environment
4. Because the environment is always changing, there is no perfectly-adapted organism.
5. There are three preconditions for natural selection.
a. The members of a population have random but heritable variations.
b. In a population, many more individuals are produced each generation than the environment can support.
c. Some individuals have adaptive characteristics that enable them to survive and reproduce better.
6. There are two consequences of natural selection.
a. An increasing proportion of individuals in succeeding generations will have the adaptive characteristics.
b. The result of natural selection is a population adapted to its local environment.
7. Natural selection can only utilize variations that are randomly provided; therefore there is no directedness or anticipation of future needs.
8. Extinction occurs whenprevious adaptations are nolonger suitable to a changed environment.
Organisms Struggle to Exist
Malthus proposed that human populations outgrow food supply and death and famine were inevitable and Darwin applied this to all organisms; resources were not sufficient for all members to survive.
Therefore, there is a constant struggle for existence; only certain members survive and reproduce.
Variations
1. In contrast to the previous worldview where imperfections were to be ignored, variations were essential in natural selection.
2. Darwin suspected, but did not have today's evidence, that the occurrence of variation is completely random.
3. New variations are as likely to be harmful as helpful.
4. Variations that make adaptation possible are those that are passed on from generation to generation.
5. Darwin could not state the cause of variations because genetics was not yet established.
Organisms differ in fitness
FITNESS is a measure of an organism’s reproductive success
A polar bear can survive and reproduce in cold climates, it is not adapted to warmer climates
Reproduction is important for evolution, only those who survive to reproduce pass on those advantageous genes
Artificial Selection
Darwin used the power of artificial selection to reinforce natural selection
Organisms Become Adapted
1. An adaptation is a trait that helps an organism be more suited to its environment.
2. Unrelated organisms living in the same environment often display similar characteristics. (Convergent evolution)
3. Because of differential reproduction, adaptive traits increase in each succeeding generation.
Drawbacks of natural selection
Can only utilize variations that are randomly provided; there is no directedness or anticipation of future needs
Extinctions occur when previous adaptations are no longer suitable to a changed environment
Some subtleties
Population is the smallest unit that can evolve—individuals DO not evolve
Has to be a heritable characteristic
Situational
Examples
Evolution of insecticide resistant insects Evolution of antibiotic resistant bacteria
Evidence of Evolution
Homology
Anatomical homologies
Vestigial organs
Embryologyical homologies
Molecular homologies
Analogous vs. Homologous
Analogous (convergent evolution)
Occurs when two organisms live in such a similar (same) location that over time certain physical characteristics are favored and the two unrelated organisms end up looking very similar:
Ex: Shark and Dolphin
Ex: fusiform body shape in tuna, marlin, penguin, seal
Homologous (divergent evolution)
Occurs when 2 or more organisms share a common ancestor but live in such different environments that they have developed different structures used in different ways
EX: forearm of many vertebrates
Vestigial structures:
Human appendix
Pelvic bone in whales
Eye sockets in blind salamander
Fossils Back Up Evolution
Transitional forms – Fossils that link older forms of an organism to the current form
Darwin predicted it and we have found many.
Achaeopteryx – transition from reptile to bird. Had some reptile characteristics (tail with vertebrae, claws on wing, teeth) Also had bird characteristics feathers, wings, but pectoral girdle was not big enough to attach flying muscles to it. May have glided.
Neil Shubin at Penn State found the Tiktaalik. It’s a transitional fossil showing an organism that bridged the gap from water to land. Front fins were under body (able to support weight) and had the classic vertebrate forearm structure of one large bone, two smaller bone and a bunch of tiny bones
Microevolution
Evolution that occurs within a population = microevolution
Population -- all the members of a single species
Population genetics – studies variations in gene pools
Gene Pool and Allele Review
Gene pool – total of all the alleles in the population
Alleles – chromosome sections that code for specific proteins (traits). You have 2 for every trait. But there may be many more than 2 possibilities in the entire gene pool
5 causes of Microevolution
small gene pool
non random mating
mutation
migration
natural selection
Small Population
A small population is more likely to have a change in allele frequency due to chance
This is called genetic drift
Two situations in the which genetic drift occurs:
Bottleneck effect – disaster which greatly reduces the population. Remaining survivors not representative of original population.
Founder effect – when a few individuals from a large population colonize a new and isolated habitat.
Non Random mating
When sexually reproducing organisms select a mate based on some trait or behavior, evolution will take place as certain traits are selected for a passed on and others are not.
Sexual selection
Natural selection for mating success
Competition amongst males for female mates
Males have ritual displays and battles as well as to attract females
Intrasexual selection is a direct competition for mates among individuals of the same species and gender
Sexual Dimorphism
Distinct physical differences between males and females of the same species (aside from reproductive organs)
Ex:
Male cardinals are red, females grey
Lion has mane, lioness doesn’t
Peacock tail, pea hen no showy tail
Mutation
Mutation creates variation
New genes and new alleles originate only by mutation
Only mutations to sex cells can be passed on
Changes the DNA
Changes amino acid sequence, changes protein, changes fitness, usually makes an individual less fit, but occasionally, more fit
Gene Flow
Any addition of genes from other populations is gene flow.
This is what we would call “migration.”
As we become more of a global community, the human race experiences more and more gene flow.
Natural Selection
Organisms in a population are not equal in our ability to survive and produce offspring.
Thus some will pass their genes on and others will not.
There are several types of natural selection:
Diversifying
Directional
Stabilizing
Types of selection: The effect of selection depending on what is “fit”
Heterozygote advantage
Heterozygotes can have a have a greater fitness
Maintain both alleles in population
Sickle cell anemia
Heterozygotes are protected from the severest effects of malaria and do not develop sickle cell anemia
Five fingered evolution
Remember the 5 things that cause microevolution using your hand:
Pinky is small population size (genetic drift)
Ring finger is non random mating
Middle finger is mutation
Index finger is gene flow (migration)
Thumb is natural selection
Hardy-Weinberg theorem
Describes allele frequencies in a non evolving population.
They remain constant because no matter how many times we shuffle the alleles in a non evolving population, we always get the same ratios
Why do we need Hardy Weinberg?
Provides a base line or comparison
Evolution is a generation-to-generation change in a population’s frequencies of alleles
So if there is no change in allele frequencies, there is no evolution
· Since allele frequencies are rarely equal, the dominants and recessives are represented by p and q.
· Frequency of alleles and genotypes
Frequency of alleles:
p + q = 1
· Suppose the p allele is for insulin production. q allele is for the mutated form where insulin is not produced. All the p and all the q added together is the entire human population.
Frequency of genes:
p2 + 2pq + q2 = 1
freq of freq of freq of
RR Rr rr
5 conditions of Hardy-Weinberg
· Large population (pinky)
· Random mating (ring finger)
· No mutation (middle finger)
· No migration (index finger)
· No natural selection (thumb)
If the allele frequencies change over generations, it must be due to one of these five things.
HW equilibrium
Keeping the same allele frequencies over generations is called Hardy-Weinberg equilibrium.
A failure to maintain HW equilibrium indicates evolution has occurred.
The pop. size is too small, there was non random mating, there was mutation, there was migration or there was natural selection.