darwin’s origin of species recognition of evolution as an explanation of life’s unity and...
TRANSCRIPT
Darwin’s Origin of Species
• Recognition of evolution as an explanation of life’s unity and diversity
• Natural selection- the mechanism of evolution
Facts and Inferences
FACT #1
All species have the potential for fertility
Facts and Inferences
FACT #2
Most populations are stable
Facts and Inferences
FACT #3
Natural resources are limited
Facts and Inferences
FACT #4
No two individuals are the same (variation in genetic make up)
Facts and Inferences
FACT #5
Most differences are heritable (genes passed to offspring)
Facts and Inferences
INFERENCE #1
There is a struggle for existence, with the survival of only a few individuals
Facts and Inferences
INFERENCE #2
Survival is NOT random
The best adapted will leave more offspring (fitness)
Facts and Inferences
INFERENCE #3
Populations will change
Populations evolve: NOT individuals
Natural Selection works at the individual level, not population or group
Natural Selection
• the process whereby organisms better adapted to their environment tend to survive and produce more offspring. The theory of its action was first fully expounded by Charles Darwin and is now believed to be the main process that brings about evolution
Questions
• Is behavior of a species driven by natural selection?
* Explain how the behavior in the video demonstrates natural selection.
Evidence for Evolution
For Darwin the proof was in the Finches!
Other Evidence
Artificial Selection
Other Evidence
Fossil Record
Other Evidence
Comparative Anat/Phys
• Homologous Structures
Other Evidence
Comparative Anat/Phys
• Vestigial Structures
Other Evidence
Embryology
Other Evidencehttp://news.nationalgeographic.com/news/2005/12/1207_051207_dog_genome.html
• Comparative study of genetic code between species reveals an extremely common genetic code – evidence that supports being copied from original ancestor (same genes for same proteins)
• Ex. Humans, mice and dogs –coding genome sequences that are shared by all three species are virtually identical
Other Evidence
• Biogeography – the geographical distribution of species
• Biogeographical evidence of the Galapagos provided Darwin with observations leading to his Common Descent (nice way of saying evolution) theory
» Explains why island species resemble species from nearest mainland versus species from similar looking islands found in different part of world.
» Australia has diverse marsupial population but could support placental species.
Related Term
• Adaptive radiation – populations spread, the new environmental conditions lead to adaptations
• Ex. – Darwin’s finches, Marsupials - colonization of island (or other isolated environment) allows rapid variation due to a decrease in environmental stress
Adaptive Radiation
Microevolution
• Population – localized group of individuals belonging to the same species
• Species – group of populations whose individuals have potential to interbreed
• Gene pool – all alleles in a population at any one time, all alleles at all loci in all individuals
• Allele frequency – the fraction or percentage that the allele occupies within the population
Sample Problem #1Total of 500 individuals
20 = white320 = homozygous pink160 = heterozygous pink
Allele frequency:a = 2(20) + 160 = 200 = 0.2
1000 1000
A = 2(320) + 160 = 800 = 0.81000 1000
Hardy-Weinberg Theory
• Theorem: Frequency of alleles and genotypes in a population’s gene pool remain constant over generations unless acted upon by agents other than sexual recombination.
Hardy-Weinberg Equilibrium
• Allele frequency is constant from generation to generation
Hardy-Weinberg Equation
p2 + 2pq + q2 = 1
p + q = 1
Hardy-Weinberg Equation
p = frequency of dominant allele
q = frequency of recessive allele
p2 = frequency of homozygous dominant genotype (AA)
2pq = frequency of heterozygous genotype (Aa)
q2 = frequency of homozygous recessive genotype (aa)
5 Required Conditions for Hardy-Weinberg Equilibrium to be true:
#1 Very Large Population
5 Required Conditions for Hardy-Weinberg Equilibrium to be true:#2 Isolation From Other Populations
5 Required Conditions for Hardy-Weinberg Equilibrium to be true:#3 No Net Mutations (lethal or altering)
5 Required Conditions for Hardy-Weinberg Equilibrium to be true:
#4 Random Mating
5 Required Conditions for Hardy-Weinberg Equilibrium to be true:
#5 No Natural Selection (all alleles must be equally viable)
Why do these 5 conditions need to be met?
• Frequency of alleles will not change
• Frequency of genotypes will not change
Are these conditions met in nature?
• No, this is the ideal
• Nature will cause changes in allele frequency – causing changes in the gene pool
• This will cause variation – natural selection acts upon variation!
Sample Problem #2• Refer to the flower population in the previous sample problem.
What are the frequencies of the three possible genotypes for the next generation of flowers.
• Determine what you know and want to know.• Know p = 0.8 and q = 0.2 (check it with p+q=1)• Want to know frequencies of 3 genotypes
– AA = p2 = ?– Aa = 2pq = ?– Aa = q2 = ?
• AA = p2 = (0.8)(0.8) = 0.64 = 64% of the next generation of flowers will be homozygous dominant
• Aa = 2pq = 2(0.8)(0.2) = 0.32 = 32% of the next generation of flowers will be heterozygous
• aa = q2 = (0.2)(0.2) = 0.04 = 4% of the next generation of flowers will be homozygous recessive
Sample Problem #3The actual results of the second generation (F1) of white and pink
flowers are listed below. Was Hardy-Weinberg equilibrium achieved? Why or why not?
GenotypesActualAA 245Aa 210aa 45
• No, HW equilibrium was not achieved from P to F1. • Expected results were:• AA = .64(500) = 320• Aa = .32(500) = 160• aa = .04(500) = 20• The expected and actual results vary, showing that HW equilibrium
did not occur. This means that at least on of the five conditions were not met.
Sample Problem #4A simple Mendelian trait has two alleles, D and d. If a population is
49% homozygous dominant, what percentage is heterozygous? What are the allele frequencies in the population?
What you know:DD = p2 = 0.49 so… p = 0.7 and q= .3 ( remember: p +q = 1)
Want to know:• Allele frequencies are:
• D = 70% or 0.7• d = 30% or 0.3
Percentage heterozygous:Dd = 2pq = 2(0.7)(0.3) = 0.42 or 42% of population will be heterozygous
III. Microevolution
• Definition:– Change in the gene pool of a population– Change in relative frequencies of alleles in
population– Evolution on the smallest scale
Causes of Microevolution
1. Mutations– In structural genes– In regulatory genes– Introduction of a new allele, variation for other
forces to act upon
Ex. Gene sequence for sickle cell anemia (structural gene mutation)
• Ex 2 – mutations in regulatory genes
http://www.findarticles.com/p/articles/mi_m1200/is_22_168/ai_n15931496
DNA clues to our kind: regulatory gene linked to human evolution
Science News, Nov 26, 2005 by B. Bower• A gene that exerts wide-ranging effects on the brain works harder in people than it
does in chimpanzees and other nonhuman primates, a DNA disparity that apparently contributed to the evolution of Homo sapiens, according to a new study.
• The gene participates in production of prodynorphin, an opiumlike protein that serves as a building block for chemical messengers in the brain known as endorphins. Studies have implicated endorphins in the anticipation and experience of pain, in the formation of intimate emotional bonds with others, and in learning and memory:
• All primates possess a virtually identical prodynorphin gene, say geneticist Matthew V. Rockman of Princeton University and his colleagues. However, a separate stretch of DNA regulates the extent to which the gene generates prodynorphin. This regulatory DNA displays a handful of mutations in people that must have evolved by natural selection and aided human survival, the scientists propose. The new findings appear in the December PLoS Biology.
• "This is the first documented instance of a neural gene that has had its regulation shaped by natural selection during human origins," says geneticist and study coauthor Matthew W. Hahn of Indiana University in Bloomington.
Causes of Microevolution
2. Gene Flow– Migration (immigration and emigration)– Genes move into and out of the gene pool– New alleles, changes frequecies– Hey – this actually decreases differences in
a population
Ex 1 – human populations “blending”Ex 2 – organic vs. genetically engineered crops
Causes of Microevolution
3. Genetic drift - change in gene pool of small population due to random chance
• Founder Effect
• Bottleneck Effect
Closer look: Founder effect
-definition – The founder effect occurs when populations are started from a small number of pioneer individuals of an original population. Due to small sample size, the new population could have a much different genetic ratio than the original one.
Ex. Today’s Amish are descendants of 30 people who migrated from Switzerland in 1720. The 30 original individuals carried a higher than normal % of genes for Dwarfism.
Closer look: Bottleneck effect
Definition: bottleneck effect – population reduced by outside forces (over-hunting, catastrophe, genocide) then population increases in number again – but all descendants of smaller representative group Ex. elephant seal off the Baja coast, Jewish population (Tay Sachs),
• Bottleneck Effect• This is a severe reduction in population size that causes
pronounced drift. For example, the elephant seal population was hunted down to just 20 individuals. Then the population rebounded to 30,000. However, electrophoresis has revealed that there is now no allele variation at 24 genes.
• In bottlenecks, some stressful situation greatly reduces the size of a population leaving a few (typical or atypical) individuals to reestablish the population.http://trc.ucdavis.edu/biosci10v/bis10v/week6/05bottleneck.html
Causes of Microevolution
4. Nonrandom mating
- Self –pollination in plants
- Sexual selection in animals (mating calls, plumage, male combat, twig selection)
Causes of Microevolution
5. Natural Selection
* Not all alleles are equally viable
* some alleles have better success in the environment, the phenotypes are selected for while others are selected against
NATURAL SELECTION – major cause of microevolution
• Adaptation of a population to the biotic and abiotic environment– Requires:
• Variation - The members of a population differ from one another
• Inheritance - Many differences are heritable genetic differences
• Differential Adaptiveness - Some differences affect survivability
• Differential Reproduction – Some differences affect likelihood of successful reproduction
V. Types of Selection
• Most traits are polygenic - variations in the trait result in a bell-shaped curve
• Three types of selection occur:
– (1) Directional Selection
• The curve shifts in one direction
• Ex - when bacteria become resistant to antibiotics
Directional Selection
Types of Selection
• Three types of selection occur (cont):– (2) Stabilizing Selection
• The peak of the curve increases and tails decrease• Ex - when human babies with low or high birth
weight are less likely to survive
– (3) Disruptive• The curve has two peaks• Ex – When Cepaea snails vary because a wide
geographic range causes selection to vary
Stabilizing Selection
Disruptive Selection
Sexual Selection
• Sneaky Male Iguanas
IV. Preserving Genetic Variability
• Industrial Melanism and Microevolution
Maintenance of Variations
• Genetic variability
– Populations with limited variation may not be able to adapt to new conditions
– Maintenance of variability is advantageous to population
• Only exposed alleles are subject to natural selection
Maintenance of Variations
• Recessive alleles:– Heterozygotes shelter recessive alleles from selection
– Allows even lethal alleles to remain in population at low frequencies virtually forever
– Lethal recessive alleles may confer advantage to heterozygotes
• Sickle cell anemia is detrimental in homozygote
• However, heterozygotes more likely to survive malaria
• Sickle cell allele occurs at higher than expected frequency in malaria prone areas
Sickle-cell Disease
B. Ways to get variation
• Sex
• Mutations
• Diploidy – two possible alleles for trait
• Polymorphism – Heterozygote advantage– Hybrid vigor– Frequency dependent advantage
Examples of Polymorphism
Species Definitions
• Species Definitions– Morphological
• Can be distinguished anatomically• Specialist decides what criteria probably represent
reproductively isolated populations• Most species described this way
Species Definitions
• Species Definitions– Biological
• Populations of the same species breed only among themselves
• Are reproductively isolated from other such populations
• Very few actually tested for reproductive isolation
Biological Species Definition
Species Definitions
• Species Definitions– Phylogenetic
• Can be shown to have genetic differences• Usually based on DNA sequence analysis• Very few species determined this way, but growing
in use
Modes of Speciation
• Speciation:– The splitting of one species into two, or– The transformation of one species into a new
species over time• Two modes:
– (1) Allopatric Speciation• Two geographically isolated populations of one
species• Become different species over time• Can be due to differing selection pressures in
differing environments
Allopatric Speciation
North Rim and South Rim
Modes of Speciation
• Two modes:– (2) Sympatric Speciation
• One population develops into two or more reproductively isolated groups
• No prior geographic isolation• Tetraploid hybridization in plants
– Results in self fertile species– Reproductively isolated from either parental species
HAWTHORNS – native to North America
APPLES – introduced by immigrants over 200 yrs ago
Gene flow has been reduced between flies that feed on different food varieties, even though they both live in the
same geographic area.
Adaptive Radiation
• Adaptive Radiation
– When members of a species invade several new geographically separate environments
– The populations become adapted to the different environments
– Many new species evolve from the single ancestral species
• This is an example of allopatric speciation
Reproductive Isolating Mechanisms
• Reproductive isolating mechanisms inhibit gene flow between species
• Two general types:– (1) Prezygotic Mechanisms - Discourage
attempts to mate • Habitat Isolation• Temporal Isolation• Behavioral Isolation• Mechanical Isolation• Gamete Isolation
Temporal Isolation
Reproductive Isolating Mechanisms
• Two general types:
– (2) Postzygotic Mechanisms - Prevent hybrid offspring from developing or breeding
• Zygote Mortality
• Hybrid Sterility
• Reduced F2 Fitness