Evolution and Natural Selection (16-18)
3 Key Observations of Life: 1) Shared Characteristics of Life (Unity) 2) Rich Diversity of Life 3) Organisms are Adapted to their Environment These observations can be explained by EVOLUTION = Genetic change over time
History of Evolutionary Thought Charles Darwin drew upon the work of others when he proposed his ideas of Evolution by Natural Selection
Charles Darwin s Life Medical School Cambridge University to become a clergyman Voyage of the Beagle
Charles Darwin s Observations aboard the Beagle Species had traits that were adapted to environment Species resembled other nearby and extinct species
Alfred Russel Wallace (1823-1913) British Naturalist Independently developed concept of Natural Selection Caused Darwin to quickly publish his hypothesis
On the Origin of Species by Means of Natural Selection (1859) 2 main ideas Descent with Modification Natural Selection http://blog.ted.com/2009/09/07/how_the_origin/
Descent with Modification Earth s many species are descendants of ancestral species Species become more adapted to their environment over time due to Natural selection
So, what evidence do we have for Evolution?
Five primary lines of evidence for evolution: 1. The fossil record 2. Biogeography 3. Comparative anatomy and embryology 4. Molecular biology 5. Laboratory and field experiments
Evidence for Evolution The Fossil Record Fossil Record= physical evidence of organisms that lived in the past Evidence for extinctions Evidence of small changes over time Evidence of transitional organisms Incomplete
Transitional fossils= fossils of species which have traits of two different groups Vestigial structures are no longer used and eventually disappear
Evidence for Evolution Biogeography Geographic patterns of species distributions reflect their evolutionary histories But continental drift also plays a role in species distribution
Evidence for Evolution Comparative Embryology Closely related species exhibit similar developmental patterns Guess the vertebrate! All have: notochord, hollow nerve cord, gill slits, tail
Evidence for Evolution Comparative Anatomy Homologous structures = shared characteristics due to shared ancestry
Homology versus Analogy Analogous characteristics = shared characteristics between unrelated species Similar adaptations between unrelated species due to similar environments = CONVERGENT EVOLUTION
Evidence for Evolution Molecular Biology Species that are more closely related will have more similar DNA, RNA and proteins The longer species have been separated (no gene flow), the more different their DNA (more time for mutations to accumulate)
Evidence for Evolution Laboratory and Field Experiments Darwin s finches on Galapagos Islands Beak size Fruit flies resistance to DDT Bacteria resistance to antibiotics Hundreds of thousands of examples
Can we get a population of fruit flies to evolve so that they can resist starvation longer?
Evolution of populations (Microevolution) = change in allele frequencies of a population over time Population = group of individuals in the same species that live in the same area and interbreed Population is the smallest unit of evolution Variation of traits (different alleles)
How do populations evolve? 1. Mutation 2. Genetic drift 3. Migration (Gene flow) 4. Natural selection
1. Mutation causes new alleles to be added to a population A change in the sequence of DNA that alters the structure or expression of a protein
2. Genetic Drift causes allele frequencies to fluctuate randomly over time (especially in small populations)
Different types of Genetic Drift Founder Effect Few individuals become isolated from the larger population - Allows for a greater prevalence of inherited disorders and loss of genetic diversity
Different types of Genetic Drift Population Bottlenecks Sudden decrease in population size can cause some alleles to become more prevalent and some alleles to disappear completely - Less genetic diversity
Genetic Drift: Bottleneck Effect bottleneck only allows a few individuals through Original population, original allele frequency. Hunting of seals in late 1800s greatly reduced population size. Surviving population had different allele frequency and little genetic diversity. This different allele frequency is reflected in today's population. Figure 17.6
3. Migration (Gene flow) causes alleles to move into or out of a population Tends to cause populations to become more similar (reduces genetic diversity) Very prevalent in human populations When migration between two populations stops, can occur
4. Natural selection causes certain adaptive alleles to become more prevalent within a population Makes populations more fit or adapted for their environment
Three conditions are necessary for Natural selection to occur: 1. Variation of traits 2. Heritability of traits 3. Differential reproductive success based on traits
Natural Selection The consequence of certain individual organisms in a population being born with characteristics that enable them to survive better and reproduce more than the offspring of other individuals in the population Over time, favorable traits accumulate in a population But what if the environment changes?
Reproductive Success is everything in Natural selection!! Fitness= how many babies you have compared to others in your population The more healthy babies you have, the more genes that you will contribute to the following generations, and you will essentially live on!
Sexual selection (nonrandom mating) = A form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates. Tends to result in Sexual Dimorphism
Types of Sexual Selection Intrasexual selection individuals of the same sex compete for mates Usually males Intersexual selection (mate choice) individuals of one sex are choosy in selecting mates Usually females
Does Natural Selection create perfect organisms? No 1) Selection can only act on existing variation 2) Adaptations are often compromises 3) The environment is constantly changing and chance events occur
Environment selecting for certain traits = NATURAL SELECTION Humans selecting for certain desired traits = ARTIFICIAL SELECTION
Natural Selection Natural selection has three modes: stabilizing selection directional selection disruptive selection
3 types of Natural Selection
Natural selection - Directional Selection Black bears in Europe
Natural selection - Stabilizing Selection Tends to be difficult to see Ipomopsis aggregata stamen length
Natural selection - Disruptive Selection Northern water snakes Can lead to SPECIATION if individuals no longer reproduce with one another (loss of gene flow)
Speciation = one species splits into two or more species (due to loss of gene flow) Geographic barrier No geographic barrier ALLOPATRIC SPECIATION SYMPATRIC SPECIATION
Allopatric Speciation 1) Gene flow is interrupted by physical barrier 2) Separated populations experience differential mutation rates and evolutionary processes 3) Reproduction isolating mechanism(s) develop
Evidence of Allopatric speciation
How long does it take for speciation to occur? Punctuated equilibrium vs. Gradualism
(a) Homology: Common structures in different organisms that result from common ancestry bat gorilla bones of wing bones of arm (b) Analogy: Characters of similar function and superficial structure that have not arisen from common ancestry litoptern horse one-toed foot one-toed foot Figure 18.12
Summary Darwin s theory of evolution describes how species change over time (Descent with modification) and this theory is supported by an overabundance of evidence Selection operates on individuals. Evolution occurs in populations. For evolution to occur, there must be variation in heritable traits Disruption of gene flow may eventually lead to speciation Natural selection is the only mode of evolution which consistently adapts a population to its environment Natural selection cannot create perfect organisms
By the end of this chapter, you should be able to: 1) Describe how Charles Darwin came to develop his theory of evolution 2) List and describe the four processes of evolution. Give one real life example for each. 3) Describe why variation, heritability and differential reproductive success are necessary for natural selection. 4) List and describe the three modes of natural selection. Give one real life example for each. 5) Discuss why natural selection cannot fashion perfect organisms. 6) Differentiate between natural selection and artificial selection. 7) Describe speciation and explain the process of allopatric speciation 8) Describe the five lines of evidence for evolution using real-life examples.