Chapter 14 - 17 - Evolution & DiversityEvolution = change

Origin of Life

1938 Aleksandr Oparin

Suggested organic molecules could have been produced from gases of the primitive atmosphere in the presence of strong outside energy sources

1953 Stanley Miller

Miller placed a mixture resembling a strongly reducing atmosphere (methane, ammonia, hydrogen, water) in a closed system, heated the mixture, & added an electric spark (lightning). In a week a variety of amino acids & organic molecules were produced

This experiment paved the way for the hypothesis that the primitive gases could have reacted with one another to produce small organic compounds, which are the building blocks for life

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & DiversityEvidence for evolution

Fossil – the remains & traces of past life, or any other direct evidence of past life

The age of fossils are determined by radiometric dating, which utilize radioactive isotopes as a clock

As we know from previous lectures, radioactive isotopes are unstable & decay to a stable form. The half-life of an isotope is the time it takes for half the isotope in a sample to decay into its stable form

By comparing how much radioactive isotope is left in a fossil to the amount that we deduced was in it at the time of its demise we can determine (within reason) the age of the fossil

Anatomical evidence

Homologous structures – structures that are similar because they were inherited from a common ancestor

Vestigial structures – anatomical features that are fully developed in one group of organisms but are reduced & may have no function in similar groups

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & DiversityEvidence for evolution

Molecular evidence

What molecule serves as the blueprint of life?

DNA

What molecules are encoded for by DNA?

PROTEINS

How do we compare organisms from a molecular point of view?

We can look at the sequences of DNA in their genes or the sequences of amino acids in their proteins

DNA comparisons

DNA hybridization utilizes the fact that base pairing is specific

DNA from two organisms are separated & allowed to interact with one another

The more similar their DNA sequences are the more likely they are to anneal with one another & form a hybrid

Chapter 14 - 17 - Evolution & DiversityNow, both DNA sequences & protein sequences can be directly determined using machines

Since all organisms utilized the same genetic code, can the proteins that they come from be significantly different?

Cytochrome c – electron transport chain protein for cellular respiration

How important is this protein to eukaryotic organisms?

20 of 104 amino acids occupy identical positions in all eukaryotic cells (19.2%)

Human vs horse cytochrome c – differ in only 12 amino acids (88.5% similar)

Human vs kangaroo cytochrome c – differ in only 8 amino acids (92.3% similar)

Human vs chimpanzee – identical

Chapter 14 - 17 - Evolution & DiversityProcess of Evolution

Macroevolution – The study of how species or higher levels of classification arerelated to one another

Microevolution – A change in gene frequencies within a population over time

Population genetics

Population – All members of a single species occupying a particular area at the same time

Hardy-Weinberg Law

As long as certain conditions are met, allele frequencies in a sexually reproducing population come to an equilibrium that is maintained generation after generation

The gene pool remains the same – All of the alleles that exist in a population

Proven mathematically

Chapter 14 - 17 - Evolution & DiversityHowever, the Hardy-Weinberg Law is only valid if the following conditions are met:

1. No mutations of alleles

2. No genetic drift – The population is very large & changes in allele frequencies of a gene pool due to chance is low

3. No gene flow – migration of individuals does not occur

4. Random mating – Individuals pair by chance & not according to their genotypes or phenotypes

5. No selection – no selective force favors one genotype over anotherWe know that these conditions are rarely met, thus the significance of the Hardy-Weinberg Law is that it tells us what factors cause evolution

Let’s look at some of these factors in more detail

Chapter 14 - 17 - Evolution & DiversityMutations

Changes in the DNA sequences resulting in new phenotypes

Genetic drift – When does it occur?

Founder effect – occurs when a few individuals found a colony & only a fraction of the total genetic diversity of the original gene pool is represented. Which particular alleles are carried by the founders is dictate by chance

Amish – Due to founders effects, 1 in 14 carry an allele for a type dwarfism. Normal population frequency 1 in 1000

Bottleneck effect – Natural disaster decimates a population down to a few individuals

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & DiversityNatural Selection, Evolution by requires:

Variation – The members of a population differ from one another

Inheritance – Many of these differences are heritable genetic differences

Differential adaptedness – Some of these differences affect how well an organism is adapted to its environment

Differential reproduction – Individuals that are better adapted to their environment are more likely to reproduce, & their fertile offspring will make up a greater proportion of the next generation

When it comes to reproduction what can create new genetic combinations in the absence of gene mutations?

Crossing over & independent assortment may create combinations that are better adapted to survive & reproduce

Chapter 14 - 17 - Evolution & DiversityCharles Darwin – Darwin’s Theory of Evolution

Darwin’s main points include:

1. Variations – Individual members of a species vary in physical characteristics, & these characteristics can be passed from generation to generation

2. Struggle for existence – The members of all species compete with each other for limited resources with certain members being able to capture these resources better than others

3. *Survival of the fittest – Natural selection by the environment determines which organisms will survive & reproduce

4. Adaptation – Natural selection causes a population of organisms & ultimately a species to become adapted to the environment. The process is slow, but each subsequent generation includes more individuals that are better adapted to the environment

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & DiversityTypes of natural selection

Stabilizing selection

Occurs when an intermediate phenotype is favored over the two extremes

Birth weigh of human infants – Death rates are highest for those extremely low (2 lbs) & those extremely high (10.8 lbs)

Directional selection

Occurs when an extreme phenotype is favored & the distribution curve shifts in that direction

Industrial melanism with moths

Pesticides & anti-biotics selecting for “super” insects & bacteria

Disruptive selection

Two or more extreme phenotypes are favored over any intermediate phenotypes

British land snails have wide habitat range– Light banded survive in one area, dark banded in another, mixtures of the bands get consumed

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & DiversitySpeciation

Species is defined as a group of interbreeding subpopulations that share a gene pool & that are isolated reproductively from other species

Different species do not exchange genes

Reproductive isolation of gene pools of similar species is accomplished by the following mechanisms

Pre-mating mechanisms – Reproduction is never attempted

Post-mating mechanisms – Reproduction may take place but it does not produce fertile offspring

Chapter 14 - 17 - Evolution & DiversityPre-mating mechanisms

Habitat isolation

Species at same locale occupy different habitats

Temporal isolation

Species reproduce at different seasons or different times of day

Behavioral isolation

Courtship behaviors differ

Mechanical isolation

Genitalia unsuitable for one another

Chapter 14 - 17 - Evolution & DiversityPost-mating mechanisms

Gamete isolation

Sperm cannot reach or fertilize egg

Zygote mortality

Fertilization occurs, but zygote does not survive

Hybrid sterility

Hybrid survives but is sterile & cannot reproduce

F2 fitness

Hybrid is fertile but F2 has reduced fitness

Chapter 14 - 17 - Evolution & DiversityHow do new species develop & therefore are not reproductively compatible?

Allopatric speciation

Two populations consisting of a single species are able to breed with one another via gene flow

Gene flow is interrupted by a geographic barrier & variants appear in each isolated population due to independent mutations, genetic drift, & selection

If the variants possess beneficial traits that select for them, & if their mutations include reproductive isolating changes, when & if the geographic barrier is removed, the two NEW species can no longer reproduce between them

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & DiversitySympatric speciation

Single population divides suddenly into two reproductively isolated groups without the need for geographic isolation

Plants are best example – due to changes in chromosome number

These plants can no longer breed with the plants around them, but can self-fertilize & create a new population of offspring

Chapter 14 - 17 - Evolution & DiversityAdaptive radiation

Proliferation of a species by adaptation to a different way of life

Best example of speciation as observed by Charles Darwin on his Galapagos Island “stop-over”

The 13 species of finches that are depicted on the following power points are believed to be descended from mainland finches via the following mechanisms

Mainland migration to island #1, population increases, some individuals migrate to other islands

The islands are ecologically different from one another to promote divergent feeding habits

Thus, the finches are physically similar to one another but differ in their beak structure which came about from natural selection of variants that appeared over time

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & DiversityPace of speciation

Phyletic gradualism

Change is very slow but steady within a lineage before or after a divergence (splitting of a line of descent)

Could explain the lack of transitional species, because the changes are subtle

Punctuated equilibrium

Speciation occurs rapidly during periods of stasis (limited change)

Could also explain the lack of transitional species, because the changes are significant

Chapter 14 - 17 - Evolution & Diversity

Chapter 14 - 17 - Evolution & DiversityPRACTICE QUESTIONS

1. Who is Stanley Miller?

2. What is the difference between a homologous structure & a vestigial structure?

3. What is DNA hybridization?

4. What is cytochrome C?

5. What is the difference between macroevolution & microevolution?

6. Define the following terms: Founder effect, Bottleneck effect

7. What are the differences between stabilizing selection, directional selection, & disruptive selection

8. What are & define the different pre-mating mechanisms that prevent different species from interbreeding?

9. What are & define the different post-mating mechanisms that prevent different species from interbreeding?

10. What is the difference between allopatric & sympatric speciation?