Evolution

Chapters 14 through 16

I.          Misconceptions about the Theory of Evolution through Natural Selection

A.        The Theory of Evolution is not about the beginning of life.  Though it is a common misconception the Theory of Evolution does not discuss the origin of life, the theory presupposes the existence of life.  The Theory of Evolution deals with the changes that occur in life over time.

B.         The Theory of Evolution is not summarized as “survival of the fittest.”  Natural Selection is not about being the fittest; it is about being fit enough to reproduce.  To use the car analogy it doesn’t matter if you drive a Ferrari or a Honda if your purpose is to get between point A and point B.

C.        The definition of ‘survival’ in the Theory of Evolution does not mean the biggest and strongest.  The definition of survival in these theories is solely the ability to pass on genes to the next generation.  You can be the biggest and the strongest, but if you can’t reproduce, you are not capable of surviving in an evolutionary sense.

D.        The process of Natural Selection is not directed.  The process of Natural Selection only works due to the conditions that an organism is in at the current time.  There is no overall design in the process that leads to an ultimate goal, simple selection of traits that are beneficial at the time.

E.         The Theory of Evolution is not doubted by the scientific community.  It is the most parsimonious scientific solution to how the diversity of life arose on Earth.

F.         The Theory of Evolution is the change in population allele frequencies over time due to the natural selection of inherent genetic variants that are adequately adapted to their contemporary environment. (know this!!!)

II.        Principles of Evolution

            A.        Why did we need Evolution?

1.         The Age of Discovery and the Age of Reason brought forward many problems with commonly held assumptions concerning the natural world.

a.         Exploration led to new types of animals that pushed the limits of the current world-view.

b.         The new science of Geology was showing that the Earth was much older than the biblical timeline allowed for.

c.         Fossils, mineralized remains of animals and plants, were being discovered and associated with the same geological layers in all occurrences.

d.         Several ideas were proposed to try and reconcile the biblical analogy with the compounding evidence for a much longer timeline and species diversity.

e.         Some scientists proposed the idea of evolution, change over time.

i.          Lamarck proposed the idea of inheritance of acquired characteristics, where the adaptations acquired during a lifetime can be passed onto the offspring.

ii.          Think of what the Lamarck idea means for the children of bodybuilders or people with long hair.

2.         The Enlightenment divided the scientific and philosophical/ religious world allowing the development of theories that no longer needed to be grounded in religious dogma.

            B.         Charles Darwin and Alfred Wallace

1.         In 1858, Charles Darwin and Alfred Wallace independently developed a theory to explain a mechanism for evolution.

2.         Both Darwin and Wallace had traveled extensively through out the tropics in their younger years, during this travel they were able to see the great diversity of animals that could exist and exploit unique environmental niches.

3.         Their works were published as separate papers in 1858 and the following year Darwin published his book On the Origin of Species by Means of Natural Selection.

4.         Without the popularity of the book these works would likely not have gathered attention, much like the works of Gregor Mendel.

            C.        From Natural Selection to Modern Genetics

1.         The kernel of Darwin and Wallace’s idea was that the vast diversity of life arose from the descent with modification.

2.         In a natural population there is always the potential for explosive growth that outpaces the need for replacement members.

3.         Even with explosive growth potential, natural populations tend to remain fairly constant

4.         Given the last two statements it is possible to conclude that not all offspring live to reproduce

5.         In a natural population there is always a level of innate variation, some more successful than others at surviving and reproducing

6.         Those variations that allow for reproduction success are going to produce more offspring and form a larger proportion of the next generation. (Natural Selection)

7.         Some of these variations are capable of being passed from parent to offspring. (descent with modification)

8.         Using the theories of inheritance developed by Mendel (not known to Darwin or Wallace) you can see how traits passage down.

            a.         Variation is based on differing alleles at individual loci.

b.         Natural Selection is about changing the frequency of alleles for a given loci, favoring those alleles that favor reproductive success.

            D.        Evidence for Evolution

1.         Fossil evidence shows lineages of descent over time with modification.

2.         Homologous structures show that common ancestry is exhibited by diverse final structures.

3.         Functionless structures continue to be passed down

4.         Anatomical similarities can arise from differing structures due to similar environments. (convergent evolution and analogous structures)

            a.         Contrast analogous structures with homologous structures

5.         Modern genetics and biochemistry has shown how down to a molecular level you can trace the descent of individual loci.

6.         Multiple examples of natural selection are present in our own experience

            a.         Antibiotic resistance in bacteria

            b.         Pesticide resistance in weeds

III.       How Organisms Evolve

            A.        How do we relate populations to genes to evolution?

1.         We are looking at a population, a group including all members of species living in a given area.

2.         Traits that are expressed arise from a mixture of environmental influences and the alleles present.

3.         All the alleles present in a population are referred to as the gene pool and the relative proportion of a specific allele for a given locus is called the allele frequency.

4.         Evolution is the change in allele frequencies over time within a population.

5.         An Equilibrium Population is a hypothetical construct that does not evolve and adheres to specific characteristics.

            a.         There must be no mutation

b.         There must be no gene exchange with any other populations.

c.         The population must be very large.

d.         All mating must random

e.         All genotypes must reproduce with equal success (no Natural Selection)

            B.         What causes evolution?

                        1.         Mutation is the ultimate source of genetic variance.

                                    a.         Mutations are rare

                                    b.         Mutations are not directed

                                    c.         Mutations are random

                        2.         Intermixed populations allow for exchange of alleles

                        3.         Small populations can adversely affect allele frequencies

a.         Small populations can experience drastic changes due to rare events.

b.         A genetic bottleneck or founder effect is one example of a population effect.

                        4.         Mating is never random

                        5.         All genotypes are not created equal

                                    a.         Natural Selection does not induce genetic change

b.         Natural Selection operates on individuals but it is the population that is changed.

c.         The idea of fitness

            C.        How does Natural Selection work?

                        1.         Natural Selection is due to unequal reproduction

                        2.         Natural Selection can only act on the phenotype not the genotype

                        3.         Competition acts to select the fit members

a.         Predator – Prey relationships can be form of competition (coevolution)

b.         Sexual competition (Sexual Selection)

4.         Selection can influence the population in three ways (know the graphs)

                                    a.         Directional Selection

                                    b.         Stabilizing Selection

                                    c.         Disruptive Selection

IV.       The Origin of Species

            A.        What is a species?

1.         The concept of the species is very difficult to get a consensus on, though slightly easier in macrobiology than an in microbiology.

2.         In macrobiology a species is an interbreeding population, in other words they must produce viable offspring.

3.         The major limitation of this definition is that it is useless for organisms that reproduce asexually.

            B.         How do new species form?

                        1.         The process of speciation relies on a population splitting in two.

                                    a.         The populations must be isolated.

                                    b.         The populations must genetically diverge.

                        2.         Speciation falls into two categories

a.         Allopatric speciation – two populations are geographically separated.

b.         Sympatric speciation – two populations remain in the same geographical area.

            i.          Ecological isolation

            ii.          Change in chromosome number (polyploidy)

            iii.         Adaptive radiation

            C.        How is reproductive isolation maintained?

1.         Isolation Mechanisms – modifications that prevent interbreeding between populations

            a.         Premating isolating mechanisms

                        i.          Changes in mating time

                        ii.          Changes in mating mechanics

                        iii.         Changes in courting rituals

                        iv.         Physical Isolation

            b.         Postmating isolating mechanisms

                        i.          Biochemical incompatibility

                        ii.          Infertile hybrids

                        iii.         Bacteria and Wasps (in class)

            D.        Extinctions

                        1.         Extinction – the death of all members of a species.

                        2.         Challenges vary that could cause extinction.

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