gsdf

Evolution

Microevolution – deals with genetic change occurring within natural populations.

-              Change in allele frequencies in a population over generations.

-              Large-scale patterns produced by microevolution become visible, like evolution of mammals from reptiles.

Polymorphism – different allelic forms of a gene in a population. (i.e. orange vs. black fox squirrels.

-              Allelic (describes allele) - one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome. Also called allelomorph

Macroevolution – evolution is that on a bigger/grand scale, includes things like

-              The original of new structures

-              Evolutionary trends

o    directional changes in the characteristics of species over time

-              Adaptive radiation

o    process in evolutionary biology where a single species rapidly diversifies into many new forms

-              Phylogenetic relationships of species

o    Phylogenetic relationships are the evolutionary connections between species, and they can be visualized in a phylogenetic tree.

-              Mass extinction.

Sources of genetic variation

-              Variation results from the unique combination of alleles that each receive through sexually reproducing organisms. They create unique variations thru

o   Crossing over

§  The swapping of genetic material that occur in the germ line of formation of egg

o   Independent assortment of chromosomes

§  Governs how different genes independently separate from one another during gametogenesis. (process in which cells undergo meiosis to form gametes.)

o   Fertilization

§  Two haploid cells (gametes) form to form a diploid zygote

o   Mutations (can create new changes and is unpredictable)

§  Introduction of new genetic variations

Hardy-Weinberg Principle:

o   Proportions of genotypes do not change in a population if:

§   No mutation happens

§   No genes are transferred to or from other sources (gene flow/no immigration or emigration)

§   Random mating is occurring

§   The population size is very large

§   No selection occur

Mechanisms of evolution

Genetic Drift: Change in allele frequency due to chance alone.

§  What Alleles does it affect? Small populations and alleles that are initially uncommon are vulnerable

§  Causes:

·       Bottleneck effect – population is greatly reduced

·       Founder effect: small subset of population that can migrate elsewhere and start a new population

§  Outcomes: High fixed rates (high rates that doesn’t change) in populations and lower diversity in allele frequency.

Gene flow: Introduces new traits into an existing gene pool. Change in allele frequency

§  Mostly affect small populations

Natural Selection: Differential and variable reproductive and survival rates inherent in populations cause the allele frequencies of some genes to differ among populations. (some can appear more than others, and less than others)

 A population of rabbits may be brown (the dominant phenotype) or white (the recessive phenotype). Brown rabbits have the genotype BB or Bb. White rabbits have the genotype bb. The frequency of the BB genotype is .35.

What is the frequency of heterozygous rabbits?  Bb = 0.3168 | 2 .1938 .8062
What is the frequency of the B allele?  p = 1 - 0.8062 |  B = 0.1938
What is the frequency of the b allele? Do .65 ^ 2 in calc since we already know that .35 = BB |  b = .8062

A population of birds contains 16 animals with red
tail feathers and 34 animals with blue tail feathers.
Blue tail feathers are the dominant trait.
                  What is the frequency of the red allele?  since we know that rr = 16/50 = .32, we do reverse of it, √.32 to get 0.566. so r = 0.566

What is the frequency of the blue allele? Since we know that r = 0.566, just do 1 – 0.566 = to get .434
 What is the frequency of heterozygotes? 2 0.566 .434 = 0.491
 What is the frequency of birds homozygous for the
blue allele? .434² = 0.188

How do new species arise?

                  Speciation: is the lineage-splitting event that forms two new species from an ancestral species. (a new species is formed from an existing species, and it's a result of evolution.

o   Speciation provide new units (species) from the ancestral species that are subsequently exposed to force of evolution by natural selection.

Isolation mechanisms: Barriers to allele flow.

o   Characteristics of species that reduce or prevent successful reproduction with other species.

How are species defined?

1.        Biological species concept

a.        group of organisms that can interbreed in nature to produce viable and fertile offspring. (i.e. a dog in Canada and Japan can interbreed and produce mixed fertile puppies, and they are considered members of the same species.)

b.       Reproductive isolation: do not mate with each other or do not produce fertile offspring. They can occur at various stages.

                                                                                             i.         Prezygotic isolation: before mating. (Cat and Mouse)

1.           Spatial, temporal, mechanical, behavioral, etc.

                                                                                           ii.         Postzygotic isolation: Failure of resulting zygote from surviving, in the event of mating.

1.           Hybrid viability (likelihood of a hybrid offspring from two different species to survive and mature into a healthy adult): zygote does not develop normally

2.           Hybrid sterility (offspring of two different species are unable to reproduce): surviving offsprings are sterile

                                                                                        iii.         This concept has some flaws:

1.           Doesn’t apply to fossils

2.           Asexual species

3.           Or geographically separated closely related species with potential for mating

2.        Morphospecies concept

a.        a way of defining species by their physical traits. This concept is based on the idea that members of the same species look similar to each other, while members of different species look different. (i.e. Homo habilis and Homo rudolfensis)

                                                                                            i.         Relevant when biological species concept is not applicable

1.           Defined as the designation of independent lineages (species) based only on morphological differences from related species

2.           Assumes differences in anatomy arises due to restricted gene flow

3.           Useful when no data exists on gene flow and applies to sexually and asexually lineages.

4.           However, it has disadvantages:

a.           Mislabeling/naming of species in relation to polymorphic (having or occurring in multiple forms)

b.           Not applicable to cryptic (mysterious) species

c.           And too subjective (bias)

3.        Phylogenetic species concept

a.        defines a species as a group of organisms that share a unique evolutionary history and ancestry. It's based on genetic data and is less restrictive than the biological species concept. (wolves and dogs)

                                                                                             i.         Relevant when biological species concept is not applicable

1.           Defined as the designation of independent lineages (species based on evolutionary history of populations

2.           Smallest monophyletic (a group of organisms that share a common ancestor) group and its descendants of the group make up the species

3.           Members of this group has shared characteristics (synapomorphies), found only in this group

4.           Applicable to living and fossil populations and is logical

5.           Disadvantages include: Limited availability of phylogenies (trace patterns of shared ancestry between lineages.) of species, and can lead to overestimation of species

                                                                                          ii.         Phylogeny: evolutionary history of a species

Nature of Species

Sympatric speciation: The differentiation of populations within a common geographical area into distinct species

-               One species splits into two at a single locality (position or site of something), without the two new species ever having been geographically separated.

o    Causes a population to contain individuals exhibiting two different phenotypes due to polyploidy (condition where an organism's cells have more than two complete sets of chromosomes)

o    Two phenotypes would have to evolve reproductive isolating mechanisms

o    Two phenotypes could be retained as polymorphism within a single population

-               Mechanisms that prevent mating among sympatric species

o   Prezygotic isolating mechanisms

§  Ecological isolation

·       Resource partitioning (divide into parts) to reduce competition

·       Competitive exclusion: the inevitable elimination from a habitat of one of two different species with identical needs for resources.

·       Lizards belonging to the genus Norops or Anolis are very similar but use different part of the vegetation

·       Evolved different patterns of coloration, feeding habits, and reproductive behavior

§  Mechanical isolation

·       Occurs because sexual organs of closely-related sympatric species are incompatible: don’t fit together

·       Thought to be a important isolation mechanisms in arthropods like insects and millipedes.

§  Behavioral isolation

·       Two sympatric species do not mate because of differences in courtship behavior

o   Visual signals

o   Sound production

o   Chemical signals: pheromones

o   Electrical signals: electroreception

§  Temporal isolation

·       Differences that arise due to different timing of biological events

·       Most common in insects and invertebrates

o   2 species of wild lettuce grow along roadsides in SE U.S.

o   Hybrids can be made experimentally and are fertile.

o   Rare in nature because one flowers in early spring and the other in summer

-               Natural selection may reinforce isolating mechanisms:

o   Formation of species a continuous process

o   2 populations may be only partially reproductively isolated

o   Reinforcement: initially incomplete isolating mechanisms that are reinforced by natural selection until they are completely effective

§  Reinforcement is not inevitable ---- hybrids may be inferior but may still be fertile, serve as a conduit (passage/tube) of genetic exchange.

-               Criticisms of the biological species concept

o   Reproductive isolation may not be the only force that maintain species integrity.

o   Interspecific hybridization

§  50% of California plant species, in one study, not well defined by genetic isolation

§  10% of world’s 9500 bird species known to hybridize in nature

o   Subspecies: within a single species, individuals in populations that occur in different areas may be distinct from one another

§  Even though geographically distant populations may appear distinct, they are usually connected by intervening populations that are intermediate in their characteristics

Allopatric speciation: The differentiation of geographically isolated populations into distinct species leading to speciation.

-              Speciation is a 2 part process

o   Initially identical populations must diverge or get separated

o   Reproductive isolation must evolve to maintain these differences

o   Speciation more likely in geographically isolated populations

-              Geographical isolation

o   Populations are isolated by geographical and ecological barriers.

Both Sympatric and Allopatric speciation can be followed by adaptive radiation

Adaptive Radiation: diversification of a group of organisms into forms filling different ecological niches.

-              Closely related species adapt to different parts of the environment

-              Occurs in:

o   Environment with few other species and many resources (i.e. Hawaiian and Galapagos islands)

o   Catastrophic event leading to extinction of other species

o   Following recent allopatric or sympatric speciation

-              What drives adaptive radiations?

o   Character displacement: Maximization of different traits among co-occurring similar species and minimization of differences when they occur in isolation

o   Process of evolutionary divergence of coexisting species

§  Competition for resources, such as food, is reduced as a result of divergence

§  Natural selection in each species favors those individuals that use resources not used by the other species

§  Greated fitness

§  Trait differences in resources use will increase in frequency over time

§  Species will diverge

§  Example is that Pied flycatcher and collared flycatcher appear very similar where they occur alone. In areas where they are sympatric, differences in color and pattern allow individuals to avoid hybridizing.

o   Key Innovation

§  New traits evolve within a species allowing it to use resources there were previously inaccessible

·      Lungs in fish

·      Wings in bird

·      Requires both speciation and adaptation to different habitats

·      Island archipelago example

The Pace of Evolution/Speciation

-              Gradualism

o   Accumulation of small changes

o   Standard view for a long time

-              Punctuated equilibrium

o   Long periods of stasis followed by rapid change

o   Stabilizing and oscillating selection is responsible for stasis

-              Gradualism and punctuated equilibrium are two ends of a continuum (endless)

Biogeography: study of geographic distribution of species

-              Reveals that different geographical areas sometimes exhibit groups of plants and animals of strikingly similar appearance, even though the organisms may be only distantly related

-              Natural selection appears to have favored parallel evolutionary adaptations in similar environments

-              Convergent evolution

o   Similar forms have evolved in different isolated areas because of similar selective pressures in similar environments

Example:

o   Desert plants

§  North American cactuses (family Cactaceae) and South African euphorbias (family Euphorbiaceae) belong to different plant families

§  Distant relatives phylogeny (representation of the evolutionary history and relationships between groups of organisms) of flowering plant

§  Share common characteristics

o   Both share

§  Succulent stems (thick steams that store water)

§  They both have spines for protection (modified leaves)

§  Both are adapted for survival in arid desert regions with low rainfall

§  Without flowers, some African euphorbias are practically indistinguishable from their north American counterparts.