EVOLUTION AND THE FOSSIL RECORD

Species are perfectly suited for their niche

Adaptations: beneficial features that allow them to perform functions
 

• Before the XVIII Century (Creationism) Species' adaptations were immutable.
• After the work of Lammarck, Mendel, Darwin & Wallace: All living organisms are evolved descendants of life forms from the past.

Evidences of past common lineage for different species

•  Geographic distribution
•  Embryology
•  Homologous structures ( = anatomical components but different functions):  bat wing, whale fin, tetrapod limb
•  Analogous organs ( different elements but = function: bird & fly wings)
•  Vestigial organs

Mendel (1860-1900): characteristics determined by genes, which cannot be modified.

 Darwin: He tried to find the mechanism by which modifications occurred
 (1859): The Origin of the Species by Means of Natural Selection.
 Observed artificial selection to introduce variation and thought about evolution through natural selection.

 Darwin-Wallace theory

•  Existence of heritable variations
•  Some variations are more favorable
•  Not all offspring survive to reproductive maturity
• The ones with favorable variations are more likely to survive and pass them on to their offspring

GENETICS (Gregor Mendel, unknown to Darwin)

• Traits are controlled by a pair of hereditary factors (we call them genes)
• From each pair of factors, one is provided by each parent during sexual reproduction
• Genes have alternate forms that control the same trait, with one form dominant over the other (recessive)
• Recessive factors may show up in younger generations

MODERN GENETICS

•  Genes are parts of the DNA (a long double helix molecule) contained within the chromosomes in the cell's nucleus
•  DNA information regulates the organism's development & function
•  # of chromosomes in each species is fixed

• Modifications =new traits are introduced by mutations

 Mutations redistribute segments of DNA
 other changes: altering the # of chromosomes

Introduction of variability through sexual reproduction: Parent cells undergo Meiosis (division), producing a gamete cell (sperm or egg) with ½ the number of chromosomes for the species.

 The offspring's chromosomes will have half of the genes for a given trait provided by each one of the parents

Chromosomes carrying the genes (genetic determinants) are passed from one generation to another.

Mutations: Only the variations in the genes taking place in the sex cells are inheritable.

Sexual reproduction and mutations provide variability for natural selection to work within the gene pool of the population

Gene pool: sum of the genetic component of the population
 

SPECIATION
 A species is a population of individuals that

• are similar in structural characteristics and functions
• can interbreed
• can produce fertile offspring

Allopatric speciation: New species arises from isolation of a segment of the population which becomes genetically distinct from the parent stock.

• Distinguish among
•  Pseudo extinctions: descendants have evolved so much that they seem unrelated to the parent. (Not truly extinct)
•  True extinctions: species dies out without evolving into anything else.
•  Mass extinctions: rate accelerated, affects higher taxonomic groups. Examples.
•  Humans causing accelerated extinction.

Evolutionary radiation (= divergent evolution):

• Surviving few species adapted to a restricted environment may diversify into many species adapted to many different environments.
• Rate is fast first (no competitors or predators) and slows afterwards
• Different adaptations are the ones that promote diversification

Convergent, divergent, and parallel evolution

• Convergent: similar adaptations to perform same functions in distantly related groups
• Divergent: single group with different adaptations (adaptative radiation).
• Parallel: similar adaptations in closely related groups.

• Phylogeny is the evolutionary history of a group. Can be constructed if sufficient fossil material is preserved. It reveals the evolutionary trends in the group.
Trends occur as adaptations to changing environments or in response of the exploitation of new habitats.
• Individuals with larger bodies. Limited by practicality
Too many adaptations leave no room for new ones. If new adaptations are needed, the species may become extinct

•    phyletic gradualism or gradualistic model: minute and continuous change allow for transitional forms (Horse foot)
•    punctuated equilibrium: species remain unchanged for a long time (stasis) and then evolve rapidly into new       species. (Adaptative radiation).

There are groups that have not changed over long time, true living fossils: the coelacanth, Lingula, the opossum.
 
 

IRREVERSIBILITY OF EVOLUTION: Given the complexity of the genetic changes leading to a new species, it is impossible for evolution to reverse its course. Therefore, when a species becomes extinct, it
has disappeared for ever.

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