Evolution: changes over time

Chapter 14

Evolution: history of ideas• Benoit De Maillet (1656 – 1738)

– Proposed that new kinds of organisms could evolve as a result of changes to the structures of an existing organism

– Creatures on the land were ultimately derived from creatures living in the seas

• Erasmus Darwin (1731 – 1802)– All living organisms originated from a single common ancestor– Species could be transformed over time to produce new species (eg. tadpole -> frog)

• Jean Baptiste Lemarck (1744 – 1829)– structures of an individual organism could change in response to environmental conditions and

physiological need (giraffe) – changes would be transmitted to the next generation

• Robert Chambers (1802 – 1871)– Supported the idea that species could evolve & published an anonymous book to that effect– Idea was not a testable hypothesis, so no scientific status

• All explain the transmutation of species (species can change over time)

• Most British of the time had contradictory views that each species was fixed in its structure & characteristics for all time - special creation of species

Evolution: The concept that species can change and give rise over time to new forms

The Darwin – Wallace view

• Before Darwin/Wallace none of the naturalists had come up with an explanation of how evolution could occur

• Theory of evolution by natural selection:– It identified a mechanism or a cause of evolution– This mechanism was testable by observation and

experimentation– Individual organisms do not evolve but evolutionary

change occurs over several generations in populations

• 1858 (then again in 1860 & 1861) Darwin's book On the Origin of Species by Means of Natural Selection

• Travelled the world on HMS Beagle including the Galapagos Islands and Australia

• Observed the variance among pigeons and saw that breeders could select different varieties through artificial selection over several generations

• First time humans were considered to be part of nature and not divine beings

• Neo-Darwinism: Combination of Darwin’s ideas andconcepts of Mendelian genetics

• Read p.511-515 Voyage of HMS Beagle

Time scales in evolution IRelative Age:• Stratigraphic (a branch of geology, studies rock layers and

layering) method of dating rocks • Principle of Superposition: the oldest stratum (rock layer) is

at the bottom and progressively younger layers lie above it• Principle of Correlation: identifying index fossils (can be

used to identify rocks as having same age)

Time scales in evolution IIAbsolute Age:

Radiometric Dating ('rock clocks')

• Based on the decay of certain radioactive elements• Present in minerals in igneous rocks

• Potassium-argon (K-Ar) dating– Most K is Postassium-39 and does not decay (provides measure of original amount of K-40)– Potassium-40 decays at a known constant rate to form Argon-40– Once igneous rock cools, no more Ar is released into the atmosphere, the 'rock-clock' begins– requires 2 samples (one for K-39 content & one for Ar-40 content)

• Argon-39/argon-40 dating– only requires one mineral crystal

• Carbon-14 dating– Proportion of stable C-12 and radioactive isotope C-14 stays constant during life of organism– After death, the proportion of C-14 decreases so we can estimate the time since death– Works up to about 60 000 years

• Electron-spin resonance (ESR)– works from ~ 50,000 to 500,000 years old (carbon dating won't work past ~60,000yrs)– encompasses the evolutionary history of Homo– When objects are buried, they are bombarded by natural radiation from the soil– Objects that are composed of minerals, some electrons move from the ground state to a higher energy level & some get

trapped at that higher energy level– The longer the material has been buried, the greater the accumulation of higher-energy electrons

Evidence of evolution• The fossil record (BZ: p. 343/44)• Transitional fossils (sheet)• Comparative anatomy (BZ: p. 345)• Comparative biochemical and genetic studies

(BZ: p. 338/39)• Bio-geographic distributions (BZ: p. 364)

The fossil recordVery rarely organisms are preserved long after death = fossilisation (normally,

microbes decompose all remains)

• Direct evidence (bones, teeth, leaves & shells)• Indirect / trace fossils (footprints, tooth marks, tracks, burrows, coprolites & pollen

grains)

• Can leave a mould or cast• Rapid burial, alkaline and oxygen-poor conditions enhance preservation

• Law of fossil succession– Certain kinds of organisms are known to be found in rocks of particular ages and appear in

a consistent order.

You should know / recognise some examples (p.527)

• This is supported by examples as seen in the evolution of the modern horse (p.529/530)

Transitional fossils• Show characteristics of both

their ancestors and the new creature

• eg: Archaeopteryx : fossil looks just like a reptile BUT has faint impressions of feathers and a wishbone

• primitive amphibians (transition from simple pelvic girdle of fish and more advance form)

• mammal like reptiles (showing reptiles conical teeth and mammals differentiated types)

Comparative anatomySimilarities can be seen amongst all mammals, regardless of their way of life.

• Homology (similar structures - forelimbs)• Analogy (similar function - fly's wing vs bat's wing)• Vestigial organs (eg: whale's reduced pelvis and vestiges of

hind limbs)• Comparing embryos (presence of notochords & gill slits in

all terrestrial vertebrates)Primitive features show up earliest in development - they show our common ancestry and

diverge as they develop into particular species. (It has been discredited by some as the changes occur so rapidly.)

Comparative biochemical and genetic studies

• Comparing proteins (all have the same 20 aa's and same genetic code)

• Comparing DNA by hybridisation - complementarity & melting temperatures (p.535)

• Comparing DNA: gene sequences (see diagrams p.537)

• Comparing Chromosomes (see figures p.538&9)

Biogeographic distributions• Native species in different isolated regions will be distinctive,

having evolved from different ancestral species

• modern species native to a given region will be more similar to species living a distant region with similar environmental conditions

• the same ecological niche in different isolated regions will be occupied by different species (that are descended from different ancestral species that once lived in that area)

• Find examples for each of these (try p.541-543 for help)

Patterns of evolution

• Divergent evolution (BZ: p. 359-362)• Convergent evolution (BZ: p. 363/64)• Parallel evolution• Co-evolution

Divergent evolution• Closely related species become more

dissimilar over time (in response to environmental conditions and different selection pressures)

• Eg. Snowshoe hare and black-tailed jack rabbit

Adaptive radiation• A special case of divergent evolution• Is the evolution of a variety of species, each

adapted for life in a different niche and each evolved over time from a single ancestral species

• Adaptive radiation occurs when an ancestral species moves into and isolated region where a variety of habitats exists but no competing species

• eg Ground finches Galapagos islands or Marsupials of Australia

Convergent evolution

• Natural selection acting on distantly related species to produce superficial similarities

• Similarities are NOT due to shared ancestry• eg: Cacti from Americas and Euphorbias from

southern Africa

Parallel evolution• Response of similar species to similar

environmental demands• eg: backward-opening pouches of marsupials

and the marsupial mole from different parts of the world but environment (or behaviour of burrowing) has created structural similarities

Co-evolution• changes in 2 different species that have a close

interaction• eg: parasite and its host or predator and prey• eg: bird-pollinated flowering plants and the bird

that pollinate them have evolved features that favour their mutually beneficial interaction

Speciation• Formation of a new species (through evolution by

natural selection)

• Phyletic evolution: one population progressively changes over time to become a new species

• Branching evolution: a population of one species splits and one part of the population evolves separately to form a new species distinct from the original

• Branching is more common then phyletic evolution• Speciation where populations become geographically

seperated = allopatric speciation

• Branching evolution typically involves:1) Splitting of a smaller group from an original population ->

smaller group becomes geographically isolated2) Over many generations, the isolated population is subjected to

different selection pressures because of the different environmental conditions and other change factors such as genetic drift

3) The isolated population changes over time such that the two population could not breed any longer if they came together again – they are now two distinct gene pools and two different species

Evolution: gradual or intermittent?• Punctuated equilibrium (‘Punk ekk’) model– changes little over time (vertical lines)– then major evolutionary events (horizontal lines)– Macro-evolution

• Gradual model– changes gradually over time– Micro-evolutionSee page 554

Extinction• Species are lost from Earth’s biodiversity• Global Catastrophes – 65 million years ago, more than 70%

of Earth’s marine and terrestrial species (incl dinosaurs) became extinct.

• Asteroid on Yucatan Peninsula of Mexico?• Volcanic Activity causing huge lava flows – Deccan traps in

India?

Evolutionary relationships

• Comparative Genomics (genome: complete genetic info of a species)

• Because living species have evolved from common ancestors, the genomes of related species exhibit similarities

• The more recent the divergence of two related species from a common ancestor, the greater the degree of conservation of DNA sequences

• By comparing the genome it is possible to:– Identify the degree of relationship between different species– Make inferences about the phylogeny or evolutionary history

of a species