EVOLUTIONWhat is evolution and what is the evidence for it?

Past and present life forms have evolved from common ancestors, the lineage of which extends back in history to simple single celled organisms

Figure 22.7 Descent with modification

The Theory of Evolution explains1. The remarkable diversity of life on Earth

2. The systemic similarities of anatomy, physiology, and the molecular basis of life and presents a paradoxical unity of life

3. The sequence of changes in fossils formed during geological history

Who evolves? Individuals do not evolve Populations do evolve which is seen as

change across generations

Remember that Individuals do not develop new traits in

response to a changing environment They do not “try to change for the

better” Individuals use or disuse of organs or

behaviors do not result in changes that will be passed on to the next generation

So what is evolution? Evolution = The change in the genetic

composition of a population over time

Changes in gene frequency over time

Theory of Evolution by Natural Selection

Evidence indicates that species evolve by natural selection over longer time periods

Evolution is validated by evidence from Comparative anatomical homology similarities

between species due to common ancestry Comparative embryological homology similarities

between species due to common ancestry Selective breeding Breeding organisms for specific

traits Molecular Biology Similarities in DNA sequences,

protein structure, etc. Biogeography distribution of living species Fossil Record Form and distribution validate the

theory

Study of Homologous Anatomical Structures Comparative anatomy of groups of animals or plants

show certain structural features that are basically similar. Homologous structures are those that are similar in

shape in different types of organisms, but are used in different ways.

Structural similarities imply a common ancestry. If species actually arose independently (naturally or

through a divine act) each organism should have characteristics uniquely suited to its nature and environment.

If species evolved, however, then their anatomy is limited by whatever their ancestors were able to provide.

They will lack some features which would be well-suited to how they live and they would have other features which aren't so helpful.

Figure 22.14 Homologous structures: anatomical signs of descent with modification

Pentadactyl (5-digit) Limb Adapted to different mode of locomotion

in particular environment. E.g. Bat’s wing and human hand Illustrates adaptive radiation since basic

plan adapted to different niches. The more exclusive the shared

homologies, the closer two organisms are related.

Other examples of homologies Plant: modification of ovary wall or pericarp to aid

seed dispersal. Vestigial organs: reduced structures that serve no

function. Structures that have lost their function and so are not selected for and are being lost gradually.

EG of vestigial organs: beginnings of teeth found in embryo baleen whales, despite adults being toothless.

Small pelvis and thigh bone found in the body wall of whales and some snakes.

Appendix in humans.

Harmful anatomical homologies Evolution dictates that these are still

present though inefficient Laryngeal nerve – from brain to larynx via

area near the heart. Direct path in fish, indirect and inefficient in giraffes for example

Human knee – rearward articulating knees are better for walking on the ground, forward articulating knees are better for arboreal life

Comparative Embryology Comparative embryology is the branch of

embryology that compares the development of embryos of two or more species.

The observed similarities and differences may be used in taxonomic and phylogenic studies

Ernst Haeckel (1866): "Ontogeny recapitulates phylogeny“ or the development of the individual repeats the development of the species.

Not literally true but common embryological paths indicate relatedness

Embryonic Homologies

General Embryonic stages1. Fertilization – union of egg and sperm2. Cleavage – cell divisions from initial

zygote3. Blastulation – ball of 100-1000 cells

vertebrates display differences at this stage

4. Gastrulation – formation of next stage involving movements of different tissues

Homologies vs. Analogies Structural or molecular The more similar the more closely related Mammalian limb bone structure, amino acid

sequences for different proteins Does not include analogous structures –

wings of insects and birds do not indicate common ancestory only common function.

The human eye and the octupus eye are analogous because they evolved independently although they are very similar.

Molecular Biology Evidence DNA and RNA are universal hereditary molecules. The genetic code is the same in nearly every

organism. ATP is energy ‘currency’ in all life. Optical isomers are molecules that are mirror images

of one another. The two forms are D- and L- (we’ll see this in chapter 2 of your textbook). All amino acids in living things are in the L-form, while all carbohydrates in DNA and RNA are in the D-form.

Physiological processes that are important to living things, such as cellular respiration, follow very similar metabolic pathways.

Table 22.1 Molecular Data and the Evolutionary Relationships of Vertebrates

Selective breeding Used by Darwin to introduce and support

his theory of evolution The ability of a breeder to select desired

traits to pass on to the next generation of a stock

Working with phenotype manipulations Used to enhance representation of

desired characteristics like high crop yields, resistance to disease, high growth rate

Domesticated animals and plants If modern breeds of livestock are

compared with the wild species that they most resemble, the differences are often huge.

EG Modern egg-laying hens and the jungle fowl of Southern Asia. Difference between modern maize (corn) and presumed wild maize (teosinte).

Used historically in agriculture

Artificial Selection vs. Natural Selection Many different breeds of sheep, cattle

and other domesticated livestock, with much variation between breeds.

It is clear that domesticated breeds have not always existed in their current form.

Only credible explanation is that the change has been achieved simply by repeatedly selecting for breeding the individuals most suited to human uses.

Artificial Selection for Different Breeds of Dogs

Effectiveness of artificial selection Shown by considerable changes that

have occurred in domesticated animals over periods of time that are very short, in comparison to geological time.

Shows that selection can cause evolution.

Biogeography evidence Species that are found geographically near

each other should be more closely related Species that are similar are found on

continental areas that were once connected Most marsupials in Australia – few others

(possums in south America) explained by continental drift

If life forms arose independently they should be found in all of the habitats around the world where they could survive, this isn’t the case

Figure 22.15 Different geographic regions, different mammalian “brands”

New World Vs. Old World Monkeys Prehensile tails,

arboreal, … Opposable thumbs,

wider habitat, …

Fossil Record Fossils found in different strata (layers) of the earth

– deeper generally = older Different organisms found in different layers /

areas Appearance and disappearance of individual forms

does not detract from overall trends seen in groups See trends towards increased complexity Remember that not all materials will fossilize well

so there will be gaps for soft bodied organisms Results seem to indicate that differentiation initially

seemed to occur more rapidly than subsequent evolution of forms

Figure 22.2 Fossils of trilobites, animals that lived in the seas hundreds of millions of years ago

Summarized evidence from fossil record The sequence in which fossils appear matches the

sequence in which they would be expected to evolve, with bacteria and simple algae appearing first, fungi and worms later and land vertebrates later still.

Among the vertebrates, bony fish appeared about 420 million years ago (mya), amphibians 340 mya, reptiles 320 mya, birds 250 mya and placental mammals 110 mya.

The sequence also fits in with the ecology of the groups, with plant fossils appearing before animal, plants on land before animals on land, and plants suitable for insect pollination before insect pollinators.

Transition to Baleen Whales

Evidence for the sequence 1. Radiometric dating relies on half-life

decay of radioactive elements to allow scientists to date rocks and materials directly.

2. Stratigraphy provides a sequence of events from which relative dates can be extrapolated.

3. Molecular clocks allow scientists to use the amount of genetic divergence between organisms to extrapolate backwards to estimate dates.

Hominid Evolution Fossils

Many sequences of fossils are known. These sequences link together existing

organisms with their like ancestors. EG horses, asses, and zebras, members

of the genus Equus are most closely related to rhinoceroses and tapirs.

An extensive sequence of fossils, extending back over 60 million years, links them to Hyracotherium, an animal very similar to a rhinoceros.

Observed evolutionary change Microevolution is change in gene

frequency Macroevolution is change in the actual

organisms or species

Observed evolutionary change encompasses examples where we see that the process is supported

http://www.phylointelligence.com/observed.html

Theory vs. Law A hypothesis is an educated guess, based

on observation

A scientific theory summarizes a hypothesis or group of hypotheses that have been supported with repeated testing.

A law generalizes a body of observations. At the time it is made, no exceptions have been found to a law.

What is theoretical about the Darwinian View

Darwin’s theory of natural selection took evolution out of the hands of amateurs and placed it in the realm of science

In science the word theory means more like a general hypothesis but more comprehensive

Accounts for many facts and explains a wide variety of phenomena

As a theory it stands up to thorough and continual testing by experiments and observations

What is theoretical is not that species have changed over time, but how the species have done this.

There is a lively debate over the roles of the other evolutionary forces that can and do change gene frequencies.