BIOL&212 Animals (Metazoans) - …faculty.seattlecentral.edu/jwhorley/Darwin_Evidence212.pdfDarwin’s postulates (hypotheses) 1. Traits in a population vary among individuals 2. Some

BIOL&212 Animals (Metazoans) The evolution of diversity in

form and function

~ 10,000 Aves

6 – 10 million Insects ~ 7,900

Squamates ~ 6,340 Amphibians

~ 1,800 Sea Stars

How should we organize this diversity of animal life?

Linneaus Darwin

• “Nothing in biology makes sense except in the light of evolution.”

– Theodosius Dobzhansky

Classification (Taxonomy) • Natural system • Often do it without understanding why

Early attempts at organization and naming

• Carolus Linneaus:Systema Naturae (1735).

Two advances • Hierarchical classification

based on “ideal of unchanging types”. – K, P,C, O, F, G, S

• Binomial nomenclature - replaced polynomials

• Assumes a static, unchanging Universe

What was Darwin trying to explain?

Observations A hypothesis

Charles Darwin • 1809-1882 • Englishman; bound for

medicine…then the clergy…sidetracked by biology

• 1831-1836 Beagle; visits isolated archipelago – 1839 Journals

• 1859 The Origin of Species! – Outlined the process of evolution

via natural selection • Demonstrated that life is

dynamic

Observations that suggested change over time

• Fossil Record – Layering of simple to complex – Extinctions – Succession & Transitional forms

• Environmental/Geological change • Vestigial traits • Apparent relatedness of species correlates with geographic distribution

Fossil record • Fossils of complex organisms occur closer to the

surface, in rocks of younger age, than those of simpler organisms. This layering was globally consistent suggesting they were different ages. – Darwin had huge collections to examine, even in early

1800’s

Extinctions • Some fossils clearly belonged to creatures no

longer walking the Earth – Irish elk – More famous examples like T. rex

Transitional Forms & Environmental Change

• “fossil sandwiches” occurred in these rock layers; middle fossils intermediate in form between fossils above and below.

• Documented changes in skeletal morphology through time

Succession of forms • Darwin’s

predecessors – Morphology of fossil

species often closely match the morphology of contemporaries

– Law of Succession

Vestigial traits • Presence of tiny,

useless traits on contemporary species

• Anal spurs; coccyx

Apparent Relatedness • Darwin’s contribution came after and

almost certainly as a result of his voyage through the Galapagos island archipelago

• 1831 - Charles Darwin (22) joins crew of HMS Beagle as gentleman’s companion (ship’s naturalist) & collects & catalogues everything. (1831-36) – Plants, insects, fossils & Galapagos “finches” – After his return he and his naturalist friends

noted some things about his collections:

Different islands have distinct (but similar) species

Weight of evidence lead to his heretical proposal:

• H: Species on neighboring islands look similar because they are all descended from a common ancestor; The small differences between them are due to changes over time. – A bold assertion & dangerous for the time. Species are not static!!

• So, he waited ~20 years after developing the idea to publish it

OK, so what’s the mechanism for change?

Differential survival & Reproduction

Variation is heritable

Beneficial variants leave more offspring

Variable traits

• Beetle color is variable, and heritable

• The “environment” selects against green beetles

On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life

-Charles Darwin, 1859 • Darwin’s “blackbirds”, “gross-

beaks”, “wrens” and finches

Darwin’s postulates (hypotheses)

1. Traits in a population vary among individuals 2. Some traits are passed on to offspring (i.e.

traits have a genetic basis and are heritable) 3. Due to environmental challenges (ex: struggle

for resources), some individuals produce more offspring than others

4. The subset of all offspring that survive are those that possess beneficial traits; these traits (and the individual carrying them) are “naturally selected” by the environment.

What was Darwin’s contribution?

• Others had proposed evolution (of some variety) as a pattern that required explanation.

• Darwin provided the process, natural selection, that explained the pattern of descent with modification.

Darwin introduced “fitness” to explain natural selection

• Fitness: the ability of an individual to produce fertile offspring, relative to that ability in other individuals. – This is a measurable quantity, and allows us to

define adaptation • Adaptation: a heritable trait that

increases the fitness of an individual relative to individuals lacking that trait. – These are time and location (environmentally)

dependent

Contemporary evidence in support of evolution

• Homologous traits – Structural/Morphological: Outward physical

appearance – Developmental: Sequence & timing of developmental

events; shared germinal tissues – Genetic: DNA sequence

• Direct observation: MRSA, soapbug beak length

• Convergent traits: Distant relatives have nearly identical traits

Structural • Limbs with radically different functions are all

built from bones of similar shape, in same orientation & same relative positions. – Why, unless this structural archetype was present in

the common ancestor of all?

Developmental • Chick, human and cat embryos have gills and tails; they

appear at similar developmental stages & in the same positions as those of fish. Why?

• Fish & human jaws look very different, but develop from same population of embryonic cells. Why should they?

Genetic • The same 64 codons specify the same building

blocks in ALL organisms. Why? • Many genes are found in organisms that are very

different and their sequences are nearly identical. – Ubiquitin is found in ALL Eukaryotes; its DNA sequence

is 96% identical between humans and yeast. Why?

Direct Observation

• Soapbugs – Host plant switch – Choice of host

correlates with beak length

– Reduced gene flow – Diversification

follows

Convergent traits • distantly related

species that share common environmental challenges have similar traits

Base groupings upon shared (homologous) traits

• Animals that share the most traits are most closely related.

• What is a trait? – Morphologic – physical appearance – Developmental sequence – physical

appearance during embryonic development; common cell fates, timing of cell migrations & gene activity

– Genetic – DNA sequence identity; chromosome number