1

Descent with Modification

Chapter 22

Descent with modification – Evolution

The earth is a dynamic place (ever-changing). In order for life to survive, it too must change.

This represents an interesting parody.Evolutionary change can not be seen at the

individual level, it is only noticed over time as organisms develop adaptations (traits which enhance survival/reproduction).

This occurs very slowly and may take many, many generations to develop.

• Evolution is the primary unifying concept in biology.

2

History of evolutionary thought

• Most are content to let Darwin have all the credit for the concepts and ideology behind evolution; however we need to look beyond (or perhaps before) Darwin to realize the true impact of the intellectual revolution that is evolution.

Early Philosophy on “Changing” Lifeforms

• Aristotle (384-322 BC) - viewed lifeforms as unchanging (fixed); however, he did understand that life could be arranged (or placed on a scale) based upon increasing complexity – scala naturae

• Not much advancement in evolutionary thought progressed until the 1700’s– Why?? Consider the science of biology – lots of

other things were happening – cells, anatomy, – Any other reasons???

3

• Carolus Linneaus (1707-1778) Sweden

• Founder of taxonomy – science of naming and classifying organisms

• Developed a hierarchical system of classification– Kingdom Phylum Class Order Family Genus Species

• Developed system of binomial nomenclature – still used today – i.e. Aix sponsa – wood duck

• This system did not infer evolutionary kinship

James Hutton (1726-1797)

• Geologist• Concept of gradualism

– Idea that profound change can take place through slow but cumulative effect of slow but continuous processes

– Mostly dealt with geologic change -i.e formation of valleys from rivers

4

Jean-Baptiste de Lamarck (1744-1829)

• Introduced to key principles– Use and disuse – idea that parts of the

body that are used extensively are larger and stronger while lesser utilized parts deteriorate

– Inheritance of acquired characteristics –organisms could pass along these traits to offspring

– Example – a giraffe’s neck is long because it continually stretched out to reach food. (not the case we now know)

Georges Cuvier (1769-1832)

• Father of paleontology – the study of fossils– Fossil – remains or traces of

organisms from the past• Cuvier studied fossils and began to

understand the deeper the strata (layers of sedimentary rock) the more dissimilar the fossils are to current life

• Made references to extinctions -catastrophism

5

• Each of these individuals had influence in the life of a young scientist from Shrewsbury, England

• Charles Darwin (1809-1882)• Trained at Cambridge as a botanist• In 1831, Darwin (22 yrs old) set

sail on the HMS Beagle to document the and chart poorly known stretches of the South American Coastline.

• A key stop on the voyage was the Galapagos Islands

Voyage of the HMS Beagle – approx 5 year voyageWhy were voyages like this done??

6

Darwin and the Galapagos Islands (GI)

• Darwin noted that most of the animals that he found on the GI resembled animals he had seen on the mainland, they were in fact different species.

• He even noted that some species on the GI very closely resembled each other however there were subtle differences in their appearance or behaviors from island to island or even habitat to habitat.

• Darwin kept detailed records of his observations

Putting it all together…

• After his return to England (1836-1840), Darwin began to compile his notes and formulated an idea that adaptation (traits) to the environment and the process of forming new species might be closely related

• He also drew upon the works of others to substantiate his findings

7

• By 1840, Darwin had formulated his theory of natural selection as the mechanism of evolution (descent with modification); where new species are formed based upon the premise that the environment selects individual members of a species based upon adaptations they possess and allows them to produce more offspring.

• He was hesitant to publish these findings due to the controversy he knew this could create and he was consulted by several other scientists like Charles Lyell

8

Charles Lyell (1797-1875)

• A geologist• Took the work of Hutton a step further• Uniformitarianism – proposed that

geologic change occurs today as in the past; at the same rate

• Both Lyell and Hutton’s concepts influenced Darwin as their ideas both supported slow change of the Earth

• Darwin surmised that the age of the Earth was in fact greater than 6,000 years and that living things might be governed by a similar process of slow change

Darwin was not alone in his ideas that slow changes to the Earth, coupled with living

things ability to change slowly over time as well could produce different species…

• We have already talked about Lamarck – use-disuse and inheritance of acquired characteristics– Provide insight to living things’ ability to change and

pass on these changes somehow to offspring

• But it was another scientist, Wallace, who had developed his own idea of “natural selection”while working in the West Indes

9

Alfred Russel Wallace (1823 – 1913)• Wallace knew of Darwin and in fact,

sent a draft manuscript of his theory to Darwin to review

• Lyell had been urging Darwin to publish his findings before someone else did…now Wallace would be first!!! (1858)

• Darwin quickly finished his work and published The Origin of Species the next year

• Wallace was a great admirer of Darwin and agreed that Darwin should get the credit as the architect of natural selection

• Two main themes to the book:– Evolution explains life’s

unity and diversity– Natural selection is the

cause of descent with modification

10

Darwin’s views portray the history of life as a tree – many branches arising from a common trunk and then continuing to branch until the young twigs appear – we know this to be true today based upon fossil evidence, comparative anatomy and

now biochemistry

Natural selection and the development of adaptations are the mechanisms that drive this process. Darwin based natural

selection on five observations :

1.1. Overproduction of offspringOverproduction of offspringoccurs for all species – if all of the offspring of a given species survived, the population’s growth would be exponential. Yet Darwin observed that this does not occur.

11

2.2. Populations tend to remain stable in sizePopulations tend to remain stable in size(relatively stable – over time)

There may be seasonal fluctuations or natural cycles where the population hovers around a stable point

time

Pop. size

3.3. Resources are limited and competition occurs.Resources are limited and competition occurs.overproduction leads to a struggle for resources

(food, water, shelter, nutrients, sunlight, etc…)Only a fraction of the population is given the

opportunity to reproduce

12

4. Variation occursVariation occurs - Members of a population vary extensively in their characteristics; no two individuals are exactly alike

5. This variation is passed from one generation to the next; variation is heritablevariation is heritable. Survival depends on inherited traits (adaptations). Those individuals that possess adaptations that increase survival are more likely to reproduce.

13

Darwin also based his theory on familiar examples of how humans had modified living

things – Artificial Selection• Breeds of dogs, livestock, and plants are created to

produce desired traits

Summary of Natural Selection

• Natural Selection is the differential success in reproduction that results from the interactions between individuals that vary in heritable traits and their environment.

• Natural Selection can increase the adaptation of living things to their environment

• If the environment changes over time, or if living things move to a new environment, natural selection may result in adaptations to these new conditions

14

Example (experimental) of natural selection in action

Evidences of Evolution

• Fossil Record

15

• Comparative Anatomy

16

Biochemistry