Unit 5 Evolution

Ch. 15 Darwin’s Theory of Evolution

The Puzzle of Life’s Diversity

• Evolution - modern organisms have descended from ancient organisms (change over time)

• Theory - a well-supported, testable explanation of phenomena that have occurred in the natural world

Voyage of the HMS Beagle

• 1831• Charles Darwin contributed most to our

understanding of evolution• He made observations & collected

evidence that led him to propose a hypothesis about the way life changes over time.

Voyage of the Beagle

• That hypothesis, now supported by a large amount of evidence, has become the Theory of evolution

Darwin’s Observations

• Darwin collected the preserved remains of ancient organisms - Fossils

• Some of these fossils resembled organisms that were still alive

• Others looked unlike any creature ever seen

Darwin’s Observations

• The Galapagos Islands influenced Darwin the most

• He observed that the characteristics of animals & plants varied among the different Islands.

Darwin’s Observations

• Each island had a different climate although they were very close.

• He studied tortoises.• Each tortoise’s shell had a different shape

depending on which island it came from.

• He wondered if the animals living on different islands were once related….

Views before Darwin

• Earth was created only a few thousand years ago.

• Since creation, neither earth nor its species had changed

• Darwin was influenced by several individuals.

An Ancient, Changing Earth

• Hutton & Lyell helped scientists recognize that Earth is millions of years old.

• They also noted that the processes that changed Earth in the past are the same processes that are changing Earth now.

Lamarck’s Evolution Hypothesis

• The year that Darwin was born, Lamarck published his hypothesis

• He proposed that by selective use or disuse of organs, organisms acquired or lost certain traits during their lifetime

• Over time, this process led to change in a species

Lamarck’s Explanation

•Tendency toward perfecton•Use and Disuse•Inheritance of Acquired traits

Population Growth

• English economist, Malthus, published a book, noting that babies were being born faster than people were dying

• He stated that if the human population continued to grow unchecked, sooner or later there would be insufficient living space & food for everyone

Darwin Presents His Case

• In 1859, Darwin published the results of his work in a book, On the Origin of Species.

• In his book, he proposed a mechanism for evolution called natural selection.

• He stated that evolution has been taking place for millions of years, & continues in all living things

Darwin Presents His Case

• Species on Earth today descended from ancestral animals in various habitats.

• Heritable variation- differences that are passed from parents to offspring.

• Variations were thought to be unimportant.

Inherited Variation & Artificial Selection

• Artificial selection - nature provided the variation, & humans selected those variations that they found useful.

• It has produced diverse plants & animals by selectively breeding for different traits.

Evolution by Natural Selection

• Struggle for existence - the members of each species compete regularly to obtain food, living space, & other necessities of life– Predators that are faster & better at catching

prey are more likely to survive

Evolution by Natural Selection

• Fitness - the ability of the organism to survive & reproduce in its specific environ.– Fitness is the result of adaptations

Evolution by Natural Selection

• Adaptation - any inherited characteristic that increases an organisms’ chance of survival– Successful adaptations allow

organisms to become better suited to their environ. & thus better able to survive

Evolution by Natural Selection

• Individuals that are better suited to their environ., with adaptations that enable fitness, survive & reproduce most successfully - Survival of the Fittest

Evolution by Natural Selection

• Since it is similar to artificial selection, Darwin referred to survival of the fittest as - Natural Selection

• In both AS & NS, only certain individuals of a population produce new individuals

Evolution by Natural Selection

• However, in NS, the traits being selected, & therefore, increasing over time, contribute to an organism’s fitness

• NS takes place without human control or direction

Evolution by Natural Selection

• NS results in changes in the inherited characteristics of a population, that increase a species’ fitness in its environ.

• Over time, NS produces organisms that have different structures, & occupy different habitats

Evolution by Natural Selection

• As a result, species today look different from their ancestors

• Each living species has descended, with changes, from other species over time - Descent with Modification

Evolution by Natural Selection

• Descent with modification also implies that all living organisms are related to each other

• Common descent - all species (living & extinct) were derived from common ancestors

Evidence of Evolution

• Darwin argued that living things have been evolving on Earth for millions of years

• Evidence of this could be found: in the fossil record, the geographical distribution of living species, homologous structures of living organisms, & similarities in early development

Evidence of Evolution

• The Fossil Record:– Darwin noticed that the sizes, shapes, &

varieties of related organisms preserved in the fossil record, changed over time

Evidence of Evolution

• Geographic Distribution of Living Species:– Darwin realized that similar animals in different

locations were the product of different lines of evolutionary descent

Evidence of Evolution

• Homologous Body Structures:– Homologous structures - structures that have

different mature forms but develop from the same embryonic tissues

– Not all homologous structures serve important functions

– Organs of many animals are so reduced in size that they are just vestiges, or traces, of homologous organs

Homologous Structures

Evidence of Evolution

• Homologous Body Structures:– Vestigial organs - may resemble miniature legs,

tails, or other structures, a trace of a homologous structure

Evidence of Evolution

• Similarities in Early Development:– The early stages or embryos, of many animals

with backbones are very similar