Ch. 15: Mechanisms of Evolution

15.1 – Evolution is both Factual and the Basis of Broader Theory15.2 – Mutation, Selection, Gene flow, Genetic Drift, and

Nonrandom Mating result in Evolution15.3 – Evolution can be measured by changes in allele frequencies

15.4 – Selection can be stabilizing, Directional, and Disruptive15.5 – 15.6 Skip

AP Biology C 2013

Charles Darwin

What do we know about him?

Charles Darwin 1809-1882• 1830’s – took a 5 year trip around the world on a

ship called the HMS Beagle• He went to the Galapago’s Islands, and was intrigued

by unique organisms such as giant tortoises and finches

• He noticed similarities and differences among the many organisms he saw as he traveled– He noticed striking differences between the species he

saw in South America and those of Europe– He noted that most of the animals on the Galapago’s

Islands were endemic – unique to that location and nowhere else

He concluded that the finches must have adapted to their habitat and food available on that continent.

George Culvier 1769-1832

• Spent years reconstructing the appearance of unique organisms from fossil bones

• Gave evidence that some organisms in the past differ greatly from any living species and that some organisms had become extinct.

• Promoted the idea of catastrophism – sudden geological catastrophies caused extinction of large groups of organisms at certain points of the past

Charles Lyell (1797-1875)

• Thought that the geologic processes that have changed the shape of Earth’s surface in the past continue to work in the same ways – concept is called uniformitarianism

• Darwin used this idea in his writings

Jean Baptiste Lamarck (1744-1829)

• Thought that simple organisms could arise from nonliving matter

• Also thought that simple forms of life develop into more complex forms

• Proposed that individuals could acquire traits during their lifetime as a result of experience or behavior – inheritance of acquired characteristics

• His ideas were rejected by scientists

Chapter 15

Click below to watch the Visual Concept.

Visual Concept

Lamarck’s Theory of Evolution

Section 1 History of Evolutionary Thought

3 Major Propositions of Darwin

1. Species change over time (evolve)2. Divergent species share a common ancestor3. Natural Selection

– Mechanism that produces change in species– The differential survival and reproduction of

individuals in a population based on variation in their traits

Darwin published his ideas in a book called, Origin of Species, published in 1859. It stirred considerable interest (and controversy) among scientists and the public.

Theory of Natural Selection (4 parts)

1. Overproduction2. Genetic Variation3. Struggle to survive4. Differential Reproduction

#1 - Overproduction

• More offspring can be produced that can survive to maturity

• Darwin used Malthus’s book as part of his reasoning– Human populations can increase more quickly-

that food supplies and that populations are often limited by conditions such as war, disease, or lack of food

– Darwin realized that the environment limits the population of ALL organisms by causing deaths or by limiting births

#2 – Genetic Variation

• Within a population, individuals have different traits

• Ex – some deer have thicker fur or longer legs than others. The deer with thicker fur will have offspring with thicker fur

• Occasionally, new traits may appear in a population

#3 – Struggle to Survive

• Individuals must compete with each other in what Darwin called a “struggle for existence”

• Some variations improve an individual’s chance to survive and reproduce, but some variations reduce this chance– Ex – thicker furred deer may survive better in the cold

• A trait that makes an individual successful in its environment, such as thick fur, is called an adaptation

#4 – Differential Reproduction

• Organisms with the best adaptations are most likely to survive and reproduce

• And through inheritance, the adaptations will become more frequent in the population, so populations may begin to differ

Survival of the fittest

• Term Darwin sometimes used• Describes natural selection• Fitness – is a measure of an individual’s

hereditary contribution to the next generation– A fit individual is one that has offspring that also

live long enough to reproduce

Chapter 15

Natural Selection

3 Patterns of Natural Selection

1. Stabilizing Selection– Individuals with the average form of a trait have

the highest fitness– Reduces variation in populations– Example – Predators can easily catch larger visible

lizards and the slower smaller lizards. The average ones best survive!

– Example – human birth weight

3 Patterns of Natural SelectionDisruptive Selection

– Individuals with either extreme variation of a trait have greater fitness than individuals with the average form of the trait

– Increases variation in population

– Example – bill sizes in black-bellied finches• Food source is two different kinds of seeds from marsh plants (soft

and hard)• The birds with large bills can easily crack the hard seeds• Birds with the small bills can crack the soft seeds• Birds with medium bills do not survive well because they are less

efficient that the other birds at cracking either seed.

3 Patterns of Natural SelectionDirectional Selection

– Individuals that display a more extreme form of a trait have greater fitness than individuals with an average form of the trait

– Average shifts in one direction

– Example – Hunters hunt for the biggest animals, therefore the smaller ones survive!

Evidence of Evolution

1. Fossil Record2. Biogeography3. Anatomy and Embryology4. Biological Molecules

Fossils

• Remains or traces of an organism that died long ago

• Show that different types of organisms appeared at different times and places on Earth

• Some fossils are of organisms that have become extinct

• Most powerful evidence of evolution

Transitional Species

• Fossil record shows that species have differed in a gradual sequence of forms over time

• Transitional species – have features that are intermediate between those of hypothesized ancestors and later descendent species

Section 2 Evidence of EvolutionChapter 15Evidence of Whale

EvolutionAncestor – land dwelling mammal

Transitional (mammal) – lived in coastal waters; could swim by kicking its legs and using tail for balance. It could waddle on land with its short legs

Transitional (mammal) – lived in ocean; resembled a dolphin, propelled itself with massive tail; had forelimbs that were flippers and tiny hind limbs that could not have been used for walking or swimming

Modern whales – have forelimbs that are flippers; They have tiny non-functioning hip bones at the rear of their bodies

Evidence #2 - Biogeography• Biogeography – study of the locations of organisms around the world• Darwin and another scientist, Wallace, observed animals that

seemed closely related yet were adapted to different environments in nearby regions

• They also observed animals that seemed unrelated but that had similar adaptations to similar environments in regions that were far apart

• Mammals of Australia provide the evidence of biogeography– There are native Australian animals that resemble wolves, cats, mice, moles or

anteaters– However, most Australian mammals are marsupials (pouches for young); – Possible explanation is that these animals evolved in isolation on the Australian

continent

Evidence #3 – Anatomy and Embryology

• Anatomy – the study of the body structure of organisms

• Embryology – study of how organisms develop

• The forelimbs are used in different ways in each animal; yet each limb has a similar bone structure

• One explanation is that an early ancestor shared by all of these vertebrates had a forelimb with a similar bone structure

• As generations passed, different populations of descendents adapted to different environments

• Bones inherited from ancestors may have become modified for different tasks.

Body Structures• Homologous Structures – anatomical structures that

occur in different species and that originated by heredity from a structure in the most recent common ancestor of the species– Might have different function– Example – forelimbs on birds, dolphins, humans

• Analagous structures – have closely related functions but do not derive from the same ancestral structure– Example – wings on bats, birds and moths – all have

different underlying structure

Vestigial Structures

• Structures that serve no function but that resemble structures with functional roles in related organisms

• Example – human tail bone, coccyx is made up of 4 fused vertebrae that resemble the bones in an animal’s tail

• Another Example – human appendix and pelvic bones of modern whales

Chapter 15

Click below to watch the Visual Concept.

Visual Concept

Vestigial Features

Development of Animal Embryos

• Stages of vertebrate embryo development are very alike

• One possible explanation for these similarities is that vertebrates share a common ancestor and have inherited similar stages of development

Chapter 15

Click below to watch the Visual Concept.

Visual Concept

Similarities in Embryology

Section 2 Evidence of Evolution

Evidence #3 – Biological Molecules

• In all species, DNA and RNA are the molecular basis for inheritance of traits

• Biologists can compare the DNA, RNA, proteins, and other biological molecules from many different organisms

Phylogeny

• Phylogeny – the relationships by ancestry among groups of organisms

• Can be modeled using different evidence

Artificial Selection

• Process occurs when a human breeder chooses individuals that will parent the next generation

• Example – humans may choose to breed oat plants that yield more grain per stalk or greyhounds that run faster

Coevolution

• Evolution is ongoing – many species may be evolving at once

• Coevolution – when 2 or more species have evolved adaptations to each other’s influence

• Examples– Animals evolved strategies to avoid being eaten– Microbes evolved to live with certain animals– Bacteria become resistant to various antibiotics

Other Terms to Know

• Gene Flow – migration of individuals and movements of gametes between populations

• Genetic Drift – random changes in allele frequencies from one generation to the next

• Population Bottleneck – Large populations pass through environmental events that only a small number of individuals survive; thus a loss of genetic variation occurs

Other terms to Know continued…

• Founder Effect – A colonizing population is unlikely to possess all of the alleles found in the gene pool of its source population

• Sexual Selection – Specific type of nonrandom mating in which an organism’s phenotype influences its ability to attract males