Evolution

Theory of Evolution

• Theory – a well supported testable explanation of phenomenon occurring in the natural world.

• Evolution – the process by which modern organisms changed over time from ancient common ancestors.• Microevolution – change in allele frequency in populations over

generations.• Macroevolution – large scale change, such as the formation of new

species.

Summary: Theory of Evolution

• Species are different due to variation in their genes (variation results from random mutation).

• Some individuals are better suited for survival, and will leave more offspring (natural selection or “survival of the fittest”).

• Over time, change within species leads to the replacement of old species by new species as less successful species becomes extinct.

• There is clear evidence from fossils, anatomy, physiology, DNA, and embryology that the species now on Earth have evolved from ancestors that are now extinct.

3 Patterns of Biological Diversity

• Species – group of similar organisms that can breed and produce fertile offspring.• 1. Species vary globally – different yet ecologically similar animals

are found in different yet similar environments.• 2. Species vary locally – different yet related species occupy

different habitats in one area.• 3. Species vary over time – fossils of extinct species are similar to

current species.

Artificial Selection• Artificial Selection – nature provides variation (variety) in

organisms’ traits, but humans choose to breed those organisms that have the most useful traits. • Example: humans breed cows that produce the most milk.• Example: humans breed trees that create the most fruit.

Natural SelectionDarwin proposed a mechanism for evolution that he called

natural selection.Individuals whose characteristics are well-suited to

their environment survive and reproduce.• By surviving, these attributes can be passed onto their

children, causing an increase of these traits in the species population, thus causing a gradual change in the characteristics of the population.

Individuals whose characteristics are not well-suited to their environment die, or leave few offspring.

Because natural selection favors a certain trait over others, more individuals in the population carry the genes for that trait.

AKA: “survival of the fittest”

Parameters of Evolution by NS• 1. Struggle for existence – more organisms are produced than

can survive.• Competition – individuals or groups of organisms compete

for similar resources (territory, mates, food, water, etc.) in the same environment.

• 2. Variation and adaptation – some variations are better suited.• Adaptation – heritable characteristic that increases an

organism’s ability to survive and reproduce.

• 3. Survival of the fittest – individuals with adaptations that are well suited to their environment survive and reproduce.• Fitness – measure of how well an organism

survives/reproduces.

Common Descent• Common descent – all species (living and extinct) descended

from a common ancestor.• Over many generations, adaptations caused a successful

species to evolve into a new species.• The fossil record provides evidence for this “descent with

modification”.

Patterns of Evolution• Divergent evolution – single species or group of species evolve over a

short period of time into different forms living in different ways due to a change in environment that makes new resources available.• Aka adaptive radiations• Ex. Dinosaurs, Darwin’s finches.

• Convergent evolution – similar structures are produced in distantly related organisms.• Ex. Mammals that feed on ants/termites evolved independently 5

times.

• Coevolution – 2 species respond to changes in each other over time.• Neither can survive without the other.

Evidence for Evolution includes:1. Geographic distribution of species2. Fossils3. Anatomy (homologous structures)4. Physiology (analogous structures) 5. Embryology6. Universal genetic code7. Biochemical homology

1. Geographic Distribution of Species

Species of animals on different continents had similar structures and behaviors.

Darwin theorized that animals on each continent were living under similar ecological conditions and were exposed to natural selection in a similar way.

Similar selection pressures caused animals to evolve common features.

2. Fossils• A fossil is the preserved or mineralized remains (bone, tooth,

shell) or trace of an organism that lived long ago.• Fossils show evidence that support the ancestry between species.• Fossils trace the evolution of modern species from extinct

ancestors.• Example: Fossils have shown that whales evolved from four legged

land mammals; 1990’s transitional forms of whales found.

3. AnatomyEvolutionary relationships can be viewed by studying and

comparing anatomy.Scientists view the anatomy of limbs and see common

similarities.Over course of evolution, vertebrates moved into environments

causing different survival needs.

• Homologous structures – parts of different organisms (that are often quite dissimilar) that developed from the same ancestral body parts.• Forelimbs of whales, bats, crocodiles, and chickens have similar

anatomy but are modified for different functions – common ancestry.

• Are similar in structure but differ in function!

Homologous structures-differ in function, similar in structure

4. Physiology• Analogous structures -- structures that are similar in

appearance and function but have different origins and usually different internal structures.• Examples: a bat’s wing and a insect’s wing--both are wings and

both are used for flight, but a bat has bones and an insect does not.

Analagous Structures-differ in structure, similar in function

• Vestigial structures – any body structure that has reduced or no body function.• Examples: A human’s appendix; a whale’s pelvis.• As species adapt to environments the change in form and behavior and

continue to inherit these structures as part of the body even though they have no function.

5. EmbryologyEmbryology shows links between different species.

Embryos of different species in early development are indistinguishable from each other.

Common cells and tissues develop in similar patterns in all vertebrates.

Illustrates descent from a common ancestor.

6. Universal Genetic Code• Genetic code – mRNA codons specify particular amino acids.• The genetic code of all organisms on Earth (bacteria, yeast,

fruit flies, humans) is the same! • Example: the AUG codon always codes for the amino acid

methionine.

7. Biochemical HomologySimilar DNA, RNA, and amino acid sequences amongst species

in same taxonomic group.Remember comparing your insulin gene DNA and amino acid

sequences to that of a cow?

• Population – mating group of organisms of the same species.• Gene pool – all genes (and their alleles) present in a

population.• Allele frequency - # of times allele occurs in a population.• Changes as population evolves over time.• Natural selection operates on individuals, but causes a change in

the allele frequency.

Allele Frequencies

Mutations: -Sources of Genetic Variation

• The main source of genetic variations in populations is mutations!!!!• These mutations occur randomly.• Not all mutations affect an organism’s fitness.• Only heritable mutations matter for evolution.• Neutral mutations don’t change phenotypes.

• Other sources of variation include:1.Genetic recombination during crossing over and

independent assortment in meiosis.

NS and Phenotype• An organism’s genotype and environmental conditions makes

up its phenotype.• Natural selection operates on variation in organisms’

phenotypes.• Higher fitness = phenotypes better suited for the environment.

Types of Selection in Populations

• When NS on polygenic traits affects the fitness of phenotypes, it leads to selection:

• Directional Selection – organisms at one end of the curve have a higher fitness than those in the middle or at the other end.

• Stabilizing Selection – organisms in the center have highest fitness.

• Disruptive Selection – organisms at the ends of curve have highest fitness.

Other Source of Changes in Allele Frequencies

• Genetic drift – change in allele frequency that occurs in small populations due to random chance.

• Genetic bottleneck – change in allele frequency following a dramatic reduction in population size.

• Founder effect – change in allele frequency following migration of a small subgroup out of the population to start a new population.

Macroevolution and Speciation• Species – population of organisms that can interbreed.• Speciation – evolution/formation of a new species.

When environments change, the process of evolution enables some species to adapt to new conditions and thrive while some species fail to adapt and become extinct.

• Niche - combination of an organism’s profession and place where it lives.• No 2 species can occupy the same niche in the same location for

a long period of time!• The more efficient species will survive and reproduce driving the

other to extinction.

Isolating Mechanisms of Speciation• Reproductive isolation occurs when 2

populations can’t interbreed - causes speciation!• Once reproductive isolation occurs, natural selection increases

the differences between the separated populations.1. Behavioral isolation – different courtship.2. Ecological/habitat isolation – can only mate in specific or

preferred habitats.3. Mechanical isolation – no sperm is transferred.4. Gametic isolation – no fertilization of egg occurs.5. Temporal isolation – reproduce at different times.

• Geographic isolation – population becomes divided (isolated) by a physical barrier.

Rates of Speciation• Gradualism – slow, steady change leading to

new species.

• Punctuated equilibrium – brief periods of rapid change leads to the formation of new species.• Rapid change occurs when a small population

is isolated from the rest of the population or migrates.