Topic 2: Intro to Evolution by

Natural Selection

Important vocabulary: Hypothesis Theory

Adaptation Evolution

Microevolution Macroevolution

Explanation based on observationsSummarizes a hypothesis or hypotheses that have been supported with repeated testing. Valid until disproven.

Inherited trait that helps an organism survive.

Living species are descendants of ancestral species that were different from present day; “descent with modification”

Change in allele frequencies in a population.

Speciation; often described as patterns over geologic time scales.

B.D. (before Charles Darwin)

Earth is very old, still changing All organisms may be related by

common ancestry Extinctions have occurred Thomas Malthus…populations Lamarck

Lamarckian evolution: main ideas

Organisms arise spontaneously in simple forms

“Use and disuse” “Inheritance of acquired

characteristics”

Voyage of the Beagle: 1831-1836

Fossils in South America

Galapagos Islands Volcanic islands Oldest: Isla San

Cristóbal (4 my) Youngest: Isla

Fernandina (<1 my)

Endemic Fauna of the GalapagosEndemic = organisms that are native to a geographic area and are found no where else in the world.

The long argument: the years after the voyage of the Beagle Observations of specimens

collected on the different Galapagos Islands

Pigeon breeding Barnacles: 8 years, “homologies” Experiments on seed survival in

salt water

Darwin’s 5 Big Observations (and related inferences)

1. All species have such great reproductive potential that their population size would increase exponentially if all individuals that are born reproduce successfully.

2. Populations tend to remain stable in size, except for seasonal fluctuations.

3. Environmental resources are limited.

Inference #1: Because more offspring are produced than can be supported by the available resources and populations are stable over time, there must be a struggle for existence amongst individuals. This leads to only a fraction surviving to reproduce.

Darwin’s 5 Big Observations (and related inferences)

4. Individuals of a population vary in their characteristics.

5. Much of this variation is heritable (passed on from parents to offspring).

Inference #2: Survival is not random. Individual survival depends on the characteristics that are inherited. Unequal survival results from a process of natural selection that favors individuals best suited for their environments.

Inference #3: Over generations, this process will lead to continuing gradual changes in populations (evolution) as those individuals that survive will reproduce, passing their genes onto the next generation.

Alfred Russell Wallace

1858 “On the Tendency of Species to Form Varieties; And On the Perpetuation of Varieties and Species by Natural Means of Selection” (Published with an abbreviated essay of Darwin’s)

Selection

“Struggle for existence”

“Survival of the fittest”

“Descent with modification”

Evolution by Natural Selection

• Overpopulation may lead to Competition for…• Predation affects both…• Selection due to…

• Variation • Due to…• Is it always stronger or faster?

• Reproduction (differential)• Speciation due to…

Key points

Individuals do not evolve…what does? Natural selection can amplify or diminish

only inherited traits Evolution is not goal-directed!

Evidence

Peppered moths Galapagos finches Divergent evolution Convergent evolution Artificial selection (breeding) Antibiotic resistance in bacteria

Peppered moth (Biston betularia)

Early 1800s population:

B. betularia typica 98%

B. betularia carbonaria 2%

Late 1800s population:

B. betularia typica 2%

B. betularia carbonaria 98%

Galapagos finches

Galapagos finchesp. 6

Finch diets and beaks

“…adaptive radiation…refers to more or less simultaneous divergence of numerous lines all from much the same ancestral adaptive type into different, also diverging adaptive zones.”

Simpson 1953

From Petren et al. 1999

Adaptive Radiation

Seeds

Bark & Crevice Insects

Buds

Nectar and gleaned insects

Evolution of many species from one common ancestor in response to variations in available environments and resources.

Convergent Evolution

• The evolution of similar characteristics in unrelated (or extremely far removed) species as a result of similar ecological opportunities and environments

• Analogous structures are due to similar selection pressures

Convergent evolution

Artificial selection

Artificial selection: wild mustard

Selective breeding: domestic dogs

How antibiotics workHow antibiotics work

Antibiotics are chemicals that disrupt bacterial processes, some…

Inhibit cell wall Inhibit cell wall constructionconstruction

Inhibit protein Inhibit protein synthesis (cell dies)synthesis (cell dies)

Inhibit DNA replication Inhibit DNA replication so that cells canso that cells can’’t t dividedivide

Mechanisms of ResistanceResistant (mutant)

bacteria can:1. Modify or destroy the

antibiotic.

2. Flush out antibiotic.

3. Alter the target.EssentialBacterialProtein

EssentialBacterialProtein

Antibiotic Resistance

Sexual selection The selection of individuals for secondary

sex characteristics

eg bird coloration, elk horns Can be intrasexual or intersexual

Male-male competition Female choice

Often appears at odds with natural selection

Pea fowl

Birds of paradise