Natural Selection and the Origin of New Species

Subtitle

Charles Darwin (1809-1882)

▪ English naturalist (interested in the natural world)

▪ Went on a journey to chart the coastline of South America▪ HMS Beagle

▪ Along the way, he was fascinated by the different plant and animals he saw▪ Collected extant (still around) organisms, and fossils of extinct organisms▪ Some organisms looked like variants of ones in other areas

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Galapagos Islands▪ One place that especially fascinated Darwin

▪ Lots of interesting organisms

▪ Different organisms on different islands or in South America appeared similar in their traits.

▪ Turtles on the islands varied▪ This is an actual tortoise captured by Darwin. It lived for ~175 years

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Darwin’s Finches▪ Also, Darwin began to capture

birds from the different islands

▪ Each had different appearance, food sources, and songs

▪ When he had them examined by a Ornithologist (bird scientist) he was shocked to discover that all were finches.

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Jean-Baptiste Lamarck (1744-1829)▪ Realized that species changed over time

▪ Thought that parents passed on traits that they had acquired during their lives▪ Ex. Giraffes stretching to reach food led to longer necks, and these were passed

on to offspring

▪ We know it is not true…

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Thomas Malthus (1766-1834)▪ Determined that populations grow exponentially, while resources grow

arithmetically

▪ Production cannot keep up with population growth▪ A.K.A. Carrying capacity

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Erasmus Darwin (1731-1802)-Charles Darwin’s Grandpa▪ Thought that species changed over time, but did not know how

▪ Only knew that Lamarck was not right

▪ Thought that perhaps little changes added up over time and gave rise to new species

▪ Shared his ideas with a young Charles

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Charles Lyell (1797-1875)▪ Geologist

▪ Came up with the idea (new at the time) that the Earth had always gone through similar geologic changes as happen today▪ Continental drift, volcanism, etc.▪ Before this, it was thought that the Earth did not often change, except for

times of catastrophes (i.e. Earthquakes and Volcanic eruptions were rare and only destroyed things)▪ We now know that volcanism leads to formation of islands

▪ Also discovered the Earth was older than thought in the day▪ Most people thought the Earth was no more than 10,000 years old

▪ Lyell’s work pushed that back to 300 million years ▪ Later findings eventually pushed that back to 4.5 billion years

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Artificial Selection▪ Darwin was also familiar with selective breeding methods to get

desired traits from plants and animals.

▪ Called artificial selection▪ Humans make gradual changes to species to get organisms we want

▪ Also known as selective breeding

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Sexual Selection

▪ Darwin also noticed that some species have traits designed to attract a mate▪ Often, these traits are detrimental to the organism, as they

require a lot of energy or make them stand out▪ Ex. Birds building complicated nests and singing and dancing to attract a

mate▪ https://www.youtube.com/watch?v=CYcs73oeFkc

▪ Or, peacocks displaying intricate tail feathers to attract a mate.

▪ In these cases, the male is putting himself in danger to attract a mate▪ However, if they are successful, they pass the trait on to their

offspring

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Darwin’s Conclusions▪ Based on the science of the day, and his own observations, Darwin

came up with 4 conclusions that led to his theory

1. Organisms reproductive potential is higher than an environment can support

2. Individuals within a species vary in their traits

3. Variations are passed from parents to offspring

4. Individuals with variations that are best suited to their environment are more likely to survive and reproduce, passing these traits on to their offspring

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Variation and Adaptation▪ Variations are differences within a species

▪ Examples:

▪ Adaptations are variations that allow an organism to better survive in their environment

▪ Examples:

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Darwin’s Theory of Natural Selection▪ Recall that a Theory in science is NOT A GUESS!

▪ It is a well-tested explanation for natural phenomena

▪ Theory of Natural Selection

▪ Nature “selects” for survival the organisms best suited to survive there.

▪ I.E. If an organism is better able to survive where it lives, it survives. If it is not, it does not.▪ Common sense…

▪ Sometimes referred to as “Survival of the Fittest”

▪ Fitness does not mean strong, smart, or good looking…

▪ Biological Definition of Fitness▪ The ability to survive and reproduce in a given environment

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Adaptations▪ Adaptations are central to Natural Selection▪ Variations that are beneficial are selected for ▪ Camouflage-blending in with surroundings to keep from being

spotted ▪ Mimicry-looking like something dangerous, which makes

predators afraid

▪ These help organisms survive

▪ Gives them an advantage over other members of their species▪ They are more likely to survive and pass the trait on

▪ If it is not beneficial, it is not an adaptation!Which of the organisms to the left can sting you?

None of them! All are flies, and are 100% harmless. None even have stingers!

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Types of Selection▪ Directional selection

▪ Disruptive Selection

▪ Stabilizing selection

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Directional Selection-An extreme phenotype is favored

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Disruptive Selection-Both extreme phenotypes are favored

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Stabilizing Selection-Intermediate phenotype is favored over extremes

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Evolution▪ Change in the frequency of certain alleles in a population over time.

▪ Also called descent with modification

▪ Over time, certain traits become more common in a population

▪ NOTE: Evolution occurs in a population, not an individual

▪ The variations are often the result of a beneficial mutation giving the organism something it did not previously have

▪ Ex. Antibiotic resistance in bacteria

▪ As conditions change, the ones with the trait survive better than those without.

▪ Over time, the trait becomes more common in the population

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Speciation▪ One species splitting into 2 species

▪ A species is a group of organisms that can reproduce and have fertile offspring

▪ Dogs are the same species (Labradoodle-fertile)

▪ Donkeys and horses are not (Mule-sterile)

▪ Usually occurs when part of the population becomes isolated from the initial population

▪ https://www.youtube.com/watch?v=YCoEiLOV8jc

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Descent with Modification▪ Darwin’s way of describing evolution.

▪ The similar species (ex. The different species of Finch on the Galapagos Islands) likely all had a common ancestor▪ A mother population of finches that changed over time into all of the different

species that we see now.

▪ Often caused by changing environmental conditions

▪ For the finches, it was availability of food ▪ Finches from the initial population that had slightly larger beaks did better than

those with smaller beaks when large nuts and seeds were the primary food source available

▪ This led Darwin to conclude that, over millions of years, all organisms descended from a single common ancestor

▪ The “Tree of Life”

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9.4.3.3.1 and 9.4.3.3.6

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The 5 factors that lead to evolution1. Genetic Drift-Allele frequencies slowly changing over time by chance

▪ Some alleles can be bred out of the gene pool

▪ Plays a very minor role in evolution

2. Gene flow-Movement of alleles from one population to another

3. Mutation-New alleles arising from mutations

4. Sexual selection-Mates choosing certain traits

5. Natural selection-Traits giving an advantage to organisms

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Patterns in Evolution ▪ Convergent Evolution

▪ Evolution towards similar features in unrelated species▪ Wings of birds, insects and mammals

▪ Divergent Evolution

▪ Evolution away from features common in ancestral species▪ Ancestor of mammals led to wide variety of features

▪ Coevolution

▪ Evolving in response to the evolution of another species▪ “Evolutionary Arms Race”

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Adaptive Radiation▪ Many species evolving from a single ancestral

species▪ Ex. Darwin’s Finches

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When Evolution Does Not Occur

▪ If environmental conditions change and species are unable to adapt, the result is extinction

▪ Extinction is the death of the last member of a species

▪ Sometimes the reason is slow changes

▪ Food or water sources being depleted

▪ Other times, it is rapid

▪ Asteroid killing dinosaurs

▪ Recall that we are currently in the midst of another mass extinction

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Misconceptions About Evolution

▪ Evolution is not random▪ It is driven, through natural selection, by environmental factors

▪ Organisms do not choose to evolve▪ It is usually a long, slow process as allele frequencies change in a

population

▪ Organisms do not give birth to other kinds of organisms▪ No dog will have kittens

▪ The changes are minor, and take many generations to cause major changes

▪ A snake may have a baby that has slightly more potent venom, giving it an advantage▪ This offspring will (maybe) pass the trait on to their offspring, giving it an

advantage▪ It will slowly move through the population

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Evidence for Evolution▪ Evolution is one of the best supported Theories in all of science

▪ Evidence for evolution comes from the following:

▪ Fossils

▪ Biogeography

▪ Embryology

▪ Anatomy

▪ Vestigial Structures

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Fossils▪ Remnants of past organisms

▪ Can be frozen organisms, mineralized skeletons, imprints, organisms trapped in amber, or other remnants

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Cool Fossils

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Biogeography

▪Distribution of organisms throughout the world▪ Different organisms live in different climates▪ Little differences, even though they are clearly related

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Embryology-study of development

At one point,you had gills anda tail!

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Anatomy▪ Homologous structures-similar structures in organisms that are not

closely related

▪ Same bones, same places, different species

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Vestigial Structures▪ Structures in an organism that once had a function, but no longer

serve a purpose for the organism

▪ https://www.youtube.com/watch?v=OAfw3akpRe8

▪ Examples:▪ Appendix, Goosebumps, tail bone, wisdom teeth

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DNA Evidence▪ All organisms, past and present, share DNA

▪ All humans are 99.9% similar

▪ Humans and chimpanzees share 98% of DNA

▪ Humans and bananas have 50% similar DNA

▪ Humans and grass have 15% similar DNA

▪ Humans and bacteria have 7% similar DNA

▪ The bases and cell structures are the same, coded from common DNA

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Why Do We Study Evolution?▪ Understanding the past is nice

▪ More importantly, it unifies all fields of Biology

▪ Why do Mendel’s Laws, discovered on peas, apply to humans?

▪ Why can we take heart valves from pigs and put them into humans?

▪ Why can we test drugs on mice and conclude that it will be safe for humans?

▪ How are bacteria becoming resistant to antibiotics?

▪ Why do prokaryotic cells have similar structures to eukaryotic cells?▪ Why are prokaryotic cells all so similar?

▪ Why are eukaryotic cells so similar?

▪ Etc.