Topic 10 Notes/Study Guide

EVOLUTION & NATURAL SELECTION

Natural Selection Leads to Evolution

Evolution is the gradual change of a species over time..

Natural Selection is the process where individuals in a population that possess better adaptations survive and reproduce more than other individuals of the same population.

Adaptions are features/characteristcs that help an organism survive and/or reproduce in its current environment.

Natural selection sometimes referred to as "survival of the fittest".

In biology, Fitness is the ability of an organisms to pass on its genetic material to its offspring. In other words, it is being able to live long enough to reproduce.

Adaptations increase an organisms fitness!

Natural selection leads to evolution since the traits of those individuals who are able to reproduce influence the traits of future generations. Over time, these advantageous traits become more common in a species, which causes the species to evolve.

For example, if a population of camouflaged white rabbits survive and reproduce more than non-camouflaged brown rabbits, then by way of natural selection, the recessive white-fur trait will increase with future generations. Slowly, the species of rabbit will evolve to have mostly white fur.

Charles Darwin - "The Father of Evolution"

Charles Darwin (1809 – 1882) was an English naturalist who first developed a theory of evolution that has laid the groundwork for modern biological thinking. Darwin grew up in a time when the scientific view of the natural world was dramatically shifting. Geologists were suggesting that Earth was more ancient than previously thought and had changed over time. Biologists were suggesting that life on Earth was also changing. In this climate, Darwin developed a scientific theory of evolution that explains how modern organisms evolved over long periods of time through descent from common ancestors.

Darwin’s Fateful Journey

In the year 1831 and at the age of 22, Charles Darwin joined a 5-year trip around the world as the official naturalist on the ship H.M.S. Beagle.

The main purpose of the Beagle’s voyage was to map the coastline of South America. The ship traveled around the coast of South America to the Galapagos Islands. From there, the ship pursued a westerly course passing the southern coast of Australia, then going north to the southern coast of Asia, and then south along the southern coast of Africa back to England.

Darwin role was to collect specimens of plants and animals and make observations about what he saw. No one knew it at the time, but this journey would become one of the most important scientific voyages in history because it led Darwin to develop his ideas about evolution.

What Darwin Observed Aboard the Beagle

Darwin filled his notebooks with observations about the characteristics and habitats of the different species that he saw. But Darwin wasn’t content just to describe the biological diversity that he saw: he wanted to explain it in a scientific way. Darwin looked for larger patterns in his data, and as he traveled, Darwin noticed three major patterns of biological diversity: Species Varied Globally, Species Varied Locally, and Species Varied Over Time.

Gallapagos Finch Example:

On the Galapagos islands, a collection of closely-spaced islands off the coast of Ecuador, Darwin observed that one type of bird, a finch, lived on all of the islands. However, finches living on each island had slightly different beaks. On one island, they finches had heavy beaks. These finches primarily ate heavy seeds that they cracked open with their strong beaks. On another island, the finches had thinner, sharper beaks. These finches probed local plants for small insects. After careful study, Darwin concluded that the finches’ beaks had adapted to the type of food available on each island.

The Struggle and the Race to Publish

For the next 25 years, Darwin studied and organized his extensive notes and specimens. Within the first 5 years, Darwin had worked out the main points of his theory of natural selection. Many of his scientific friends considered Darwin’s ideas to be brilliant and urged him to publish them. Although Darwin wrote a complete draft of his ideas, he didn’t publish for another 20 years. Why?

Darwin knew that many scientists, including some of Darwin’s teachers and colleagues, had ridiculed the ideas of the French biologist Jean-Baptiste Lamarck’s. Lamarck’s theory of evolution proposed that helpful adaptations developed within an organism’s lifetime as they were used more and more. In addition, Lamarck proposed that organisms evolved in a straight line fashion from “less perfect” to “more perfect”. Both ideas were later proven to be incorrect. Still, Darwin felt that his own theory was just as radical as Lamarck’s. Darwin wanted to gather as much evidence as he possibly could to support his ideas before making them public.

Lamark's Theory of Evolution vs. Darwin's Theory of Evolution

Meanwhile, anonther scientist named, Alfred Russell Wallace (1823 – 1915) had independently begun writing a book based on ideas very similar to Darwin’s using his observations of species in Southeast Asia. In 1858, Wallace presented his early manuscript to Darwin for review. At this point, Darwin realized that he needed to publish his own work immediately or his theory would be overshadowed by Wallace’s. In 1859, Darwin published On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life. Today, this book is referred to by a shorter title, The Origin of Species. In this famous book, Darwin’s theory of NATURAL SELECTION was clarified.

Darwin’s Theory of Natural Selection

Darwin’s did not invent the idea of evolution – that had been described multiple times by multiple individuals in history. Darwin’s great contribution was to describe a process in nature – a scientific mechanism – that could explain how evolution occurred. Natural selection is the name of this process.

Darwin’s theory of natural selection explains how large numbers of new plants and animals are produced by nature. Many of these do not survive because nature “weeds out” weak and feeble organisms by killing off those that cannot adapt to the local environmental conditions. Only the strongest and most efficient survive and produce offspring, which carry on the traits of their parents. These traits become more common in the general population.

Darwin created four specific tenets, or postulates, for his theory, which are listed in the following table.

Types of Natural Selection

Mechanisms of Evolution

Natural selection is not the only source of evolutionary change in a population!

Any event that changes the gene frequencies in a populations gene pool can cause the population evolve!

There are FIVE primary mechanisms of evolution. They are:

Genetic Drift - Population Shrinking

Nonrandom Mating

Mutations

Gene Flow - Migration/Movement

Natural Selection --> Adaptations

Genetic Drift

Genetic drift—along with natural selection, mutation, and migration—is one of the basic mechanisms of evolution.

Genetic Drift is a change in the genes of a population due to random chance events that decreases the size of the population.

In each generation, some individuals may, just by chance, leave behind a few more descendants (and genes, of course!) than other individuals. The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals.

In this hypothetical cartoon above genetic drift affects the genetic makeup of the population but, unlike natural selection, through an entirely random process. So although genetic drift is a mechanism of evolution, it does NOT work to produce adaptations.

There are two types of genetic drift: The Bottleneck Effect and the Founder Effect.

Bottleneck Effect

Sometimes, a disaster, such as a disease, can kill many individuals in a population.
Just by chance, the smaller, leftover population’s gene pool can be different from the original population’s.
The bottleneck effect is a change in gene frequency following a dramatic reduction of a population’s size.
The bottleneck effect usually results in a population with reduced genetic diversity

Founder Effect

Genetic drift can also occur when only a few individuals colonize a brand new habitat.
These founding individuals may carry alleles that differ than those from the main population that they came from.
The new gene pool therefore stars out with different frequencies and grows in size.
When a small group of founding individuals colonize a new location, reduced genetic diversity usually occurs.

Nonrandom Mating

Nonrandom Mating: individuals choose their mate based on specific characteristics or behaviors.

In a randomly mating population the alleles in the gene pool assort at random. BUT when mating is nonrandom, gametes and alleles assort according to specific behaviors or characteristics.

Mutations

Mutations are random changes in an organism's DNA.

These random changes produce new genetic variation in a populations gene pool since they can introduce new types of alleles (versions of a gene).

Gene Flow

Gene Flow is a form of evolution that takes places when individuals migrate in out of a populaton changing the frequency of alleles (forms/versions of genes) in the population.

Gene Flow can cause a population to gain new alleles or lose existing alleles.

EXAMPLES:

The wind can carry pollen or seeds from one plant population to another.
Young wolves leave one wolf pack and join another pack of wolves.

EVIDENCE FOR EVOLUTION

CLICK HERE to read about the Evidence for Evolution on Khan Academy!

Evidence for Evolution

SPECIATION & EXTINCTION

SPECIATION = The formation of a new species.

CLICK HERE to read about Speciation on Khan Academy!

In allopatric speciation, groups from an ancestral population evolve into separate species due to a period of geographical separation.

In sympatric speciation, groups from the same ancestral population evolve into separate species without any geographical separation.

Sympatriac Speciation can occur due to pre-zygotic or post-zygotic barriers! (See table on right.)

Prezygotic barriers prevent members of different species from mating to produce a zygote, a single-celled embryo.

Postzygotic barriers keep hybrid zygotes—one-celled embryos with parents of two different species—from developing into healthy, fertile adults.

Note: Ecolgoical Isolation is the same as HABITAT ISOLATION!

EXTINCTION = The permanent loss of all individuals of a species.

PHYLOGENTIC TREES & CLADOGRAMS

CLICK HERE to read about Phylogenetic Trees and Cladograms on Khan Academy!

Phylogenetic Tree Notes

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