Natural Selection and the Evidence for Evolution Julieanne Quigley 2008

Food for Thought… Learning the principals of evolution make it easier to understand modern biology Charles Darwin had ideas that are supported by fossil evidence and his ideas are now the basis for modern evolutionary thought

Biologists have used fossils since the 1700’s and started forming evolutionary ideas back then They wondered how fossils formed, why they were extinct and how they were related to modern species When geologists provided evidence that earth was billions of years old, biologists began to suspect that life evolves slowly over time

Charles Darwin English scientist- 1809-1882 He took a job at the age of 21 on the HMS Beagle (ship) as a naturalist A naturalist studies and collects biological specimens He sailed to The Galapagos Islands near the equator He started to see evolutionary patterns there

What’s Next? For 22 years, Darwin worked to find an explanation for how species changed over time He came up with the idea of NATURAL SELECTION Natural Selection is: a mechanism for change in populations

The Rules of Natural Selection OVERPRODUCTION-Each species produces more offspring than can survive. Not all offspring can survive because there is not enough food or living space for all

The Rules of Natural Selection COMPETITION There is a struggle for existence. The offspring of each generation compete for things that they need to survive (food, water, shelter, space, a mate). Only a few will live long enough to reproduce, the others will die.

Rules of Natural Selection VARIATION The offspring of each generation are not exactly alike. For example, some organisms are faster or stronger than others. Differences in traits among individuals of a species are called variations

Rules of Natural Selection SURVIVAL OF THE FITTEST Some variations make organisms better suited for survival in their environments. These organisms are more likely to survive and reproduce

Evolution of a New Species Individuals with favorable variations survive and reproduce. They pass their favorable traits to their offspring. Therefore, their offspring are more likely to survive and reproduce in the next generation. Unfavorable variations eventually disappear. In this way, favorable variations remain in the species. Over many generations, these changes can result in the appearance of a new species.

Evidence for Evolution Adaptations- Some organisms have thorns and others do not. Some organisms have distinctive coloring and others do not…these adaptations help in the survival of organisms

Adaptations Mimicry: a structural adaptation that enables one species to resemble another species. A harmless species resembles a harmful species. Predators avoid it

Adaptations Camouflage- an adaptation that enables a species to blend with their environment. This helps them not to get eaten.

Adaptations Physiological Adaptations- changes in an organisms metabolic processes. This happens often in bacteria. When antibiotics are not used properly, bacteria become resistant to treatment.

Other Evidence for Evolution Fossils- Provide information on early life and evolutionary history. It is possible to conclude the ancestors of organisms and when the evolution of that species took place.

Other Evidence for Evolution Homologous Structures: structural features with a common evolutionary origin. The features can be similar in arrangement, function or in both Example: Animal Limbs

Other Evidence for Evolution Analogous structures- Body parts of organisms that do not have common evolutionary origin, but similar function Example: wings of a bird verses wings of an insect

Other Evidence of Evolution Vestigial Structures- a body structure that no longer has a function in a present day organism, but was probably useful to an ancestor Example: Eyes of blind mole rats or cave fish (the animals are blind but still have eyes) or wings on flightless birds

Other Evidence for Evolution Embryology- Looking at the embryo (developing baby) at the earliest stages of growth. The embryos of fish, reptiles, mammals and birds are all very similar. All start forming with gill slits and tails! They develop into different things.

Other Evidence for Evolution Biochemistry- reveals information about relationships between individuals and species by comparing the DNA and RNA

Mechanisms of Evolution Population Genetics- populations of organisms evolve, not individuals. A population is all the members of a species that live in a specific area A populations genes can change over time…

Population Genetics Gene Pool- All of the alleles (gene choices) in a specific population Allelic Frequency- The percentage of a specific allele or gene shows up in that population Genetic Equilibrium- A population whose frequency of alleles or genes stays the same over generations

Population Genetics A population that is in genetic equilibrium is not evolving. Genetic Drift- A mechanism that disrupts a populations genetic equilibrium. This can cause an increase in rare alleles. Example: Small Amish population in Pennsylvania often have 6 fingered offspring

Natural Selection Acts on Variations Variations increase or decrease an organisms chance for survival. If a variation is helpful, it is passed on to offspring. If it is harmful, the organism dies and does not pass on the variation/trait to its offspring.

Different Types of Natural Selection Different types of natural selection act over the range of a trait’s variation. PLEASE OPEN YOUR BOOK TO PAGE 416

Stabilizing Selection Natural selection that favors the average individual. Consider a population of spiders where average size is the survival advantage. Too big and birds can see them, too small and they can’t find food. In this environment the average sized spider is more likely to survive and pass on its genes

Directional Selection When natural selection favors one of the extreme variations of a trait A population of woodpeckers feed on insects living deep in the bark. Only woodpeckers with the longest beaks could feed on that insect. Long beaks birds would survive better than average or small beaked birds.

Disruptive Selection Natural selection that favors individuals with either extreme of a trait’s variation are selected for. For example- a population of clams with shells of white, tan and dark brown live on a beach. All the rocks on the beach are either very light or very dark. All the light clams survive on the light rocks, and all the dark clams survive on the dark rocks. So it is a strong disadvantage to be tan. They can be seen on both the light and dark rocks.

The evolution of the species Speciation-The evolution of new species. It occurs when members of similar population no longer produce fertile offspring within their natural environment Species- group of organisms who look similar and can interbreed.

Causes of Speciation Geographic Isolation- when a physical barrier (water, mountain, city) divides a population. A new species can evolve when a population has been isolated if the conditions are different

Causes of Speciation Reproductive Isolation- When formally interbreeding organisms can no longer mate and produce fertile offspring. Example: one population of tree frog mates in the fall, the other in the spring. These two populations will not mate with each other.

Causes of Speciation Changes in the chromosome numbers can cause speciation in plants. When a mistake happens in meiosis it can result in a polyploid individual (too many chromosomes). This often creates bigger fruit and flowers in plants. If this happens speciation might occur.

Speciation can occur quickly or gradually… Gradualism- a new species originates out of a slow gradual change of adaptations. Such as camels. They evolved very slowly over millions of years. Punctuated Equilibrium- Species occur rapidly in bursts with long periods of genetic equilibrium in between. This usually happens when the environment changes. A new species can form in 5,000-10,000 years

Patterns of Evolution Biologists have observed different patterns of evolution that occur throughout the world in different natural environments. These patterns support the idea that natural selection is an important agent for evolution

Adaptive Radiation When an ancestral species evolves into an array of species to fit a number if diverse habitats. Example: Hawaiian Honeycreeper birds- Differ in beak shape and color depending on the flowers of the area

Divergent Evolution Pattern of evolution when species that once were similar to each other become increasingly different

Convergent Evolution A pattern of evolution in which distantly related organisms evolve similar traits. It happens when unrelated organisms share similar environmental pressures, they share similar natural selection