The Theory of Evolutionby Natural Selection

Part 2: Natural Selection

7 Points to Remember:

1. Overproduction2. Competition3. Variation4. Adaptation5. Natural Selection6. Inheritance7. Speciation

1. Reproduction

• Every species is capable of producing far more offspring than are needed to maintain a stable population.

• Yet, generally, populations of organisms are stable within an environment.

• What are some ways that species can reproduce?

2. Competition

• Living space and food resources are limited, so offspring from each generation must compete in order to live.

• They compete with each other, and with other species for the resources.

• Only a fraction will survive long enough to reproduce.

• These environmental pressures are what helps keep population sizes stable.

2. Competition - 2

5. Competition - 3

• MEANING: The idea that species adapt and change by natural selection with the best suited mutations becoming dominant

• ORIGIN: This expression is often attributed to Charles Darwin and although it appears in the fifth edition of his Origin of Species (1869), it is there attributed to Herbert Spencer:“The expression often used by Mr. Herbert Spencer of the survival of the fittest is more accurate..”

• Spencer had published The Principles of Biology in1864. In that he referred to ‘survival of the fittest’ twice:

– "This survival of the fittest, implies multiplication of the fittest."– "This survival of the fittest... is that which Mr. Darwin has called 'natural selection, or the

preservation of favoured races in the struggle for life'.“• By 'fittest', of course, Spencer and Darwin did NOT have in mind the

commonly used meaning of the word now, i.e. the most highly trained and physically energetic. The 'fittest' referred to here are those animals which are the most suited to their environment, i.e. those which are best fitted to survive.

““Survival of the Survival of the Fittest”Fittest”

3. Variation

• Individuals of any species vary in their characteristics. These differences are variations within a species.

• What produces these variations?

• What form of reproduction mixes genetic material from two parents?

3. Variation - 2

How do you account for the variation in a zebra’s stripes?

How do you account for the regularity in a zebra’s stripes?

3. Variation - 3

People around the globe have taken advantage of those variations, and selectively bred organisms of domestic plants and animals to accumulate the characteristics they were looking for.There is a big difference between selective breeding and natural selection (can you explain the difference?), but the result is the same – gradual change in characteristics over time.

4. Adaptations

• An adaptation is any characteristic that improves an organisms’ chance of survival and reproduction in its environment.

• What molecule codes for an organism’s characteristics?

• How could that molecule vary from individual to individual?

4. Adaptations - 2

What adaptations make these organisms well-suited to their environments?

4. Adaptations - 3

• What is the difference between:– adapting to one’s

environment and

– having an adaptation to that environment?

5. Natural Selection

• The environment (nature) selects for organisms most suited for each environment by eliminating the unfit. Adaptations that are not suitable for an environment are weeded out.

• Organisms most suited survive & reproduce.

• Selective Pressures– Climate/Weather– Resource availability– Predation– Sexual Selection

Source: http://evolution.berkeley.edu

5. Natural Selection - 2

• The fittest survive to pass on their DNA to the next generation. (“Survival of the fittest”)

• Offspring inherit these better characteristics and as a whole the population improves.

5. Natural Selection - 3

• Individuals do NOT evolve.

• Populations--and therefore species--do.

IMPORTANT POINTS IMPORTANT POINTS #1#1

5. Natural Selection - 4

• Natural Selection does not move in a pre-determined direction!

• Organisms don’t “get” what they “need” to survive.

• Organisms don’t get to choose their DNA or characteristics that would help them survive.

• The changing earth determines what will and can survive.

IMPORTANT POINTS IMPORTANT POINTS #2#2

Source: http://sci.waikato.ac.nz

5. Natural Selection - 5

5. Natural Selection - 6

• The environment (nature) selects for organisms most suited for each environment by eliminating the unfit. Adaptations that are not suitable for an environment are weeded out.

• Organisms most suited survive & reproduce.

• Selective Pressures– Climate/Weather– Resource availability– Predation– Sexual Selection

6. Inheritance

• Those best adapted to their environment will survive to reproduce.

• They pass on their genetic information (DNA) to the next generation.

6. Inheritance - 2

• The environment (nature) selects for organisms most suited for each environment by eliminating the unfit. Adaptations that are not suitable for an environment are weeded out.

• Organisms most suited survive & reproduce.

• Selective Pressures– Climate/Weather– Resource availability– Predation– Sexual Selection

6. Inheritance - 3

• Lamarck thought that a trait acquired during an organisms life could be passed on.– Example: Giraffes that

stretched their necks to reach higher branches for food would pass the longer necks to their offspring.

• We now know this is wrong.

6. Inheritance - 4

6. Inheritance - 5

• Darwin did not know anything about DNA or genes or genetics or inherited characteristics.

• He inferred everything about it from his careful observations from nature.

• Darwin's theory of descent with modification was accepted by most scientists worldwide within ten years of its publication in 1859.

• However, his theory of natural selection was widely criticized, and by the turn of the 20th century was widely considered to be dead.

• However, the work of Gregor Mendel a Czechoslovakian monk, who discovered the foundations of what we now call genetics, provided a mechanism by which Darwin's theory could be revived and expanded.

• Gregor Mendel published his work with peas in 1865 (Six years after Darwin’s Origin of Species)

7. Speciation

• Over many generations, favorable adaptations gradually accumulate in a species and bad ones tend to disappear.

• Eventually, accumulated changes become so great, the result is a new species.

• Formation of a new species is called “Speciation”

• Speciation takes many, many, many generations to occur.

7. Speciation - 2

• Speciation may occur due to:– Geographical isolation– Reproductive isolation– Genetic drift

Genetic Drift

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

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. That, in a nutshell, is genetic drift. It happens to ALL populations—there’s no avoiding the vagaries of chance.