Ch. 16: Evolution of Populations

Ch. 16: Outline

• 16-3: The Process of Speciation– Isolating Mechanisms– Testing Natural Selection in Nature– Speciation in Darwin’s Finches– Studying Evolution Since Darwin

Variation and Gene Pools

• Genes control heritable traits. • Changes in genes produce heritable variation

on which natural selection can operate.

Variation and Gene Pools

• Genetic variations is studied in populations.– Population: Group of species in a specific area

• Gene pool: – All genes (including different alleles) that are in

population.

• Relative frequency:– The Number of times that an allele occurs in a gene

pool, compared to the number of times other alleles for the same gene occur

Sources of Genetic Variation

• Two main sources of genetic variation:– Mutation– Genetic Shuffling (from sexual reproduction)

• Mutation: any change in a sequence of DNA– Caused by a mistake in replication or radiation and

chemicals in the environment– Some mutations are harmless, but others affect an

organism’s fitness• Fitness: The ability to survive and reproduce

Sources of Genetic variation

• Gene shuffling:– MEIOSIS!– Review

• Segregation• Independent Assortment • Crossing-over

– Thus, Sexual reproduction is a major source of genetic variation

Isolating Mechanisms

• Because the definition of a species is a group of organisms that interbreed to produce fertile offspring, the gene pools of two populations must become separated enough to become new species.

• As new species evolve, populations become reproductively isolated from each other.– Reproductive isolation: when the members of two

populations can no longer reproduce with each other.

Isolating Mechanisms

• Three ways for reproductive isolation to occur:– Behavioral Isolation– Geographic Isolation– Temporal Isolation

Isolating Mechanisms

• Behavioral isolation: Two populations have differences in courtship rituals or other reproductive strategies that involve behavior.

• Geographic isolation: Two populations are separated by geographic barriers such as rivers and mountains.

• Temporal isolation: Reproduction occurs at different times of the year.

Concept Map

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Reproductive Isolation

Isolating mechanisms

Behavioral isolation Temporal isolationGeographic isolation

Behavioral differences Different mating timesPhysical separation

Independentlyevolving populations

Formation ofnew species

Testing Natural Selection in Nature

• Darwin’s finches in the Galapagos Islands are the perfect example on natural selection happening in nature.– Natural selection shaped the birds beaks on

each island depending on the food source– Figure 16-16.

Chapter 17

• The History of Life– 17-1: The Fossil Record

• Fossil and Ancient Life• How Fossils Form• Interpreting fossil evidence• Geologic Time Scale

– 17-4: Patterns of Evolution• Extinction• Adaptive Radiation• Convergent Evolution• Coevultion• Gradualisms vs. Puntuated Equilibrium

Fossils and Ancient Life

• Scientist who study fossils are called paleontologists

• Fossil Record: the information about past life, which includes structure of organisms, what they are, and their environment

Fossils and Ancient Life

• The fossil record provides evidence about the history of the Earth.

• The fossil record indicates that species have change over time (evolution). Certain fossils can only be found in older rocks, for example (and are not found in any recent rocks).– Extinct: a species that has died out.

How fossils form

• Most fossils form in sedimentary rock (formed when existing rock is broken down into sand, silt, etc.) Layers of earth build up over time on top of the fossil and some stay preserved.

• Forces inside the earth lift rocks up into mountain ranges and wind and rain erode the rock. Then old fossils are exposed

Extinctions

• Reasons for extinction: competition for resources, changes in the environment

• Mass Extinctions: wipe out entire ecosystems– Possible causes asteroids, volcanoes,

changes in sea level.

Adaptive Radiation

• Adaptive Radiation: the process of one species evolving into diverse forms that live in different ways.– Darwin’s finches had one common ancestor

Convergent Evolution

• Sometimes unrelated organisms evolve to appear similar. The process of this is called convergent evolution.

• It happens when organisms are in the same type of environment. – Ex: Many water animals have fins to help them swim

even if they are totally unrelated (shark and dolphin)

• Analogous structures: structures that are similar in structure and function (but in unrelated species)

Coevolution

• An evolutionary change in one organism can sometimes result in a corresponding change in another organism

• Coevolution: the process by which two species evolve in response to changes in the other over time– Ex. Flowers and pollinators

Gradualisms vs. Punctuated Equilibrium

Modern scientists disagree with Darwin about the speed of evolution.

• Darwin believed that the speed of evolution is a slow, gradual process (gradualism).

• The fossil record indicates that this is true for some species. But other species are produced by short periods of rapid change (also supported in the fossil record).

Gradualisms vs. Punctuated Equilibrium

• Puntuated equilibrium: the pattern of long, stable periods interrupted by brief periods of more rapid change

• What causes punctuated equilibrium?– Migration– Isolation– Mass Extinctions

Developmental Genes and Body Plans

• Puntuated equilibrium: the pattern of long, stable periods interrupted by brief periods of more rapid change

• What causes punctuated equilibrium?– Migration– Isolation– Mass Extinctions