Unit 7c - Evolution

Population Genetics

Population Genetics

How genes affect populations is called “population genetics.”

Gene Pool = all the alleles from a population.

Allelic Frequency = the % of any allele present.

Genetic Equilibrium = the allelic frequency remains the same through many generations.

Genetic Equilibrium can be disrupted:

Mutations – lethal ones don’t survive. Useful ones give the organism an advantage and they survive (i.e. Galapagos turtles).

Genetic Drift – allelic frequencies are changed by chance events (usually isolation). Ex: Amish communities have increased genes for short limbs and extra fingers.

Gene Flow – the movement of an individual organism into or out of the population.

Three Types of Natural Selection

Disruptive Selection – Both extreme ends of variation are selected for (light or dark, but NOT medium colors).

Stabilizing Selection – favors the average individual (middle colors of camouflage).

Directional Selection – one extreme variation of a trait is selected for (long beaks reach insects).

Speciation

When members of similar species no longer interbreed to produce fertile offspring.

This usually happens in two steps:1. Geographic Isolation – a physical barrier

divides the population.2. Reproductive Isolation – formerly

interbreeding organisms can no longer mate and produce fertile offspring.

Speciation can be FAST or SLOW!

Gradualism – new species originate through gradual (slow) change in adaptations.

Punctuated Equilibrium – new species (speciation) occurs quickly (in spurts) with a long period of equilibrium in between.

Succession

If you didn’t cut your lawn, what would it look like in a year? 10 years? 50 years?

Grass gets taller, weeds grow and a meadow forms.

Bushes grow, trees appear and different animals move in.

Trees and bushes block light, grasses disappear. 30 years later, the area is a forest.

Succession

Succession

Primary Succession = the colonization of barren land by organisms (Ex: after volcano dies)

Pioneer species are the first ones to take hold in the new area. Usually lichens and mosses.

Succession = the orderly natural changes and species replacements that take place in a community. It can take decades or centuries.

Succession

Secondary Succession = is the sequence of changes that takes place after an existing community is disrupted some way. Soil already exists so secondary succession can take less time than primary.

Climax Community = A stable, mature community that undergoes little or no change. It may last for hundreds of years.

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Population Density

Population is determined by using simple addition: total number of organisms + births – deaths.

Population Density – the number of individuals per unit land area (or water volume).

Population Density

Growth Curve – is shown as a “J” shape on a graph

Exponential Growth – rapid growth, usually unrestricted (sometimes a sharper J- curve).

Carrying Capacity

Carrying Capacity – the largest population of a species that a certain environment can support.

These pressures will cause an “S” shaped (stabilized) growth curve.

What limits growth? Food, disease, predators, lack of space are limiting factors

Predator/ Prey Relationships

Predator/Prey Relationships – A growth curve sharply increases and then decreases. Some populations go through repeated and regular periods of boom followed by bust. The numbers of predators and prey cycle closely with each other.

Population Demographics

Demography is the study of population characteristics such as growth rate, age structure and movement of individuals.

Birth Rate is the number of live births per 1000 people in a given year.

Death Rate is the number of deaths per 1000 people in a given year.

To see if a population is growing or declining, subtract the death rate from the birth rate

Population Demographics

Age Structure – refers to the population of certain age groups. Also shows male or female.

http://www.census.gov/ipc/www/idb/ These rates vary among different countries.

Population Demographics

Immigration (In) – People (or animals) moving in…

Emigration (Exit) – People (or animals) moving out…

PGR - Population Growth Rate = I – E to see if a population is growing or declining…