SEL - Selection and Population Genetics [OVERVIEW]

Selection and Population Genetics

Introduction

In this unit, you will learn about the history of evolution as a branch of biology and the scientific evidence that supports this theory. You will examine how groups of species evolve through the five different forces that can cause a change in populations. This unit will also review natural selection and teach you how to use the Hardy Weinberg equation to analyze how populations evolve.
As we begin, let’s take a moment to review the difference between a scientific law, a theory, and a hypothesis.
The hierarchy of scientific knowledge can be described as follows:
  • A hypothesis that is supported by experimental observations may become part of a theory, which is an explanation of scientific phenomena.
  • A theory has been tested many times and is reliable.
  • Bits and pieces of a theory may be revised and updated as new technologies and data emerge
Scientific laws are known to be simple, true, universal, and absolute, they are accepted at face value based upon the fact that they have always been observed to be true. Some scientific laws, or laws of nature, include the law of gravity, Newton's laws of motion, the laws of thermodynamics, and Mendel's Laws regarding inheritance. We are now understanding that laws are not always universal – for example, Newton’s laws of motion do not explain what we see at a subatomic (quantum) level.
The biggest difference between a law and a theory is that a theory is much more complex and dynamic. A law describes a single action, whereas a theory explains an entire group of related phenomena. A theory is developed through the scientific method, meaning it is the final result of a series of rigorous processes. Note that theories do not become laws – they will always remain theories. Scientific laws must exist prior to the start of using the scientific method because laws are the foundation for all science. Very often, a law is mathematical, and a theory is an explanation. You have already learned about lots of theories in this course. 
Here are some common ones:
  • cell theory – theory that things are made of cells and it is the basic unit of life.
  • gravitational theory – theory that it is gravity causing objects to fall toward Earth.
  • germ theory – theory that disease is caused by microorganisms that transmit infection.
In science, the word theory means a well-tested explanation that unifies a broad range of observations. There is a tremendous amount of evidence for a scientific theory. A theory serves as a model that explains something. A theory helps scientists to make predictions about new situations.

List of Lessons

In this module, we will study the following topics:

    1. Natural Selection – this lesson will describe the requirements and results of the process of natural selection.

    2. Evidence for Evolution – this lesson discusses the anatomical, biochemical, and geological data which support evolutionary theory.

    3. Population Genetics – this lesson covers the modern definition of evolution and the five forces that can cause a change in allele frequencies.

    4. MATH: Hardy-Weinberg – this lesson discusses the five requirements for Hardy-Weinberg equilibrium and how to solve the two mathematical. equations

Module Objectives

By the end of this module, students will be able to:

    • Describe the types of data that provide evidence for evolution.
    • Explain how morphological, biochemical, and geological data provide evidence that organisms have changed over time.
    • Describe the fundamental molecular and cellular features shared across all domains of life, which provide evidence of common ancestry
    • Describe structural and functional evidence on cellular and molecular levels that provide evidence for the common ancestry of all eukaryotes.
    • Describe the causes of natural selection.
    • Explain how natural selection affects populations.
    • Describe the importance of phenotypic variation in a population.
    • Explain the relationship between changes in the environment and evolutionary changes in the population.
    • Explain how random occurrences affect the genetic makeup of a population.
    • Describe the role of random processes in the evolution of specific populations.
    • Describe the change in the genetic makeup of a population over time.
    • Describe the conditions under which allele and genotype frequencies will change in populations.
    • Explain the impacts on the population if any of the conditions of Hardy-Weinberg are not met.
    • Explain how humans can affect diversity within a population.

Key Terms

Adaptation ­‐ a trait that makes a living thing better suited for survival in a particular environment.

Biogeography - geographic distribution of species based on continental drift.

Bottleneck effect ­‐ results from a drastic reduction of a population, often caused by a catastrophic event.       

Charles Darwin -1809 - 1882; first developed the basis of evolutionary theory from observations of fossils, living organisms, and characteristics that enhance an organism's chance of survival, and the environment.

Coevolution ­‐ process whereby two species evolve in response to each other over time.

Descent with Modification - organisms come from ancestors that lived in the past and that had various adaptations based on the environment in which they lived. 

Directional Selection - natural selection which favors one extreme or the other. 

Disruptive Selection - natural selection which favors the extreme phenotypes. 

Evolution ­‐ a change in the frequency of an allele in a population over many generations.

Fossils ­‐ remains or traces of organisms provide evidence about the history of life on Earth.

Founder Effect ­‐ results from a small population colonizing an area away from the larger population. 

Gene Flow ­‐ the gain or loss of alleles from a population caused by the immigration or emigration of individuals.

Genetic Drift - refers to evolution occurring through random changes in allele frequency within a gene pool due to chance.

Genetic Variation ­‐ the variety in alleles of genes within and among populations.

Hardy-Weinberg Equilibrium Equation ­‐ simple equation used to discover the probable genotypic frequencies in a population and to track changes from one generation to another.

Homologous Structures – structures that are similar in closely related species due to common ancestry.

Homology - anatomic and or molecular similarities present in an ancestral organism; passed down to descendants.

Microevolution ­‐ the change of allelic frequency in a population over generations.

Mutations ­‐ any change in the nucleotide sequence of an organism's DNA.

Natural Selection ­‐ the concept that individuals in a population exhibit variation in their heritable traits, and those organisms with traits that are best suited for the environment tend to produce more offspring than those that are less well suited.

Species ­‐ a group of organisms whose traits are so similar that they can successfully mate with each other and produce viable, fertile offspring.

Stabilizing Selection - natural selection which favors the intermediate phenotypes.

Vestigial Structures ­‐ remnants of structures once used in extinct organisms are found in extant organisms.

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