Lecture AP Biology Chapter 23 The evolution of populations

In this chapter, you should be able to: Explain why the majority of point mutations are harmless; explain how In this chapter, you should be able to: Explain why the majority of point mutations are harmless; explain how In this chapter, you should be able to: Explain why the majority of point mutations are harmless; explain how

Ch 22/23 Warm-up

1 List 5 different pieces of evidence for

evolution

2 (Review) What are the 3 ways that

sexual reproduction produces genetic diversity?

3 What is 1 thing you are grateful for

today?

Ch 23 Warm-up

1 In a population of 200 mice, 98 are homozygous

dominant for brown coat color (BB), 84 are heterozygous (Bb), and 18 are homozygous (bb).

a) The allele frequencies of this population are:

B allele: _ b allele: _

b) The genotype frequencies are:

BB: _ Bb: _ bb: _

2 Use the above info to determine the genotype

frequencies of the next generation:

B (p): _ b (q): _

BB (p 2 ): _ Bb (2pq): _

bb (q 2 ): _

Chapter 23

The Evolution of

Populations

What you must know:

• How mutation and sexual reproduction

each produce genetic variation

• The conditions for Hardy-Weinberg

equilibrium

• How to use the Hardy-Weinburg equation

to calculate allelic frequencies and to test whether a population is evolving

Smallest unit of evolution

Microevolution: change in the allele frequencies of a population over

generations

• Darwin did not know how

organisms passed traits to offspring

• 1866 - Mendel published his paper on genetics

• Mendelian genetics supports Darwin’s theory  Evolution

is based on genetic

variation

Sources of Genetic Variation

• Point mutations: changes in one base (eg sickle cell)

• Chromosomal mutations: delete, duplicate, disrupt, rearrange  usually harmful

• Sexual recombination: contributes to most

of genetic variation in a population

1 Crossing Over (Meiosis – Prophase I)

2 Independent Assortment of Chromosomes (during meiosis)

3 Random Fertilization (sperm + egg)

Population genetics : study of how populations change genetically over time

Population : group of individuals that live in the same area and interbreed, producing fertile offspring

• Gene pool: all of the alleles for all genes

in all the members of the population

• Diploid species: 2 alleles for a gene

Hardy-Weinberg Principle

genotype frequencies of a population will

remain constant from generation to

generation

…UNLESS they are acted upon by forces

other than Mendelian segregation and

recombination of alleles

Equilibrium = allele and genotype

frequencies remain constant

Conditions for Hardy-Weinberg

Allele Frequencies:

• Gene with 2 alleles : p, q

p = frequency of dominant allele (A)

q = frequency of recessive allele (a)

Note:

1 – p = q

1 – q = p

Allele

frequencies

Genotypic frequencies

Strategies for solving H-W Problems:

1. If you are given the genotypes (AA, Aa, aa),

calculate p and q by adding up the total # of

A and a alleles.

2. If you know phenotypes, then use “aa” to

find q 2 , and then q (p = 1-q)

3 Use p 2 + 2pq + q 2 to find genotype

frequencies.

4 If p and q are not constant from generation

to generation, then the POPULATION IS

EVOLVING!

Hardy-weinberg practice problem #1

The scarlet tiger moth has the following genotypes

Calculate the allele and genotype frequencies (%) for a population of 1612 moths.

Hardy-weinberg practice problem #2:

Causes of evolution

Conditions for Hardy-Weinberg

Minor Causes of Evolution:

Major Causes of Evolution:

• Natural selection, genetic drift, gene flow

(#3-5)

Major Causes of Evolution

#3 – Natural Selection

• Individuals with variations better suited

to environment pass more alleles to next generation

Major Causes of Evolution

Genetic Drift

Founder Effect

Polydactyly in Amish

population

Major Causes of Evolution

How does natural selection bring about

adaptive evolution?

Natural selection can alter frequency

distribution of heritable traits in 3 ways:

1.Directional selection

2.Disruptive (diversifying) selection

3.Stabilizing selection

Directional Selection:

eg larger black bears

survive extreme cold

better than small ones

Disruptive Selection:

eg small beaks for small seeds; large beaks for large seeds

Stabilizing Selection:

eg narrow range of human birth weight

Sexual selection

M compete with other M)

showy M)

Preserving genetic variation

Running Time: 14:03 minHHMI Video:

Natural Selection in Humans

Natural selection cannot fashion

perfect organisms.

1 Selection can act only on existing variations

2 Evolution is limited by historical constraints

3 Adaptations are often compromises

4 Chance, natural selection, and the

environment interact

Sample Problem

Define the following examples as directional, disruptive, or stabilizing selection:

represent a species distasteful to birds

frequently than drab birds of same species

over time