Chapter 12 ~ Forces of Evolutionary Change

How evolution acts on population

• traits are selected for or against over generations

• a population’s genetic makeup changes over time

• mutation, migration, genetic drift, & natural selection drive evolution

• EVOLUTION = change in allele frequencies over time

DARWIN helped shape the development of the theory of evolution by:

developed the theory of natural selection

MALTHUS helped shape the development of the theory of evolution by:

he said population growth is limited by resources → inspired Darwin

CUVIER helped shape the development of the theory of evolution by:

studied fossils which led to conclusion of extinction occurs

VON BAER helped shape the development of the theory of evolution by:

showed embryos of different species are similar early on which concluded to

→ life isn’t a straight series

LYELL helped shape the development of the theory of evolution by:

(a geologist)

proposed Earth changes slowly 

→ supported long-term evolution

All theorist came with the common descent

all species come from common ancestors but have changed over time

Natural selection

process where individuals with beneficial traits survive and reproduce more

• which acts on PHENOTYPES, not genotypes

Artificial selection

selective breeding strategy in which a human allows only organisms with desired traits to reproduce

selective pressures generally not affecting embryos

• temperature

• food

• predators

Allele Frequency formula:

or also know as genotype frequency

allele frequency’s = number of individual of one alleles DIVIDED BY total number of all alleles in population

describe Hardy-Weinberg Equilibrium and the formula

• situation in which allele frequency’s and genotype frequencies do not change from one generation to the next

• describes a non-evolving population

• no mutation, migration, selection, or drift; random mating

FORMULA:

p² + 2pq + q² = 1

p = ?

q = ?

p² = ?

2pq = ?

q² = ?

p = frequency of dominant allele

q = frequency of recessive allele

p² = homozygous dominant

2pq = heterozygous

q² = homozygous recessive

Directional Selection

one extreme phenotype becomes more prevalent in a population

EX. darker moths survive better

Disruptive Selection

form of natural selection in which TWO extreme phenotypes are fittest

• EX. birds with small or large beaks

stabilizing selection

extreme phenotypes are less fit than the optimal intermediate phenotype

• average trait favored

• EX. human birth weight

Sexual Dimorphism

difference in appearance between male and females

• maintained by sexual selection

• traits that help attract mates

• EX. bright feathers in male peacocks

Five agents of Evolution

1) mutation - introduces new alleles

2) gene flow (migration) - movement of alleles between populations

3) genetic drift - random changes (especially in small populations)

4) natural selection - traits that help survival are favored

5) nonrandom mating - some individuals reproduce more than others

Founder effect

genetic drift that occurs when a small group of individuals separates from a larger population to establish a new community

Bottleneck Effect

genetic drift that occurs as a result of a sudden reduction in the size of a population

Selective Pressure

any factor that influences survival

• predators

• food

• weather

Artificial Selection

Humans choose which traits get passed on

• dog breeding

Definition of “common descent with modification”

all species share ancestors but have changed through evolution

Polyallelic

a gene has more than two possible alleles

what makes an offspring “viable” ?

can survive and reproduce

definition of heritable

a trait that can be passed from parents to offspring

definition of gene pool

all the alleles present in a population