AP bio evolution

analogous

• Different structure, similar function 

• Not from a common ancestor 

• Ex. bird wing and insect wing

homologous

• Similar structure, different function

• From common ancestor

• Ex. bat wing and mouse leg

prezygotic barriers

block fertilization from occuring by:

-habitat isolation

-temporal isolation

-behavior isolation

-mechanical isolation

-gametic isolation

habitat isolation

They live in different places, so they don’t run into each other to mate.

Temporal isolation:

They reproduce at different times (different times of day, seasons, or years), so they never mate.

Behavioral isolation

• They act differently when trying to find a mate (like different dances or calls), so they don’t recognize each other as partners

mechanical isolation

Mechanical isolation means two organisms can’t physically mate because their body parts don’t fit together properly.

Gametic Isolation

Sperm of one species may not

be able to fertilize eggs of another species

Postzygotic barriers

prevent the hybrid zygote

from developing into a viable, fertile adult:

Reduced hybrid viability

Reduced hybrid fertility

allopatric speciation,

a population forms a

new species while geographically isolated

from its parent population.

For example, the flightless cormorant of the

Galápagos likely originated from a flying

species on the mainland

sympatric speciation,

a subset of a

population forms a new species without

geographic separation.

Gradualism-

changes occur slowly and steadily

over long periods of time

Microevolution

consists of changes in allele

frequency in a population over time

Speciation

the origin of new species, is at the

focal point of evolutionary theory

Divergent

Two or more species diverge from a common ancestor (ex. elephants and wooly mammoths) 

Convergent

Two or more species share traits NOT due to a common ancestor (ex. birds, bats, butterflies) 

What are the five conditions for non-evolving populations?

1.  No mutations 

2.  No gene flow (no migration)

3.  Random mating 

4. Extremely large population size (no  genetic drift)

5.  No natural selection

Three mechanisms cause allele frequency change:

• Natural selection

• Genetic drift

• Gene flow

Natural Selection

• Differential reproductive success results in certain alleles being passed to the next generation in greater proportions

Genetic Drift

Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next

• The smaller a population, the greater the impact of chance events on allelic frequencies

• (Chance events lead to changes in the gene pool of small populations)

• Genetic drift tends to reduce genetic variation through loss of alleles

The Founder Effect-an example of genetic drift

• The founder effect occurs when a few individuals become isolated from a larger population

• Allele frequencies in the small founder population can be different from those in the larger parent population

The Bottleneck Effect-
an example of genetic drift

• The bottleneck effect is a sudden reduction in population size due to a change in the environment 

• The resulting gene pool may no longer be reflective of the original population’s gene pool

• If the population remains small, it may be further affected by genetic drift9

Effects of Genetic Drift: A Summary

1. Genetic drift is significant in small populations

2. Genetic drift causes allele frequencies to change at random

3. Genetic drift can lead to a loss of genetic variation within populations

4. Genetic drift can cause harmful alleles to become fixed

Gene Flow

• Gene flow consists of the movement of alleles among populations (migration)

• Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)

• Gene flow tends to reduce variation between populations over time

Natural selection is the only mechanism that consistently causes adaptive evolution

• Evolution by natural selection involves both change and “sorting”

  • New genetic variations arise by chance

  • Beneficial alleles are “sorted” and favored by natural selection

• Only natural selection consistently results in adaptive evolution

Reproductive success

• is generally more subtle and depends on many factors

Relative fitness

 is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals

Directional selection

• favors individuals at one end of the phenotypic range

Disruptive selection

• favors individuals at both extremes of the phenotypic range

Stabilizing selection

• favors intermediate variants and acts against extreme phenotypes

• Intrasexual selection

• is competition among individuals of one sex (often males) for mates of the opposite sex

Intersexual selection,

often called mate choice,occurs when individuals of one sex (usually females) are choosy in selecting their mates

gene drift

Genetic drift (random changes in allele frequencies, especially in small populations) usually reduces genetic variation. Over time, alleles can be lost completely by chance, making the population less diverse.

Reduced hybrid viability

is a type of reproductive barrier in evolution where offspring (hybrids) from two different species or populations are weak, unhealthy, or fail to develop properly, so they often die before reaching maturity.

vestigial structure is

is a body part that has lost most or all of its original function through evolution, but is still present in an organism.

Hybrid breakdown

is a type of postzygotic reproductive barrier where:

👉 The first-generation hybrids (F1) are healthy and can reproduce,
but their offspring (F2 or later generations) are weak, sterile, or have reduced survival or fertility.

In simple terms:

• Generation 1: mostly fine ✅

• Generation 2+: problems start ❌

Why it happens: